Electromagnetic Testing Classroom Training Book: Publications

The ASN T PERSONNEl TRAININ

PUBLICATIONS

ELECTROMAGNETIC TESTING CLASSROOM TRAINING BOOK

The American

for Nondestructive

T he ASNT PE RSONN EL T RAIN I NG

PUBLICATIONS

ELECTROMAGNETIC TESTING CLASSR()OM TRAINING BOOK

Written for ASNT by Hussein Sadek .hnologies Consulting International, Inc.

The American

for Nondestructive Testing, Inc.

Published by The American Society for Nondestructive Testing, Inc. 1711 Arlingate Lane Columbus, OH 43228-0518 Copyright © 2006 by The American Society for Nondestructive Testing , Inc. All rights reserved . ASNT is nm responsible for the authenticity or accuracy of infoml1.tt ion herein, and puhlished opinions or statements do nOl necessaril y reflect the opinion of ASNT. Products or se.rvices that are advertised or ment ioned do nut carry the endorsement or re<.:olIunendation or ASl\'T.

lRRSP. Materials EvaluatiulI, NDT Handbook . Nondestructive Tl~Stil'lg Handbook. The NDT Tech nician and <www.asnt.org> are trademark s of The American Society for Nondestructive Testing. Inc. ACCt>. ASNT. Level III Study Guide, Research in NondestrZl ctive Evaluation and RNJ)£ are registered trademarks of The American Soc iety for Nondes tructive Testing. lnc.

ASNT exiS I ~ to create a safer world by promoting the

profe~~ion

and technologies of llondestnJctive testing .

rSBN-IO: 1-57117- 122-3 ISBN-13: 978-1-57117-122-1 Printed in the United States of Ame rica Library of Congress Cataloging- in- Publication Data

Sadek, Hussein. Electromagnetic testing class room tra ining book I written for ASNT by p. cm. -- (Personnel training publications serie s) Includes bibliographical references and index.

Hussein Sadek.

ISBN-I O: 1-571 17- 122-3 ISBN-13: 978-1-57117-122-1 1. Electromagnetic testing. 2. Eddy current testing. 1. American Society for Nondestructive Testing.li. Title. TA4I 7.J.S23 2006 620. 1' 127--dc22 2006039336

First printing D ecem ber 2006

ii

Acknowledgments A special Lhan k YOll goes to the fo llowing technical editor who helped with this pub lication:

Dave Russe ll. Russell NDE Systems, Tne.

A special th ank you goes to the following reviewers who helped wi th this publ icati on: Ri ck Cah ill. GE inspection Technologies Jim Cox, Zetec Nat Faransso . Kellogg, Brow n & Root. Inc. Darrell Harris, ASRC Energy Services Don Locke, Karta Technologies Michael McGloin. Hellie r Mike Mester, Consullant Alan Pardini, PacHk North west National Laboratory Frank SaltIer, Sattier Consultants. Inc. Roderick Stan ley, NDE infom, .. ion Cons ultants A.M. Wcnzig, Jr., Industrial Testing Laboratory Services. LLC

The Publications Review Committee includes: Chair, Sharon 1. Vukelich, University of Dayton Research Tnst itute Mark A. Randig. Cooperhcat-MQS.lnc. Joe Mackin , lnlernalional Pipe inspectors Assoc iat ion

Ann E. Spence Educational Materials Editor

iii

Foreword The American Soc iety for Nondes tructive Testing, Inc . (AS NT) has prepared thi s series of Personnel Train ing Publicalivm to present the major areas in each nondestructive lesting method. Each classroom training book in the series is organi zed to fo llow the Recommended Training Course Outl ines found in Recommended Practice No. SNT-TC- I A . The Level l and Level 11 candidates should use [his classroom training book as a preparation tool for nondestructive testing certification. An ASNT NDT Level Tor Level 11 may he ex.pected to know additional information based on industry or employer req uirements.

iv

Table of Contents Acknowledgments .............. .. .... . ..... . .. . . ... . ......... . ..... .iii Foreword Table of Contents . ... .. ............ ... . . .... .. ........ . .... . .. .... ... v

Level I Electromagnetic Testing ................. . ....... . ......... 1 Chapter 1 - Introduction to Electromagnetic Testing ... . . ....... . ......... 3 Early Observations of Magnetic Attraction ..... . ....... . .. . .. . . . ... . ..... 3 Development of Induced Currents ...... . ............. . . . ....... . .... 3 Oersted's Discovery ............................... . ...... . ... . ... .4 Faraday's Law of Electromagnetic Induction ......... . ....... . . . ....... 5 James Clerk Maxwell . .................... . ... . .. . .... ..... . ..... . .5 Pried rich Forster ............................. . . ........... . ..... .. 6 Basic Principles of Eddy Current Testing ..... ........ ...... . ....... . .... 6 Principles of Flux Leakage Testing ........ . ...... . .... . ................. 7 Personnel Qualification ............................. . . . .. . ........... 8 Levels of Qualification . . . . . . . . ..... . ....... . .. . ....... . .... 9 Qualification for Level I ..... . .... . .. . ..... . . . .. . . . . .. . ........... 9 Qualification for Level IT ..... . .. ... .. . .. . .... . .. . .... .. ... .. . ... .9 Qualification for Level III . ....... ... . ............... 10 Challenges .................. . .... . .. . .. ... . . . .... . ...... . . . .... 10 Personnel Certification ........... .. . . .. . ...... .. . . .......... . .. . ... 10 Chapter 2 - Eddy Current Theory ... .... . ... . ............ . . . . .. . .. . . .. 13 Generation of Eddy Currents .... ... ....... . ..... . . .. ...... . ......... 13 Electromotive Force ...... .. .... . . . ....... . .... . ....... . .... . .... . .. 14 Resistance ....................................... . ... .. . . .. .. . .. . .15 Alternating Current ............. . ........... . ....... . .. .. . .. . .... 16 Sine Wave ............... . . .. ...... . . . . .... . ....... . . . .......... 16 Frequency .......... .. .. .. .... . . . . . .. .......... . ....... . . ....... 17 Self Inductance .................. . ....... . . . . .. .. . ....... . . . . .... 17 Inductive Reactance ....... . .... . ... . .. . . . .. . .. . .... . ...... . . ...... .18 Impedance ............. . . . ...... .. ....... .. ...... . .... . ....... . .. 18 Arctan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . ............ 20 Resistance ............. . .... . . .. .. ... . .. .. ..... . ................ 20 Inductive Reactance ............................. . . .... .. .. .. .. . .. 20 Chapter 3 - Eddy Current Instrumentation ..... .. . . .................... 21 Introduction ....... . . ... ............ ... ..... . . . . . . .. . .. . .... . ..... 21 Eddy Current Instrument Circuits .......................... . .... . . . ... 21 Impedance Bridge Circuit (Wheatstone Bridge) ... .... ... ..... .... .. ... 21 Internal Functions of Eddy Current Instrumentation .... . ....... . ..... .. .23 Signal Excitation .. .. ........................... . ....... . .. . ..... 23 v

Signal Modulation .............. . ..... . ..... . . . ........ .. ........ 23 Signal Preparation . . . . ..... .. ...... .. ... . .......... . .... . . .. ..... 24 Signal Demodulation and Analysis .... .. .......... .. .......... . ..... 24 Signal Display ... . ... . .... . ..... . . . . . . ......... . .... . .... ... . ... .24 Signal-to-Noise ratio ... .... ... . . . ......... . ... .. . . . . . ... ... . ...... .24 Improving Signal-to-Noise Ratio ................................... .24 Chapter 4 - Readout Mechanisms ......... . ........... ... .. . .. . . . .. . .. 25 Introduction ......... ............ . .... ..... , ...... .. __ . _ . ......... 25 Analog Meters ............. .. . ...... .......... .. . ........ . . . . ..... 25 Audio Alarms ....... , ....... . .... , .. . ........ .. .. .. .. .... . . . . ... 25 Strip-Chart Recorders. , . ... , ...... . , .............. _.. _. _.......... 25 Digital Displays . ... . . . ... .. , . . .. . .. , ............ . ... . . . . .... . . .... 26 Cathode Ray Tubes ........................ , . ......... .. . . .. ..... .26 Digital Data Storage ..... . . ..... . ..... . .. . . .. .. . ................ . .26 Digital Mixing .... . .... . .... . .. . .... _ ............... . .... . . . . ... 26 Liquid Crystal Display ... ....................... ..... ............ .27 Chapter 5 - Eddy Current Sensing Elements .... ...... .. . . .... . . . .. . ... .29 Introduction ......................... . .. .. .... . .. , .............. .. 29 Surface Coil . ... . ....... , .. ... ... . ...... . .......... .. .. . . . . . .... 29 Applications ... ... . .................... .. . .......... . .. .... . .. 30 Encircling Coil ........... . . .. ... . . . . . . . ...... _........ ... .... . .. 31 Applications ... . . .. .. ................... . . . .. . ... .. . . ... . ..... 32 Internal Co il .......... . . . ........... . ........................... 32 Applications ..... . , ... . , . ..... . ..... . .. . ... . ....... .. .... . .... 33 Test Coil Arrangements . .. ... , . ...... , .... .. ..... . . . . . . . ... ......... 33 Single Co il (Absolute Arrangement) .. , ... .. ................... . . . ... 33 Double Coil (Absolute Arrangement) ., .... , . ... .. ................... 33 Differential Coil (Self-Comparison Technique) ............. . ........... :14 Differential Coil (External Comparison Technique) ............. . ....... 35 Hybrid Coil Arrangements (Th rough Transm ission) ..... . . . _ .. .. ..... _.. 35 Factors Affecting Choice of Sensing Elements .... . .. . ................ . .. 36 Frequency ........... . . . ....... . ........... . .. .. ...... _.... . ... .36 Excitation ..... . .... _.. ____ ...... .. ....... . ........... ..... ..... 36 Gain Linearity ... . ... . ........ _........... . . .... . . ... . . . ..... . .. :16 Horizontal and Verti cal Deviation .. . . ............ , . ..... , .. . ..... . . .36 Quad ratu re Accuracy . .... ... ................ . .... . ... . ... . ... .... 37 Digitization Rate ... . ..... , . ...... , ....... , ..... . .. . .. . . . . . . . . ... 37 Sample Rate , ...... . , .... , . . .......... , .... _ .... __ . __ ........... 37 Bandwidth ....... .. .... . . . ... ... , .... , . .. . ... . . . . .. . . .. ........ 3 7 Chapter 6 - Flux Leakage Theory ...... . , .... , ....... . _. _. . _........... 39 Introduction ... . ... , . ...... .. ..... . .... . , .............. . . . . . . ..... 39 Band H Curve ..... , . ...... .. .................. .. ... . ............. 40 Lines of force ...... _...... . .... . ............. . .. ............... . .. 42 Law of Magnetism ...... . ....................... .. .. . .. .... .. . ..... 42 Flux Density ... . ............ . . .. .... . . . .................. . ..... . .. 43 Right Hand Rule . .. .. . . .. . . . .... .. .... ... .. .. .... . . .. .. . . . .. . . .... .43 vi

Personnel TrainiuJ!, Pllblicaf;ol1.\·

Magnetic Properties of Materials ...... .. . .... . .. . ........... .. ... .. ... 44 Magnetic Domains .. .. .... .. ..... . . . ... ..... .. . . . . .. . . .. . ..... .. .44 Magnetic Hysteresis ..... . ... .. . . ..... .. ........ . .. . .... .. ... . ... .45 Magnetic Permeability .................... . .. . .... ..... ... . .. ... .. 47 Chapter 7 - Flux Leakage Sensing Elements ...... ... ..... ... .. .. ... .. . . .49 Inductive Coil Sensors .... .... .... .. ......... . ... . ... . ...... .. . . ... .49 Hall Effect Sensors ............. .. . .. ..... ... . ....... . . .. ... . .. . .. 50 Flux Gate Magnetometer . . .. .. .. .. .... ... ... . . ... . ... . . ...... .. ... 51 Magnetodiode .. . ... . . . . . . ... . . ......... . .. . . . . . . ...... . .. ....... .5 1 ApplicatiOns of Magnetodiodes ......................... . . . . .. . . . . .. 53 Other Methods of Magnetic Leakage Field Detection ... . .. . .... .. . . .... .. 53 Magnetic Tape System .... . ................ . . . .... ... . .. ..... . . . . .53 Magnetic Particles .. .. .. .. . . . . .. . . . ..... .. .. ... . .... . . ..... ..... .54 Magnetic Resonance Sensors ........ . ... ... . . .. .. ... .. . .. .... . . . ... 54

Level II Electromagnetic Testing . . . .... .. .... .. . .... . .. .. ... . . . . .55 Chapter 8 - Coil Impedance ........... . ......... ... . ... . . . . . ......... 57 Test Object ... . ... ... ... . .. .. .... . ..... . ..... .. .. . .... . . .. . . .... .. 57 Conductivity ........ .... .................. . ................ . . . .57 Factors Affecting Conductivity ... . ..... . . .... . ..... . . .. . . . .... ... 58 Alloy Composition .... ............ . . . . .. . . . ...... .. .. ....... . ... .58 Hardness ....................................... .. ... .... .... ... 59 Temperature and Residual Stresses ....... .. ... . . . ... . . . . . . .. . ....... 60 Conductivity Coatings . . ....... . .... ... ....... . . . ... ... ... . . . . .... 60 Edge Effects . . .... . ... .. .. . . ....... . ............ . .... .. . . .... .. .60 Skin Effect . .. . . .... . .. ... . . ..... ........... . ........ . .. . . .... ... 61 End Effect ......... ... ..... .. ... .. .. . . . . .. ... . . ......... . .. . .... 62 Permea bility Factors .. . . . . ... . ... ... ... . ........ . . . ... . ............. 62 Dimensional Factors . ....... . ............. ... . . .... . . . ... . . .. . ..... 62 Test Object Shape an d Thickn ess . ... ...... . . . .. .. ....... . ... . . .... .. 63 Discontinuities ..... . ... ... .. .. .......... .. .. ... .. . . . ... . . . . ....... 63 Chapter 9 - Eddy Current Test Systems and Analysis .. . .. . .... . .. . . .... . .65 Impedance Test ing Systems .... ........ . . . . . .. .... ...... .. . . ......... 65 Phase Analysis Systems ........... .. .... .. ........ . . . ..... . . .. . .. ... 65 Conductivity on the Impedance·Plane Diagram .. ..................... 65 Effect of Frequency on Impedance-Plane Diagram ..... . . .... . ....... . . .68 Effect of Material Thickness .. .. ... ..... ... ..... . .......... .. .. . .... 70 Effect of Frequency on Thickness Measurements ... . . .. .... .. ...... .. . . 71 Suppression of Nonrelevant Variables .... . . . .. . ...... . ... . ..... .... . . 72 Suppression of the Lift-Off Variable .. .... ... . .. .... . .. ....... . ...... 72 Suppression of the Conductivity Variable . . ........ . . .. .. . .... ..... . . . 73 Conductivity and Permeability ........... .. . . ... .. . .. . . . .. . . .. . .... 74 Cathode Ray Tube Methods .. ... . ....... . .... . . . .... .. . . . .. . . . ..... . 79 Cath ode Ray Tube Vector Point Meth od .. .... . . . . . . . . . . . .. ... . . . . . . . . 79 Cathode Ray Tube Ellipse Display Meth od .. .. . .. . . . .. .. . . ... .. .. .. ... 80 Modulation Analysis ... . . . . . ... .. ....... . . .. .. . . . . . . . ..... .. ....... 80

Classroom Training Series: Electromagnetic Testing

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Chapter 10 - Selection of Test Frequency ......... . .. ... .. .. . . . . . .. .. .. 83 Test Frequency .... . . .. ... .. . ...... . ... . .. . .. . . .... ....... . . .... ... 83 Depth of Pen etrat ion ... ... ......................... ..... . . ... . ..... 83 Single Frequency Systems .... . ..... ...... . ... . .. . .. . . . . . . . . . . . .... 86 Multi-Frequency Systems . ... .. . . . . .. . . . . ... . . . ...... . ............. 86 Chapter 11 - Coupling . .. . ..... . . ... . . . . ... ... . . ... . . .. .. . . . .... ... .89 Lift-Off and Fill f actor . .. . .. . . . .. . . . .. . . .... .... ... ... . .. . . ... .. . . . .89 Lift-Off .......... . . . .. . ....... . .. . ......... ... . .. .. . . . .. . . ... .. 89 till factor .... ... ...... . ..... . ... .... .. . . . .... .. .. . . . ...... ..... 89 Calculation of Fill Factor .. .......... .... .. . . . . ... . . ... . ..... .... 90 Chapter 12 - Electromagnetic Testing Applications .. . .. . .. . .. . . . .... .... 91 Eddy Current Applications . ... .. ...... . ..... . .. .. .. . . . . ............ .91 Aerospace Applications . . ... . ....... . .... . ..... . .... ....... .... . .. 9 1 Meas urem ent of Metal and Coating Thickness ...... . .... . .. . .. . ... . .91 Reference Standards for Thickness Testing ....... . ..... . .... ... ... . . . ... 92 Metal Thickness . .. . . . ........ .. ....... ... . .... ..... .. .. . . . ...... 92 Conductive Coating Th ickn ess ................... . . ...... . ..... ... .92 Metal Spacing ..... . . .... . . ... . . .... .. ... .. ......... . . . . . ... . .... 92 Tests of Metal Conductivity ........... . . .. . .. ... ... ... . .. . ... . . ...... 93 Testing of Bolt Holes .... . ... .. .. ..... . ....... . . . . .. ....... .. . . . . .93 Testing of Aircraft Structures ... .. . . .. . . . .. .. ... . ....... . .. . . . ..... .95 Testing of Jet Engines . .................. . ... .. .. . . .. .. ......... .95 Surface Tests .. . . ..... . . . ...... . . . ... .. ... . .. ....... .. .. . ...... ... .9S Chemical and Petroleum ApplicatiOns ... ... .. ...... .. .. ... . . .. . . .... .. 96 Electric Power Appli cations . . ....... ... . ... .. . . .. ..... . . .. ....... .. . .96 Steam Generators ... .... ... . ..... ... . . ..... . .... . .............. .. 96 Balance-of-Plant Heat Exchangers .... .... .... . ..... . .... . . .. ... . . . .. 97 Industrial Air Condition ing Chi llers Applications .... . .......... . ...... 97 Material Sorting Related to Co nductivity ...... . . . .... ... . . .. ... ...... 98 Electromagnetic Testing in Primary Metals Industries .... . . .. . . . .... . . ... .99 Testing of Hot Rolled Bars .. . . . . . . . . . . . . . ...... . ...... . ...... . . .. .99 Testing of Square Billets .. .. ....... . ... . .... . ........ . . . ........... 99 Testing of Hot Steel Rods and Wires . .... . .... ... . ... .. ..... . .. . . ... 100 Chapter 13 - Factors Affecting flux Leakage Fields .... .......... .. . . . . . .101 Defect Geometry, Location and Orientation ....... . .. . .. . . . . .. ........ 101 Subsurface Discontinuities ...... . .. . . . . . . . ... . . .. .. .... ..... . ... . . 102 Degree of Initial Magn etization ..... . .................... . . . . . ..... 103 Chapter 14 - Selection of Magnetization Method .... .. . . . . ... .. .... . .. .105 Introduct ion ... .. . . . . ............... .. . ... ... ..... .. .. . .. . . .. . . . .105 Permanent Magnets ..... . ....... . . . . ..... . . . . . . . . . . .. .. ..... . .. .105 Electromagnets . . ... . ........ . .. . ..... .. ............. .. .... . . . . . 105 Magnetizing Coils ...... ... . ... . .. .. ............ . . . . . .. . ... . . . . .106 Testing in Residual Field ... . . . ..... ... . . . .. . ....... . ....... . ... 107 Magnetizing by Direct Current ...... . ........ ... .... ...... ......... 107 Magnitudes of Magnetic Flux Leakage fi eld s ... . . ... . .. . . .. . . . . . . . . .. .. 108 viii

Persunnel Training Publica/ions

Chapter IS - Flux Leakage Applications ............... .. . . ...... . ..... 109 Introduction .... . ............ ... . .. ............ . ...... . .......... 109 Heat Exchanger Tubing Applications ........ . ....... . .......... ... . 109 Wire Rope lnspection ............. . .............................. 110 Round Bars and Tubes .......... ... .... .. ...... . ........ .. . .. .... 111 Petroleum and Gas Pipelines ... . ............................. . .... 111 Billets .. ..... ..... . .......... . ........... . . ......... .. . . .... .. 112 Installed Heat Exchanger and Boiler Tubes . ..................... . .... 113 Above-Ground Storage Tank Floors ...... .. .. . . . ... ... .. ............ 113 Chapter 16 - Remote Field Testing ............. . ........... .... ....... 1l5 History . .... . . ....... . ................. .. . ........ . .. . .. . .... . . . 1l 5 lnstrumentation ............... .... .. . .... ... . . . . . ... ... .... . . .. .. 115 Probe Configuration .......... .. . ..... . .. . ....... . .............. 116 Effects of Probe Speed ...... ... ......... . .. .. .. . .. ... .. . .... . .... 116 Frequency Selection . ....... ... ... ........ . ....... .. .. . ...... . ... 1l 7 Features of Remote Field Testing ..... . ............... . ............... 1l7 Applications .................... . ...... . .. . . ..... . . . . ... . .. . . . . 117 Sensitivity ............... . . .. ......... ... ....... . .............. 117 Signal Analysis and Data Presentati on .. .... . . ... ..... ... . .. . ..... . .117 Reference Standards ............. ....... ......... . ...... . .... . . .... 120 Chapter 17 - Alternating Current Field Measurement . ... . ....... . ...... 121 History ............ . . . . . .. . . .. ..... .... ...... ................... 121 Principle of Operation .... . .......................... . ............. 121 Probe Configurati on ... . ....... . .... . .. . .......... ... . . ......... 123 Advantages and Disadvantages .... .. . . ........... . ....... . ... . . . .... 123 Advantages ............. . ............ . . . . . . . . .. . ..... .. ....... .124 Disadvantages .. . . ... ... .......... . ..... . . . .... . . ... .. . .. . . ... . . 124 Alternating Current field Measurement lndications .. . . . . .... • .......... 124 I-'atigue Cracks ... ... .......................... .. ........ . . . ... 124 Stress Corrosion Cracking ... .. .. . .. . ........... .. ....... . . . .. .... 124 Hydrogen Induced Cracking . . ...... . ..... . .. .. . . ......... ........ 125 fatigue Cracks in Rail Heads ....... .. .. . ...... . .. .. . ... ...... . ... 125 CorrOSion Pitting .... ... . ... . ..... . ..... . .. .. ................ . . 125 Chapter 18 - Electromagnetic Testing Standards and Procedures ...... . ... 127 Introduction ......... . .......... .. ... . .... . .. . ................... 127 Calibration Standards ..... . ........ . . . . . . . ...... . ..... . ...... .... . 127 Reference Standards .. . . . ..... . ......... ... . .. ... .... . . ......... . .. 128 Conductivity Reference Standards .......... . ........ .. ............. 128 Coating Thickness Reference Standard s .. . .. .. ........ . ... . .. . ... . . .129 Discontinuity Reference Standards ............. . .. . ......... . . . .... 129 Natural Discontinuity Reference Standards .................. . . . . ... 129 Artificial Discontinuity Reference Standards ..... ... ........... . . . . . 130 Lift-Off Reference Standards ......... . ... . .. . ... .... . .. . . . ...... 130 Sorting Reference Standards ........ . ................... .. ....... 13 1 Standards and Specifications .......... . . .. . . .. . ....... . . . . .... .. .... 132 Standards and Industry Specifications . ...... .. .. . ........... ....... 132 Classroo/}/ 'Jj-ainin g Series : Electromagnetic Testing

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The American Society for Testing Materials (ASTM) .. ... . ... ......... 132 American Society of Mechanical Engineers (ASME) .. . .. ... .......... 133 Military Standards (MIL-STD) ...... . ............... . . ... . . .... .. 133 Glossary ............ . .... ...... .... . ...... . ....... . ............... 135 Appendix - Units of Measure for Electromagnetic Testing ... .. . .. . ....... 145 Bibliography . . . .. . .. . ...... . ....... . .. . ....... . . ... .. .. . . . .. ...... 148 Index . ... ........... .. . . . . .. . .. . .... . . . ............ .. ........... 149

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Persollnel Training Publications

The ASNT PERSONNEL TRAININ

PUBLICATIONS

LEVEL

I

1

Chapter 1

Introduction to Electromagnetic Testing EARLY OBSERVAnONS OF MAGNETIC ATTRA CTION Electromagnetic tesling is one of the oldest nondestructive testing method. Thales of Mi1et us (GOO S .C.E.) first recorded th at rubb ing amber induced a state in which the ambe r would aUract other light objects. The Greek word for amber is electron. Thales also mentioned the remarkable powers of lodestone (iron oxide) . also known as magnetite . Democritu s (400 S .C.E.) provided concepts of an alomic structure or matter. Demoeritus' fifth principle states that , "varieties of all things depend on the varieties of their ato ms, in number, size and aggregatioll." Many electromagnetic lests intend to identify the specific atoms in materials and the discontinuities that occur in these materi als when needed atoms are missing or separated from their neighbors. By 1200 A.D., the use of the magnetic compass was reported in China. At about the same time, Englishman Alexander Neckam also repOIied the use of the compass in navigation. Tn the year 1600 , the physician William Gilbert wrote a com prehe nsive descrip tion of his 18 years of expel;ments and hi s theory of magnctism in the book De Magnete.

Development of Induced Currents Electromagnetic induction had not been ob~erved nor explained before the nineteenth century. James Clerk Maxwell, shown in

Figure 1.1, sUlllmarized the first 50 years of e.lectromagnetism in the 011 Electricity and Magn etism.

book, A Treatise

Figure 1.1: James Clerk Maxwell.

3

Oersted's Discovery Max well explained that. conjectures had becn made as to the relationship between magnetism and electricity, but the laws of these relationships remained entirely unknown. Hans Ch ristian Oersted (shown in Figure 1.2) observed that a wire connecting the ends of a voltaic hattery affected a compass in its vicinity. In hi s published account in 1820, he explained that the current itself was the cause of the action. and that the electric conflict acts in a revolving manner; that is, a compass placed near a wire transmitting an electric current tends to set itself perpendicular to the wire, and always points toward the wire as the compass is moved around the wire. The space in which these forces act may therefore be cons idered a mag netic field (shown in Figure 1.2b). Oersted's discovery meant that the Jines of magnetic force are at rig ht angles to the wire, and are therefore ci rcles perpendicular to the wire.

Figure 1.2: Hans Christian Oersted: (a) with stude nt Oersted d iscovers electri c current's magnetic effect on compass when circu it is completed; and (b) Oersted's observation that compass needle near electric current moves to position perpendicular to direction of current.

(a)

(b)

4

Personnel Training Publications

Faraday's Law of Electromagnetic Induction Elccrron1agnetic testing originated when Michael Faraday discovered the effect of electro magnetic induction while experimenring wi th coi ls of wi re and a bauery. He noticed that by connecting one coil to a battery, there was an instant electrical

CUlTen! through a second coil placed ncar the first coil whe n he sw itched the battery on and off. He also concluded that the second cu rren t was in the opposite direction of the first c urrent .

Similarly. Faraday found that moving the secondary circuit toward the primary induced a CUlTent opposite to the primary current in the prim ary coi l. A lso, moving the secondary c irc uit away from

the primary induccd a cutTent in the same direc ti on as the primary current in the primary coil. Max well explained that , "the direction of the secondary ClllTcnl is such that the mechanical action between the

two conductors is opposite to the direction of motion, being a repulsion w hen wires are approaching! and an attraction whe n they

arc receding." This electromotive force was observed by Faraday but was given more systemat ic treatment by Heiru-ich Lenz . Figure 1.3: Electromagnetic induction in a direct current circuit.

Switch

-L Battery

- -----

-==-

• Primary coil ~ ~

Secondary coil

t

Ammeter

David E . Hughes pelt'onned the first recorded eddy CUITent test in 1879 . He was able to d istinguish bctween different metals by observing and noting a change in excitation frequency resul tin g from effects of a test material's resistivity and magnetic permeability.

James Clerk Maxwell James Clerk Maxwell conceived and published the compre hens ive group of relations to the electromagneti c fi eld known as Maxwell's equations, which mathematically represent the entire present knowledge of the principles of electromagnetic testing. Maxwell's remarkable ach ievement of imegrating the available knowledge conceming electromagneti c circuits and fields provides the basis for analysis of al l basic eddy cutTent and electromagnetic induction problems and most of the modern electromagnetic theory. Several ideas Cor electromagnetic methods of detection emerged in the 1930s, but most of these relied on inductive measurements of the leakage field. This required either a high frequency magnetjzing current. \vhich limited the technology to near-surface disconti nu.ity detection, or coi ls moving at constant ve10city over the surface. which was too cumbersome for the technology of that time.

Classroom Traininx Series: Eleormnagncric 'testing

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[n 1946, the first pract ical system fo r the automatic. electromagnetic measurement of flux leakage fields was designed by Hastings . Hastings demon strated that he could detect both surface and subsurface discontinu ities near the bore surfaces of steel tubes . He also noted an empi rical correlation between the amplitude of leakage s ignals and the depths of su Ii'ace cracks.

Friedrich Forster The first eddy current instrument was developed in 1926. In [ 950, Forster developed the first instruments with impedance-plane signal displays. which we re used to discrimin ate between different parame ters . The introdu ction by Forster of sophisticated, stable quant itative tes t equipme nt , and of practical techniques fo r analysis of quantitat ive test signals on the impedance-plane, were important factors co ntributing to the rapid development and accepta nce of electromagnetic ind uction and eddy current tests from 1950 ro 1965. Since 1965, electromagnetic testing and eddy c urre nt lcsting appl ications ha ve developed and are in use dai ly in nearly every tech ni cal industry.

B ASIC PR[NCIPLES

or E DDY

C URRENT TESTING

Eddy c urre nt testing is a nondestructive testing method that is based on the principles of electromagnetic induction. Electromagnetic simply means that electricity and mag netism are used. Flow of electricity, under certain circumstances , can cause magnetism. Magnetism , under certain circumstances, causes the flow of electricity. Whe n an altcrnating cun'ellt (AC) is passed thro ugh a coil , a changing magnetic field is generated . As the coi l is placed near a cond ucti ve tcst object, the magnetic field induces curre nt (eddy currents) as illu strated in Figure 1.4.

Figure 1.4: Basic eddy current test syste m. Indicating

instrum~=:l==;-I;'/"';'\j) Test coil

Di rection of coil's fiel d Di rection of eddy current's field Conductive material Eddy currents

6

Personnel Training Publications

The now of eddy cunems depends on the physical and electrical characteristics of the test object. As an eddy cunent flows in the test object, it causes a fluctuating magnetic field of its own . The magnetic field from the eddy cunems is always in opposition to the coil 's magnetic field, as illustrated in Figure lA . Thus, when the test co il is pl aced on a conductive material . the strength of the coil's lllagnctic field is lesse ned. This change in the magneti c field causes a change in thc cun-ent flowing through the coil that , in turn , causes a change in the impedance of the coi l. This change in impedance is detected by the readout disp lay pl aced ill the test circ ui t. Eddy cunent instruments measure and displ ay these impedance coil changes and allow the technician to evaluate val uable information regarding the properties and condition of the test object. Eddy cunent testing, like other nondestmct ive test methods , in vol ves the application of elec tromagnetic energy to evaluate the cond it ion of te:=;t objects . The e nergy interacts wit h the 1l1aterial and the interaction process is analyzed to asceltain the conditio n of the material. In princ ipl e, electromagnet ic methods cover a wide range of techniques, in c1uding eddy current testing and magnetic nu x leakage testing. Al though all e lectro magnetic methods are governed by

Maxwell 's equations . the distinctive nature of each method stems from differences in excitation frequcnc ics , the nature of sensors used and the signal analysis teChniques for characterizing the state of the test object. For cxamplc. eddy CUlTcnt techniques use excitation frequencies from aboUl 100 Hz to aboUl 1 Hz, while magnetic flux leakage uses excitation frequencies near 0 Hz. As the excitation frequency increases from zero, the undcrlying physical process grad ually changes . Bel ow abo ut I Hz, the magnetic field is said to he l)uasistatic, whi ch means that the displacement current is negligihle. As the frequency increases heyond quas istatic val ues, the energy propagates in the fo rm of waves into the tes t

material. Di ffe rences in the underlying processes associated with each freq uency make it possible for electromagnetic techniq ues to test a wide range of materials.

P RINCIPLES OF F LUX L EAKAGE T ESTING Magnetic nux leakage testing is an electromagnetic techniq ue that can provide a quick assessment of the integlity of felTomagnetie material. Thi s technique involves magnetization of the test object by a permanent magnet or by passing a direct CUlTent directly through a co il, creating an electromagnet . The presence of a discontinuity on

or near the surface of the sample disturbs the magnetic n ux lines and results in a local leakage field arou nd the discontin uity. The magneti c flux leakage field can be detected using a variety of tec hniques. Tn magnetic particle testing, the leakage field is ind icated by dusting the snnace of the test object with magneti c

Classro olll Trainillg Series: ElectromagneTic Testinx

7

pa!1iclcs. These particles can be either dry or wet. Forces exel1ed by the magnetic leakage field around a crack altracts the parti cles to line up along su rface cracks . The magnetic flux leakage can also be detected using no ncontact sensors, such as a Hall effect probe or a simple ind uction coil. A Hall effect probe using an element oriented parallel to the sample surface is sensitive to the normal component of the magnetic flux leakage field and generates a typical signal . as shown in Figure 1.5 for a rectangular notch .

Figure 1.5 : Typical leakage field signal.

J

if Scan position or time (relative scale)

P ERSONNEL Q UALIFICATION It is imperative thai personnel responsible for electro magnetic testing are trained and qualified with a technical understanding of the equipment and materials, the test Object and the test procedures. The American Society for Nondestructi ve Testing (ASNT) has published guidelines fo r traini ng and qualifying nondestructive testing personnel since 1966. Thcse are know n as Personnel Quali/icarion and Certi/ication in Nondestructive Testing: Recommended Practice No. SNT-TC-I A. The Recommended Practice No. SNT-TC-JA describes the knowledge and capabilities of nondestructive tes ting personnel in terms of certification levels. MiNT CP-189 was appro ved by the ASNT Board of Directors in 1989 as a standard for the qualification and certification of nondestructi ve testing personnel. T he inte nt was to produce a new docu me nt that provided strict requirements rathe r than simply guidel ines. ASNT obtained ANSI accreditati on to process thi s document through a consensus balloting process that would recognize ASNT CP -l li9 as a national standard. The first successful consensus doc ument became ANSIIASNT CP-J89- J 991.

Personnel Training Publications

ANSI/ASlVT CP-1R9 is similar to SNT-TC-IA in terms of training_ experience and exam inations. Several significant differences were

introduced to strengthen the NDT personnel qualitication and certification program, which include the following. I. 2.

Employer certification requirements and ASNT I\'DT Level III certification in the method. Instructor for training must meet qualifications of the standard. a. ASNT Level 1Il celti ficate. b. Bachelor of Science in engineering, physical science or technology with knowledge of nondestructive testing method. c. NOT Le vel IT with at least ten years of experience .

Levels of Qualification There are three basic levels of qualification applied to nondestructive testing personnel anclused by companies that fo llow Recol1lll1ellded Praclice No. SNT-TC-l A and ASNT CP-189: Level l , Level Il and Level TIL An indi vidual in the process of becoming qualified or celtified to Level T is considered a trainee. A trainee docs not independently conduct tests, interpret, eval uate or report test res ults of any nondestructive testing method. A trainee works under the direct guidance of certified individuals. Qualification for Level I Level l personnel are qualified to perform the followi ng tasks: 1. 2.

3.

Perform specific calibrations and nondestructive tests in accordance with specific written instmctions. Record test results . Normally, the Level l does not have the authori ty to sign off on the acceptance and completion of the nondestructive test unless specifically trained to do so with clearly written instructions. Perform nondestructive test ing job activities in accordance with written in structio ns or direct supervision from Level TT or Level 1Il personnel.

Qualification for Level II Level II personnel are qualified to pelform the following tasks: I. 2. 3. 4. 5.

Set up and calibrate equipment. Interpret and evaluate results with respect to applicable codes , standards and specifications. Organize and repon the res ults of nondestructive tests. Exercise assigned responsibility for on the job training and gui dance of Level J and trainee personnel. Be thoroughly familiar with the scope and li mitations of each method for which the individual is certified.

Classroom Trailling Series: ElectromagneTic Testing

9

Qualification for Level III A Level 111 is responsible fo r nondestructi ve testing operations to which assigned and for which certified . A Level III must also be generally fam iliar with appropriate nondestructivc testing methods other than those for which speCi fically certified, as demonstrated by pass ing a Level lIT Basic examination. Level III personnel are qualified to perform the following tasks : I.

2. 3.

4.

Develop. qualify and approve procedures; establish and approvc nondestructive testing methods and tech niques to he used by Level I and Lcvelll personnel. Interpret and evaluate test res ults in terms of applicable codes. standa rds, specifications and proced ures. Assist in establ ishing acceptance criteria where none are available, based on a practical background in applicable materials, fab rication and product technology. In the methods for which certified, be responsible fo r. and capable of, training and examination of Level I and Level U personnel for certification in those methods .

Challenges The major challenge facing nondestructive testing personnel is to lea111 all that can possibly be learned during thc qualifi cation processes. Another challenge involves developing the mindset that there is something else to learn each time the nondestructive testing method is used. There is no substitute for knowledge . ancl nondestructive testing personnel must be demanding of themselves. The work performed in the nondestructive testing field dese rvcs the velY best because of the direct effect of protecting life or endange ring life.

PERSON NEL CERTIFICATION

It is important to understand the di fferen ce between two tc1111' that are often confused within the field of nondcstructi ve testing: qualification and certification . Qualification is a process that should take pl ace before a person is certified . According to Recommended Practice No. SNT-TC- I A. rhe qualification process for any nondestructive testing method should involve the fo llow ing. I. 2.

10

Training in the fundamental principles and applications of the method. Experience in the appl ication of the method under the guidance of a certified individual (on the job training).

Personnel Training Publicatiolls

3.

Demonstrated ability to pass written and practical (hands on) tests that prove a comprehensive understanding of the method ancl an ability to perform actual tests using the specific

4.

The ability to pass a vision test for visual acuity ancl color perception or shades of gray, as needed [or the method.

nondestructive testing method.

The actual certification of a person in nondestructive testing to a

Level I , Level II or Level III is written testimony that the individual has been propcrly qualified. It should contain the name of the individual being certified, identification of the method and level of certification, the date and the name of the person issuing the

certification. Certification is meant to document the actual qualification o[ the individual, Proper qualification and certification is extrcmely important because the process of testing performed by certifiecl nondestructive test.ing personnel can have a direct inlpact on the health and safety of every person who will work on~ in, or in proximity to the

equipment or as sembi ies being tested. Poor work performed by unqualified personnel can cost lives.

Mouern fabrication and manufacturing projects challenge the strength and endurance of the materials of construction, Preventive maintenance activities also present a chaUenge to nondestructive

test ing personnel. The industries thal depend on nondestructi ve testing cannot tolerate nondestructive testing personnel who are not adequately

gualified and dedicated to good performance. Too much depends on the judgments of nondestructi ve testing personnel macle in the work pCrfOTI11ed every day.

Classroom Tl'l.lininM Series : Eleclromagneric Testing

11

Chapter 2

Eddy Current Theory GENERATION Of EDDY CURRENTS The principles of eddy cunenttesting depend on the process of electromagnetic induction. This process includes a test coil through which varyi ng or alternating current is passed. A varying CUITent flowing in a test coil produce~ a varying electromagnetic field around the coil. This field i ~ known as the pr;lIIwy field. Figure 2.1 presents a ~chemat ic view of an exci ted te~t coil. The electromagnetic field produced arou nd the unloaded test coil can be de~cri bed as decreasing in intensity with distance from the coil and also varying across the coiJ"s cross-section. The electromagnet.ic fiel d is most intense near the coil's surface. fig ure 2.1: Electrom agnetic field produced by alternating current.

Generator

)

The field produced around th is coil is directly proportional to the magn itude of applicd current , rate of change of cunent or frequency and the coi l parameters. Coil parameters include ind uctance, diameter, length. thickness , number of tum s of wire and core material .

Electrical Clln'ent i~ defined as the movement of electrons through a cond uctor. T he unit of current is the am pere. Its quantitative val ue is established as the tlow of 6.25 x J018 el ectron~ per second past a given point in the circ ui t. Electrons are negati vely charged panic les that are part of the basic building blocks of any material (the atom). A conductor i ~ any

13

material that is capable of carrying electrical currem . Some materials are conductors. others are not. Whether a material can conduct electricity or not depends on the structure of the individual atoms in the materiaL

ELECTROMOTIVE FORCE Faraday's major contribut ion was the discovery of electromagnetic induction. His work can be summarized by the example shown in Figure 2.2. Coil A is connected to a battery through a switch S. A second coil B connected to a galvanometer C is nearby. When switch S is closed producing a current in coil A in the direction shown, a momentary current is induced in coil B in a direction (___ a ) opposite to that in A. If S is now opened, a momentary current will appear in coil B having the direction of C - b) . In each case , currem flows in coil B only while the current in coil A is changing . I = current flowing in coil.

Figure 2.2: Induced current.

-

b a

R

G

-

~

Ih

Electromagnetic force is the electrical energy derived from mechanical , chemical or other form of energy that must be applied across the material to force the electrons to mo ve . The unit of measurement of the Electromagnetic force is called the volt. Some materials, due to the ir molec ul ar structure, require more energy than others to cause electrons to move. These are said to have more resistance to the flow of electrical current. The amount of

resistance in a material is the factor that limjts the amount of current that tlows through the material for a given applied electromot ive force. The electromagnetic force (voltage) induced in coil H of Figure 2 .2 can be expressed as follows : N/),cfJ

Eq.2.1 E = - K/),t

where E is average induced voltage t N is number of tun1S of \vire in

coil B , /),cfJ / /),/ is rate of change of magnetic lines of force affecting coil B , and K is lOR.

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Personllel "hainillg Puh licarions

RESISTANCE In an alternating current circuit containing only resistance , the resistance simply limits the amount of current that /lows through the circuit. It does oat change the phase relatio nship between the voltage and the current. T he current is exactly in phase with the voltage. Resistance is present in all circuits . The total resistance in a c ircuit includes the resistance of the wiring as well as the resistance of the coil. The unit of resistance is called the ohm. Its value is dcfined as thc resistance through which an electromotive force of I V will producc a current of 1 amp. Tn a direct current (DC) circuit , tbe voltage, current and resistance are related to each other through the mathematical expression known as Ohm's law.

Eq.2.2 E=lxR

Or

E 1=R

where E is voltage (volt), T is current (ampere) and R is resistance (ohm). Example Problem : What is the value of the current flowing in a 5 ohm resistor connectcd across a 10 V battery? Solution: E = 1 X R Given: E = 10 V and R = 5 ohm IO=Tx5 1= 10/5 = 2 amp The resistance of a coil is dete rmined by the length of wire used to wind the coil. The specific resistance is determined by the wire typc and the cross-sectional area of the wi re. .

Specific resistance . Length

Eq.2.3 Reslsianec = - ' - - - - - - - - --"--Area

where resistancc is in ohms, specific resistance is in ohms/circular mil-foot , area is in circular mil s, and length is in feet. Example Problem : The resistance of a 10ft length of 40 gage wire (area of 40 gage wire = 9.888 circular mils) with a spccific resistance of 10.4 circular mil-foot at 20° C wo uld be found as follows: 10.4 . 10 Eq.2.4 R = - 10.518 ohll1 9.888

Classroom Training Series: Electromagnetic Testing

IS

Alternating Current When a coil of wire is placed in the open end of a magnet between the north and south poles and given a spin , electricity is induced in the coil. The current produced does not travel in the same direction through the coil at all times. nor is it of a constant value . Instead the CUlTent starts out at zero, ri ses to a max im um value, decreases to zero, rises 10 a maximum value in the opposite direction and then returns to zerO. This cycle repeats itself as long as the coil keeps spinning. Note that one revolution of the coil produces one cycle of current (Figure 2 .3). Figure 2.3: Induction by m ovem ent of a coil through a magnetic field. Axis of rotation

Magnet _ _ _

iVlagne[ic field (lines or rorce) J

Sine Wave A sine wave is the form commonly produced by alternating etment generators. Since one turn (360 0 rotation) of the generator coil produces one cycle of the sine wave . the sine wave can be marked into corresponding degrees of rotatio n, as shown below in Fi gure 2.4. This method of des ignating posi ti ons on the sine wave serves as an excellent way of show ing the timing between specific occurrences.

Figure 2.4: Gen erator coil position versus curren t produced .

Generawr

-;-r / j I 's

coil pos ition ~ 21/1..::.

Max +0 I

8tII

~tl}

180 I

360 I

I

I

o f-------~--------~~------~--------~­ I I

--,

Max -

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Personnel Tl'uining Publications

Frequency The frequency of an alternating current is defined as the number of cycles of current that occur in one second. One complete cycle is shown in Figure 2.4. Its unit is the hertz, I Hz being one cycle per second. Current at 60 cycles per second has a frequency of 60 Hz.

Self Inductance When working with the induction of current in a secondary coi l, Henry went even fUl1her than Faraday in that he discovered that the changing magnetic field also induced a current in the primary co il that opposed the original currcnt. Thi s opposing current was the result of the magnetic field cutting ac ross windings in the primary coil. The magnet ic field created by each tum of wire in the coil affected all of the other turns in the same coil. This is known as the principle a/self inductioll. Figure 2.5 shows an alternating current source connected to a coil. A voltmeter is provided to measure the voltage applied LO the coi l, and an ammeter is provided to measure the current thro ugh the coi l. If the instantaneous values of voltage and current are plotted on a graph, the cu rrent is found to lag behind the voltage in Lime as shown in the lower portion of the figure. The angle between the two curves is called the phase QlIgie or phase lag.

Figure 2.S: Alternating current voltage and current pilot. A Ammeter

Alternating current

Coil

source

Max

Classmom Training Series : Electromagnetic Teslillg

17

I NDUCTIVE REACIAKCE The opposition to changes in altemating current flow through a coil is calJc.d indlicTive reactance and is designated by the letters XL' The inductive reactance of any coil is also a fu nction of the frequency of the alternating current. Since the higher frequencies cause the magnetic field to change more rapidly, the inductive reactance increases as the frequency increases (assuming a constant

altenulting current voltage) . The increase in the inductive reactance due to the increase in frequency causes the cun'ent through the coil to be reduced. thereby reducing the strength of the magnetic field of the co il and is explained in Eq. 2.5.

Eq.2.5 XL

= 2nfL

or

where XL is inducti ve reactance (ohm).fis frequency (hcrtz). L is inductance (henries) and W equals 2:'Tf.

It has been determined that in an alternating current circuit containing only inductive reactance (no resistance) that the current will lag behind the voltage by exactly 90'. Example Prohlem: What is the inductive reactance of a coil with an inductance of 5 micro-henries at 60 kHz? Sollltion:XI, = 2JlfL or XL = WL L = 5 micro-henries f= 60 kHz XL = (2 x 3. 14) x (60 x 1000) x (5 x 111000000) ohm XI. = 1.884 ohm W = 2Jlf

I'v1PEDANCE Tn an al[emaring current ci rcuit. impedance (symbolized by the letter Z) is the name given to the combination of resistancc, capacitive reactance and inductive reactance. for most testing applications. the capacitive reactance can be dropped from the eq\lation since most eddy cu rrent probes have little or no capacit ive reac tance. Impedance in an alternating current circui t is the total opposition to the cu rrent flow through the cirenit. The impedance unit is the ohm. Since the resistance and the inductive reactance cause results that occur 90° out of phase with each other, they cannot be simply added together to determine the impedance. The simplest way to combinc the resi stance and the inducti ve reactance values to obtain the impedance value is through a vector diagram. A vector is a line whose length represents its val ue and the direction represents its phase relationship . figure 2.6 shows

18

Personnel Training Publicarions

resistance and inducti ve reactance vectors 90° apart in direction, By adding these two vectors together. a rectangle can be constmcted and the diagonal from comer to comer represents the impedance (Z) and phase angle, as shown in Figure 2,6b,

Figure 2.6: (a) Voltage·plane diagram; and (b) impedanceplane diagram, (b) (a)

9 °

o

t xR

R

A Greek philosopher named Pythagoras developed what is known today as the pyrhagorean rheorem, which states that in a right angle triangle the sq uare of the hypote nuse is equal to the sum of the squares of the other two sides. Using Figure 2,6b, it is determined that:

E q.. 26

z' -- R' + X L' Z=

or.

J(R' + X,')

The concept that must be understood is what is meant by phase angle, Refer back to Figure 2,6, The phase angle between the resistance vector and the inductive reactance vector has been 90°. The Greek letter alpha (A) is used to denote the phase angle of the impedance vecto!". Phase angle (A) = arctan Xd R Example Problem: An eddy curre nt test is carried out at a test frequency of 60 kHz, The coil resistance is 10 ohm and its inductance is 5 micro-henries. Calculate the follow ing: 1, 2, 3,

The inductive reactance of the test coil. The impedance of the test coil. The phase angle between the toral impedance vector and the resistance vector.

Solution 1: XL = 2 nIL XL = 2nx (60 x 1000) x (5 x l)fl 000000 ohm XL = 6.28 x (60 x 5)/1000 ohm XL = 1.884 ohm

ChHsroom Trainin g Series: Electromagnetic Te.ftillg

19

Solution 2: Z =

J( R' + X r' )

z = J(]() x ]() ) + ( 1.884 x I .884) ohm Z =,jI03.5 ohm Z = approximately ]() ohm Solution 3: Phase angle (L'1) = arc tan (XdR) Phase angle ( L'1) = arctan (1 .8 84/10) Phase angle (L'1) = lOS

Arctan Tangent is defined as the ratio of the opposite side of a righ t angle triangle to the adjacent side. This is called the tangent of 9. tan 9= length of opposite sidelle ngth of adjacent side. The angle whose tangent is kno wn is written

taW I

or arctan.

Resistance With all other facto rs held constant, changes in resistance will affect the impedance of the ci rcuit. As the resistance increases . the impedance inc reases and the phase angle decreases .

Inductive Reactance With all olher factors held constant, changes in inductive reactance will affect the impedance of the circ ui t. As the inducti ve reactance increases, the phase angle ancl the impedance increases. This is illustrated in Figure 2.7.

Figure 2.7: Current vector diagra ms. I:

Current

R:

Resistance

XL: R eactance

IX , L

Iz

o.~--~---------- oo

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Persollnel Training Pubfica/;olts

Z: a:

impedance Phase angle

Chapter 3

Eddy Current Instrumentation INTRODucn ON This chapter presents a general description of the principle types of test circuits that are applicable to eddy current testing, a general description of the types and arrangements of the coils used, and a gencral desc ription of the types of indicating instruments (readout apparatu s) that might be encountered.

E DDY C URRENT I NSTRUMENT CIRCUITS There are many differe nt types of cddy current instruments on the market toda y. They are similar in principle, but vary in fu nction and accessories. All cddy current instruments have somc method of detecting the impedance or change of impedance in the test coil. lt is the electronic circuitry that is the greatest variation between instrume nts,

A basic test circuit consists of an altell1ating current sOllrce supplying power to the testing coil. A voltmeter is connected across the testing coil to measure the voltage across the coil. When the coil is placed on or near a tes t sample, the impedance of the coil changes. This changc in impedance is reflected by the change in the reading oftlle meter (Figure 3.1 ). Figure 3.1: Basic test circui t.

Alternating current rv

source

r I I I I

IL_

-, <

R I

I

I XLI

V Voltmeter

I

__ J

Impedance Bridge Circuit (Wheatstone Bridge) The alternating current Source supplies power across the bridge that consists of two balanced res istors (R l and R 2), the testing coil , a balancing coil and an ammeter. Thesc units are connected in a bridge format: the res istor and the testing coil in one leg of thc bridge, a resistor and a balancing coil in the other leg and the ammeter across

21

the two legs. When the bridge is in balance (t he impedance on both sides of the bridge are equal). the meter w ill read 0 amps. W hen the testing coil is placed on or near a test sample ! there is a change in im pedance in th at leg of the bridge . T he bridge becomes unbalanced and the ammeter will indicate a current that is prop0l1ionai to the imbalance (Figure 3.2). A differential amplifier can replace the bridge circuit.

Figure 3.2: Basic bridge circuit .

Alternating current

r\...J

E----{

A ~

source

/ /

/

/


/ /

~/

)

/

/

Balam.:ing

i rnpcdancc

A nother bridge c ircuit consists of two identical coils, one in each leg of the bridge (if the coils arc ide nt ical , the bridge wiJJ be in halance) . One of the coils is caJJed a lestillg coil ancl Lbe other is called a reference coil. If Lbe reference co il is pl aced on one sample of the material a nd the testing coil is placed on anot her sample of the material, the meter will show an imbal ance only if there is some d iffe rence in the two samples (Figure 3.3).

Figure 3.3: llridge utilizing an inspection coil and a reference standard.

:::::;-3 Tnspecrion L3 coil Alternating

Test sample

rv current

~ Reference

source

U

impedance

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PersoJlnel Training Publica/ions

coil

Reference standard

INTERNAL FUNCTIONS OF EDDY CURRENT INSTRUMENTATION For an eddy system to provide information to a technician , five functional steps have to be performed, as shown in Figure 3.4. I. 2. 3.

4. S.

Signal Signal Signal Signal Signal

excitation. modulation. preparation. demodulation. display.

Figure 3.4: Internal functions of eddy current test instrument.

Signal

Signal

excitation

modulation

Signal

Signal demodulation

:----T~s; -~bJ;;~; -h~;;dl;~g , ,

,,

,'

,- - ----- - -- - - -- ~

Signal Excitation The excitation portion of an eddy current instrument consists of a signal generator and amplifiers to drive the test coils. The signal generator (or oscillator) provides sine wave excitation for the test coil. Single frequency systems have one fixed frequency, whereas multi-frequency systems can apply several frequencies to provide multiple parameter options. The application determines the required frequency and the number of frequencies to be used.

Signal Modulation Signal modulation occurs in the electromagnetic field of the coils assembly. It is the magnetic field created by the primary coil that provides the energy transfer into the test object. This magnetic energy is modulated by the test object, and the resultant magnetic field from the eddy currents opposes the primary field and is sensed by the instrument for processing .

Classroom Training Series: Electromagnetic Testing

23

Signal Preparation After modulation , the signal is processed fo r demodul ation and anal ysis, The purpose of thi s step is to ampl ify the p robe signal and reject extraneous noise . This part of the instru ment may consist of a single-ended a mplifier in an absolute or simple dri ver pickup system, or d ifferential am plifiers fo r a differential bridge or a more sophisticated system of dri vcr and pickup. The bandpass filter is selected to dctect d iscontinui ties at the inspection speed and could be called a speed jiller,

Signal Demodulation and Analysis Tn this step and after the sig nal has bee n demod ul ated, the signal can be analyzed by many means, The signal may be dirccted to an analog mete r or bar graph for cUsplay or it may be dig itized fo r fu rthcr man ipulation and analysis ,

Signal Display The signal display section is the key link between the test equipment and its inte nded purpose, The signal can be d isplayed many d iffe re nt ways , Common d isplays incl ude mete rs , cathode ray tu bes (CRTs), liquid crys tal display (LCD) or computer screens ,

SIGNAL-TO-No ISE RAllO Sig nal- to-noise ratio is the ratio of signals of interest to un wa nted Sig nals, Common noise sources are tes t object variatio ns of surface rough ness , geometry and homogene it y. Other electrical noises can be caused by external sources , such as welding machines , e lectric motors and generators, Mechanical vibrations can increase test systcm noise by p hysical moveme nt of the test coil o r test object. Tn other words, anything that interferes w ith a test system's abil ity to define a measureme nt is considered noise ,

Improving Signal-to-Noise Ratio Signal-to-noise ratios can be improved by sevcral methods. If a test ohject is dirty or scaly, the signal-to-noise rat io can be improved by cleaning the test object via shot blasting, pickling in acid, wire blUshing or light abrasive belts. Electrical interfe rence can be shielded or isolated , Phase discri mination and fil tering can improve the signal-to-noise ratio, It is com mon prac ti ce in nondestructive testing to requi re a mini m um signal-to-noise ratio o f 3: I, This means tha t a signal of interest must have a respo nse at least three times that of the noise at that point. T he absolute noise level and the absolute stre ngth of a signal from a discontinui ty depcnds on several fac tors . such as the search coil ty pe a nd size, the freq uency, thc inspection path and distance, object surface CDndition and mi crostr uctu re, in add iti on to the discontinuity size , loc atio n and orientat io n,

24

Personnel Training PublicatiOJls

Chapter 4

Readout Mechanisms I NTRODUCTION An imp0l1ant par1 of the eddy current test system is the part of the instrume nt that quantifies the change in impedance . There are several different ty pes of devices used . The device may be an integral pat1 of the test set; it may be a module that is plugged into the test set; or it may be a separate un it connected to the test set with a cable. The indicating device used should be of adequate speed , acc uracy and range to meet the require ments of the t.est system . Test reco rds may require storage on large in service components so that corros ion or disco ntinuity rates of change can be monitored and projected . The displays of analog instruments are relatively simple, enabling the user to view and perform simple manipul ations of raw data. Generally. controls for phase. gain, alarm levels and some filtering arc available. Analog recording of data via magnetic tape and strip chan recorde rs was common in the 20th century, but has largely been replaced by di gital data storage. Analog instrume nts arc uscd in a few niche ap pli cations.

A NALOG M ETERS An analog meter is an indicating dcvice whose visual outp ut varies as a continu ous funct ion of the input to the meter. A meter has a needle that moves in respo nse to the input. The response is immediate, and scales can usuaJly be calibrated to read spec ific values directly.

Audio Alarms Audio alarms only indicate an abnormal condition. Alarm lights and audi o alarms arc commonl y incorporated in eddy current test equipment. The indicator light and audio alarm give only qualitative info rm ation about rhe item , whether a condition is prcsent or not.

Strip-Chart Recorders Snip-chan recorde rs providc an analog recording of values at reasonably high speeds. The strip-chan reco rde r is one method that produces a pel1lHlIlent, fairl y accurate record. Several chan nels can be recordc·d at the Same time. Strip chart recordings are CO llll11on in testing tubi ng where the discontinuilY's location down the tubc is 25

cri tical. The strip chart length is indexed to time or distance and indicates nonnal or abnonnal conditions. Although useful, strip charts can qui ckly acc umulaIe and create storage problcms. Computcr memory is replacing strip charts.

D IGITAL DISPLAYS A digital meter is one whose visual output is shown in di screte steps in ti mc. The meter measurcs the input at a given moment, and the value of the measurement is displayed in llumerical form. Since the readout is in num bers, the chance of tech nician en'or is less than when analog meters are used, but the output is relatively slow. Digital meters prov ide greater accuracy and range than analog meters.

Cathode Ray Tubes Cathode ray tubcs (CRTs) can be used to displ ay the output of a test. circuit. They give instantaneous, contin uous presentation; are highly accurate; provide calibrat ion capabilities so that val ues Illay be read e1irectl y; have a broad range; anel presentation is adj ustable and stored so that parameters of particular interest may be studied more closely.

Digital Data Storage Digital e1ata are generally displayed in a complex plane presentation with supporting suip chali and C-scan displays , as required by the application. The point described by the in-phase and out-of-phase compone nts of the signal is displayed as a flying dot, and the digital capabllities of the in strument allow variable persistence, centering of the dot , rotation of the signal and scaling of the display. D igltal systems allow setup of cal ibration curves constructed from stored data and automated analysis of sig nals as compared to these curves. Digital conductivity meters, calibrated fro m conductivity refe rence standards, feed suhsequcntl y acquired data into algo ri thms that calculate conductivity and thcn dis play in a numerical format.

Digital Mixing The comb ination of components from different test frequencies allows the suppression of unwanted paramcters or signals from structures, suc h as support plates in tubing applicat ions, while retaining the signature of discontinuities beneath those structures. Alanm can be constructed digitally in any way, for example as amplitude levels, boxes or ellipses, for rejection or accepta nce of test objects. Alarms from various frequcncies or coi ls can be tagged to allow discrimination of differe nt test Object conditions .

26

Persollnel Training P"blication s

Liquid Crystal Display A liquid crystal display (LCD) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of it I ight source or reflector. It uses very small amounts of electric power, and is suitable fo r battery-powered electronic devices, Important fac tors to cons ider when evaluating a LCD monitor include resolution, viewable size. response time, matrix type , viewing angle, color support luminos ity, contrast ratio and input

ports (for example, DVI or VGA). A computer screen is a typical LCD that is widely used in state of the an eddy CUITem instruments.

Cla .rsroo/'H Training Series: Electrumagnet;c· Te stillg

27

Chapter 5

Eddy Current Sensing Elements INTRODUCTION Inspection coils arc available in a variety of form s and can be an'anged in a variety of ways. The choice of the arrangement of the inspection coil depends on the test situation. There are three types of inspection coils related to their physical structure, arrangement and testing they perform. I. 2. 3.

Surface coil (probe). Encircling coil (through). Bobbin coil (internal).

Surface Coil SI/rface coil , probe coil,flat coil or pancake coil are all common tenns used to describe the same type of test co il. Probe co ils provide a convenient method of examining the surface of a test object. Figure 5.1 is an illustration of a typ ical surface probe . View A shows the coil mounted in a probe, while view B shows the coil mounted in a spring loaded housing . The spring load in g ensu res that a constant pressure is applied at all times to prevent separation (liftoft) of the coi l from the surface of the test object. The surface probe may be hand-held Or may be mounted in automated scanning equipment. The coil, mounted in the end of the probe, is provided with a protective coating of epoxy to serve as a wear surface. The magnetic field produced by the coil is app rox imately the size of the coil. Probe coils and probe coil form s can be shaped to fit particular geometries to solve co mplex testing problems. As an example, probe coils fabricated in a pencil shape (pencil probe) are typically used to lcst threaded areas of mounting slllds and nuts or serrated areas of turbine wheels and turbine blade assemblies. Probe coils with added coil shielding may be used where high resol ution is required. A variation of the surface probe is shown ill Figure 5.2. The coil is mounrcd in a holder shaped and sized to serve that special func tion. Holders may be designed to serve any particular requirement (such as the round surface of a lUbe. or bar), or position the coil at a particular place (such as the leading edge of a turbine blades). Spring loading ensures that constant pressure can be obtained and preve nts separation.

29

Figure 5.1: Surface probes.

Cal

Coil leads

Coil recessed and

epox y rilled Figure 5.2: Hole probe. L Cad recessed and epoxy filled

(II'" - C---If' I c~:'j,\ Coil leads

J==·f~ ~

r@

~----'lfi-~====JI-==~ ~ \

Depth adjust collar

Applications When using a high-resolution probe coi I. the test object surface must be carefully scanned 10 ensure complete test coverage. This careful scanning is very time consuming . For this reason , probe coi l tests of large objects are usually l imited to critical areas. Probe co ils arc used extensively in aircraft testing for crack detection near fa steners and fastener holes. In the case of testing fa stener holes (bolt holes, rivet holes) and critical areas of heat exchanger tubes. the probe coil is spinnin g whil e being withdrawn at a uniform rate.

31)

Personnel Traininx PublicatioJls

This provides a hel ical scan of the hole using what is referred to as a spinning prohe technique.

Encircling Coil Encircling coil . outer diameter coil andjeed-lhroufi" coil are terms commonly used [0 describe coi ls that are used primarily to tes t outside diameter surfaces of objects that pass through the coil. Although the coil is the same in encircling and inside diameter probes , the field distribution is somewhat different . The flux den sity gradient tends to be more un iform inside the coil and decreases to zero at the center of the tested material. The width of the coil (Figure 5.3) is a function of the appl icatio n. Wide coils cover large areas, so they respond mostly to bulk effects. e .g., conductivity; whereas, narrow coils sense small areas and so are more respo nsive to small changes produced by disco ntinuities or small thickness changes. The magnetic Ileid of the coil extends slightly beyond the ends of the coil.

Figure 5.3: (a) Wide enCircling coil; and (b) narrow encircling coil. (a)

(b)

'-----iI~Ir----' fig ure 5.4 shows the eddy currents produced in a test rod by an encircling co il. Since the primary magnetic field intensity in the coil is normally cons idered to be constant across the diameter of the coil, it might seem reasonable to expect that the density of the eddy currents induced wou ld also be constant across the rocl. This is not true.

Figure 5.4: Eddy curren ts produced by an enCircling coil. Encircling coil Eddy current

paths

Classroom Trainillg Series: EleClrm11l1l
31

The eddy cun'ent', ~trength is greater near the surface and decreases toward the center of the rod. If the diameter of the rod is large e nough , there is no curre nt at the center. The phenomenon is known as the center effecr and is the result of the opposition of the eddy current fie ld . The eddy current magnetic fi eld at the surface opposes the primary field and reduces it . Thi s reduced field is aga in reduced by eddy currents just below the sUlf·ace. Ultimately the strength of the primary field is reduced to zero, and no additi onal eddy currents are induced. Note that the eddy currents flow in the same plane as the currents flow in the co il; i.e., around the circumference of thc rod. Thus the enc ircling coil is espec iall y adapted to locating discontinuities that are parallel or longitud in al to the length of the rod.

ApplicatiOns Encircling coils are primarily used to test tubular and bar-shaped products. The tube or bar is fed through the coil (feed-through) at relatively high speed. The cross-section of the test object within the test coil is simu ltaneously interrogated. For this reason, circumferential orientation of di scontinuities can not be detected with thi s application .

Internal Coil Bobbin coil, inner diamerer coil and inside probe are terms that descri be coils used to test from the inside diameter or bore of a tub ul ar test object. Fi gure 5.5 illustrates a type of coi l that can be inselted into tubing to test for discontinuities and thi ckness changes in the tu be . The internal coi l induces currents that encircle the entire circumference of the tu be so that the entire section surrounding the coi l is tested. Because the CUITents induced in the material arc strongest near the coil, the internal coi l is more sensitive to discontinuities lying on or ncar the inner suti-ace of tubing . while extern al coils are more sensitive to discontinuities lying on or near the outer surface. The intemal coil may he either wide or natTOw, and the magnetic fiel d extends slightly beyo nd the ends of the coil . The coil may be shielded or unshielded.

Figure 5.5: Internal coil.

32

Personnel TraillllllS PliblicGriolts

Applications Intemal coil s are primary used to test tubular prod ucts from the inside diameter or the bore of a tubular test object; fo r example, tubes examined in heat exchanger applications.

T EST COIL ARRANGEMENTS There are three bas ic co il arrangemem that can be applied in surface, inlema1 and encircling co il s. 1. 2. 3.

Absolu te coil arrangement.

Differential coil arrangement. Hybrid coi l arrangement (through transmission).

Single Coil (Absolute Arrangement) Figure 5.6 illustrates the single coil arrangement. Tn this arrangement, the same coil is used to ind uce eddy currents in the test Object and to sense the test object's reaction to the eddy currents. The single coil will test only the area under the coil and does not direct reference or compare itsel f to a reference standard. Because it tests the object without a com parison, it is called absolUTe.

Figure 5.6: Single coil - absol ute arrangement.

f---{rv}---I Indication

Double Coil (Absolute Arrangement) Figure 5.7 illustrates the double coil absolute arrangement. In thi s it is possible to use two co ils; one to cst.ablish the magnetic field and induce eddy currents into the test object, and another to detect changes in eddy cu rrent flow. Note that the secondary coil has the indicating clevice connected across the coil and it is not connected to an alternating current source. Normally the secondary coil is located inside the pri mary coil. and the t.wo coils are referred to as double co;l. This double coil alTangement may appear in all three of the coil cl asses: surface, inside and encircling.

Classroom Training Series: Eleclromagnetic TestiUM

33

Figure 5.7: Double coil- absolute arrangement. Primary' coi l

Indicator

Secondary coil

Note: secondary coi l i~ located inside primary coil .

Differential Coil (Self-Comparison Technique) The differential coi l arrangement shown in Figure 5.8 illustrates a means of balancing out effects that are the same . The two coils are wound and electricall y connected so that the output of one coil cancels the output of the other coil (oppose each other). This occurs whe n the test object properties are the same under both coils. In other words. so long as there is no difference in material properties under coils, there is no indication on the indicating device; but when a discontinuity or inhomogene ity is located under either one of the coils, an imbalance OCCurs which is indicated on the indicating device. This differential coil arrangement is know n as the selfcomparison technique. The self-comparison tech nique is insensitive to lest object variables that occur grad ually. Variables such as gradually changing wall thickness, diameter, temperature or conductivity that affect both coi ls are not detected with the self-comparison differential coil. Figure 5.8: Double coil - differential arrangement .

cCf---\-=-16 I I

Self I------II'.[A:--.It-------l comparison

,=-,

L----11

Indicating

11-__-1

I inslnlment I

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Personne.l Training Publications

Differential Coil (External Comparison Technique) The coi l arrangement illustrated in Figure 5.9 is exactl y the same as the self-comparison , except that the differential coil arrangement is set up on an external reference standard and the test object. A carefully chosen, discontinuity-free test reference object is held stationary in one coil while the test object moves through the other coil. Here coil C2 and the discontinuity-free test reference standard or object are set up as a reference standard. As the test object passes through coil C/, a compari son is made with the reference standard. No indication is observed unless a discontinuity or other change such as thickness , permeability or conductivity appears in the test object. If a discontinuity passes through coil C/ , the output of the coils become unbalanced and an indication is obtained. Thi s differential coil arrangement technique is used to detect differences between a calibration standard and test object. It is mostly used to compare conductivity, permeability and dimensional measurements. The problem with this method is the large number of calibration standards required and the tolerances that exist in diameter and metallurgical composition. Figure 5,9: Differential arrangement - external comparison technique.

Ct \ \

Test object

Indicating device

External comparison

Reference standard

Hybrid Coil Arrangements (Through Transmission) Hybrid coils mayor may not be the same size and are not necessarily adjacent to each other. Common types of the hybrid coil are driver/pickup, through transmission or primary/secondary coil assemblies. The through transmission technique involves inducing eddy currents into the test object by a transmitting coil placed on onc side of the material , and the presence of eddy currents is sensed by a recei ving coil placed on the opposite side of the material , as shown in Figure 5.10. This arrangement requires that the two coils be placed exactly opposite each other. The voltage developed in the

Classroom Training Series: Electromagnetic TesTing

3S

sensing coil is a function of the current magnitude and frcquency applied 10 the excitation coil, coil parameters of the exciting and scnsing co ils , and the test object characteristics. Only thin materials may be tested with through transmission coi ls. Figure 5,10: Through transmission arran gement. Transmitting circuit A Itemming current source

Transmitting

coil

M aLerial

Receiving co il Receiving circuit

FACTORS AFFECTING CHOICE O F S ENSING ELEMENTS Several critical factors and specifications must be carefully considered when selecting eddy current sensing elements.

Frequency The frequency determines eddy current depth of penetration and the ampli tude and phase of a di scontinuity response .

Excitation The amplitude of the exc itation signal detennines the ampl itude of the response . It should be well controll ed and its frequency respo nse should be specified.

Gain Linearity The amplitude and phase characteristics of gain stages must be qualified for adherence 10 a standard or specification appropriate to the appl ication .

Horizontal and Vertical Deviation Gain of the in-phase and out -ot~ p hase components of a signal must be controlled to prevent unwanted distortion of a signal. In a typical heat exchanger test, a flattened appearance would cause misrepresentation of the data whereas, in the rolating test of a rivet hole for surface breaking cracks, it can be used to minimize lift-off noise and accentuate a crack signal.

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Personnel Tl'oillill}: Publications

Quadrature Accuracy The phase of reference signals mUSI be well conIrolled andlhe response of the display mUsl be designed 10 ensure that Ihe in-phase and oul-of-phase components are truly al 90" electrically and Ihey are displayed orthogonally.

Digitization Rate The digiti zalion rate for a digital system is the number of samples per unit of probe travel. Thi s rate is critical for determ inin g the response to a disco nti nuity. Digit ization rate must be determined li'om the application cri teria.

Sample Rate The sample rate for a digital system is the number of interrogations per unit time, often give n as samples per second. To calculate the required sample rate for a test, the probe speed is multiplied by the desired digitization rate.

Bandwidth Bandwidth governs the response of the syste m as a function of frequency and is measured in hertz. A variable frequency, external modulator is often used to measure anel characterize bandwidth through all stages of an instrument.

Classroom Training Series: eleCTromagnetic Te,\'ring

37

Chapter 6

Flux Leakage Theory I NTROUUCTION Magnetis m is a property pos sessed by certain materials by which these materials can exert a mechanical force on other like material. Magnetic tl ux leakage testi ng is an electromagnetic tech11 ique th at can provide a quick assessment of the integri ty of felTOmagnetic material. This teclmique involvcs magneti zati on of the test object by a permanent magnet or by passing an excitation cun'em directly through an electromagnet. Thc presence of a discontinuity or thickness change on or ncar the surface of the material di sturbs the magnetic nux lines and rcsults in a local leakage fie ld around the discontinu ity. The magnetic flux leakage can be detected using noncontact sensors, such as a Hall cffcct probe or a simple inducti on coil. A Hall effect probe using an element oriented parallel to the sample slllt'ace is sensiti ve to the normal component of the magnetic flux leakage field and generates a typical signal, as shown in Figure 1.5 in Chapter I for a rectang ular notch. A leakage field at an air gap in a longitudinally magnetized test object is shown in Fi gures 6.1 and 6.2. This disruption is produced by the al ignme nt of magnetic domains in the stretched mctal crack site. For this reason, cracks in the formatio n state are highly detectable and normally produce sharp. well-defined ind ications. Open cracks that havc been subject to large thermal, chemical or mechanical forccs may have had their magnetic fie ld disruptio n characteristics greatly or entirely reduced and may not be detectable using magnetic particlc testing. For high detectability of fom1ing surface cracks, magnetic particle testing uses induced altemating field magnetization techniques as the primary method to test ferromagnetic objects in service. Using wet alternating current fl uorescent techniq ues will generally produce the highest sensitivity obtainable with this tech nology. Figure 6.1: Leakage field at an air gap in a longitudi nally magnetized test object. Leakage field ~..,..:;:::::", Test

39

Figure 6.2 : Magnetic particles attracted by a leakage field from a subsurface discontinuit y. Particle buildup at leakage i

BAND

H

CURVE

To understand the operation of magnetic flu x leakage, it is userul to consider the physics of permanent magnets. A permanent. magnet can be conside red an agglomeration of do mains. These domains are elementary magnets obtained as a result of the dipole moments of uncompensated electron spin s contained within the domain that arc held parallel. In the demagnetized Slate, the domain s orient themselves randomly (Figure 6.3a) so that closed paths for the mag netic flux ex ist in the material. The m agneto~ tatjc energy under the condition is a minimum . This state is indicated by point 0 on the hysteresis loop (Figure 6.4). When an extern al magnetic field is appl ied. the domains tend to al ign with the direct ion of the applied field . thereby increasing B. The operating point now moves into region OA in Figure 6.4. The size and orientation of the domains arc affected by the potential energy arisi ng out of the interaction bet.ween neighboring atoms , as sociated with the anisotropy energy and associated with external field energ y. As the external magnetic field is increased , the operating point moves into region AB of Fi gure 6.4. The domain walls start shifting and ultimately reach a state when each crystal represents a single domain. FUlther increases in the magnetic ficld intensity results in magnetic saturation . a state in which the domains rotate against the forces of ani sotropy until all the do main s get aligned in the direction of the applied fie ld (Figure 6.3b). Thi s state is represented by the region Be on the curve of B vers us H (Figure 6.4). If the applied magnetic field is then withdrawn, the doma ins relax. As a res ult , the parallel alig nment of the domains is disturbed (Figure 6.3c). The residual flux B represents a new minim um energy at point D where magnetization H equals O. If a gap is then introduced as shown in (Figure 6.3d) , the material self-demagnetizes. The imbalance created by the gap res ults in a realignment of the domain s closest to the gap. These domains take up orientations that are 180" from the original ori entation. The mechanical energy injected into the system to introduce the gaps is used to transfer the operating point from D

40

Persolllll:1 Training Publicmiol1s

Figure 6.3: Physics of permanent magnets : (a) random orientation of domains in un magnetized state; (b) domain s aligned in direction of applied field; (c) relaxation of parallel al ignment of domains when magnetic field is removed; (d) self-demagnetization of material after magnetic field is removed; and (e) reversion of do mains to random orientation when gap is removed. See Figure 6.4 for characteristic curve . (a)

,B

Ili{~,~ I (c)

T

.r~

j

L

.

D 7

..,

I '-

... ~+

0

(e)

L

Lq

I

-+

2 0

c H

(d)

c

~I:-:;::. ~ ;:11 1

H

,/tZ 0

~ ,

_-

1:~"'-.............

I

H

C-- +

I:' -. , . . _

(b)

"-./

c

H

~ c / G

H 0 H Legend B; magnetic flux densi ty (relative scalel measured in gauss (field strength) H= magnetic field intensity (relative sea e) measured in oersteds (force)

Figure 6.4: Typical characteristic curve of magnetic flux density B versus magnetic field intensity H. B

H

Legend B = magnetic flux den sity (relative scale) measured in gauss (field strength) H= magnetic field inten si ty (relative sca le) measurea in oersteds (force

Classroom Training Series: Eleclromagnetic Testing

41

to E.lf the air gap is then reduced to zero . as shown in Figure 6.3e. the operating point moves along the m inor or recoil loop to F and the domains revert back very nearly to the same orientation as before. If the gap is once again restored , the operating point then moves toward E along the recoil loop FOE. Repeated cycles of opening and dosing the gap cause the minor recoil loop to be traced.

LINES OF FORCE If a bar magnet is covercd with a sheet of paper and iron filings are scaltered over the paper, the filings al ign themselves along definite lines that pass from the poles of the magnet, as shown in Figure 6.5. The alignment of the iron particles indicates that these lines form a field around the magnet, and any mag netizable material that enters this field is attracted to the magnet. For this reason the lines are called lines afforce. Figure 6.5: Magnetic field surrounding a bar magnet.

L AW Of MA GNETISM When like poles of magnets are brought togcther they repel each other. but whcn unlike poles are brought together, the magnets attract each other. Since the lines of force around a magnet seem to flow [rom the north to the south pole, they are often callcdjlLLx lines. Flux is defined as a !low or l1owing.

42

Personl1el noininx PublicaTio1ls

FLUX DENSITY The flux lines that surround a magnet are close together ncar a bar magnet. Figure 6.6 shows where the magnetic force is strongest. When spread fUl1her apart away from the magnet, the magnetic field grows weaker. Thus the density of the flux in a particular area dete rmines the strength of lhe field in that area. The flux density can be determined by measuring the strength of the field in that area. Flux density is defined as the number of lines of force that pass through a given area at right angles to the lines of force. The unit is the gauss . One gauss is one line of force passing through an area of one square centi meter.

Figure 6.6: Distribution of flux arollnd a magnet. __ ~ Lines of force

---~

-----..... -~-

-- -

RIGHT H AND R ULE It was discovered that when an electrical current flows through a wire . a magnetic field exists arouncllhe wire . The direction of the magnetic field around the wire depe nds 011 the direction of CUlTent flow tllrough the wire. This relationship may be determined by the right hand rule. as illustrated in Figure 6.7. Figure 6.7: Magnetic field surrounding a bar magnet. Field direction

\\Tire

Current direction

Classroom Training Series: Electromagnetic Testing

43

If the wire is grasped in the rig ht hand with the thumb pointing in the direction of current flow. the fingers will point in the direction of the magnetic field. If st raight wire lS wound into a coil. the lines of force encircl ing the wire form the magnetic field inside and outside of the coil, as illustrated in Figure 6.8. This field th us created is simi lar to the field of a har magnet. The strength of the magnetic fie ld is dependent upon two factors : the number of turns in the coil. and the magnitude of the current. Increasing either one increases the strength of the magnetic field.

Figure 6.8: Magnetic field of a co il.

-----F.l +-

Lines offorce ~.,- -~-_,

(,I.

-'h

-j ___ 1-:--:\

f ,"

~

_

r;r) )

+-1 t-,,::t.. ... ) .....

, - - -_- - J .... -.....---'"

-

t

- Coil

MAGNETIC PROPERTIES OF MATERIALS Electric current can be used to create a magnetic fie ld in materials. If a coil is wrapped around a bar of iron and direct ClIITent passed through the coil, a mab~letic field is established in and around the bar due to the magnetic field caused by the curren! flowing through the coil . Materials like iron that are magnetizahle are calledferrol11{fglletic materials. Ferromagnetic materials are capable of retaining some part of the magnetic field induced in them . With in the feTI'omagnetic group of materials some are more easily magneti zed than others. Those that are easily magnetized retain relatively little of the magnetic field afle r the curren! is shut off. Those that are difficult to magnetize retain more of the magnetic r;eld after the current is shu t off.

Magnetic Domains Some materials that can be magnetized possess atoms that are classified as submicroscopic regions. called magnetic domains . T hese domain s have a positive and negative polarity at opposite ends hecause of internal magnetic alignment. jf the material is not cons idered to be magnetized , the domains are randomly aligned, usually parallel to the crystall ine axis of the material. When the material is subjected to a magnetic field, the domains align themselves parallel 10 the extemal magnetic fie ld. The material then acts as a magnet. Figure 6.9 illu strates the domain alignment in nonmagnet.izecl and magnetized material.

44

Persolln el 'fj·oining Publications

Figure 6.9: Alignment of magnetic domains: (a) in an unmagnetized material; and (b) in a magnetized materia l.

(a)

(b)

01

s0B;;>.ct>0 s0 100 0 0B000ct>000B s

'~BNBsB'B'B"B'BBB N

s

BBBBBBBBBB

Magnetic Hysteresis All ferromag netic materials have certai n magnetic properties that are specific to that material. Most of these properties are described by a magnetic hysteresis loop. The data for the hysteresis loop are collected by placing a bar of ferromagnetic material in a coil and applying an alternating current or direct current. By increasing the magnetizing field strength H in small increments. and measuring the flux density B at each increment, the relationship between magnetic fie ld strength and flu x density can be plotted. The relationship between magnetic field strength and fl ux density is not li near for ferromagnetic material s. A specific change in H may produce a smaller or larger change in B, as shown in Figure 6.10 , the initial curve for an un magnetized piece of steel. Starting at point o (zero magnetic field strength and zero magnetic flu x) and increas ing H in small increments, the flux density in the material increases quite rapidly at first, then generally slows until point A is reached. At point A, the material becomes magnetically saturated. Beyond the saturation point, increases in magnetic fie ld strength do not increase the flu x dens ity in the material. In diagrams of full hysteresis loops, the curve OA is often drawn as a dashed line since it occurs only duri ng the initial magneti zation of an unmagneti zed materi al. It is referred to as the virgin curve of the material. When the magnetic field strength is reduced to zero (point B in Figure G.l ~b), the flux density slowly decreases. It lags the field strength and does not reach zero. The amou nt of flux density remaining in the material (line DB) is called residual magnetism or remanence . The abi lity of ferromagnetic materials to retain a certain amount of magnetism is called retentivity. Removal of residual magnetism requires the application of a magnetic field strength in the opposite or negati ve direction (see Figure G. IOc). When the magnetic field strength is first reversed and only a small amount is applied, the flux density slowly decreases. As additional reverse field strength is applied, the rate of reduction in flu x density (line BC) increases until it is almost a straight line (point C) where B equals zero .

Classroom 7i-aining Series: Eleclromagnetic Testing

45

Figure 6.10: Hysteresis data for un magnetized steel: (a) virgin cu rve of a hysteresis loop; (b) hysteresis loop showing residual magnetism; (e) hysteresis loop showing coercive force; (d) hysteresis loop showing reverse magnetism; (e) hysteresis loop showing reverse residual magnetism; and (f) complete hysteresis loop. (a)

(b) B+

B+

Zero flux density

,. ~

Zero magnclic \ fi eld strength

"

/ I

Saturation point

I I

B

H+

I

o

,.

'\

I

H-

___-~=A

Residual magneti sm

Saturation point

"

/

/

_A

I

H---------------'/''-------------H+

o

B-

B-

(d)

(C)

B+

B+

,-

,.

,.-

__

A

"

/

,

/

H-

/

,,

/

C

/

(0

B+

(e)

___

I

,, "

Saturation -- point

o

Reverse magnetization saturati on point

B-

I

H- --------~~-f~ ' ------------H+

H+

0

Coerc ive rorcc

I

A

B-

B+

~- A

Saturation

point

I

I

H- ------------~4f'--------------H+

l~

H - ----------~~_+~--------H+

Residual magnetism

D .-;:;...-Reverse

D"":::::"'--

8-

magnetization point

46

Personnel Training Publications

Reverse residual

point

B-

If the amount of magnetic fie ld stre ngth is increased beyond point C, the mag netic fl ux changes its polarity and initi ally increases quite rapidly. It then graduall y slows until point D is reached (Figure 6.lOd). This is the reverse polarity saturation point, and add itional magnetic field strength will not produce an increase in flux density. When the reversed magnetic field su·ength is reduced to zero (point E in Figure 6.1 Oe) the flux densities of the residual magnetis m from the straight and reversed polarities are equal (line OB is equal to line OE). Removal of the reversed polarity residual magnetism requircs application of magnctic field strength in the original direction. Flux density drops to zero at point F in Figure 6.lOf with the application of coercive force OF. Continuing to inc reasc the field strength results in the magnetic polarity changing back to its original direction. This completes the hysteresis loop ABCDEF (note that the curve CDEF is a mirror image of CUrve ABCF).

Magnetic Permeability One of the most important properties of mag netic materials is pcnneability. Penneability can be thought of as the easc with which materi als can be magnctized. Air is assigned a permeabi lity of one (I). Permeability is the ratio between the flux de nsity and the magnetic fi eld strength (BIH). It is also the rate of change of fl ux density (B) with respect to the magnetizing force (H), and it varies wi th position around the Band H curve.

Eq.3.1 i'

~

BlH

Or

B ~ Jt H

where i' is permeability (pronounced "mu"), B is magnetic flux density and H is magnetic field imensity. Magnetic properties and hysteres is loops vary w idely between materials and material condi tions. They are affected by chemical compositions, microstruct ure and grain size. Fig. 6.11 a is a hysteresis loop for hardened slcel, and the loop is typical of a mate rial with low permeabi lity, high reluctance, high retentivity and high residual magnetism. That requires high coercive force for removal. Figu re 6.1 1b is the hysteresis loop for an annealed low carbon steel. It is typical of a material with high penneability, low reluctance. low retenti vity and low residual magnetism that requires a low coercive fo rec for removal.

Classroom TraiJlinx Series: Electromagnetic Testing

47

Figure 6. 11: Positive field strength hysteresis loops:

(a) hardened steel hysteresis loops; and (b) annealed low carbon steel hysteresis loop. (a) (b) Residual

Residual ?;-

magnetism

"§ il

"~ fI:

48

Personnel Training PlIblicm;olls

Coerc ive force

Chapter 7

Flux Leakage Sensing Elements INDUCTIVE COIL SENSORS In order for an inductive coil to detect perturbations in a static magnetic field, there must be relative motion between the co il and the field such that the flux within the coil changes. Ferrites are useful in pickup coils because they do not only provide support for the wire turn s but they also concentrate the flux density through the coil windings by a value equal to the effective permeability of the ferrite. For small pieces of ferrite (Figure 7.1) where the dimensional ratio is small , the effecti ve permeability of the ferrite may vary from the low teens to the thousands. The advantage of using ferrite occurs not only in thi s application but also in the fact that ferri tes have very low electrical conductivities, minimizing detrimental eddy current effects in them. Figure 7.1: Ferrite cored magnetic flux leakage detector co il systems.

(a)

(b)

Input

....----l

1

';r,

0.3 to 0.5 em

0

..... 'd

\..1 •....---

21--~P

J

I

Magnetized material

It is important to note in the inductive sensor approach that the flux density must be changing through the coil in order to produce a signal. It is essential that pickup coils are used to generate voltages and not currents. Once a current is allowed to flow in a co il, it creates its own magnetic field - one that can interfere with the field under investigation. The output of such co ils is therefore generally fed to a high resistance operat ional amplifier.

49

Hall Effect Sensors Hall eleme nts are crystals of semico nductor material. When a current is passed through them and they are placed in a magnetic field, a voltage develops across two of the faces of the crystal. The voltage is prop0l1ionai to the strengtb of the magnetic field. Bulk Hall elements are generally bismuth doped semiconductors such as indium antimonide (InS b). These are produced by solid-state crystal growth technology, cut into small rectangular blocks and have cu rrent and voltage leads attached before being encapsulated. Typical size s are as small as O.OB cm (0.03 in.) long by 0.04 cm (Om S in .) wide by 0.05 cm (am in .) thick. Vapor deposited Hall elements have been reported for use in the testing of ball bearings by the magnetic flux leakage technique. In this application, bismuth was evaporated onto an alum ina substrate. A newer development is to combine the Hall sensor, its power supply and an amplifier on one chip. Figure 7.2 and Table 7.1 show configurations of typical Hall sensors and their specifications. Figure 7.2: Typical Hall element probes: (a) flat; (b) high linearity; (e) miniature; (d) subminiature; and (e) axial (see Table 7.1). 0.2 x 0.5 em (0.08 x 0.2 in.) (a)

(b)

0.24 x 0.64 em (0.1 X 0.25 ill .)

..... Brass holder

(c)

Cd)

:

4!J~~'-1 ?=----]]'?l Jif . _-_-_-_-_-_-_-_-_-_-_] ==~ UJ 0.075 x 0.15 cm (0.03 x 0.06 in .)

0.075 X 0.15 em (0.03 x 0.06 in .) ~~~.~ . -~ .. ·~~~!ll!:··::In'm::::::::::::J

..... Epoxy coatcd (with slide protector)

0.06 x 0.2 em (0.025 x 0.08 in.) (e)

50

Per.wmnel Training Publicalio/Is

Table 7.1: Specifications of typical Hall element probes (see Figure 7.2). Probe Type

Flat or lransverse

High linearity Miniature Subminiature Axial

Hall Output Voltage (millivolts)

Nominal Current Control (milliamperes)

Temperature Coefficient

Operating Temperature

(0C)

(HC)

340 350 200 200 100

200 350 25 25 100

--0. 1 --0.1 --0.25 --0.25 --0.1

- 65 - 65 - 65 - 65 -65

to 85 to 85 to 85 to 85 to 85

Since Hall effect sensors do not depend on motion for their sensiti vity. they can be scanned at any rate that is mechanically convenient, which may present an advantage over inductive coil sensors depending on the application. However, Hall effect sensors are difficult to fabricate , arc somewhat delicate and require more complex aux ili ary electron ic apparatus than inducti ve coil scnsors.

Flux Gate Magnetometer The flux gatc magnetomcter, also referred to as aferro-probe or Forster probe, is a de vice that measures magnetic field s by utilizing the non-linear magnetic characteristic of ferromagnetic core material s as its sensing elcment. A drivc coil and sense coil are wo und onto an easily saturated core. The corc characteri stics and drive current are such that the magneti zation changes induced by the leakage field affect the filter harmo ni c output of the sense coil. In operation, the ferromagnetic core of the sensor is driven cyclically tc saturation by means of a periodic current of suitable wave shape in the dri ve coil wind ings . In the absence of a signal field , which is usually direct current or vcry low frequency alternating cllrrent, and is generally a very small fraction of the peak value of the driving field , the voltage induced in the sense winding is symmetrical. In other words, it contains only odd harmonics of the fundamental of the drive current. In the presence of a signal , the sense winding voltage becomes asymmetrical. This asymmetry is sensiti vely relatcd to the signal field and can be detected by various techniques.

MAGNETODIODE The magnetodiode is a solid-state de vice, the resistance of which changes with magnetic field intensity. It consists of (p) zones and (n: zones of a semiconductor, separated by a region of material that has been modified to create a recombination zone, as illustrated in Figure 7.3 .

Classroom Trainin g Series: Electromagnetic Testing

51

Figure 7.3: Schematic of magnctodiodc. p+

n+

H

Legend II. ;;;; !nagnc,lic field 1 = IntrInSIC zone n = nzone

p =p zone. . r = recomb1l1atlon zone

Active areas typically measure 0.3 x 0.06 x 0.04 em (0.12 x 0.024 x 0.016 in .). and output signals arc generally larger than for Hall elements. although the response to field intensity is not so linear for higher fields, as shown in Figure 7.4. Figure 7,4: Response of magnetodiode is linear up to about 40 kA ·m- 1 (SOO Oe) at ambient temperature of 2S °C (77 OF) and potentia l of 6 V. 1.6

I

1.4

1.2 1.0 0.8

:>

I-

0.4 0.2

.:3

0 -0.2 -0.4 -0.6 -0.8

&.

__L

0.6

-

1

-I -\

-1.0

I

I I

I

1---

I

I

I I

-1.2 - 1.4 - 1.6

I I/

v::: J.....-""

Y

I

-11/

-200

-120 (-IS)

1

-

I

j;1

-

-

t-- t--

-l-

I

I I

I

-40 0 40 (-05) (0.5)

I

I

Personnel 'Ii'aining Puhlications

_I

120

200

( I .5)

(2 .5)

Magnetic fidd in tensit)' H. A-m- 1 (kOe)

52

-

II

1

(-25)

----+-

-

Applications of Magnetodiodes Figure 7.5 shows the use of magnetodiodes for detecting magnetic flux leakage from discontinuities in tubes. The magnetic flu x leakage is excited by alternating current electromagnets arranged to detect either internal or external surface breaking di scontinuities. The system illustrates the general principles of magnetic flux leakage testing . Sensors are connected differently to eliminate signals from the applied field and from relatively long range variations in surface field strength. This system and magnetic flux leakage systems like it are used to rap idly evaluate the sUiface condition with a depth of only om cm (0.004 in.). Figure 7.S : A magnetodiode testi ng system for tubes: (a) alternating current magnetizing method; and (b) electrical block diagram. (b) (a) Differential amplifier

Magnetization electric source

Sensors .--:""7"",Le-.:akage nux

.

Detection section

,

'-;'-~

Selc(;{ing cradle

Hot rolled steel bar Marker

Pipe

Controller

OTHER METHODS OF MAGNETIC LEAKAGE FIELD DETECTION

Magnetic Tape System For the testing of flat plates and billets, it is poss ible to scan the surface with wide strips of magnetic recording tape. Discontinuity signals are taken from the tape by an array of tape recorders heads. Scale , dirt or oi l on the test surface can contaminate the tape. Surface roughness can tear the tape.

Classroom Trainin g Series: Electromagneac Testillg

53

Magnetic Particles Magnetic panicles are finely ground high permeabi lity mag netic material, sometimes dyed for visible contrast with the test surface . Ideal test conditions occur when a fine spray of particles is intercepted by a magnetic flux leakage field and some of them stick to the field. An ad vantage over other forms of magnetic indicators is that the particles have zero li ft-off from the discontinuity. A critical facror in successful magnetic particle testing is the proper choice of magneti zing cu rrent level. With too liltle magnetizing current , the field gradient aro und discontinuities will not be sufficient magnitude to hold the pal1 icies in place . On the other hand, if the magnetizing curre nt is too high. a field gradient may be strong enough even in flawless areas to attract and hold particles over the entire surface , thus obscuring genuine discontinuity indicati ons . For each situation, the opti mum current levcl must usually be found experimentally. but guidelines put you in thc general vicini ty.

Magnetic Resonance Sensors Nuclear magnetic resonanCe magnetometers are based on the fac t that the characteristi c atomic frequencies also depend on the strength of magnetic field. In operation , when an atomic nucleu s is placed in a constant magnetic field and is subjected to a high freq uency alternating magnetic field. resonance abso rption of energy from the alternating current fiel d take place . T he ahsorption always takes place at a fixed rat io of constant field strength to the alternating field frequency. Hence, if the freq uency at which nuclear magnetic reson ance take place is measured , the constant field strength can be determined. Resonance magnetometers include a large family of devices that can measure magnetic fie lds of any strength and are subject to wide variety of appl ications in the engineering field.

54

Personnel Tra iJlillX Publications

The ASN T P E RSONN EL TRAINING

PUBLICATIONS

LEVEL

II

55

Chapter 8

Coil Impedance TEST OBIECT There are three fundamental properties of material that affect the eddy currents induced in the test part. 1. 2. 3.

The conductivity of the test object. Thc permeability of the test objec t. The dimensions of the test object.

Conductivity Conductivity of a material is defined as the ability of the material to carry elec trical current. i.c .. the number of amperes of current that will flow through a given size (cross-sectional area) of the material when a given voltage is applied to the material. Thi s del'i nitio n is too cUlllbersome to use with ease in eddy current testing. Instead, the Intern ational Annealed Copper Standard (lACS) system is used. The symbol for conductivity is I (a ) and the units are expressed in percent lACS. In the TACS system , the conductivity of unalloyed (pu re) an nealed copper was selected as the standard , and the conductivities 01' all other materi als are expressed as a percentage of this standard. Unalloyed annealed copper is assigned a rati ng of 100% and a material that conducts electrical current only half as well is rated at 50% l ACS . For example , a wire made of alum inum can carry onl y 01 % of the current that can be can'ied by the same size wire made of pure alloyed copper at a given voltage . Table 8.1 lists the lACS of several materials. The table ill ustrates that materials do have different abi lities to cond uct electrical current. Good conductors include copper and silver, poor conductors include nickel and steel, and non-conductors include wood and glass. Differences in the conductivity of di fferent materials are detectable by eddy current testing due to the effect that the conductivity of the material has on the magnetic field of the exciting co il. Resistivity is defin ed as the ability of mate rial to resist the fl ow of current. The symbol for resistivity is p (rho) and the uni ts are expressed in micro-ohm centimeters (JI Qcm). Conducti vity and resistivi ty work in opposition to each other. Conversions between conductivity and resistivity can be made using simple divisio n. The original copper bar used to establish the standard had a direct current resistance of 0.017241 ohms with a conversion factor of 172.41. 57

To convert to either un it. simply follow Equation 8 .1. 172.41 . %IACS = .,--____-'--..::-'--..:.--E q. 81 Resistivity in micro ohm·cm Table 8.1: Relative conductivity of var ious metals and alloys.

Metal or Alloy

Conductivity, % IACS

Silver Copper, annealed Gold Aluminum Aluminum alloys: 6061·T6 7075-T6 2024-T4 Magnesium 70-30 Brass Phosphor bronzes Monel

105 100 70 61 42 32 30 37 28 11 3.6 3.4 2.4 3.1

Z irconium

Zircaloy-2 Tiraniulll

Ti-6AI-4V alloy 304 stainless steel Inconel 600 Haslelloy X Waspaloy

I

2.5

1.7 1.5 1.4

As resistivity increases. conductiv ity decreases and vice versa. Example Problem: Convert resisti vity of 5.5 micro-ohm centimeters to cond uctivity in % IACS. Solution : %IACS

= 172.41/5.5 micro-ohm = 31.3% lACS

Table 8.2 lists the e lectrical conductivity and rcsistivity of common metals and alloys.

factors Affecting Conductivity While the inherent conductiv ity of a pure material is always the same , there arc intem al fac to rs that can cause what appears to be a

change in the inherent conductivity.

58

Per.wJllI1eI '/i'a;ning Publications

Table 8.2: Electrical resistivity and conductivity of selected metals and alloys.

Metal

Aluminum, pure

Aluminum (99.99%) Antimony

Bronze, commercial annealed

Cadmium

Calcium Chromium

Cobalt Copper Gold Iron, pure

Iron ingot (99.9%) Magnesium , pure Molybden um Nickel Selenium

Si lver, tin solder

Steel, high alloy Tin , pure Tin foil Tungsten Zinc, commercial ro lled

Conductivity MS'm- 1

(%IACS)

35.38 37.67 2.55 25.52 14.62 28.25 5. 10 16.0 1 58 40.60 10M 9.05 22.39 19.14 14.62 8.35 9.63 1.68 8.70 2.44 18.21 16.24

(61) (64.94) (4.4) (44) (25.2) (48 .7) (8.8) (27.6) ( 100) (70) (18) (15.6) (38.6) (33) (25.2) (14.4 (16.6) (2.9) ( 15 ) (4.2) (31.4) (28)

Resistivity Q'm 2.83 x 2.65 x 3.92 x 3.92 x 6.84 x 354 x 1.96 x 6.25 x 1.72 x 2.46 x 9.58 x 1.11 X 4.47 x 5.22 x 6.84 x 1.20 x 1.04 x 5.94 x 1.15 x 4.10 x 5.49 x 6.16 x

10- 8 10- 8 10- 7 10-8 10- 8 10- 8 10- 7 10- 8 10- 8 10- 8 10-8 10- 7 10-8 10- 8 10- 8 10- 7 10- 7 10- 7 10- 7 10- 7 10- 8 lO- 8

(jtQ·cm) (2.83) (2.65) (39.18) (3.92) (6.84) (3.54) (19.59) (6.25) (1.72) (2.46) (958) (J 1.05) (4.47) (5.22) (6.84) (11.97) ( 10.39) (59.45) (J I .49) (41.05) (5.49) (6 .16)

Alloy Composition Alloys are combinations of other metals and/or chemical elements with a base metal. Each metal or chemical element has an individual affect on the conductivity of the base metal. The conducti vity of the base metal changes to a value relating to the composition of the alloy. Thus it is possible to identify basic metals and their alloys by measuring the ir conductivity, but ranges of al uminum alloys, for example , overlap. Examples include copper alloys (90-10 Cu-Ni , 70-30) and 300 series stainless steel (304-SS , 3 16-SS and 316L-SS).

Hardness When a metal or alloy is subjected to heat treatment , the metal wi ll become hardcr or softer depending on the material. This change in hardness is brought about by the internal change in the material that also affects the conduct ivity and/or permeab ility of the material. Thi s change can be detected by eddy current test methods. An improper heat treatment can be detected in this manner.

Classroom Trainin g Series : Electromagnetic Testing

59

Temperature and Residual Stresses The ambient tem perature and intemal residual stresses of a test material al so have an effect on the conductivity of the mate rial . Thcse changes Can also be detected by eddy current testing. An increase in the temperaturc of the material nomlally results in a decrease in the conductivity of the material. Residual stresses cause an un predictable. but detcctable. change in conductivity.

Conductivity Coatings The prese nce of a conductive coating on a co nductive material changes the inherent conductivity of a base meta! just an as an all oy would. However, if the thickness of the cladd ing varies, the conductiv ity will vary. Thi s change in thickness can be detected using eddy current testing methods. As the test coil is influenced by different conductivities , its impedance varies inversely to conductivity. A higher conducti vity causes the test coil to have a lower impedance value . Figure 8. 1 illustrates this concept.

Figure 8 .] : Measured conductivity locus.

t\ I'--

o%

C~nd~ctiLty

i'..

( ai r)

I\,

~. \

\ 2%

si

"

I

10%

1

1\ 1"1

l- l

100% tACS I .....

ReSistance

Thc coil's inductive reactance is reprcsented by the Y ax is, and coil resistance appears on the X ax is. The 0% conductiv ity point , or air point , is when the co il 's cmpty reactance (X LO) is max imum. Figure 8. 1 represents a measured conductivity locus. Conductivity is influenced by many fac tors.

Edge Effects Edge effects can be demonstrated by moving the eddy current probe toward the edge of a test sample and observ ing the change in instrument reading that results. Figure 8.2 is an example of instrument change resulting from edge effect. Test samples must be

60

PersUlme/1i-aillillg Publicarions

large enough to prevent thi s edge interference or probes must be shielded to collimate the f ield . In this panicular case, an edge di stance of 0.5 cm (0 .2 in .) or more must be maintaincd to avoid errors. If test objects have a narrow width, a centering jig or holder should be useel lo maintain the probe on center. Edge distance curves are used to apply correction factors to co nductivity readings on production pans. The same changes occur as the coil approaches the end of a tube. Even shieldecl probes have some edge effect. Figure 8.2: Influence of edge effect o n conductivity m easured by edd y curren t probe: (a) probe at edge; and (b) signal. Eddy

(a)

/

CUlTonL

probe

~N"_"""='

H (b)

A92024 wrought alul~inum alloy,

0 (+)

1Vf?j I / I· ~

17.4 (30)

-, E V,

16.8 (29

) / I

rl

:2: (/) ~

i;.U ._ <

r

>~

.- ~

-g .:....

'"s:

0

temper 6

-

I

)~ II

16.2 (2 8

-

I

-

II

I I

-I 0 1 234~6 (4) (8)( 12)(16)(20)(24) H ) Distance from edge, mOl (10- 2 in.) Skin Effect In many applications , electromagnetic tests are most sensitive to tcst object vari ables nearest to the test coi l clue to sk in effect. Skin effect is a result of mutual interactions of eddy currents, operating frequency, test object conductivity and permeability. The skin effect (the concentrati on of eddy currents in the test object nearest the test coil) becomes more evident as test frequency, test object conductivity and permeahility arc increased. Classroom Training Series: Elecfromagnetic Tesrillg

61

End Effect End effect follows the same logic as skin effect. End effect is the signal observed when the end of a product approaches the test coil. Response to end effect can he rcdueed by coil shielding Or reducing co il length in outer diameter encircling or inner diamcter bohhin co ils. End elrect is a tenn most applicable to the testing of bar or tube products .

PERMEAl3ILlTY FACTORS

Permeability origlnatcs fro m the word permeate, meaning to sp read through. Permeability is the ease with which a material can beco me magnetized and it occurs mainly in the XL component of Z. Soft iron and j ron with low carbon content arc very easy to magnctize and are highly permeable. These magnetic materials readily conduct thc lines of force. Magnetic materials that are hard to magnetize have low permeability. Harde ned ferromagnetic steel with high carbon content is hard to magnetize and has low permeability. Although hard. femllnagnetic steel has low permeability and is difficult to magnetize. It will hold some of the magnetism afte r the magnetizing current is sh ut off. That is how a permancl1l magnet like thc horse-shoe magnet is made. The magneti sm retained in a magnet is call ed residual magnetism. When an energized test coil is placed on ferromagne tic materials, the field is greatly intensified by the magnetic properties of the material so that a large change in the impedance of the test coil occurs . If the magnetic field strength at various locations varies even slightly, these small variations have a large effect on the impedance of the coil. These changes in the impedance of the coil are often so large that they mask all other changes, such as conductivity and dimensional changes. Thi s effect may be overcome by magnetizing the Inaterial to sat.uration using a separate coil energized by a direct current source . Magnetic saturation el iminates or reduces any variations in thc residual magnetic fie ld caused by magnetic variables , and thus all ows other variatio ns to be measured. Eddy cun-ent tests on carbon steel materials where saturation is not used limits the depth of penetration to very small depths. During saturation, the slope of the Band H curve is almost horizontal , and the permeabil ity re lative to air is approximately one (I).

D IMENSIONAL FACTORS Dimcnsional fac tors of the matcrial that are of concern fa ll unde r two t.ypes. 1. 2.

62

The dimension and shape of the test object. The presence of di scontinuities in the test object..

Personnel Training PlIblicari()I1s

Test Object Shape and Thickness Eddy currents do not penetrate throughout thick material but tend to be concentrated near the surface. Thus there is a finite, or limited , depth of penetration. For mathematical reasons , it has become useful to define the standard depth of penetration as the di stance from the surface of the test object to the point where the current density is about 37% of the current density at the surface. The depth of penetration of eddy currents in a nonmagnetic test object depends on the conductivity of the nonmagnetic material (the greater the conductivity, the less the penetration) and the frequency of the alternating current used to energize the test coil (the lower the frequency, the greater the penetration). When the material is thin enough (as shown in Figure 8.3) so that all of the coi l's magnetic field is not used in creating eddy currents , the strength of the eddy currents is reduced. This appears to the test circuit as an apparent difference in conducti vity from that of the thicker piece of the material. Figure 8.3: Effect of material thickness on eddy current tests. AC source

Magnetic

tield

AC source

Magnetic fi eld"" / - \ f

I

I

.... \~

/

1/-.. .

,-, I,... ..... \

If/ ~\'

I ~

1,1

I

Thick

Thin

material

material

\

I

I I,

, : III I \

\\

I' \

-

'/

"

';:::.'/1 /1

" ,/

''/ /

DISCONTINUITIES A discontinuity is defined as any interruption in the normal physical structure or configuration of an object. The flow of eddy currents within the material is affected by the presence of discontinuities , such as cracks, pits , vibrational damage and corrosion. Discontinuities in a test object disturb the normal eddy current flow and this results in a change in the coil impedance . The magnitude of the indication caused by a discontinuity is primarily dependent on the amount of current disrupted by the discontinuity. In other words, the depth, width and length of a di scontinuity determines the change in the eddy current flow, as shown in Figure 8.4. Discontinuities open to the surface are more easi ly detected than subsurface di scontinuities. Discontinuities open to the surface can be detected with a wide range of frequencies; subsurface in vestigations require a more careful frequency selection.

Classroom Train ing Series: Elecrromagneric Testing

63

Figure 8.4: Distortion of eddy currents by a di scontinuity.

I lndicating Source

instrument ~

,.-

/'

\

Test coil

I

\

_______ _- L_ _____

~~,~\

/

'-" Direction o/" coi l's field Direction of eddy current's field _ ___ Discontinuitv ---)I ~ -,-'-i-- Eddy CU ITcnts

Persollnel Training P//hlicmiol1s

Chapter 9

Eddy Current Test Systems and Analysis I M PEDANCE TESTING SYSTEMS Impedance testing systems are the simplest to operate. Most of the portable conducti vity tes ters and di scontinuity detectors s imply dctcct a change in impedance. They do not detect phase shifts. Impedance testing , in spite of the many variahles that can callse changes in impedance, is actually fa irly simple and direct. The most difficul t task fo r the technician is the abil ity to recognize with any degree of certainty whethe r a change in impedan ce is due to a change in li ft-off or due to a change in conductivity. The presence of discon tinuities in the materi al will also cause changes in the impedance. Since discontin ui ties arc indicated by sudden changes in indications as opposed to gradual changes for all the other conducti vity factor s. the technician has no difficulty in differe ntiating the two types.

PHASE ANALYSIS SYSTEMS Because a change in impedance is accompanied by a shi ft in phase, it is possible to observe the phase shifts rather than the impedance change to dete rmine the conditions that exist in the material. The impedance of a co il may be represented by a vector. whose length represents the impedance value and whose direction represents H phase angle (the angle by which the cunent lags behind the voltage). These vectors may be measured and plotted on a chart know n as the impedance-plane diagram. The impedance-plane di agram may be pl aned either by knowing rhe impedance value and the phase angle o r by knowing the resistive component and the reactive component.

Conductivity on the Impedance-Plane Diagram Figure 9 .1 is a typical impedance-plane representation of sever al obta ined by eddy currcnt testing from several different types of non magnctie metal s with the same test set and at the same frequency with diffe rent conducti vity . The lag angles and impedance values ohtained for each of these materials, including the val ues obtained when the coil was held in air, are shown by their individual

impeda nce ~

vectors .

65

Figure 9.1: Impedance vectors shown on impedan ce-plan e diagram. 0% lACS

1 8

o

Coil

rcsi,~ tance

In Figu re 9.2. a curve is drawn connecting the impedance value of each or the vectors. Thi s curve then is the locus of all the impedances that will result from changes in conductivity. If all other factors are held constant . a change in conductivi ty of any material will result in an impedance value that will fan somewhere on this curve . An increase in conductivity will cause the impedance to move to the ri ght and down along the curve, while decrease in conductivity will cause the impedance to move up and to the left. Figure 9.2: Conductivity locus o n impedance-plane diagram for no nmagn etic materials. 90'

-

t-

"u v"" "u

-

~

u >

V

=>

-

~

e

l-

u

I

0

66

I

I

Coil rcsi.stance

Per.wmnel Training P/lblicaliolls

I

I

Figure 9.3 illustrates the locus of all the impedances that will res ul t as the coil is lifted off the 100% lACS material (all other factors held constant) . When the coil is held in contact with the 100% lACS mate ri al. the locu s is on the conductiv ity curve . As the coil is li fted off the material. the impedance moves in the direction indicted by the dashed line. As the coil is fUlther removed , the impedance movcs furt her along the dashed line until the material no longer affect s the coil (the co il is in air). At this point the impedance is back on the conducti vity curve at 0 % lACS as shown.

Figure 9.3: Lift-off locus on impedance-plane diagram. 90'

0% lACS (Air)

i

"u

Conductivity locus

u"" ~

" >

<:; ~

] '0

u

100% lAC ~~~

0

__~~~~__~__~__- L__-J

O'

Coil resistance

Figure 9.4 illustrates a family of 1itt-off curves - each one starting at a differe nt conductivity. Note the angles (A and B ) formed at the junctions of the lift-off locus curves and the conductiv ity locus curve. In the areas of low conductivity on the diagram. the angle (8 ) is small. 1n the areas of high conductivity. the angle (A) is larger. Thus the impedance-plane di agram shows that in materials with hi gh conductivity. it is e,"sier to detect a change in lift-off. 1n areas of high conduc tiv ity on the diagram, a change in cond uct ivity is mostl y a change in the coi l's induc tive reactance (venical). whi le changes in lifl-off are 1110stl y a change in the coil's resistance (horizontal). Due to thi s large difference in direction. the effect of lift-off may be distinguished from the effect of a change in conductivi ty.

Classroom Trainillg Series: Electromagnetic Testing

67

Figure 9.4: Family of lift-off loci on impedance-plane diagram.

90 ' 0% l ACS

i Q

V

::

'" "~

U

">

u

Angle A

"::

."

8 ~--~--~--~--~---L

0

Coil resistance

__

100% lACS

- L_ _~_ _~ O '

•

Effect of Frequency on Impedance-Plane Diagram Tf thc frequency of the voltage applied to the coil is changed , there is a change in the resisti ve and react ive facto rs of the impedance. If an impedance-pl anc diagram of conductivity is then prepared at the new freque ncy. the conductivity loc us curve will be d ifferent than that obtained at the old freq uency. Although the specific values are different , thc two curves are very simi lar in appearance. Figure 9 .5 shows three typical conductjvity loci that might be obtained at three different operating frequencies. Note that at higher frequencies, the locus of thc point representing any given material shifts toward s the lower end of the conductivity curve. Also note that at lower test frequency, coppcr and al uminum are widely separated on the conductivity loc i curve whi le at the highe r test frequencies, copper and alumi num are much closcr together on the lower end of the curve .

68

Personll el Training Publications

Figure 9 .5: Frequency effect o n impedance-plane diagram: (a) low frequency (20 KHz); (b) med ium frequency (100 KHz) ; and (c) high frequency (1 MHz). (a)

90'

Ai r Graphite

Bron ze

2024

'0

CU O·

U '---'-~'-:-;-""""---'_-L-::-'-_-'--.....J

o

(b)

90'

Coil resistance ----"-

Air

t1:l

u" ~

~

~

Bronze

~

>

:i

~~

(j Coil resislance~

0

(e)

90'

Air Graphite

t ~

u

~

304

~

u @

">

'B =>

.:: "" '0

u

0

Coil resistance----+

Classroom Training Series: Electromagnelic Testing

69

Effect of Material Thickness Figure 9.6 shows the effect of thickness changes as applied to brass. The thickness loc i may be plotted by measuring the resistance and reactance of a very thin sheet of brass and repeating the proccss for steadily increasing thicknesses. Ultimately the depth of penetration limit is equaled and the resistance and reactance val ucs reach the cond uctivity curvc. Further thickness incrcases will havc 110 effcct.

Figure 9.6: Thickness variations of brass on the impedance-plane diagram. 90'

A~ll \ ~~~=-____~T~hj1ckness

\

r

\

\

\

'\

'\

"-

"-

"-" ( "//j,

"-

o/(.

4

""

'0

u

"-""............................ 120kHz Th.ickness in mils L---~~--~--

o

__

~

____

~

____

~

__

~

Brass __

____-L__

Coi l resistance- - - - -•• Figure 9.7 shows thickness loci for several different types of metals. Since the eddy currents gradually become weaker and weaker as they approach the depth of penetration limit, thickness variations have less and less effect (note how the distance between po ints on the cu rve become shorter for the same change in thickness as the material thickens). Hence, the sensitivity to vary ing thickness becomes less and less as the material thickness increases.

70

Persol1nid Trainin g Publications

AL CU ~O '

Figure 9.7: Thickness variations of several metals on the impedance-plane diagram.

i ,..

"= E '"",..~

--

>

u = "= 0

U

" l20 kHz Thickness in mils

Coil resistance

Effect of Frequency on Thickness Measurements Figure 9.8 shows the effect that a change in frequency has on thickness measurements of brass. Since a changc in frequency changes the depth of penetratio n, a lower frequency will give a greater depth of penetration when the conductivity eurve is reached. The thickness points that are plotted will be located higher on the thickness curve wi th more distance between them . Raising the frequency will have the opposite effecL The depth of penetration will not be as great as the points that are plotted . These will be located further down on the curve and closer together.

Cla ssroom Trahling Series : Electromu;,:neric TeSTin g

71

Figure 9.8: Effect of frequ ency on th ickness measurements. Air

Air

i 2

3 4

60 kH z Thickness in mils

120 kH z Thickness in mi ls

Coli resistance- -

Coi l resistance--+-

Suppression of Nonrelevant Variables In the type of eddy current equipment where the bridge networks may be balanced (or nulled) by adjustment of resistance and reac[ance, the bridge may be set to operate from any point on the impedance-plane . Figure 9.9 illustrates the basic concept of this approach.

Suppression of the Lift-Off Variable Figure 9 .9a shows the selection of test poinls that could be used suppress the lift-off vari able. Suppression of Ihe lift-off variable is accomplished by simply selecting a tesl poi nt D so that the distances D-C and D-A are equal. Thus a slight Change in li ft-off will not affect the overall reading . The test po int selected could lie anyw here along the line D-E. If Ihe test point selected is too far away from the conductivity curve it is poss ible that the meter will be driven off scale , but selecting a test point conside rably away from the conductivity curve will produce the best overall results. If test point D is selected as the lest point and the probe is moved [rom alloy A to alloy 8, the meter reading will decrease. If point E is selected as the test point , the meter reading will increase when the prohe is moved from alloy A to alloy B. It is possible that better sensitivily can be obtained from a test poi nt located on one side than

10

72

Persollnel Training Publicariull.'i

on the other. In practice, the best procedu re is to evaluate test points On both sides . then choose whichevcr gives the best results.

Suppression of the Conductivity Variable Tn measuring the thickness of nonconducti ve coatings on conductive materials, it is necessary that the lift-off variable be measured and that the conductivity variable be suppressed. To accompl ish thi s, a test point is selected (sec Figure 9.9b) that lies on a line that is perpendicular to the conductivity curve althe point of interest.

Figure 9.9: Test point selection. (a)

90' I

Conduc tivity

iD

1ft III

/I I ,II

,I! II

Lift-off

, Alloy B

--''-I-.l- A lI oy A CI

I

I

EJ

~~~--r---~~~--~--~--~--~ O '

o

(b)

Coi l resi,tan c e - - - -

90'

Conductivity

Conductive

,uppresslOns line Lift-off

l.-

B __ - - -:::'=;-~D

--- -

----::.--

E

Bare metal

A

':0U ---,--,----,C;;;~;,;;;;:;;=~==------, O' Coi l resistance Classroom Training Series: Electromagnetic Testing

73

Conductivity and Permeability For magnetic material s . the lirt-off and mag netic pe rmeability loci curves arc virtuall y superimposed (Figure 9.10a) b ut their res pective values increase in oppos ite direc tions , Figure 9. 11 shows that the reactance component of the test coil impedance is decreased by the presence of non mag netic materials. Thi s reactance reduction occurs because induced cun ents Il ow in the conduc ti ve and non magnetic object ancl set up a secondary field that panially cancels the p ri ma ry field of the coil. The opposite is true when a magnetic material such as iron or feuite is placed within the fiel d of the coil. This happens because the presence of the magnetic field intensity of the primary coil field causes atomic mag netic clements of the magnetic material to become al igned with the field . increasing the Dux de nsity, The magnetic pcnneabi lity fI is the rati o of nux densi ty B to magnetic field intensity H: Eq. 9.1

~

~

R

H

where B is magneti c flu x dens ity (tesla) and H is magnetizing force or mag netic field intensity (A-m- l ). T he nickel zinc fen-ite cores (F igure 9.IOb) were chosen as examp les because they have a low conductivity and two different values for permeability, The effect of the inc reased flux density gives a greater induced voltage in the test coil that in turn raises the iIl1pcdance. The increase in impedance is in the reactance direction except 1,)1' the effect of a smaU amount of energy loss resulting from hy steresis. The nickel zinc fenite cores may have an initial penneabil ity of 850, high on the pe rmeability line of Figure 9. 1Oa, Usually practical engineering materials also have an associated elecuical conductivity that affects the impedance, as sho wn for 422 steel and 4340 steel in Figure 9.IOb. The relative relationship of the permeability loc i lines and the conductivity curveS for three material s are shown in Figure 9. 10c. The vec tor or phasor va lues of inductive reactance and resistance fo r different material cond itio ns yield un ique loci or phasor plots on the impedance-plane at particular operat ing frequencies . The phase angle of the im pedance vectors wiU change at different frequencies because the inductive reactance value is a function of inductance and frequency. Hence vector points may move relative to one another along the conditional loci curves whe n the operating frequency is Changed, This shift in phase is shown in Figure 9.12 for the conducti_vity val ues of nonmagnetic materials. Sim ilar phase angle cha nges for the permeability of 4340 steel are shown in Figure 9.13 as the frequency changes from 7S to 300 kHz . These changes in phase sh ift at diCferent freque ncies do not interfe re with impedaneeplane analysis . provided that the operator is aware of this factor. In some cases. test results may be improved by changing the freque ncy to cause phase shifts.

74

Persollnel Trainillg Pllblicmiol1S

Figure 9 .10: Permeabili ty, lift- off an d conduct ivity loci on im pedan ce-plan e: (al permeability and lift-off locus; (hl permeability loci for d ifferent m aterials; and (c) loci for permeability p and con ductivity <5.

(a)

Permeabili ty JI

/

Lift-off I

lA.ir

Magnetic Nonmagnetic

Copper

25 kHz

Resistance R (ratio)

(b)

Nickel zinc (JI = 125 or 190) JI = 125 (nickel zinc)

~

Unified N umbering System

.J91422 alloy steel casting

Air

Unified Numberin a System G43400 nickel chrome mofy bc1enum alloy steel

· M . T·ltanmm I .uanetlc

&Brass

Nonmagnetic \' Aluminum Copper 25 kHz

Resistancc Ii (ratio)

(c)

Nonmagnetic

Legend

Res istance R (ratio)

PI,ITl =fcnite~ 1' 0.<52 =steel

fij ; 0"3 =nickel

Classroom Training Series: Electroma/1netic Tesling

75

Figure 9 .11: Lift-off an d edge effect loc i on impedan ce-plan e: (a) li ft-off loci; and (b) edge effect loci.

(a)

S

Lift~ Off

U ~ ~

Unified Numbering System 043400 steel alloy 0013 em (0.005 in.)

a 025 em (Om in.)

R Ai r

oms em (0,015 in.)

0.05 1 elll (0.02 in.) Ma"nelie 0 .1 em (0.04 in.J )\lolllllagnelie 0 .076 em (0.03 in.) 0.05 1 em (0,02 in, ) 0.025 em (0 .0 1 in.) Lift-off Un iricd Numbering System A97075 wrought aluminum alloy

~

Resistance R (rat io)

(b) Uni fied Numberi ng System 04340 sleel alloy ~

.!!

1i ~

,.<.> 0:':

Magnetic

"TI

~ ~~~--------------~--­

Nonmagnetic

Lin-off Un ified Numberin. System A97075 wrought al uminum alloy Resistance R (ratio)

76

Personnel Trainin g Pllhlicario1!.\'

Figure 9.12: Movement of material points by frequency changes: (a) low frequency, 20 kHz; (b) medium frequ en cy, 100 kH z; and (c) high frequency, 1 MHz.

(a)

Graphite Air ~ co ~

§!

:R." '<

Conductiv ity

(b) Air

UNS R56401 UNS S30400

t -:;; ;;

~

Lift-

off

' Conductivity Grap hIte

t

Lif t-

off

Bronze

~ ~

"

.'3 ~

~

UNS A97075 , temper 73 Co er Resistance R (relative scale) (e)

-g

UNS A97075, temper 73 Co er Resistance R (relative scale)

ConductivilY

Air

Legend Graphite

?

UNS R56401 UNS S30400 8r nze

NS A92024, temper 3 UNS A97075. temper 73

UNS A92024 = Unified Numbering System A 92024 heat treatable wrought aluminum alloy UNS A97075 = Unified Numbering System A97075 heat treatable wrought aluminum alloy UNS R56401 = Unified Numbering System R5640 I titanium alloy UNS S30400 = Unified Numbering System S30400 austenitic chromium nickel stainless steel

Re sistance R (relati ve scale)

Figure 9.13: Phase angle changes on impedance-plane caused by frequency changes.

"

'"u

Steel

~

">-

!" "u

"u E

.=" '0

250 ~~::::::::"""'----1300

kH z

Air

Resistance R (relative scale)

Classroom Training Series: Electromagnelic Testin g

77

With phase analysis eddy current instruments , an operator can produce impedance-plane loci plots or curves automatically on a fl ying dot oscilloscope or integral cathode ray tuhe. Such impedance-plane plots can be presented [or the following material conditions , as shown in Fi gure 9.14. 1. 2. 3. 4. 5.

6. 7.

Lift-orr and edge effects. Cracks. Material separation and spacing. Permeability. Specimen thinning. Conductivity. P lating thickness .

Figure 9. 14: Impedance changes in relation to one another on impedance-plane.

Lift-ofr "0

..::..

Ma!lnetic Airf...=----+------~5~~-

Nonmagnetic

'" Pa/t' I

~

Copper

I

Aluminum

' pc

Resi stance R (relative scale) Legend c. l = crack in almninum

C:<.; ::; crack in st.eel p~ ::; plating (aluminum on copper)

P~ ::; plating (copper on aluminum) P = plaring (nonmagnetic) ~ :;::; spacing between alum inum layers T = thinning in aluminum jI = permeability Om ::; conductivity for magnetic materials On ;;;; conductivity for nonmagnetic materials

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Personnel Trainill g Publications

Evaluation of these plot, shows that ferromag netic material conditio ns prod uce higher values of inducti ve reactance than values obtained from non mag netic material conditions . Hence the magnetic domai n is at the upper quadrant of the impedance-plane . whereas nonmagnetic materials are in the lower quadrant. The se paration of the two domains occurs at the inductive reactance values obtained with the coil removed from the conductor (sample); this is proportional to the value of the coiJ"s self-inductance L. Linear material values do not prod uce linear responses on the impedance-plane loci . With the eddy currcnt probe halanced on the mctal specimen, the loci values for linear material conditi ons arc displayed as fo llows. I.

2.

3. 4. S. 6. 7.

Magnetic and no nmag netic lift-ofr conditions arc displayed logarithmically (in X). Magnetic and nonmag netic edge effects arc displayed logarithmica II y. Magnetic and nonmagnetic conductiv ities vary with test frequency. Magnetic permeability varies wi th tcst frequency. Metal thinning varies exponentially. Nonmagnetic plating thickness is displayed logarithmically. Material spacing or separation varies exponentially.

Electromagnet ic induction effects arc not easy to understand. Nei ther the mag netic field s no r the eddy CllD'ents can be seen. In a proble m solving situ ation. impedance-plane analysis is a useful tool because it improves the ability to detect various conditi ons and provides a better understandjn g and interpretation of the eddy current test results.

CATHOD E

RAy

T UBE METHODS

Cathode Ray Tube Vector Point Method When a cathode ray lUbe (CRT) js provided as pan of the tcs t equipment , the equipment may be set up to show on the tube the locus of all the points in which the technician is interested. Thus, the operator may construct. point by point, the impedance-p lane diagram directl y on the tube rathe r tha n on a separate sheet of grap h paper. During actual testing of spec imens, the impedance of the coil will cause a dot to appear at some point on the screen . Its pOSition wi th respect to the impedance-plane diagram tells thc technician what has occu rred within the test object. An advantage of using a cathode ray mbe is its extreme fle xibility. For example. the equi pment may be set up so that the display is rotated to a position where a change in lift-off would move the dot left or ri ght, while a change in conduc tivi ty would move the dot up or down . The prese nce of a

Classroom Training Series: Electm11laglleric Testing

79

disconti nuity would cause the dot to movc up and to the left , as shown in Figure 9,1 5,

Figure 9,15: Rota tion of cath ode ra y tu be display. Conductivity

Conductiviry

Liftoff Lift-

off

Cathode Ray Tube Ellipse Display Method A CRT may also be set up to compare a test object with a reference standard . The ellipse method uses an in spection co il in conj unction wi th a reference coiL When a standard is pl aced unde r the reference coil and the tes t object is placed under the inspection coil, the CRT shows the phase relationshi ps between the signals obtained . This com parison between the signals provides indications of the dimensio n variable and the co nductivity variable , T he d imension vari able and the conductivity variable arc shown on the CRT by the width of the elli pse and the angle tilt of the axis , F igure 9 .1 6 show s CRT displays fo r dimension and conductivity .

MODULATION A NALYSIS A modulation analysis system , shown schematically on Figure 9_17 , adds a mOdulating device between the test set and an indicating device - a strip chart recorder. Tlle mod ulating de vice is simply an electronic l1lter that will pass only certain frequencies . In modulation analysis a differentia] coil is used so that two adjacent areas of the art icle are compared.

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Personnel Training Publications

Figure 9.16: Cathode ray tube displays for d imension a nd conductivity.

Dimension

Small change

Large change

Small change

Large change

Small change

Large change

Conductivity

Both dimension and conducti vity

Figure 9.17: Modulation analysis system. Test set

Modulating device

Indicating dev ice

Vertical marks on paper

9111111

Classroom Training Series: Electromagnetic Testing

81

As the test specimen passes under or th rough the coils at a constant rate , various variables being sensed cause the equi pment to register a signal. A discontinuity such as a crack will be ind icated by a sharp rise in signal, followed immed iately by a shalV drop. A dimension change. on the other hand , is most likely to occur graduall y. Thus by using ei ther a low frequency or high freque ncy til ter, the effect of one variable or the other is eliminated fro m the strip chart readout. Figure 9.18 illustrates this .

Figure 9.18: Elimination of n oise.

No filters

More filters needed

Crack signal Good fi Itration

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Personnel Traillinx Publications

Chapter 10

Selection of Test Frequency TEST fR EQUENCY T hc freq uency of an altcmating cun-ent is defined as thc number of cycles (one complete cycle of cun-em) that occur in one second. Its unit is the hertz - one hertz ( I Hz) being one cycle per second. Thus, house cun-ent at 60 cycles per second has a frequency of 60 Hz. The mos t important parameter of thc test system that affects the depth of pcnetrat ion is the test frequency. The type of alloy involved and the varia bles to be measured or suppressed detenlline thc best frequency. The depth of edd y cun-ent penetration within test mate rials is strongly affected by test frequency, penlleability and conducti vity. For a given alloy, higher frequenc ies normally limit the eddy cun-ent test to inspection of the excited mctal surface nearest the primary coil wind ing. Lowe r frequencies permit deeper eddy current penetration. A given test freque ncy wil l all ow eddy CUITents to penetrate deepe r in lower conductivity all oys tha n in higher conductivity alloys. High test f requenci es arc no rmally used for detecting small surface cracks or surface contamination and for gaging thin coatings. Med ium frequencies are useful for conductivity measurements such as alloy sOl1ing. Low tes t frequencies arc us ually required for tcsting thicker materials (for oppos ite side con·osion, for example), for thickness gaging and for penetrating into magnetic materials. Penetration depth . however. is on ly part of the process for selection of opti mum eddy cun-ent tes t frequenc ies. The geometric relation ship between the impedance curves for the variable magnitudcs (di fferent conductivity points or lift-off po ints) along Lheir respective impedance curves is impol1ant.

DEPTH OF P ENETRATION Eddy currents are not unifo rml y distrihuted throughout a test object. They arc mostl y de nse at the sud·ace closest to the coi l. and become progressively less dense with increasing d istance below the surface of the materi al. At some di stance below the surface of a thick material there will be essen tiall y no curre nts fl ow ing. The depth of penctration is affected by the freq uency. conductivity and permeabi lity of the material.

83

j.

2. 3. 4.

The depth of penetratio n decrcases as the frequency increases . The depth of penetration increases as the frequency decreases. The higher the conductivity, the less the penetration. The highe r the permeability, the less the penetration

Note: When testi ng ferromagnetic materi als, the pelmeability factor will have no effect on thc depth of penetration if thc test Object is magnetized to saturation by a separate direct current coi l. The standard depth of penetration is defin ed as thc depth at whi ch the current strength has dropped to 37% of the ClJn'ent de nsity that exists at the surface. Fi gure 10.1 shows the distribution of eddy curre nts in a material. Figure 10.1: Variation in eddy current de nsity.

1.0

I

--1-

O.81+--+- + Standard depth of penetration where density of eddy current 0.6 f--\--t--j - 37% of de nsity at the surface 0.4

I

\ 11 __

I

j

=

1

LJ _ -1-----7

0.2

o

2

3

4

5

6

Units of depth (in multiples of the standard depth of penetration)

If the standard depth of penetration exceeds the thickness of the matcrial under tes t, the restrictio n of the eddy current paths appears as a change in the conductivity of the matetial. The coil response then retlects the thickness of the material. Edd y cu rre nts do not cease to exist beyond one standard depth. Normall y the material must have thickness of two or three times the standard depth before thickness ceases to have any effect on the test coil. A depth of penetration formula using res isti vity, frequcncy and permeabi lity can be expressed as:

Eq.l0.l i5 ~ K ~P I (jXJ.lIT,) where i5 is depth of penetration , K is constant (1.98). P is resist ivity in m.icro-ohm-cm ,f is frequency in hertz , and Jt,d is I for nonnlagnelic materi als.

84

Personnel Training Publications

The K factor beco mes 1.98 (if working iil inches). Assuming this will be 2, the formula then becomes simplified as foll ows:

Eq.l0.2 8=2JPI(JX P",) Example Problem: Calculate the standard depth of penetrati on in a pure aluminum sample at a test frequency of 1DO kHz. Conductivity of aluminum SDP

= 2.83 micro-oh m centimeter = (ReSistivity I Frequency) = « 2.83 x 0.00 1) I 100) x 1000 = 0.0283 cm

Another approach to freque ncy selecti on uses argument "A" of the Bessel function where argument "A" is equal to unity or I .

Eq.l0.3 A = f p,,,a d' 5066 where f is frequency, fl rei is relative permeability, a is cond uctivity in meter/ohm-mm 2 and d is diameter of test object in centimeters. A freq uency can always be selected to establish factor " A" equa l to I . This freq uency is knowh as the limit f requency and is noted by the term fg. By substitu ting 1 for factor "A" andfg fo r f, the equation becomes: fg p ad'

Eq. 10.4 1=

"I

5066 5066

jEi = u a d' ,

r~1

wherefg is limit frequency in hertz,Prel is relative permeability, d is diameter of test object in centimeters andais conductivity in meter/ohm-nun 2. Limit frequency (fg) is then establi shed in terms of conduct ivity. permeab il ity, dimension and constant (5066). Since limit frequenc y is based on these parameters, a method of freq uency determination using a test frequ ency to limit frequency ratio.f!fg can be accomplished. High.f!fg ratios are used for ncar surface lests, and lower jlfg ratios are used for subsurface tests. Figure 10.2 shows the standard depth of penetration fo r several materials with different conductivities at various operating frequcncie.s.

Classroom Trainin g Series: ElectromagneTic Tes/inK

85

Figure 10.2: Standard depth of penetration versus frequency for different types of material. 10

I""'-

.....

i'-- :---

,..,

l""'- i..[

.......

v'"

.s'"

Gr~~hite'

"'-I'-

i'--

0. 1 ::::....

.....

l""'-

I

Titanium

./. .....

....... l::"-

Stainless steel

ii-

I Aluminum

:;

~

~

~

0 -'=

.......

"'- ~

::::....

am

----

Ingot iron

Q.

II

OJ

Q

I'--

:::::-

High- alloy steel/

Copper

I I III 0.0001 10-2

I I II

10-1

10 1

----

::--

-

----

........ ~!-

0.001

......

"'-

----

i'--

----

...... ;;::::

1:::--

102

Frequency (kHz)

Single Frequency Systems These systems are capable of energizing the test coil with a single frequency. Frequency selection often becomes a compromise. It is common practice in in-serv ice in spection of thin wall, non ferromagn etic tubing to establish a standard depth of penetration just past the midpoint of the tube wall . This permits about 25 % of the available eddy current to flow at the outside surface of the tube wall. In addition, this establi shes a phase difference of approximately 1500 to 1700 between the inside and outside surface of the tube wall. The combination of 25% outside, or surface current, and 1700 included phase angle provides good detectability and resoluti on for thin wall tube inspection.

Multi-Frequency Systems The frequency choice discussed previously deals with coil systems driven by only one frequency. Test systems driven by more than one frequency are called multi-frequency or multi-parameter systems. It is common for a test co il to be driven with three or more frequencies. Although several frequencies may be applied simultaneously or sequentially to a test coil, each of the individual frequenc ies fo ll ow rules establ ished by single frequency methods. Signals generated at the various frequencie s are often combined or mixed in electronic circuits that algebraically add or subtract signals to obtain desired result. The technician must have a good working

86

Personnel Training Publicatiolls

lOS

knowledge of current density and phase relationships in order to make intell igent frequency choices. Modern multifrequency eddy current systems allow the techn ician to perform disco ntinuity detection using the differential mode and absolute mode simultaneously. These systems generally use the time sharing technique to allow testing at four frequencies at the same time. Each frequency channel is capable of generating differential or absolute data. The vector analyzer used for multifrequency eddy current examination has the ability to display the signals from all four frequencies and the two mixers si multaneously. This allows the interpreter to evaluate all informat ion available from the examination at the same time .

Classroolll Trainiflg Series: ElectromagneTic Testing

87

Chapter 11

Coupling LIn-Off

AND FILL FACTOR

Lift-off and fill factor are terms used to describe any space that occurs between the test object and the inspection coil.

Lift-Off Lift-ofr is a term used to describe any s pace that occurs hetween the lest object and the inspection coi l. \\lhen a surface co il is energized and held in the air above a conductor. the impedance of the coil has a cerlain value . As the coil is moved closer to the conductor, the initial value will change when the field of the coi l begins to intercept the conductor. Because the field of the coil is ::.;trongest close to the coi ll the impedance value will continue to change until the coil is directly on me conductor. Con versely, once the coi I is on the conductor, any small variation in the separation of coi l and conductor will change the impedance of ti1e coil.

Fill Factor Fill factor is a telm used to describe how well a test object will be electromagnetically coupled to a test coil that surrounds or is inserted into the test object. Fill factor pertai ns to tests that use bobbin or enc ircling coils. II is necessary to maintain a constant relationship between the

diameter of the coil and the diameter of the test object. Small changes in the diameter of the test object can cause changes in the impedance of the co il, This can be useful in detecting changes in the diameter of the test object, but it can also mask other indications, Fill factor can be described as the ratio of rcst object diamerer squared to coil diameter squared ro r a circling coil, or vice versa for a bobbin coil where the inside diameter is used . Eq.11.1 _d'

D' The fill factor will always he a number less than one, Tt is necessary to have a means to guide the test object or coil through the

center at all times to maintain a fill factor of constant value , as shown in Figure 11. 1.

Figure 11.1: Distortion of eddy curren ts by a discontinuity. Material

lmpedancc indica tion

Lift-off

... Conductive material

Calculation of Fill Factor Inside cuils: \vherc:

Fill fac tor = d 2/0 2 d = co il windings inner di amcter D = tu be internal dia meter

Outside coils:

Fill factor = d 2/D 2

where:

D = coil windings in ner diameter d = tes t object outside dia meter

Example Problem : What is thc fill factor for inspecting 0.75 in. 0.049 in . of wall thick ness copper tu bes using a 0.610 in . out er diameter bobbi n probe?

Solution:

Fill factor (bobbin coil) = d 2/D 2 d = 0.610 0= 0.75 - (0,049 x 2) 0= 0.75 - 0.098 = 0 .652 Fill facror = (0.610)2/ (0.652)2 Fill factor = 0.372 x 0.425 Fill facto r = 87%

Example Problem :Wbat is the coi l winding's in ne r diameter for ins pecting I in. (2.5 em) solid rods using a fi ll fac tor of 90 %?

Solution:

Fill factor (encircling coil) = d2/0 2 Fill fac tor = 90% or ().9 d=l 0 .9 = 12m2 0 2 = 110.9 = 1.11 1 0 = JI.llI = 1.05 in.

Persunnel Training Puhlications

X

Chapter 12

Electromagnetic Testing Applications EDDY CURRENT ApPLICATlO S Applications of eddy current testing in industry arc numerous and widespread. Eddy current testing can detect discontinuities that lie in planes transverse to the eddy currents present in the material or detect thickness changes and measure nonconducti ve coating thicknesses . Di scontinuities include cracks, scams. laps, pits and laminations at cut edges of sheet or plate. The presence of a discontinuity is characterized by a sudden, brief change in impedance as the test coil moves across the discontinuity. Readings arc calibrated with standards hav ing known discontin uities , either natural or manufactured. in the same kind of material.

Aerospace Applications Applications of eddy current tests in the aerospace industry include the follow ing.

Measurement of Metal and Coating Thickness In the aerospace industry, numerous metal parts are coated or plated to obtain special slIIface properties such as corrosion resistance, wear resistance or improved appearance. These coatings arc prepared by various means, incl uding electron-deposition. hot clipping, cladding and spraying. For controlling the thi ckness of the deposit and also for testing purposes, reliable and rapid measurements of coatings arc necessary. Four general types of coating and base materials lend themselves to eddy current testing. The operatin g procedure used with the. test instrument is determined by (he specific combination of materials. These material combinations are classitied as fo llows.

J. 2.

3. 4.

Metal foil and sheet or metal coating on a nonconductive base material , such as metall ic film on glass, ceramic or plastics. Metal eladding with higher conducti vity than rhe base metal. such as copper. zinc on steel or pure al uminum on an alumin um alloy. Metal cladding with a lower cond uctivity than the base metal, such as nickel on al uminum. Nonconductive coating on a metallic base materi al, such as anod ic film or paint on aluminum, or other organic coatings on metals.

91

REFERENCE STANDARDS FOR THI CKNESS TESTING For eddy current thickness tests . at least th ree objects with known metal thickness are needed as reference standards to calibrate the equipment. One reference stan dard represents the minimum acceptable thickness. the second represents the maximum acceptable thickness and the third is from the middle of the range . All calibration standards must have the same conducti vity, permeability, substrate thickness and basic geometry as the test objects.

Metal Thickness Tests for measuring metal thickness are generally used on chemically milled sheet stock, thin wall tubing, metal foil bonded to nonmetallic materials and any objects that may experience thinning from corrosion.

Conductive Coating Thickness Claddi ng thickness measurements are used in two types of situations where conductivities of two metal layers are very diffe rent . The first situation, in which the cladding is a better conductor than the base metal, pertains to coppe r, zinc or cadmium coatings on steel base materials . The second situation , in which the claddi ng is the poorer conductor of the two metals, pertains to lead coatings on cop per or to nickel coatings on aluminum .

Metal Spacing There are times when a gap separates two metal sheets . The gap may be tilled with a nonmetall ic shim or may be purposely produced as a fi xed dimension. If it is desired to measure the gap or spacing, an operating frequency must be chosen so that eddy cu rrents will be generated in the second (subsurface) layer. The frequency chosen should produce eddy currents that penetrate both metals to a depth of 3 x conductivity, as shown in Figure 12. 1. Gap or spac ing measurements are best made with the probe located over the thin member; thh reduces the problem with depth of penetration. If both members are equally thick but one has a lower conductivity, the probe should be placed on the low conductivity test object.

92

Per.mnllel Training Publicarions

Figure 12.1: Probe coil impedance curve showing effect of metal spacing between two flat, parallel, metallic conducting sheets.

g-.l

:2

B ~

i

!

Air

Spacing

I

J.
" >

";:;

1!"

J.

(maximum gap) Conductivity locus /

Lift-off locus

-+-- Thinning

/P' ----

locus

Metal spacing (zero gap)

_____ Conductivity locus

(infinite thickness) Resistance R (relative scale)

TESTS OF M ETAL CONDUCTIVITY Eddy current conductivity meters usually differ with respect to operating frequency, lift-off. temperature. sensiti vity and probe size compensation or means of presenting test results . Most conductivity meters can provide results as a percentage of Internat.ional Annealed Copper St.andard (%IACS). Other instruments can provide conductivity measurCll1cnts in sicmcnsper-meter or as cu stomized displays of signal amplitude or phase angle (Figure 12.2).

Testing of Bolt Holes Small cracks in material located next to fastener holes may go undetected until the cracks grow til a size that allows detection at the sULface not covered by the head of the fastener or nut. A crack of this magnitude and under high stresses may continue to grow to failure at a rapid rate. Eddy current testing is a reliable way to detect cracks in material adjacent to fastener holes. Eddy cu rrent tests are well known for detecting small fatigue cracks, particularly in aluminum fastener holes . Automated bolt hole scanners provide reliable and repeatable test results. Finding slllali cracks at fastener holes sometimes requires removing the fastener and perfonning an eddy eunem check. Automatic scanning Illay be used to detect cracks as small as 0.013 CIll «(l.ODS in .). Figure 12 .3 illustrates an eddy current bolt hole probe.

Cla.rsro01Il- Traillillg Series: Electromagnetic Testing

93

Figure 12.2: Relative conductivity of metals and alloys shown by eddy current meter readings. Unified Num bering System (UNS) is used here for wrought aluminum a lloys.

-- ISH ~~~-=:==:=:=tl ~

!

~

8" b

80

~

__Sil ~er. coppcr~ I

-Pure aluminum:: Go1J ..... I

~g r-

-1-UNSA91IdOalumin~ I

JMagnesium. rhouiurn----cr-I

50 -

I

- --UNS A95052 alu,';illllm-i-...'>. .

40

Cadmium'____. - ",- Zinc

Berylhum copper::-...

20

~

Chromium \ ~

10 9

8

7,

5

Platinum

,

F

".t

~ Tin I --- ~luminum bronze

~SiliconI bronze _~_

other maone tic

metals are in region to IeIT

,

1

1

-1 ~ ' _

1_Niekel steel and=r:- -'_----,_

4 - -

I

Yellow brass , cobalt

'-I~:~~p~'~r c~;~~

Lcad -+- Nickel silver I

' I

I ~

- l\llolybdcnum

30 Tungsten, UNS A92024 aluminulll-J..

I

- ,

I

~

,

-250 -200 -150 - 100 -50 0 Meter reading (relati ve unit)

50

100

Figure 12.3: Eddy current Ilole probe . .....-----Coil asscmbly Coi l wires sealed in body probe with

epoxy cement

Adjustable

Probe body

.K~'YI'"

collar""""'- -

Selscrew

Insulating sleeve

~

Coil w ire soldered to microdot tenninal

Seal connector and shield in probe body with epoxy cement con wi re soldered

Shield

94

Personnel Training Publicalions

Microdot connector

Testing of Aircraft Structures Airl ines and airframe manufacturers have used eddy current crack detection techniques since the early 1900s. Most of the eddy current equipment and procedures were designed to operate at high frequencies to detcct very small surface cracks . Around 1973 , damage tolerance studies detennined that eddy current testing was more sensiti ve than radiography for detecting fatigue cracks. It then became desirable to use eddy current testing instead of radiography [0 dctect subsurface cracks. Eddy current testing is a primary nondestlUctive test method for detecting cracks and corros ion of aircraft stlUcture and engine hardware during maintenance overhaul. These tests are performed in accordance with procedures de veloped by the manufacturer. Such tests can be used to detect fatigue cracks resulting from cyclic loading during flight , take-off or landing . They can also be used to detect stress corrosion cracks to indicate the ex tent of corrosion damage or to identify portions of aircraft structures damagcd by firc. Testing of Jet Engines Eddy current testing is a well-establ ished techniqnc for the detect ion of snrfacc discontinuities in conductivc test objects. The United Statcs Air Force and other organizations have used automated eddy current testing to ensure that criti cal engine componcnts are free of rejectable surface discontinuities . Eddy cun'ent lests of mi litary aircraft engines can be categori zed as manufacturing or depot tests. When aircraft engine com ponents arc initially produced. eddy currcnt tests of the manufactured objects arc often required hy specification to ensure that the new engine components are free of undcsirable material propelties or machining discontinuities. Inscrvice parts sim ilarly require dcpot tcsts at predetermined intcrvals to ensure that parts are free of small di scontinui ties in critical, predetermined zones. Manufacturing tests and depot tests arc performed as preventive screening. SURFACE TESTS

Surface tests are also conducted using eddy cu rrent techniques to detect cracking in a variety of components that are dillicult to test with other surface methods. Components typicall y tested include turbine rotors, turbine blades, socket welds and stainless steel pipes susceptible to corrosion cracking of the outside surface . These applications typically rcquire fiat (pancake) coils or flexib le array coils. The coi ls are usually operated in the absolute mode , in eithcr a cross wound or a transmit/receive configuration.

Classroom Training Series: Elecrromagneric Testing

95

CHEMICAL AND PETROLEUM ApPLICATIONS Heat exchangcr lUbe lesting is an im portant pan of mai ntenance fo r the refining and petroleum industry. Heat exchange rs and condensers arc designed 10 keep products in the lUbes separate from products in the vessel (Figure 12.4). A leaking lUbe could cause a significant impacI on production and could also cause a catastrophic fa ilure and loss of life. Bobbin probes are used to test for discontinui ti es in the matelial such as cracks . pitting or wall loss. They will dis rupt the flow of eddy currents and thus be detected by the in strument. Figure 12.4: Cutaway ima ge of typical h eat exchanger, showing t ube bun dle. ,,[()aUCI

in

Product circulates

~i~::;~~~:=:~ ",) to below divider Product out

Steam

ELECTRIC P OWER ApPLICATIO NS

Steam Generators The most common use of electromagnetic testing in the power industry is tube testing in heat cxchangcrs. Tubes in a nuclear steam generator are important for safcty because they carry the primary cooLing water. Degradation of steam generator tub ing results from corrosion and mechanical mechan isms; intergranular corrosion , stress corrosion cracking, thinning, impingement and fatigue. ~1ost tubes are fabricated from iron nickel chromi um and nickel chromium alloys . For electromagnetic testing, multi-frequency bobbin coil techniques arc used to tesl the full length of the tuhes . It is recognized that rhe bobbin coil is not quali fied for all tuhe locations, so other techniques such as rotating coil technology and array probes are also used .

96

Pen01l11el Trainin g Publications

Balance-of-Plant Heat Exchangers In a nuclear power plant , heat exchangers that do not carry reactor-cooling water from the containment vessel are commonl y referred to as balance-oj-plant heat exchangers. Most of them have counterparts in fossil fuel plants and are tested with multi-frequency eddy current tec hniques in the differential and absolute modes using bobbin coil probes. Table 12.1 lists rou tinely inspected heat exchangers, potential damage mechanisms and the tubing material. Table 12.1: Heat exch anger materials and damage mechanisms.

Heat Exchanger Feedwater heat exchanger

Tube Material

Potential Damage Mechanisms

carbon steel copper nickel alloy

steam erosion

stainless steel

vibrat ional wear condensate groov ing loose part damage

pitting

circumferential cracki ng

roll transi tion cracking axial cracking Generator stator cooling system

alloy of 90% copper and 10%

inside surface pitting

nickel

Generator hydrogen cooler

aluminum brass

inside surface pitt ing and outside surface damage

Lubricating oil cooler

admiralty brass

inside surface pitting

Main condenser

stain less steel copper nickel

inside surface pitting and vibrational wear

alloys; admiralty brass; titan ium

Industrial Air Conditioning Chillers Applications Cooling for large buildings and manufacturing processes is most commonl y provided by centrifugal chillers. Condenser and evaporator tubing have fins on their outside diameters to increase heat transfer surface , as shown in Figures 12.5 and 12.6.

Classroom Trainin.g Series: Electromagnetic Testing

97

Figure 12.5: Typical centrifugal chilleI.

Fi!{Ufe 12.6: Cross-section of sample h eat exchange tube.

•

~"..,.."'"

1.9 cm (0 .75 in.) ~.,.""".~

0.13 em (0 .053 ill. )

f 1.6 em (0.64 ill.) t

f

,.Ii

. ,

""01' •....,.... ..'\"."""""""""

1.4 em (0.56 ill .)

.....

*

~

Material Sorting Related to Conductivity Identify ing or separating materials by composition or structure is referred to as material sorting. Impedance values are established on reference known material, and then the readings obtained from test references are compared with the reading obtained fro m the test object. Eddy current can also be used to idenLify and control heat treatment conditions and evaluation of fire damage to metall ic structu res. Thi s can also be accomplished by determini ng depths of case harde ning of steel s and some ferrous alloys. During age harde ning of aluminum or titanium alloys , for example, the hardness and conductivity of the material change simultaneously so that the degree of harde ning may be obtained by measuring the conductivity of the test object and comparing it with a standard of that material with a known hardness.

98

Personnel Training Publications

ELECTROMAGN ETIC T ESTI NG IN P RIMARY METALS IN DUSTRIES Electro magnetic testing tec hniques find their appl ication in all stages of formi ng, shapin g and heat treating of metals and alloys, where the effective ness of processing steps can be quickly evaluated . Materials damaged during processing can be detected and removed from production without incurring further processing costs. Themlal treatments such as annealing, normalizing , harde ning and ot her heat treating processes can be directl y monitored in many instances.

Testing of Hot Rolled Bars There are several requirements fo r an effecti ve bar testing system.

I. 2. 3. 4. 5.

High discontinuity sensitivity. Ability to classify bar quantity. F ull automatic operation . R ugged construction for use in mills . The ability to test bars as received without spec ial preparation.

A rotating sUiface probe eddy cUITent technique has been used for the tes ting of hot rolled bars . With this technique, the eq ui pment rorates an eddy current probe arou nd an advancing bar. The probe is held a preset minim um distance from the bar surface. The equipment mainta ins the selected level of test sensiti vity regardless of changes in di scontinuity signal amplitude caused by varying surface spacing from probe to bar. It marks only those disco ntinuities that exceed a preselected length and depth. Bars with discontinuities are automati call y separated from discontinuity-free bars.

Testing of Square Billets Eddy cUITent testin g can auto matically inspect 100% of the surface of steel billets w ithout the need of an operator's judge ment for interpreting tes t results . The system described here can detect seams, evaluate their severi ty and mark the location of those that exceed an accepta ble depth. The key component is a scanning head assembly that keeps an eddy current probe in contact with, and tangent to, the billet surface at all locat ions arou nd the periphery, incl uding the com ers. The mac hine is des igned to test round cornered , square billets as they are rolled. This integration with the manufacturing process is an important step in the development of integrated automatic testing and conditioning systems.

Cla,\'STOO1l1

Twining Series: t.:lecrmmagnetic Testing

99

Testing of Hot Steel Rods and Wires Surface disco ntinuity testing is essential in the qual ity assurance of iro n and steel products. In many mills, quality control of hot rolled rods is provided through eddy current and flux leakage testing carried OUI after the rolling, shearing and cooling process. Wires and bars are usually coiled immediately after they are hot rolled. When discontinuity detection is performed on cold products, they must be uncoiled for testing. Eddy current tes ting using an encircl ing coil has been applied to hot rolling afbars. Steel mills usc enci rcling coil eddy current systems to test hot wires. Generall y, encircling coils are in the differential mode eoi] arrangement. This testing system can detect shOlt di scontinuities, slIch as scabs and roll marks. A diagram of a single-strand steel rocl control system is shown in Figure 12.7. Figure 12.7: Diagram of Si ngle-strand steel rocl control system, s!1owing four stages: (1) signal acquisition; (2) data d isplay; (3) data sorting; and (4) reporti ng. Stages

Photoce lls (presence of product)

r;=.:====:::::::J 0<: ROd

'0 Sensor

>

Rod speed

0

mca~lIrement

£

Remote i1Hec(iOn and ampl i l eI' box

- - - - - - ~J,.- Raw eddy current data Cathode ray tube 2

ffi

Q

Programmable lultiehannel processor Analog processing of

IAnalog-to-digital l

Graphic recorder

IF-

COnVCJ1er

--- - -Photocells. in formalion~

3

Q

eddy current signals

-

a

---- ----- ----

-

Rod speed information U

Preprocessmg computcr Digital, signa1 processmg L1nlt

---- - -

-- - - - - - - - - - - - - -

Centra] compute-r

Qualily managemenl 4

,;>

I

Printer I I Results I

10(1

Per.wJ11J1el Training Publications

e11101

¢::) If'and 1 dISI

Chapter 13

Factors Affecting Flux Leakage Fields DEFECT GEOMETRY, L OCATI ON AND ORIENTATION Thc presence of a discontinuity causes a reduction in the crosssectional area of the test object, thereby resulti ng in a local increase in the magnetic nux dens ity. A reduction in the pemleability. I.Ogether with an inc rease in the magnetic nux dcnsity, causes the fl ux to leak into the surrounding medium. Magnetic leakage fields can be subdivided fU l1her into acti ve or residual leakage fields . To undcrstand the origin of the leakage fields and choice of initial mag netization for the active leakage lield technique, consider an unmag netized steel billet with a surface discontinuity, as shown in Figure 13.la. Let A represent the cross-sectional area of the b illet, and let a represent the cross-sectional area of the discontinuity. The crosssect ional area of the sound pOltion of the bill et in the vicinity of the discontin uity is reduced to (A - a) un its, as shown in Figure 13.lb. H is the magnitude component and so is the scalar qua ntity. Then place thc billet in a uniform mag netic field H and represent the induced flux density in the sound portion of thc billet by B) (Weber! meter2) . This magnetic fl ux density corresponds to a point P to the right of ,UII"" on the permeability curve of the material, as illustrated in Figure 13.lc. Poin t Q is the corresponding point on the initial magnetization curve in rigure 13. 1c. Thc magnetiC nux density passi ng through the sound part of the billet is B1 (Weber/meter2). If it is assumed that this same magnetic flux is to pass through the reduced billet area in the vicini ty of the discontinuity, then the Ilu x density present in this section is greater than B ]A (A - a) , namely B2. This local increase of magnetic flux density results in a change of the operating point on the magnetization curve from Q to Q' and the corresponding decrease of local permeability from P to P '. However, this results in conflicting demands in the vicini ty of the discontinuity. The magnetic flux density must increasc with the reduction of the cross-sectional area. This change drivcs permeability in the restricted region of the billet to a value of less than that present in the sound regions. Consequently, some of the flux leaks into the surrounding med ium near the discontinuity, which is why this is call ed a leakage field (Figu re 13.ld). The detection of this leakage field is the basis for magnetic fl ux leakage testing.

101

Figure 13.1: Billet with discontinuity: (a) view of billet; (b) cross·section through discontinuity; (c) magnetic characteristics of billet material; and (d) billet in magnetic field showing discontinuity leakage field. (a)

(b)

Area a of discontinuity

Discontinuity t

Area A Area (A - a)

(d)

(e)

Discontinuity ~

Leakage field

-;; " '-' ~

T

.:::;;"

---------- --

I

'"C

'iO

ii

Magnetic Permeability It

/

B1

"'><"

"

0::

'-'

.~

---------- --

--- ----~ -

-~t-===~:=:=~=5....=~~-=-=-=-

-------.... -------------- --- -------~--------~--

----~-----------------

Initial S

magnetization curve

So

2"

Magnetic field intensity H (relat ive scale)

Subsurface Discontinuities If a discontinuity is farther below the surface, the difficulty of detecting the magnetic leakage fie ld is much greater. The reason for this difficulty is that the surrounding material tends to smooth out the field distortion caused by the subsurface discontinuity, thus resulting in a small field disturbance on the surface of the hillet. Because most detectors used to monitor the magnetic leakage field s rely on a sharp change of field gradient to record the presence of the ]02

Personnel Training Publications

Magnetic field H

field , it is naturally difficult to sense the location of subsurface discontinuities, as i1lustrated in Figure 13.2.

Pigure 13,2: Billet with subsurface discontinuity, showing resultant leakage field. Suhsurracc discontinuiLY

I-

leak agC field

-~

Degree of Initial Magnetization The initial operating point on the permeability characteristic of the material is very important. For example, if this point should lie to the left of I1ma.1" as illustrated by the point T in Figure lO.lc, an area reduction caused by a discontinuity would drive the local permeability higher than the permeability of a matcrial free of discontinuities. Thus, there is a possibility that the discontinuity may go undetected in these circumstances. Moreover, if the initial magnetization of the material should locate the operating point ncar saturation, then the difference between the magnetic flux density in the material and the leakage magnetic field in the surrounding medium decreases with the increasing disco ntinu ity cross-sect io nal

area. Therefore, the problem of quantitatively detecting the discontinuities is magnified because it becomes increasingl y difficult

to discriminate between the severities of the various heterogeneities. Atso, because the degree of magnetizat ion is so great, the surface roughness is easily mistaken for acmal discontinuities and results in unwarranted rejection of the test objects. Thus , there exists an upper and lower limit of magnetization to which a test object should be subjected if the magnetic leakage field teChnique of nondestlUctive testing is to be successful. Magnetiz.ation of the test object lies on the linear pan of the magnetization curve in such a way that the mate rial permeability is maximum. lvJagnetization should not approach saturati on but should have a value of tlux density that locates the initial operating point of the material on the steepest pan of the initial magnetization curve. If the degree of magnetization is too low. discontinuities may go unno ticed and, if the magnetization level is too high, a lack of discontinuity discrimination may result in false indications.

Classroom Training SerifS: Elfctromagnctic Testillg

103

Chapter 14

Selection of Magnetization Method I NTRODUCTION Successful testing requires the test object to be magnetized properly. This can be accomplished using one of several approaches. I. 2. 3.

Perma nent magnet. E lectromagnets. Magnetizing coils

Permanent Magnets Excitation systems that use permanent magnets offer the least flex ibility. Such systems use high-energy pClmanent magnet materials such as neodymium iron boron. samarium cobalr and alumin um nickel. The major disadvantage wirh such sysrems lies in the fact that the excitation cannot be switched off. Due to the fact that magnetizatio n is always lUmed on. it is difficult to inscI1 and remove the tes t object from the rest instru ment. Although the magnetization level can be adjusted using appropri ate magnetic shunts, it is awkwa rd to do so. Consequently, pe rmanent magnets are very rarely used fo r magnetization. except in rhe case of in-line inspection tools (called

smarl pigs).

Electromagnets Electromagnets are used extensively to magnerize test objects. Figure 14 .1 shows an excitation syste m where the test object is p'1l1 of a magnetic circuit energi7.ed by current passing through an excitation coil. Figure 14.1: Electromagnetic yoke for magnetizing of test ob ject.

Air gap \I,lhere rest! obJect i:-; in serted

105

The magnetic circui t passes through a yoke made of so rt magnetic material and through a test object placed between the poles of the yoke. When the coil wo und on the yoke canies cu rrent. the resuhin g magneti c force drives magnetic Dux through the yoke and the test object. The total magnet ic flux E (webe r) is given by:

Eq. 14.1 E = NI S

where I is cun'ent in (ampere) in the co il. N is the number of lUms in the coi l and S is the reluctancc (ampere per weber) of the magneti c circuit.

To obtain maxim um se nsitivit y. it is necessary to ensure that the magnetic flux is perpendicular to the d iscontinuity. Becausc the orientation of the discontinuity is unknown. it is necessary to test twice with the yoke, in two di rect ions perpendicular to each other.

Magnetizing Coils A conmlOnly used encircling coil is show n in Figure 14.2. The field direction foll ows the right hand ru le . The right hand l1Iic states that if someone grips a rod , holds it and imag ines an electri c cun'em fl owing with the thumb. the induced circular field in the rod would flow in the direction that the fingcrs point. Figure 14.2: En circlin g coil using di rect current to produce magnetizing force.

Legend I ;;;; eleclric current

P, Q ~ points or discontinuities in examp le R ;: poi nt at which magnetic fie ld intensit y H is measured S = point at wh ich magnetic nux density 1) is measured

106

Personnel Trainillg Publicatio1ls

Introduction of the test object into the field of the coil changes the field. The metal becomes part of the magnetic circuit, with the result that close to the surface of the test object magnetic field intensity H is lower than it would be if the test object were removed. A Hall element tesla meter will show the field intensity at the test object is not the same. Testing in Residual Field Test objects are first passed through the coil field and then tested in the resulting residual field. Elongating thc coil and placing the test object next to the inside surface of the coil will expose the test object to the largest field that the coil can produce. This technique is often used in magnetic particle testing. The main problem to avoid is the induction of so much magnetic flux in the test object that the magnetic particles stand out like fur along the field lines that enter and leave the test object , especially close to its ends. The technician should experiment to optimize the coil field requirements for the test object because this field depends on the test object geometry.

MAGNETIZING BY DIRECT CURRENT If an electric current is used to magnetize the test object, it may be more advantageous to orient the direction of current in a manner where the presence of a discontinuity impedes the current flow as much as possible. Bars, billets and tubes are often magnetized by applying direct current to their ends, as shown in Figure 14.3. Other methods of magnetizing using direct current includes passing the current directly through a tubular test object to magnetize the test object circularly, as illustrated in Figure 14.4.

Figure 14.3: Circumferential magnetization by application of direct current: (a) rectilinear bar; (b) round bar; and (c) tube.

Figure 14.4: Current carrying clamp electrodes used for testing ferromagnetic tubular objects with small diameters.

(a)

~C5il====i}~H==========-'>."""I'~ I

(b)

4-'.L)_---LfH~ _ _ _~r- I

(c)....o

} H

L

Magnetic flux lines

, Clamp

I Current / out

Legend H = magnetic field inten sity I = electric cun"ent

'Current I in

Classroom Training Series: Electromagnetic Testing

107

Figure 14 .5 shows a central conductor energized by a curre nt source (1) , again to establish a circular magnetic field inte ns ity H (ampere per meter) in a tubular tes t object:

Eq. 14.2 H = _ T_ 21ra

where J is the current in (amperes) and u is the area (square meters). Figure 14.5: Central cond ucto r using d irect current to produ ce magnetizin g force.

t

(('\

r

-t

-

\\_-j

---------tH

T

1:-:-=

r---

Current source

Legend H;:;: magn~tic fleld IntensIty I = electnc current

r = tube radius

M AGNITUDES O F M AGNETIC FLUX L EAKAGE FIELDS The magnitude of the magneti c flux leakage field unde r acti ve direct current excitat ion natu rall y depends on the applied field. However, in the case of residual magnetization, the magnetic fl ux leakage fi elds may be only a few hu ndred micro-tesla (a few gauss). Furthermore, with residual field excitation, an interesting field reversal may occur depending on the value of the initial active field excitatio n and the dimensions of the discontinu ity.

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Chapter 15

Flux Leakage Applications INTRODUCTION Magnetic flux leakage testing is a commonl y used technique. Signals from probes or sensors are processe d electronically and

presented in a manner that indicates the presence of discont in uit ies . Although some techniques of magneti c flux leakage testing may not be as sophisticated as others , it is probable that more ferromagnetic material is tested with magnetic fl ux leakage than with any other techn igue .

Heat Exchanger Tubing Applications Magnetic fl ux leakage testing is based on the influence of discontinuities on a magnetic field. The technique is limited to felTomagnetic material. The magnetic llux leakage prohe consists of a magnet wi th two types of magnetic pickups : coil and Hall element. The coil picks up the flux rate of change , whereas the Hall effect detector picks up ahsolute flux. The coil detects small discontinuities tha t cause pel1urbations in the flux . M agnetic flux leakage co ils are more sens iti ve to sharp discol1linuities than to grad ual wall loss. In fact, the coi ls can totally miss long areas of wall loss if the changes in wall thickness are grad ual. A Hall effect detector is therefore used to detect gradual wallinss . Figure 15.1 shows a probe con figuration fo r flux leakage test ing. Figure IS.1: Bobbin coil probes for magnetic flux leakage testing. .Di SC(Jnt i Iluity Tube.

Ferrite

end plate North

FcnilC core

enen?:l zmg coil ~ ~

enepJizinQ, e

coil

109

Hall effect detectors measure the absolute flux and can be used for s izing wall loss, not pits, But the output of the Hall effect detector depends on the orientation of the sen>ar in the probe relative to the discontinuity and whether the location of the discontinuity is on the inside or outside surface . In side surface discontinuities produce larger signals than outside surface discontinuities because the fie ld strength on the inside sUIt"ace is hi gher than on the outside surface .

Wire Rope Inspection Because the reliab ility of wire ropes is cruc ial for the safety of many mining, o il indus try, crane and ski lift o perations , concern with thcir integrity is a co nstant preoccupat ion of users and safety authorities. Etfeetive procedures, combined with a good understanding of degradation mechanisms and discard criteria, can notably increase wire rope safety. For example, advanced electromagnetic wire rope test equipment of the magnetic flux leakage type has been developed since the 1960s. These instruments provide an important , and in many cases indispensable. clement of wire rope testing. Such ropes are used in the construction, marine and oil production industries, mining applications, elevators and bridge cables. Testin g is performed to determine cross-sectional loss caused by currosion and wear, and to detect inte rnal and external broken wires. The type of fl ux loop used (electromagnet or permanent. magnet) can depend on the accessibi lity of the rope. Permanent magnets might be used where taking power to an electromagnet might cause safet y problems. The cross-sectional area of the rope can be measured by sensing changes in the magnetic fl ux loop that occur when the rope gets thinner. The air gap becomes larger, so the val ue of the field intensity falls. This change can be easily sensed by plac ing Hall effect probes anywhere within the magnetic ci rcui t. Modelll dual-function electromagnetic rope testers (show n in Figure 15.2) allow simultaneous tests for loss of metallic cross· sect ional area and localized

di~c{)n tin ui ti es .

For thi s tester, strong

permanent magnets induce a magnetic flux at the satu rat ion level

In

the rope in the ax ial (long itudinal ) direction. Various types of sensors close to the rope (such as co ils, Hal l se nsors or fl ux gate sensors) sense and measure the mag netic flux. Any discontinuity. such as broken wire or corrosion pitting , distorts the magnetic flux in the rope and causes it to leak from the rope. For localized discontinuity tests, the radial component of the leakage flux is measured by radial sensors.

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PersolUll'l Trainillk Publications

Figure 15.2: Elect romagnetic testi ng of mooring rope.

Round Bars and Tubes In some test systems, round bars and tubes are magnetized by an alternating current magnet and rotated under the magnet poles. Because the flux leakage from surface discon tinuities is very weak and confined to a small area. the probes must be very sensitive and small. The system uses a differential pair of magnerodiodes to sense leakage flux from the discontinuity. The diffe rential output of these twin probes is ampli fied to separate the leakage !lux from the background flux. In th is system, pipes are fed spirally under the scanning statio ns to increase the test rate. In one similar system, rou nd billets are rotated by a set of rolle rs while the billet surface is scanned by a transducer array mo ving straight along the billet axk Seamless pipes and tubes are made from the rou nd billets.

Petroleum and Gas Pipelines Pipelines connect field production (gas and oil extraction) with refineries and petrochemical plants where gas and crudc petroleum are processed into usable prod ucts . Because pipelines cross state lines in the Uni ted States. they are govel11ed hy the DepaJ1ment of Transportation. The construction, maintenance and testing of these pipelines arc critical to the safety of the environment and the general public. Buried pipelines not only have the potential for catastrophic fa ilure but could contaminate water sources if leakage occurs. Flux leakage testi ng can be applied to detecting cracks and corros ion pits in the walls of underground pipelines. In pipe factories cracks and hairline cracks. O.QlS cm (0.006 in .) deep on the outside diameter and 0.03 em (0.0 12 in.) deep on the inside diameter, are satisfactOlily detectcd by magnetic flux lcakage methods for wall thicknesses up to 0 .8 cm (0.315 in.). Based on pipelines past t"ilures, the di scontinuities that caused these failures me much larger, and thus

Classroom Training Series: Electromagnetic Tes/ing

III

can be detected more easily in principle . However, the technological difficulties to be overcome are often great. For example, the entire testing device must be transponed inside th, pipe for long distances. Magnetizing conditions close to saturation must be achieved to reveal outside diameter discontinuities with inside diameter probes , requiring very power efficient magnetizing systems. The entire pipeline must be tested with only one pass of the probes and associated magnetizing device. Recording systems must be capable of giving definitive positional information . For the pipeline inspection tool, a recorder package is added and the signals from di scontinuit ies are recorded. When the recordi ngs are retrieved and played back , the areas of damage are located. Pipe welds provide convenient magnetic markers. Smart pigs are test vehicles that arc pushed through the pipeline by product flow (Figure 15 .3) . The technique got its name from the sound of the original scraper pig moving through the pipe. At the end of the line or run , the pig is retrieved and the on-board data are then processed and analyzed. The pigs are similar to the mag netic flux leakage probes used in tube testing , but pigs are constructed to be propelled down pipelines and co llect the required test data.

Figure 15.3: Eq ui pm ent for magneti c flu x leakage testin g of pipes and tubes: (a) pig tool; and (b) data acquisiti on from pig sensors. (b)

Pickup coils

Billets A relatively common problem with square billets is elongated surface breaking cracks. By magneti zing the billet circumferentiall y, magnetic flu x leakage testing can be performed to detect such defects. Magnetic flux leakage systems for testing tubes exhibit the same general ability to classify defect depth. It is generally accepted that even with the lack of correlation between some of the instrument readings and the actual discontinuity depths, the automatic readout of these two systems still rep resents an improvement over visual or magnetic particle testing .

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Installed Heat Exchanger and Boiler Tubes Flux leakage testing is widely used to test installed ferromagnetic heat exchanger and boiler tubes. The !lux leakage probe consists of a magnet and two inductive coil sensors. The magnets set up a fl ux field in the lube wall as it passes through the tube. The field fluctuates when it encounters a discontinuity. The nux rate nuctuation effect is picked up by the coils and displayed On the display ap paratu s and chaIt recorder. A Hall effect sensor is also used in this applicatio n to detect abrupt defects and gradual wall loss. The output of the Hall effect detector depends on the orientation of the sensor in the probe relative to the disco ntinuity. and whether the location of the discontinuity is on the inside or outside tube surface. I! should be noted that the output of the magnetic nux leakage coil is related to the change of flux caused by the discontinuity but not directly to Ilaw depth. Tubes up to 8.9 cm (3 .5 in.) in diameter and 0.03 em (0.0 12 in.) wall th ickness can be tested.

Above-Ground Storage Tank floors Tank floors of above-ground storage tanks (shown in Figure 15.4) are subject to corrosion where they touch the ground. Magnetic flux leakage testing has been widely used in the oil field industry for over 25 years for the examination of pipe , tubing and casting, both new and used. It is only in the past ten years that this testing technique has been applied to above ground storage tank floors in an attempt to provide a reliable indication of the overall floor condit ion within an economical time frame. For the purpose of this application, a magneti c blidge is used to introduce as near a saturation of flux as is possible in the test material between the magnetic poles. Any significant reduction in the thickness of the plale will res ult in some of the magnetic flu x being forced into the air around the area of reduction. Flux leakage sensors that can detect these flux leakages are placed between the poles (see Figure 15 .5a). Figure 15.4: Above-ground storage tank for petroleum products.

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113

[t is important that magnetic fl ux leakage equ ipment produced for this part icular application be designed to handle the environmental and practical problems always present. Figure IS.Sb shows a mobile floor scanning unit. Powerful rare earth magnets are ideally su ited for thi s application. They are more than capable of introducing the required flux levels into the material under test. Electromagnets by comparison are excessively bulky and heavy. They do have an advantage in that the magnetic fl ux levels can be easily adjusted and turned off if necessary for cleaning purposes . Hall effect and coil sensors can be used in this application ; however, it was demonstrated that coil sensors are more sensitive, stable and reliable. Hal l effect sensors are proven to be too sensitive when surrace conditions are less than perfect, which results in an unrel iable test. Magnetic flux leakage in this applicat ion cannot differentiate between the response from top side and bottom side indications. Contrary to what is expected, the fl ux leakage response from a top side indication is significantly lower in amplitude than that from an equivalent bottom side indication. Magnetic flux leakage is a qualitative , not quantitative , testing tool and is a reliable detector of corrosion on tank floors. Figure 15.5: Magnetic flux leakage test: (a) schematic of bridge; and (b) tank floor scanner incorporating magnetic flux leakage test bridge.

(a)

(b)

Test object

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Chapter 16

Remote Field Testing HISTORY The remote field effect was firs t noted in 1940s and was patentcd by W.R . Maclean in 195 1. In the late 1950s, Thomas R. Schmidt independently red iscovered the techniq ue while developing a tool for testing oil well casings. Schmidt spearheaded the development of the technique and named it remote fle ld eddy current tesling. The techn ique used by the industry is now refer red to as femOle field testing. Several test cqui pment manufactu rers recogn ized the value of this nondestruc tive electromagnetic techniq ue for the testing of ferrou s heat exchanger tubes and began manufacturing remote field test inmuments starling in 19X5. Developments since 1990 include the testing of tlat plates and steel pipes using external probes thaI use a technique simil ar to remote field testing. Modem insU1lInents use computers to display and store data, and more aci vanced systems also have automated signal analysis.

I NSTRUMENTATION The typical remote field testing instruments contains four major components . 1. 2. 3. 4.

An oscillator is used as the signal source for the exciler coil and as a reference for the detector signal. A power ampl ifier increases the powe r level from the oscillator signal so that it can be used to ciri ve the exciter coil. The phase and amplitude detector measllres the dctector coil signal. A computer based storage device processes and stores the data ,

Figu re 16. 1 shows how the different electronic components

interact.

llS

Figure 16.1: Electronic components of remote field test system. Detector Phase and ampl itude coil '----L-="-':.'.'.'.O.._ detector

-..

D

Computer

Reference signal

Exc~~~;

,-I 1.1 - D

Oscil lator

P()\\Icr amplifier

Probe Configuration Figure 16.2 ,hows the configuration of a basic remote field eddy CU!Tent probe. There is one exc iter coil and one receivi ng (detector coil). Both coils are wnund coaxially with respect to the tested tuhe and are separated by a distance greater than twice the mbe diameter. If' the exciter and receiver were to be placed close together, the detector would measure on ly the field generated by the exc iter in its vicin ity. l'igure 16.2: Simple probe for remote field testing.

To observe remote field testing 's unique through-wall transmission effect, the detector needs to be moved away [rom the exc iter. The detector meas ures the electromagnetic field remote from the exciter. Although the fill factor of the coils can be as low as 70% it will usually be similar to the till factor for eddy current probes at 80 % or more. A lowcr fill factor reduces sens itivity to small discontinuities but docs not affect the quality of remote field testing data . The ability to function with low fill factor makes remote field testing attractive for pipes with internal coaling and tight bends.

Effects of Probe Speed The ability of remote field testing to detecl anel quantify discontinuities relies on the quality of the signal received. The probe must be in the vic inity of the discontinu ity for at least one exc itation cycle in order to detect it. The probe pull speed should be slow enough so that the digital sample rate allows the field profile near the prohe to be accurately recorded. It is equally important that the speed of testing be as constant as possible. Sudden changes in speed can result in anomalous signal s.

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Frequency Selection Remote field testing operates at relatively low frequencies typically ranging from 40 to 500 Hz. In general, a lower frequency (up to 250 Hz) is used for thick walled and high permeability pipe . For higher sensiti vity and hi gh test speeds, a higher frequency is used. The frequency is chosen as high as possible while minimi zing noise and remaining in the remote field zone . It is important to check for the presence of any electromagnetic noise sources such as welders, electric motors and pumps that tend to generate noise in the frequency range ofremote field testing. F EATURES OF REMOTE FIELD TESTING

Applications Remote field testing can be used for all conventional carbon steel material specifications. diameters and wall thicknesses . The test speed can be up to 60 ft per minute depend ing on wall thickness and test frequency. Remote field testing is a noncontac! technique . The probes have minimal friction with the tube or pipe wall and require no couplant.

Sensitivity The accuracy for remote field testing in the straight part of tubes is about 5% of wall thickness for general wall loss. The accuracy is less (20% of wall) for highly localized di scontinuities and in bends or near external conducting Objects because of the changes in magnetic properties of the tube in the bend area and due to shielding effects of external objects. Remote field testing can detect both inside and outside surface discontinuities with equal sensitivity, but in most cases cannot di fferenti ate between them without using near field coils. Remote field testing is insensitive to scale and nonmagnetic debris. Internal and external magnetic de posits do affect the signal to a small degree. A large fill fac tor is not required for remote field testing , and centralization is not as critical as with other nondestructive testing techniques [or similar applications.

Signal Analysis and Data Presentation Re mote fie ld test data are recorded in computer memory or hard drive. Phase am plitude diagrams (voltage planes) are displayed during the test on instrument monitors in real time. The raw data from the detector are stored either in phase amplitude format or as in phase and quadrature components. The data can be recalled for display, analysis and final report preparation. Figure 16.3 chart (a) shows phase and log amplitUde signals for an absolute coil. Chart (b) indicates x-y voltage signals; chart (c)

Classroom TrailJing Series: Eleclromagnelic Testing

117

shows phase and log amplitude from a differential coil and chart (d) shows the mixed frequency signals . Figure 16.3: Strip chart recordings: (a) phase and log amplitude signals for absolute probe; (b) x,y voltage signals; (e) differential signals; and (d) mixed frequ ency signals.

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The voltage plane and x-y displays pro vide maps of the detector coil output in polar coordinates. On polar displays, signals are drawn as vector points with the angle represeming the phase and the radius representing the amplitude. Remote [ield testing signals on the voltage plane or x-y display are scaled and rotated to a convenient position for final viewing . Figure 16.4 shows a typical phase amplitude diagram; voltage plane recording , while Figure 16.5 shows a phase amplitude diagram: x-y plane recording.

liS

PerSOIl1li.'l Training Publicutions

Figure 16.4: Phase amplitude diagram: voltage plane recording. (lI A1x;2 12 ~JOHz

Reference curve ~ of thickness to ~\ ampl itude \

Zero

\

/I .~ Nominal

0,0

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point

Figure 16.5: Phase amplitude diagram: x,y plane recording.

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Classroom Training Series: Electromagnetic Testin g

119

By observing the signal shape. phase angle and relative size on the voltage plane. many discontinuities can be characterized and sized for depth and circumferential extent . The axial length of a discont inuity can also be measured by recording the data on a strip chart as the probe is pulled through the tube. All remote field test instruments display the data as strip charts and voltage planes. In addition, some instruments have automatic depth sizing and repol1ing software.

REFERENCE STANDARDS All nondestructive testing methods use reference standards to compare discontinuity signals with those from knovm machined or a.t1ificial discontinuities. Remote field testing requires reference standards for each variation in tube diameter, wall thickness, conduct ivity and pemleability. Reference discontinuities must be machined to closely simulate the discontinuities expected in the tube or pipe being examined. ASTM £-2096-00 lists two recommended standards that can be used for remote field testing .

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Chapter 17

Alternating Current Field Measurement HISTORY In rhe 1980s there were moves to develop nondestructive testing techniques for detecting and siz ing fatigue cracks underwater in welded offshore structures. The development foc used on two existing techniques: eddy current testing for detection and altenlating cu rrent potential drop testing for sizing. The alternating current field measurement tech niq ue was developed out of work on the potential drop techniques. Potcntial drop test applications in the 1980s rended to use direct current rather than alternating current. Work in potent ial drop testing in the early I 990s studied the surface electric fields to describe the assoc iated magnetic fields. As with alternating CUlTCnt potential drop testing, field measurement made it possible to estimate crack depths without calibrati on. As it tumeu out, making a noncontacting technique from alternating current potential drop meant that the probe could be scanned along a weld very easily, Thus, although altern ating current field measurement was developed for noncontact sizing, it was also useful fo r discontinuity detection.

PRINCIPLE OF O PERATION The alternating current field measurement tec hnique involves inducing a locall y uniform CUlTent into a test object and measuring the magnet ic tl ux de nsity above the test ohject surface, The presence of a surface discontinuity peI1urbs the induced current and the magnetic flux densily. The method uses an instrument and a hand-held probe contain ing a uniform field induction system and two magnetic field sensors. Software on the external personal computer is used to control the instrument and to display and analyze the da ta . The required locally uni form magnetic field is ind uced us ing one or more horizontal axis solenoids, with or without a yoke . The direction of thi s electric field E, as shown in Fi gure l7.l is designated as the Y axis and the direction of the associated uniform magnetic dens ity B (at right angles to the electric field ancl parallel to the test surface) is dcsignated as the X ax is. The Z axis is then the direction normal to the surface .

Classroom Training Series: Electromagnetic Testing

121

Figure 17,1: Coordinates conventionally used in alternating current field measurement, Electric field E

J-y X

Figure 17,2 sho ws the effect of a surface break ing discontinuiry on the magnetic field, The presence of a discontinuity diverts current away from the deepesr parts and concentrates it near the ends of a crack, Figure 17.2: Effect of surface breaking discontinuity on magnetic field,

-'~" ¥~ I

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B:z. =magnctic fl ux com ponent nonnal to te st surface

r =time or scan di.stan ce (relative scale)

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Personnel Trainin g PublicaTions

Amplitudes of components of the mag netic flux density are used to minimize variations caused by material properties and instrument calibration. These relative ampl itudes are compared with values in sizing tables prod uced from a mathematical model to estimate discontin uity sizes without the need for calibration using artificial disconti nuities such as slots , notches , holes, etc. The sizing tables have been produced by re peated running of the model for semi-elliptical cracks in a wide range of di fferent lengths and depths . The model is called the forward problem fo r which the discontinuity size is known and the signals are then predicted.

Probe Configuration Figure 17.3 shows components arranged in a typical alternating current fie ld measurement test. The exact parameters used in a probe vary according to the application. The larger dimensions are used where possible because they gi ve the most uniform field and allow the two sensors to be wo und concentrically, which gives clear symmetric loops in the butterfly plot. In probes designed for tight access applications or for higher senSiti vity , the smaller dimensions are used. Figure 17.3: Typical al tern atin g current field measurement probe layout.

I

Solenoid , 1.5 to 3 em (0 .6 to 1.2 in.J long

2 to 4 em (0.8 to 1.6 in.)

1

A

Probe coils. 0 .1 W 0.5 em (0.04 to 0.2 in.) in diameter

ITIIl§

/ / / /Test / /object //// ADVANTAGES AND D ISADVANTAGES The alternating current field measurement tech nique uses a un iform input field to allow comparison of signal intensities w ith theoretical predictions. A uniform field has advantages and disadvantages compared with conventional eddy cun-ents.

Classroom Traininx Series: Electromagnetic Testing

123

Advantages The main advantages include the following. I. 2. 3.

The ability to test through coatings several millimeters (0.25 to 0.5 ern [0.1 to 0 .2 in.]) thick. T he abil ity to obtain depth information on cracks up to 2.5 cm (1 in.) deep. Easy testing at material boundaries, such as welds .

Disadvantages The main di sadvantages include the following. I.

2.

3.

Lower sensitiv ity to smaller discontinuities. This reduction in sensitivity is of little consequence on welded or rough surfaces where sens itivity would be reduced anyway. On smooth, clean surfaces however, altel11ating current field measurement is less sensitive to short or shallow discontinuities than conventional eddy current. Geometry changes such as plate edges and corners can produce signals that may confuse the technician. even though these signals do not have the same form as a signal from a discontinuity. Discontinuity's orientation relative to probe. The signals obtained from a discontinu ity depend on the orientation of the di scontinuity. The uniform fie ld theoretical model wo uld suggest that no signal be produced when a probe Scans across a transverse discont inuity, because the current tlow is then parallel to the discontinui ty and would not be perturhed.

ALTERNATING CURRENT FIELD M EASUREMENT I N DICATIONS

Fatigue Cracks Alternating current field measurement was designed for the

detection and sizing of fatigue cracks, Fatigue cracks arc generally sUlt'ace breaking d iscontinu ities that tend to grow at defined stress concentration, and are well suited for the linear scanning path of alternati ng current field measurements probes. Fatigue cracks also tend to grow in a semi-elliptical shape and at right angles to [he surface.

Stress Corrosion Cracking Stress corrosion cracking can take the form of a series of parallel cracks acting as a colony. The detection of stress corrosion cracking by altcrnating currenr field measurement is reliable, and depth values obtained by treating isolated cl usters as single discontinuities agree reasonably well with the typical discontinuity depth.

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Personnel Training Publications

Hydrogen Induced Cracking Alternating cun-ent field meas urement techniques have also been used to detect sul fide stress concentration cracking , hydrogen induced cracking, hydroge n sulfide cracking and stress oriented hydrogen induced crackin g in the base metal adjacent to the heat affected zone.

Fatigue Cracks in Rail Heads Non semi-elliptical discontinu ities occur also in railroad rails . Head checking (also called gage corner cracking) is craCking that initially grows into the top sulface of a rail at a highly inclined angle . As they grow below a certain depth, they tum to a steeper angle but also start to grow sideways, so their length is greater beneat h the surface . Altemating current field measurement can be used to detect this type of cracking.

Corrosion Pitting The nondircctional currents used in alternating cun-ent field measurement arc most strongly pert urbed by planar discontinuities. However, surface corros ion pitting also perturbs CUlTent flow to some extent and can also be detected. The deg ree of cun'ent pelturbation is much lower than fo r a crack of the same depth and length, so on initial scan a corros ion pit looks l ike a shallow crack. However, the dis tinguish ing featu re of a pit is that unl ike a crack , it will produce the same signal regardless of the orientation of the interrogating current.

Classroom Training Series: Electromagnetic Tesring

1, ~,

Chapter 18

Electromagnetic Testing Standards and Procedures INTRODUCTION Procedures, specifications and standards are produced to provide a means of controlling product or services quality. Written instructions that can be used as guidance to companies or indi viduals to a desired end resuit and are acceptable to industry are the basis of standards, specifications and procedures. CALIBRATION STANDARDS An important requirement for successful electromagnetic testing is the use of an accurate reference standard for equipment calibration. The reference standard is used to adjust the electromagnetic equipment's sensitivity to various test object parameters (cracks, surface roughness, conductivity/permeability variations and other material conditions). Success in electromagnetic testing depends on the proper use of available reference standards. The development and use of eddy current reference standards requires a thorough understanding of the test to be performed. Reference standard considerations should include: I. 2. 3. 4. 5. 6. 7.

The material tested. Si ze and shape. Discontinuities of interest. Means of producing art ificial discontinuities. Non-relevant indications that might be encountered, such as material property variations. Instrument capabilities and limitations. Criteria for relevant indication.

In view of many types of artificial discontinuities that can be produced , an understanding of their relationship to the di scontinuities of interest is critical. For example, if transverse discontinuities in a tube are of interest , it might seem obvious to use a transverse notch. However, that notch could be either straight , wh ich is much easier to fabricate , or curved to match the radius of the tube . Figure 18.1 illustrates this configuration. Another similar case is that of cracks in a fastener hole , which can be simulated by several notches, as shown in Figure 18.2, all of which produce different signals. 127

Figure 18.1 : Transverse notches in tube: (a) straight; and (b) curved . (a)

Notch

~NotCh

(b)

~-......: Tu be

Tube

J'igure 18.2: Notches to simulate cracks as might be fou nd in fasten er holes. Notch

t Kole side

Hole side

/

/

Hole side

R EFERENCE STAN DARDS

Conductivity Reference Standards In edd y curre nt testing , frequent use is made of measurements of conductiv ity based on thc Intemational Annealed Copper Standard (lACS ). Two standard melal blocks are supplied with conductivity meas uring in struments. One block represents a high level of cond ucti vity. while the other re presents a low level of conductivity. The percemage value in l ACS is stamped on the blocks. as illustrated in Figure I ~.3 .

Figure 18.3: Conductivity standards. 101.0%

High

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Persol1nl'i Training Publications

13.5'k

Low

Conductivity reference standards should be tested with a relallvely small coil to detennine the uni fo rm ity of electrical conductivity over the surface of the standard . Both the fron t and the back surface should be tested for conductivity differences.

Coating Thickness Reference Standards Coating thickness calibration reference standards of uniform thickness are available in two types. 1. 2.

Foils or shims of known thicknesses laid on an appropriate substrate. Actual coatings affixed to prepared substrates as supplied or recommended by the instrument manufacturer or standardizing organization (Table 18.1).

Table 18.1: Standard reference materials from National Institute of Standards and Technology for calibra tion of instrumen ts used in measurement of organic and nonmagnetic in organic coatings over steel. Each 4.5 x 4.5 cm (1.8 x 1.1> in.) block consists of fin e grained copper electrodeposited on low carbon steel substrate. Material N umber 135Ra 1359b 1361b 1362b 1363b 1364b

Coating Thickness ]lID

RO, 255, 1000 48 , 140.505,800 6, 12,25,48 40.80, 140,205 255 , 385 , 505 , 635 800,1000,1525. 1935

(in. x 10. 3) (3. 1,9.8,39) (2.0,5.5,20,32) (0.2,0.5, 1.0,2.0) (1.6. 3.1, 5.5.7.9) (9.8 .1 6,20 ,26) (32,39, 59,79)

Discontinuity Reference Standards A discontinuity reference slandard should duplicate the test situation for material type and geometry and , in addition , include discontinuities that establ ish the maximum discontinu ity that is acceptable and also establ ish the sensitivity of the test system . Discontin uity reference standards fall under two types: natural discontinuity reference standards and artificial discont inuity reference standards. Natural Discontinuity Reference Standards Natural discontinuity standards consist of duplicates of the test piece configuration that contain natural discontinuities of a known size and shape. At least one of the discontinuities should be at the limit of acceptability. Natural discontinuities can be developed or accumulated. Fo r example, cracks can be developed by submitting a material to cyclic stresses until a natural fatigue crack is generated. Cla.}·srooHl Training Series: Electromagnetic Testing

129

This can then be machined to produce a surface or hole crack of know depth . Figure 18.4 illustrates an example of how a nat ural fatigue crack is developed. figure 18.4: Developing a natura l fatigue crack. Location of induced

Small slot to induce fat igue at this point

fat igue crack

b----r--r,7I / '--------- -'71 f----------------c//' f/~/~r------------{

o o

A"'-+----"""

w--_

'-_ -_ -4 _ 0_ _ ..Y /

Fatigue specimen

o

,

,Z V ::::::::::::~+H--,l'

--tp/,LL:....... =:=1___

crack on tosUlface Machine leavejI'-

SeC[ion con taini ng fatigue crack

machined from fatigue specimen

Artificial Discontinuity Reference Standards Artificial discontinuities may be machined into a duplicate of the test object configuration. Several samples may have to be n111 through the lest system to find one that does not prod uce any appreciable indications of natural discontinuities from the tcst piece prior to machining. Standard reference disco ntinuiti es that are perti nent to the required specification are then fabri cated into the sample. Types of standard reference discontinuities that can be used to simulate test object discontinuities incl ude longitudinal notches, circumferential notches , drilled holes , nat bottom holes , diameter steps and geometry variations.

When external compari son techniques are used, the refe rence standard must be free of discontinuities. The standard must be representative of impelfections that may be fou nd in the test object.

Lift-Off Reference Standards Lift-off reference standards are easi ly constructcd by the application of known thicknesses of a nonconductive material to a sample of the test object. Paper, polyethylene terephthalate and cellophane arc examples of nonconducrive materials alien used. When it is required to measure the thickness of nonconductive coating, the standa rd does not have to be coated with the same nonconducrive coating material since , to eddy eurrenttcsting, one nonconductive coating is exactly like any other. The firm 130

Personnel Training Pub/ict1lions

req ui rement is that the thickness of the coating used on the standard be known. If measuring thickness of a non conducti ve coating over a conductive test object, lift-off standards need to be constructed that represent both the max imum and the minimum acceptable standard. Sorting Reference Standards When sorting with the absolute encircling coi l tcchnique, a known acceptable calibration standard and a know n unacceptable standard are required. When using the comparative encircling coil technique, usually two known acceptable specimens of the test object and one unknown unacceptable specimen are required. For a three-way sort, it is best to have three calibrati on reference standards, including either two for the high and low limits of acceptability fo r one group or one each for two unacceptable groups. The third reference standard represents the acceptable lot of material. Although electro magnetic sorters can be useful for limited appl ications , sorting steels is better done via spectroscopic equipment. A change in product diameter, for example, can simulate a change in composition to electromagnetic sorters. Figure 13.5: Types of reference standards; (a) notched tubes; (b) calibration block; and (c) block with graduated h oles. (b) (a) 0.05 em (0.02 in.) typical .., 0.02 crn (0.008 in.) '" typical ~

0.05 cm (0.02 in.)

Transverse notch

-0.005 em (-0.002 in.)

0.1 em

(0.04 in.)

(c) B

A, '>..

e 8' / 11.5 em

C

08g oo;:O~Q °0

(4.6 1 in .) y

5.8 em (2.32 in.)

Legend A. Made of same material as part to be inspected , 0.64 em (0.25 in .) thick , 32 root mean square finish.

B. Holes reamed to ±O.o05 em (±O.002 in.) tolerance and 32 root mean square fini sh. C. V ibration etched hole sizes and materiaL

Classroom Training Series: Electromagnetic Testing

131

STANDARDS AND SPECIFICATIONS In nondestmctive testing . a specification is ofte n written by a commercial organization , usually one of the primary parties in the purchasing agreement. A specification is product specific and may be considered a tailored form of a standard. A spec ification can req uire morc stringent limits than a related standard's limit. In practice , a specification provides a clearl y organized list of testing parametcrs (a spec ially wlillen procedure) that describes the technique for locating and categorizing discontinuities in a specific test object. A typical specification includes acceptance criteria and is required by the designer, buyer. manufacturer and users of the test object it covers. For electromagnetic testing , thc tenu procedure refers to a set of brief generalizcd guidelines that show the technician how to perform an accurate rcst fo r a given contract. A procedu re often includes details about the in house setup . Specifications are writtcn to eliminate the variahle characteristics of human operators and system designs, to prod uce an accurate re,ult regardless of who is performing the electromagnetic test. Specificatio ns must be written with full knowledge of the electromagnetic test technique, its sensitivity, tcst object design, and its matcl; al characteristics. In addition to knowledgc of discon tinuities criti cal to rhe test object's service life. Testing specifications are working documents that in struct how to locate discontinuities in a specific test object. Speci ficati ons are in regular need of review and revision.

Standards and Industry Specifications Procedures, specifications and standards are produced to provide a means of contrOlling product or services quality. Written in struct ions that guide a company or individual to a desired end res ult and are acceptable to the industry are the has is of procedures, speci fications and standards . Several pUblications arc availahl e to guide or instruct.. Some of the most frequently used references include the following. The American Society for Testing Materials (ASTM) The American Society for Testing Material s (ASTM) puhl ishes several standards pertaining to the electromagnetic testing techniques. These standards are numbered, for example E268-81. E268 refers to the standa rd, and 81 refers to the year of ori gin or the last rev isio n. The following list includes some ASTM standards that peltain to electromag netic techn iques . I.

132

E309-77: Eddy CU1Tent Examin ation of steel Tubular Products using Magnetic Saturation .

Personnel Training PuhlicatiolJs

2.

3. 4.

5. 6.

E426-76 (1981): Electromagnetic (Eddy Current) testing of seamless and welded and welded Tubular Products, Austenitic Stainless Steel and Si milar Alloys. E703-70: Electromagnetic (Eddy Current) Sorting of Nonferrous Metal s. E566-82: Electromagnetic (Eddy Current) Sorting of Ferrous Metals. £376-69 (1979) : Measuring Coaling Thickness by Magnetic Field or Eddy Current (Electromagnetic) Test Methods. £570-69: Flux Lcakage Examination of Ferromagnetic Stcel Products.

American Society of Mechanical Engineers (ASME) In 1911 the American Society of Mechanical Engineers (AS ME) set up a committee to establish rules for safety for design, fabrication and testing of boilers and pressure vessel;;. Theses n Il es have become known throughout industry as ASME code . The ASM£ Boiler and Pressure Vessel Code is divided into eleven sections. ASME Section V is Nondestructive Examination. Scction V is divided into two subsections,A and B . Subsection A deals with nondestructi ve methods of examination . Article 8 is Eddy CutTent Examination of Tubular Products . Subsection B contains documents ado pted by Section V. Eddy Current standards arc described in Article 26. In this case , the ASME E215 documcnt has been adopted by ASME and reassigned the designation S£215 . Military Standards (MlL-STD) The United States Military uSes the military standard document to control testing and materials. Standard procedures are provided by a series of MIL-STD-XXXX documents. Special requirements arc specified by the military spec ificati on system. For example, MIL-STD-J537A refers to Electrical Conductivity Test for Measurement of Heat Treatment of Aluminum Alloys, Eddy Currcnt Method. MlL-STD-2032A is Eddy Current Inspection of Heat Exchanger tubing on ships of the United States Navy. The mi litary standard usually contains several parts and is very descriptive. These parts nonnally include scope, applicable doc uments , definitions, general requirements , detail requirements and notes.

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Glossary Absolute coil: Co il that responds to the electromag netic properties of that region of the test part within the magnetic field of the coil , withou t CIlmpari son to the response of a second coil at a di fferent location on the san1C or similar mate ri al. Absolute measurement : (1) Measurement made with an absolute coil. (2) Measurement of a property without reference to another measu re ment of that property. Acceptance criterion: Bench mark against which test res ults are to be compared for purposes of establishing the fu ncti onal acceptabi lity of a pan or system being examined. Acceptance level: Measured value or values above or bclow which test objects arc acce ptable in contrast to rejection level. Acceptance limit: Test signal value used in electromagnetic testing, establishing the group to which a material under test belongs. Acceptance standard: Specimen, simi lar to the product to be tested, containing natural or artificial discontinuities that are well defined and similar in size or extent to the maximum acceptable in the produc!. Acceptance standards are available also for material properties such as conductivity and hardness. Alternating current: Electri cal current that reverses its direction at regular intervals. Altcl'11ating current field: Varying magnetic field produced around a conductor by altern ating CUlTenl fl ow in g in the conductor. Altcl'11ating current magnetization: Magnetization by a magnetic field generated when alternating cun ent is fl owing. Ampere (Al : SI unit of electric CUtTen!. Amplitude response: Property of a te~t system whereby the amplitude of the detected signal is meas ured without regard to phase. See also phase analysis . Analog-to-digital converter: Circuit whose input is information in analog fo rm and whose output is essentially the same information in digi tal form. Anisotropy: Havi ng properties that differ according to the direction of measu rement. Artificial discontinuity: See discontinuity, artificial. Artificial discontinuity standard: See acceptance standard. ASNT : American Society for Nondes tructive Testing. ASNT Recolllmended Practice No. SNT-TC-I A: See Recolllmended Practice No. SNl~TC-IA.

135

Attenuation: Decrease in signal amplitude Over distance, often called loss: can be expressed in decibel s or as a scalar ratio of the input magnitude to the output magnitude. Automated system: Acting mechanism that perfOlms req uired tasks at a determined time and in a fixed seque nce in response to certain conditions. Band pass filter : Frequency filter that has a si ngle transmission band between two cutoff frequencies. ne ither of the cutoff frequencies being zcro or infinity. Bandwidth: Difference between the cutoff frequencies of a bandpass fi Iter. Calibration, instrument: Adjustment of insU1Jment readings to known refercnce standard. Central conductor: Electric conductor passed through the opening in a part with an aperture. or through a hole in a test object, for the purpose of creating a circular magnetic lield in the object. Certification: With respect to nondestructive test personnel, process of providing written testimony that an individual is qualified . Sce also certified and qualified . Certified: With respect to nondestructive test personnel, hav ing wrincn testimony of qualification. See also certification and qualification . Circular magnetization: Magnetization in an object resulting from current passed 10ngiLUdinally through the object itself or through an inserted central conductor. Circumferential coil: See encircling coil . Coil: One or more loops of a conducting material; a single coil may be an exciter and induce currents in the material or it may be a detector or both simultaneously. Coil clearance: See lift-off. Coil spacing: In electromagnetic testing, the axial distance between two encircling or inside coils of a diH'erential or remote lield test system. Conductance: Transmission of electric current through material. Measured in siemens (S). Inversely related to resistance R (ohm). Conductivity: Ability of material to transmit electric current. Measu red in siemens per meter. Inversely rclated to resi stivity r. Contact head: Electrode asse mbl y used to clamp and support an object to facilitate passage of electric current through the objcct for circular magnetization. Coupled : (1) Of two electric circuits, having an impedance in conlIDon So th at a current in one causes a voltage in the other. (2) Of two coil s, sharing parrs of their magnetic flux paths. Coupling: Percentage of magnetic flux from a primary circuit that links a secondary circuit ; effectiveness of a coil in inducing eddy currents in the test object . Current flow technique: Magnetizing by passing current through an object using prods or contact heads. The current may be alternating current or recti fied alternating current. 136

Persollnel Training PublicQtions

Current induction technique: Magnet izati on in which a c irculating current is induced in a ring component by a fluct uating magnetic field. Cycle: Single period of a waveform or other variable. See period. Defect: Discontinuity whose size , shape, orientation or location make it detrimental to the useful service of its host object or whi ch exceeds the accept/reject critcria of an applicable specifi cation. Note that some discontinuities may nor cxceed specif ications and are the refore not defects . Compare discontinuity and indication . Demodulation: Process wherein a carrier frequency modulated wi th a signal of lower frequency than the carrier frequency is converted to a close representation of the ori ginal mociulating signal. See modulation . Depth of penetration: See skin effect and standard depth of penetration . Differential amplifier: Amplifier whose output signal is proportional to the algebraic di ffe rence between two input signals. Differential coils: Two or more physically adjacent and mutuall y coupled coils connected in series opposition such that an imbal ance betwee n them, causing a signal, will be produced only whe n the electromagnetic conditions are different in the regions beneath two of the coils. In contrast, comparator coils are not adjacent or mut ually coupled. Differential measurement: In elecu·omagnetic testing, the measurement of system imbalance by using differential coils, in contrast to absolute and comparati ve measurements. Differentiated signal: In electromagnetic testing, an output signal proportional to the input signal's rate of change. Direct current: Electric current flow in g continually in one direction without variation in amplitude through a cond uctor. Direct current tield: Active magnetic field produced by direct Clm-ent fl owing in a conductor or coil. Discontinuity: r;terrupt ion in the physical structu re or configurati on of a test object. After nondestructive testing , unintentional discontinuities interpreted as detrimental to the serviceab ility of the host object may be called flaws or defects. Compare defect and indication . Discontinuity, artiticial : Reference discontinuity such as hole , indentation , crac k, groove or notch introduced into a reference standard LO provi de acc urately reproduc ible indications for determining sensitivity levels . Domain: Any of numerous contiguous regions in a ferromagnetic malerial in which the direction of spontaneous magnetization is uniform and different fro m that in neighboring regions. Eddy current: Electrical CUlTent induced in a conductor by a time varying magnetic field.

Classroom Training Series: Electromagnetic Testillg

137

Eddy current testing: Nondestructive test technique in whi ch eddy CUlTcnt fl ow is induced in the test object. Changes in the fl ow caused by variations in the s pecjrnen arc reflected into a nearby coil , coils , Hall etfect device or other magnetic Il ux sensor for subsequent analysis by suitable instmmcntation and techniques, Edge effect: In electromagnetic testing. the dis turbance of the mag netic fie ld and eddy currents because of the proxim ity of an ab rupt change in geometry. such as an edge of the test object. Sometimes called elld ~ffect. T he effect generall y results in the masking of discontinuities wi thin the affected regio n. Effective depth of penetration: In electromagneti c testing. the nrinimum depth beyond which a test system can no longer practically detect a further increase in specimen thickness, Electric field : Vector field of either the electri c f ield inte nsity (volt per meter) or of the electric nux dens ity (coulomb per meter squared ) , E lectroma gnet: Ferromagnetic corc sUlTounded by a coil of wire that temporari ly becomes a Tnagnct when an electric c urre nt flows through the wire . Electromagnetic acoustic transducer (EM AT ): Electromagneti c device u:;;ing lorentz forces and fi1agnetost ricti on in cond uc ti ve and felTomagnetic materials to generate and rece ive acoustic signals for ultraso nic nondestruc ti ve tests . E ncircling coil : In e lectromagnetic test in g, a coil or coil assembly that sUlTounds the lest object. Such a coil is also called an ann ular coil . circumferential coil or fee d-through coil. ET: Electromagnetic testing . Evaluation : Review following inte rpretati on of indications , to determine whether they meet spec ified acceptance criteria , Excitation coil: Coil that carries the excitation CutrenL A lso called primary coil or lvinding. External discontinuities: Di scontinuities on the outside or exposed surface of a test object. False indication: Test indication that could be interpreted as orig in ating from a discontinuity but which actually orig in ates where no d iscontinui ty exists in the lest Object. Di stinct fro m non re levant indication . Compare Defect. Ferromagnetic material : Material such as iron , nickel or cobalt \vhose relative permeability is consi de rably greater than unity and depends on the mag netizing fo rce and ofte n exhibits hysteresis , Materials that are most strong ly affected by mag netism are called f erromagnetic . Fill factor: For encin.:1 ing co il electro magneti c testing , the ratio of the cross-sectio nal area of the test object to the effective cross sectional core area of the primary enc ircling coil (outside diameter of co il fo rm. not ins ide diameter that is adjacent to the object), For internal probe electromagnetic testing, the ratio of the ellecti ve cross sectional area of the primary internal probe co il to the cross-sectional area of the tube interior,

138

Personnel Training Publication'S

Fill factor effect: Effect of fill factor on coupling between coil and test object. Flaw: Rejectable or unintentional anomaly. See also Defect and Discontinuity. Flux density : See Magnetic flux density . Flux leakage: See Magnetic flux leakage field ; Magnetic flux leakage technique ; Magnetic flux meter. Flux meter : See Magnetic flux meter . Hall detector : Semiconductor element that produces an output electromotive force propoltional to the product of the magnetic field intensi ty and a biasing current. Hall effect: Potenti al difference developed across a conductor at right angles to the direction of both the magnetic field and the electric current. Produced when a current nows along a rectangular conductor subjected to a transverse magnetic field. Hertz: Measurement unit of frequency, equivalent to one cycle per second . Heterogeneity: The qu ality or state of being nonuniform or dissimilar. Horseshoe coil: Probe coil in which the ferrite core of the coil is horseshoe shaped . Also called a V-shaped coil. Hysteresis: Apparent lagging of the magnetic effect when the magnetizing fo rce act.ing on a ferromagnetic body is changed; phenomenon exhibited by a magnetic syste m wherein its present Slate is influenced by its previous history. Hysteresis loop : Curve show ing flux denSity B plotted as a fu nction of magnetizing force H as magnetizing force is increased to the saturation point in both negative and pos itive directions sequenti ally. The curve forms a charactcristic shaped loop. lACS : International Annealed Copper Standard. Impedance: Opposition that a circuit presents to the flow of an alternating cUlTent, specifically the complex quotient of voltage di vided by current. Impedance analysis : [n electromagnetic testing, an analytical teChnique that consists of correlating changes in the amplitude, phase, quadrature components or all of these of a complex test signal voltage to the condi tio n of the test object. impedance-plane diagram : Graphical representation of the locus of points indicating the variations in the impedance of a res t coil as a fu nction oj' a parameter, such as conductivity or lift-off. Indication: Nondestructive test equipment response to a di scontinu ity that requires interpretation to detennine its relevance. Compare Defect, Discontinuity and False indication. Indication, discontinuity: Visible ev idence of a material discontinuity. Subsequent interpretation is required to determine the sign ificance of an indication. Indication, false : See False indication .

Classroom Troil/ing Series: Electromagnetic Testing

139

Indication, nonrelevant: Indication due to misapplied or improper testing. May also be an indi cati on caused by an actual discontinuity that does not affect the usability of the test object (a change of section, fo r instance). Indication , relevant: Indication from a discontinuity (as opposed to a nonrelevant indication) requiring evaluation by a qual ified inspector, typically with reference [Q an acceptance standard, by vi rtue of the di scontinuity's size, shape , orientat ion or location. Induced current technique: See Current induction technique . Inductor: Device consisting of one Or more associated windings, with or without a magnetic core, which impedes the flow of current. Initial permeability: Slope of the inducti on curve at zero magnetizing force as the test spec imen begins to be magnetized from a demagnetized condition (slope at the origin of the B,H curve before hysteres is is observed). Inserted coil: See Inside diameter coil. Inside coil: See Inside diameter coil. Inside diameter coil: Coi l or coil assembly used for electromagnetic testing by insertion into the test piece. as with an inside probe for tubing. Also called illserted coil. Intcmational Annealed Copper Standard (lACS): Conductivity measurement system in which the conductivity of annealed, unalloyed copper is arbitrarily rated at J 00% and in which the conductivities of other materials are expresscd as percentages of this standard. See also Conductivity and Percent Intemational Annealed Copper Standard. Leakage flux: (1) Magnetic flu x of the coil that does not link with the test object. (2) Magnetic tlu x that leaves a saturated or nearly saturated speci men at a discontinuity. Level, acceptance: See Acceptance level. Level, rejection: See Rejection level. Lift-off: Di stance between the probe coil and the test object. Lift·off effect: In an electromagnetic test system Olltput, the effect observed due to a change in coupling between a test object and a probe whenever the distance between them is varied. Longitudinal magnetic field: Magnetic field wherein the flux lines traverse the component in a direction essentially parallel with its longitudinal axis. Magnetic field : Di strib ution of a vector quantity that is a measure of an exeI1ed magneti c force. Magnetic field indicator: Dev ice used to locate or determine relati ve intensity of a tlux leakage field. Magnetic field intensity: Strength of a magnetic field at a specific point. Measured in ampere per meter. Magnetic flux density: Normal magnetic flux per unit area, measured in tesla (T). Magnetic nux leakage field: Magnetic field that leaves or enters the surface of an object. 140

Personnel Training Publication s

Magnetic flux leakage technique: Electromagnetic test techniq ue for the detection and analysis of a surface discontinuity or near surface discontinuity using the flux that leaves a magnetically saturated, or nearly saturated , test object at a discontinuity. Magnetic flux meter: Electronic device for measu ring field strength magnetic tl ux leakage. Magnetic flux leakage: Excursion of magnetic lines of force from the surface of a test object. Magnetic particle testing (MT ): Nondestmetive test method using magnetic leakage fields and indication materials to disclose surface and ncar surface discontinuities. Modulation: Process of imparting information to a carrier signal by the introduction of amplitudc or phase perturbation. Multifrequency: Two or more frequencies applied sequentially or simultaneously to the test coil. Multifrequency technique: Use of the response of a test object to more than one frequency, usual ly to separate effects that would be ind istinguishable at a single frequency. NDT: Nondestructive testing. Noise: In electromagnetic testing. any nonrelevant signal that tends to interfcre with the normal reception or processing of a desired discontinuity signal. Such noise signals may be due to an cxtraneous sourCe or generated by heterogeneities in the test part that are not detrimental to the use of the pmt. Nondestructive testing (NDT): Determ ination of the physical condition of an object without affecti ng that object's ability to fulfill its intended fu nction . Nondestructive test methods typicall y use an appropriate form of energy to determ ine material propert ies or to indicate the presenec of material discon tinuities (surface, intemal or concealed). Nonferromagnetic material: Material not magnetizable and essentially not affccted by magnetic fields. Percent International Annealed Copper Standard (% IACS): Measurement of conductivi ty as a percentage of the conducti vity of pure copper, arbitrari ly rated at 100%. See also International Annealed Copper Standard . Period: Absolute value of the minim um interval after which the same characteri sti cs of a periodic waveform or a periodic feature repeal. Permeability: Ratio of magnetic ind uction to mag netizing force. This relationship is eithe r ( I) absolute pel111eability, in general the quotient of magnet ic ind uction di vided by the magneti zing fo rce, or (2) relative permeabi lity (or specific permeability), a pure number that is the same in all unit systems. The value and dimension of absolute pel111eabi lity depend on the system of units used. In anisotropic media, permeability is a matrix .

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Phase analysis: Anal ytical technique that discriminates between variables in a part undergoing electromagnetic testing by the different phase angle ancl amplitude changes that these conditions produce in the test signal. See also Phase detection . Phase angle: Angular equivalent of the time displacement between con'esponding points on two s ine waves of the same frequency. Phase detection: Derivation of a signal whose am plitude is a fun ction of the phase angle between two alternating currents, one

of which is used as a reference. Pulse technique: Mul tifrequency technique in which a broadband excitation such as an impulse is used. Either the frequency components are ex tracted and analyzed or the interpretation is based directly a ll characteristics of the ti me domain waveform . Qualification: Process of demonstrating that an individual has the required amount and the required type of training , experience, knowledge and capabilities. Qualified: Having demonstrated the required amount and the required type of training , ex perience , kno wledge and abilities. Recommended practice: Set of guidelines or recommendations. Recommended Practice No. SNT-TC-IA: Set of guidelines publi shed by the American Society for Nondestructive Testing , for employers to establish and conduct a nondestmctive testing personnel qu ali ficat ion and certification program. Reference standard: Refe rence used as a basis for comparison or calibration. In tube testing. a rube with artificial discontinuities used for establi shing the tcst sensiti vity setting ancl for pclioclically checking and adjusting the sensitivity setting as required. See also Acceptance standard. Rejection level: Value established for a test signal above or below which test specimens arc rejectable or otherwise distinguished from the remaining specimens. This level is ditlerent from the rejecti on level as defi ned for ultrasonic and other test systems. Relative permeability: Ra tio of the permeability of the material to the permeability of a vacuum. Sensing coil: Coil that detects changes in the flow of eddy currents induced by an exc itation co il ; sensing and excitation coils can be one and the same. Also called detector coil . Signal: Physical quantity, such as electrical voltage, that contains relevant informati on.

Signal-to-noise ratio : Ratio of signal val ues (responses that conta in relevant information) to baseline noi se values (responses that contain Ilonrelevant infomlation). See Noise. Skin depth : Standard depth of penetratioll. See also Skin effect . Skin effect: Phenomenon wherein the depth of penetration of electrical currents into a conductor decreases as the freq uency of the current. is increased . At very high frcquencies , the current flow is restricteel to an extremely thin outer layer of the conductor. See Standard depth of penetration. SNT-TC-IA: See Recommended Practice No. SNT-TC-IA . 142

Personnel Training Publicatiu11.\"

Specification: Set of instructions or standards invoked by a specific customer to govel11 the results or performance of a specific set of tasks or products. Standard: (1) Physical Object with known material characteristics used as a basis for comparison or calibration; reference standard . (2) Concept established by authority, custom or agreement to serve as a model or rule in the measurement of quant ity or the establishment of a practice or procedure . (3) Document to control and govern practices in an industry or application, applied on a national or international basis and usually produccd by consensus. Sec also Acceptance standard and Reference standard. Standard depth of penetration: In electromagnetic testi ng , the depth at which the magnetic field intensity or intensity of induced eddy currents has decreased to 37 % of its surface value. The square of the dcpth of penetration is inversely proportional to the frequency of tne Signal. the conductivity of the matetial and the permeability of the material. See also Skin effect. Test coil: Section of a coil assembly tnat exc itcs or dctects the magnetic field in the material under electromag nctic test. Test frequency : In electromagnetic testing. the number of complete cycles per unit time of the alternating currcnt applied to the primary test coil. Three-way sort: Electromagneti c sort bascd on a test object signal response above or below two levels established by three or more calibration standards. Threshold level : Setting of an instrument that causes it to register onl y those changcs in response greater or less than a specified magnitude. Through-transmission: or or pertaining to electromagnetic techniques where the excitation field penetrates thc test Object so that the detected signal is responsivc to features external to or on the opposite surface. The primary coil and secondary coil are positioned on oppositc sides of the product to be inspected. Volt (V): Measurement unit of electric potential.

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Appendix

Units of Measure for Electromagnetic Testing Table 1: 51 base units.

Quantity

Unit

Length Ma" Time E lectric current Temperature Amount of substance Lu minolls intensity

Symbol

meter kilogram second

m kg

ampere

A K mol cd

s

kelv in

mole candela

Table 2: S[ derived units with special names."

Quantity

Units

Capacitance Catalytic activity Conductance En ergy Frequency (periodic) Force ]m.hLctance llluminance

Luminous tlux Ekclric ,·harge Elcctri<: potential e Elcctrk resistance M agnetic flux Magnetic flux densi ty P lane angle

Symbol

farad kata l

siemens

F kat S J Hz

Relation to Other 81 Units b C-V-I

s- ). mol A·V-I

N·m

joule hertz newton henry lux lumen coulomb volt ohm weber tesla

Wb

V·~

T

N H Ix 1m

C V Q

h -I

kg·m· s-2 Wb·A-I Im·m- 2 cd·sr A· s W ·A- l V·A- I

radian

rad

Wb.m 2 I

Power

waft

W

1->- 1

Pressure (stress) R ad iatio n ab sorbed dose R ad iation dose equ ivalent R ad ioactivi ty Solid ang le Temperatu re. celsius Timcll Vo lumea

pascal gray sievert becquerel steradi an degree celsi us hour liter

Pa

N 'nr2 Jkg- l

Gy Sv Bq sr

'C

J-kg- 1 1·,1 1 K

h

3600

L

dOl)

s

Legend u. Hour and liter are not SI un it!) but are accepted for use with the S1. b. Numbe r onc (1) expresses dimen sionless relat ionship . c. Electromotive fo rce.

145

.... ..,.

'"

''',ble 3: Example of conversions to SI units,

Quantity

~

S

"

A ngle

~

:;-J S·

A rea

'"

Distam.:c

:P

Energy

~.

<3-

~ ~

:;"

C

~

Power Specific hei1!

~

Force (Iorque, couple)

Pressure Frequency (cycle) Illuminance LUll1inJllce

Radi oactiviLy [o ni z ing rad iati o n exposure

Mass Tem perat un.: (d iffe rence) Temperature (sca le) Temperature (scale)

Measurement in Non SJ Unit m in ute {min) degree (deg) squ are inc h (in .2) angs trom (.6..) inch (in ,) British therm al unit (BTU) calori e (ca l), thcnnoc he miccil Bril ish th erm al unit per hour (BTU 'h- I ) Bri tish thermal unit per po un d degree Fahrenheit (BTU ·lbm- LOF- I) foot· pound (n. lb r) pound fo rce l>Cf sq uare inc h (Ibr in.- 2) cycle per minute rootcandle (l"Ie) phot. (ph) candel a per square loot (cd·n- 2) candela per square inch (ctl-in .- 2) foot lambeI'I (ft l) lambe rl nit (nL) stilb (sb) curi e (C i) roentgen ( R) pOllnd (Ibm) dcgrcc Fahrenl1 ei t (OF) deg ree Fahrenh ei t (OP) deg ree Fahrenh e it (° F)

Multiply by 2,90~~82

x 10 4 1.745329 x 10- 2 645 0, 1 25,4 1.055 4.1 84 U,293 4. 19

1.36 6.89 60- 1 10 ,76 10000 10.76 1550 3.426 3 183 (= I a aOO/n) I 10000 37 0.258 0.454 0 .556 (OF - 32)1 1.8 C F - 32)1 1,8) + 273 .15

To Get Measurement in SI Unit rad ian (rad) rad ian (rad) squ are millimeter (m m2 ) nan ometer ( nm)

millimet.er (111m) ki lojoule (kJ ) j oule (J) wa tt (W) kilojoule per kilogram per kelvi n (kJ-kg- I.K- I ) joule (J)!' kilopascal (kPo) hertz (1-11.) lux (Ix) lux ( Ix) cande la pe r square me.te r «xl'm- 2) cande ltl per square meter (cd ' m- 2) candela per square me te r (cc..hn-2) ca nde la p CI' sq ua re mete r (cd' m- 2) cand cla pe r sq uare metcr (cch n-2) ca ndel a per sq uare meter (cd 'rn- 2) gigabct.:q ucrel (G Bq) mi llicoulolllb per kil ogram ( mC kg- l ) kilogram (kg) kel vi n ( K) or deg ree Ccl 5illS (OC) degree Celsius (OC) kelv i n (K)

Table 4: SI prefixes and multipliers.

Prefix

Symbol y

yotta zett3

10 2+ 10 21 10 18 lOi S lOll 109

Z

exa tera

E P T

giga

G

mega

M

kilo hecto:l deka (or decal"

k

pCLa

cenlP milli

h da d c m

micro

II

nano p ica

Il

deci;.J

106 103 10 2 10 10-\

10- 2 10- 3 10-6 10-9 10- 12 10- 15 10- 18 10- 2 1 10- 24

p f a

fcn1to

aUo :lepta yocto a.

Multiplier

Z

y

Avoid these prefixes (except in dm 3 and cm3) for science and engineering.

Table ~: Units from the centimet e r-gram-second (CGS) system of units and not accepted for use with 51. Factor to convert each CGS unit to SI unit is given. ~ical

Quantity

CGS Unit

Busic CGS Units Magnetic field intensily oersted (Oe) Magnetic tlux maxwell (Mx ) Magnetic nux density gauss (G) Mag potential difference gi lbel1 (Gb) Electromagnetic CGS Units Capacitance abfarad Charge abcoulornb ConduClanL:e abmbo ahampere Current Inductance ab henry Mag field intensity abampe re/em POlential abvo lt Resistance abohm Electrostatic CGS Units Capacitance stalJarad Ch;Jrge statcoulornb st::u mho Conuuclancc statamperc Current Inductance stathenry Potenli,,1 Sial vol t s[atohm Resistance

Multiply by 10)'(4,,)-' 10- 8

SI Unit

10-4

ampere per meter wehe r tesla

JO· (4,,)- 1

ampere

109 10 L09

farad coulomb siemens ampere henry ampere per meter voll ohm

10 10 -9 103

10- 8 10-9

1.11 2650 x 10- 12 3.3356 x 10- 10 1.]] 265 x 10- 12 3.335641 x 10- 11 8.987 552 x 10" 2.997925 x 102 8.98755 x lO"

SI Symbul A'm- J Wb T A

F C

S A H

A·m- I V Q

farad coulomb siemens ampere henry

F

voir

V

ohm

Q

Clas.'1room Trainil/g Series: Elcctromagnclic Teslillg

C

S A H

147

Bibliography 1.

A5NT Level III 5/Udy Guide: Eddy Current Testing Method. Columbus, OH: Amcrican Society for Nondestructive Testing (1983) .

2.

Nondestructive Testing Handbook, third edition: Vol. 5. EieCll'Ol1Iagnetic Testing. Columbus. Ohio: American Society for Nondcstnlctive Testing (2004).

3.

Nondestructive TeIting Handbook, second edition: Vol. 6, Magnetic Panicle Testing. Columbus , Ohio: American Society for Nondestructive Testing (1989).

4.

Nondestructive Testing Classrool1l Training Handbook, second edition: Eddy ClIrrenr l esting. Fort Worth , Texas: Convair Division of General Dynamics Corporation (1979) .

148

Persolln el Training Pllblicmions

Index A above-ground sTorage t~~nk floors n ux leakage testing applications, 11 3-114

absolute arnmgement, of eddy current :::;cnsing demenIs, 33-34 active n ux leakage tields, 101 adm iralty brass heat exchanger tubing, 97 aerospace indu stry eddy current testing applications. 91 aircraft st ructures eddy current testing applications. 95 probe coils for crack detection. 30 alarm lights, for eddy current testing, 25

alarms audio,25 digital mixing. 26

alloys composition effect on conductivity. 59 conductivi ty of selected, 581Gb/e. 59table primary metals industries eddy current testing applications. 99 -1 00 rdativc conducti vity of selected by eddy current meter rcadin gs, 94 resistivity of selected, 59wble alloy sorting. 83 altemating current, I () electromagnetic field produced by, 6, 13 alternating cun'e m field measuremenl lt:t:hn ique, 121 -1 25 alternati ng; t: urrcnt generators , 16 altern ating Current potential drop testing , 121 aluminu m, 57,59.78 conductivity, 58lable, 59table degree nfhardcning determination, 98 depth of penetration , !-I5. 86 effeet o f frequency On impedance-plane diagram. 68-69 frequency effect On impedance-plane diagram, 68 pemleabiliry. lift-o f(, and conductivity loci on impedance-plane diagram, 75 relative t:onductivi ty by eddy current meter reading, 94 resistivi ty, 59/Ubh~

thickness loci On impedance·plane diagram. 70, 7 1 thickness testing of coati ngs on, 9 1,92 al um inum alloy 2024 effect of freq uency OIl impedance-plane diagram, 69 aluminum alloy A91100 relative conductivity by eddy Cllrrent meter reading , 94 aluminum a!loy 2024-'1'4 conductivity. 58rable aluminum alloy 606 1-T6 conductivity. 58tuf,Ie aluminum alloy 707 5~ T6 conducti vity. 58table aluminum alloy UNS A92024, 77 edge effects in conductivity measuremen t, 61 relative conductivity by eddy cu rrent meter reading, 94 aluminu m alloy UNS A95052 relative conductivity by eddy (,'urrent meter reading, 94 aluminum alloy U . S A97075, 77 lift-off and edge effect J.:.x;i on impedance-plane diagram . 76 aluminum brass heat exchanger tub ing, 97 al uminum bronze re lativ~ conductivi ty hy eddy cum :nt meter reading. 94 aluminum nit:kcl in permanent magnt!ls , 105 amber. 3 American Society for Nondestructi ve Testing (ASNT) , 8 American Society for Testing M aterials (ASTM) standards. 132- 1:13 American Society of Mechani cal En gineers (ASM E) standards. 133 ammete r. in eddy currcnt test circuits, 21-22 ampere, 13 anal og meters , for eddy current tcsting. 24, 25-26 anneali ng eddy current. testing application s. 99

ANSJIASNT CP-189-J99/, 8-9 antimony conducLiv ity and reSistivity. 59rahle arctan, 20

149

art itici al discontinuity reference standanls, 130

effeu of freq uency on thickness measurements, 72 heat exchanger luhing. 97

I~SlI!IE

Roiler alld Pressllre Vessel Code, 133 tl5M£ £-215 , 133 115M£ 51'-215. 133 .1SNT CP- IN9 , g ASTM £309-77, 132 tlSTM £376-69 (1979). 133 ASIM C426-76 (1981). 1.13 AST,."I F.566-82. 133

permeability. lift -otT, and conductivity loci on impedance-plane diagram, 75 rdative conductiv ity hy eddy current meter reading. 94 thickness lod on impedance-plane diagram , 70 , 7'1 thickness testing or (,.:oatings Oil, 92 bronze conducti vity and re sistivity of commercia l annealed,

59Jable

A5TM £570-69.133 ,ISTM F.703-70 , 133

ASIM £2096-00. 120

condu ct ivity of phosphor, 58rahle effect of frequcncy Oil impel/anee-plane diagntm , 69

audio alarms. for eddy cmrenl test ing , 25

relmive conductivity by cddy current T11~(er reading . 94

halance-or-plant heal exchanger tubes, eddy current

C

testing, 97

cadmium

B bull

bearing~

Ha H effect se lN lr for, 50 bandwidth. and choice of eddy current sensjng clements , 37 bar magnets, 42 , 43 bars direct current Imlgnetizarion, 107 eddy current tes ling or hot rollt'.d, 99 c-.ncirdillg coil applications, 32

end effect. 62 flux It!akage testing app1ic."utions, III hillets

conductivity and resistivi ty, 59Ulble calibration 5-tandards, 127 ~ 12X use with external comparison carbon steel h<::c)t e;.;changer tubing. 97

t~chI1ique.

rcmote fidd testing, 11 7

cathodc ray tuht: vector point method. 79-&0 ccl loph ane

elkly cu rrent testing applications. 99 factors alTecting: flu x leakage fields , 101-I 03 tlux leakage tit'lds. 101 -102

center effect. 32 centrifugal chillers , eddy currenl testi ng , 98

flux leakage testing app lications for square , 112 magnetic tapc system appl icmions. 53 hobbin coil s. 32

35

cathode ray tube ellipse display mcthod, ~O cathode ray lubes (CRl\), for eddy current te~;ti n g , 24 , 26

direct cu rrent magnetizution , 107

flux leakage t.esting app lications for round. 111

for lift -off n.'Jcrcnct:. stand ard. 130

CGS units. for electromagnet ic testing. 147rable chemical industry eddy current tesring applications , 96

cht:mical and petroleum industry application s, 96

ehromhllll conductiv ity and resistivity. 59r,dJfe

electric power applications, 96 end effec t retluclion. 62

coatings ac field meaSllremcnt technique application. 124

fill factor. 89 for hcat cXch,mger tubing applications , [09 test ("'oil urrung<.'mcllts, 33~36

optimum eddy current testing rreql1ency. 83 thickness testing for , 91 , 92 coaling thickness reference !:'tamlards. 129

uoilC'J' tubes flux lC
boh holes eddy c urrent lesling app lications . 93 ~lIr fi:\ce co il
150

c\.mductivity and resistivity. 59table thickn ess testing of c:oating!'i , 92 calcill m

Personnel Trainill/{ Publications

cobalt conduc ti vi ty and resistivity . 59table relative conductivity by eddy (,.'urrcnl meter reading. 94 coercive force , 46 coil imped ance , See impedance co ils. See bobbin coils: encircling coil s: smface coils conductive coatings, 60

conductive coating thickness. 91 conductivity. 57·58 and depth of penetration of eddy currents. 83. 84. R5 eddy current tcst.ing applications. 93-94 effect on impedance-plane diagram. 63-67 factors affecting. 58·62 and impedance-plane diagram. 74·79 se lected metals and alloys, 3Mrable, 59rahle suppressi()n nf condllctivity variable on impedance· plane diagram, 73 conductivity measurement. 93-94 and impedance·plane diagram. 78, 79 optimum eddy current testing frequency. X3 conductivi ty reference standanls. 1 2~- ! 29 conductors. 13-14,57 copper. 78 conductivity and resistivity. 39rable conductivity of annealed. 58table depth of penetration versus frequency, M6 effect of freque ncy 011 impedance-plane diagram. 68-

69 frequency dfect on impedance-plane diagram. 68 as good condul.:tof. 57

llltemariona l Annealed Copper Standard (lACS) Sys tt'IIL 57, 128 pellm~~lbiljty, lift-ofr, and comluctivity lOC:I on impedance-pl ane diagram, 75 relaLive conuw.:ti vity by eddy cunent meier rcnding, 94 thicknt:ss loci on impedance-plane diagram. 70. 71 thickness testing of cOeltings. 91 . 92 thkkncss ll:sting of coatings OIl. 92

copper nickel c()nos inn

~illoys.

59

opli mum eddy current testing frequency. 83 steam generators . 96 wire rope inspection. 110

COlTosion pitting ac fie ld mea~uremen t iIldications. 125 uIlderground pipelines. III in wire rope . 11 0 cracks, See also fmigue cracks: subsu rface discontinuitit'~ ac fi eld measurement indications . 121, J 24-125 calibrmion standard s. 127, 128 eddy current testing application.'., C) 1 flux leakage delectability. 39 flux leakage te ~{ing for dett!clion. X and impedance-plane diagram. 78. 79 Hamral discontinuity reference standards. 129, 130

optimuIll eddy current testing frequency for detection, 83 petroleum and gas pipelines. II I surface coil applicmions, 30 C· scan displays. 26 current. 4, 5 . 13

D demodulation and allaly~is, eddy currc.nt instrumentation, 24 depth of pe netration, or eddy currents. X3-X7 ditferential coil arrangement cxtemal comparison lcdmiyue, 35 scl f·c omparison tcchnjque, 34

digiml conductivity mcters. 26 digital data storage fo r eddy currefl( testing, 26 for remote field testing. 115. 117-118 djgital displays, for eddy current testing. 26-27 digital meters. for eddy current testing. 24. 26 digit.11 mixing. for eddy Cllm::nt lt~ting. 26 digitiz<\tioll rate , and choice of eddy current sc:nsing dt:ment~. 37 dimensional factors. effect on coil impedancc , 62-63 direct CLirrent magndizalion. 106. [0 7- IOH discontinuities ac fidd me<\:H1rement teChnique applicatiOIl. 121·125 eddy curre nt s distorted by, 64. 90 eddy current testing applications. 91 effect ()n coil impedance, 63·64 effect on nux leakage fields, 101-103 effects on coi l impedance. 63-64 nux leab,ge testing for dctect.ion, 8 hem t'xchnnger tuhes , 109- 110 magnemdiode applications for LUbeS. 53 optimum eddy currenl testing frequency for detection. X3 discontinuity reference standards. 127. 129-131 douhle co il alTangcmcllt , of eddy current sensing clements. 33-34

E eddy current hole probe, 94 eddy currents depth of penetration, 83-87 distortion by dio.;conlinuity. 64. 90 generation. 6-7.13-14 eddy current !\ellsing dements. 29·33 factors affecting choice of. 36-37 te~t coil arrangements. 33-36

Classroom Trail1il1X Series: EIe.ctrollluXfletic Testing

151

eddy current testing. See also conductivity: rill factor: impedance; lift-off alternating current field measuremenl tel:hniqut: , 121-125 applications, 91 - 100 basil: principles , 6- 7 basic test system setup . 6 cathode fay Lube methods , 79-XO

standards and spe.ci fications , 132 ~ 133 units for, 1451ab/e, 146rable, 147raMe elec tromagnetic yokc. 105 electromagnets. 105-106 electrom{ltivc forcc, 5. 14 electrons. ]3 encircling coi ls , 31-32 , 106 end effect reduction. 62

fill fac.:tur, 89

coil impcdam:c ,57-62 coupling, 89-90 excitation frequcncy. 7 generation, 13-14 impedance testing systems . 65 instrumentmion , 21-24 modulation analysis systems . 80-82 phase analysis systems, 65-79 rC<-Idout lllcch<-lnisms . 25-27 remOTe field. 115-120

sensing c1crncnts . 29-37 lcst f'rc 4ucncy ~cJcctiOll. 83-87 theory, 13-20

edge dfccts effect on nc field mellsurement technique. 124 effect 011 conductivity, 60-61 loci on impedance-plane diagram, 76. 7S, 79 elecfrical cow.iucti-..rity. Scc conductivity electrical CUlTcnt. 4 . 5, 13 electric:11 resi.stiviry, See resistIvity electric power industry cddy eurrent tcsting applications . 96-98 electrornagnetic fields. 13 electromagnetic induclion, 3, 5, 13 and c\el:tmll1olive forct'. 14 ~elf induction , 17 electromagnetic testing:. Sec also eddy current tesling: flux leakage testing applications of cddy current testing, 91-100 applications of tlux leakage tcsting . 109-1 14 calibration standards. 35 , 127-128 coupling. 89-90 introduction. 3-11 Level I personnel. 8, 1 I Level Il personnel, 8.11 I.eyellli personnel. 9.11 magnetization method selection, 105-108 pt:rsolllle.J certification . 10-11 personnel qualification . 8- 10 reference standards, 92-93. 120,127 .1 28-131 standards and procedun!s . 127-133

152

Per:w11Iu:1 Training Publicaliolls

end effect , 62 cx: tCl11aJ comparison technique, .,5

F Faraday. Michae l. 5 Faraday's law of elec tromagnetic induction. 5.14 fastener holes CfllibTation slanuan.1s JilT cracks, 127. 12;.; surface coil applications,.10 fatigue cracks ae field measuremem indicm.ioJls , I'll , 124 , 125 aircraft structures. 95 natural dist:ontinuity reference stan dards, 129, 130 feed-through coil. ., I

fenites in illllU(:tivc coil scm-ors. 49

ni(.:kclz lJle fcrrite cores, 74. 7S ferromagnctiL' materials, 44 . See also iron; steel hysteres is, 45 ferro-probe,5 1 fill facLor , i-l9-90 remote field testing. 116, 117 tlar coiL 29 nux density . 43,10 1 flux-gate magnetometer, 5 1 tlux leakage fields . Sf:e. alsu magnetization detecting , 7-8 fw.:tors atfcctjng . 3Ot-103 magn.itude s . 108 flux leakage sensing elements . 49-54 tlux leakage testing applications. 109-114 bask principles. 7-8 excitation frequency, 7 magnetization method sclection . lOS-lOS sensing clements . 49-54 theory. 39-48

typical signal. X flux lines . 42

fanning eddy current

chcmil.:ai and petroleum applications. 96 te~ting ~pplications .

99

Fors ter. Friedrich , 6 Forster prohe, 51 frequency, 7 alt.crnating currcl][. 17 and choke of eddy current sensi ng dements. 36 and depth of penetration of t:!ddy currents. 8)·87 effect on impedance· plane diagram. 6X-70 effcct on thickness measurements, 71- 72 limit frequency. 85 frequcncy selection . 83 multi-frequency sy stems . 86-87 for remote field eddy current testing. 117 single frequen cy systems, X6

G gage corner cracking . 125 gain linearity. and choice of eddy current sens in g

elc::ments.36

gas pipelines

nux leakage testing app licat ions, 111-112 gall <;s.43

flux leakage testing npplications. 109- I 10 tlux leakage testing applications for in~talled. I J 3 intcrnal coil applications. 33 su rface coil applications. 30 hc.at treating eddy currelll testing app lication ~. 99 hertz (Hz). 17 . 83 high -alloy steel. See Steel. high-alloy hole probe.)O holes anlt1cial discontinuity reference st.:lndards. 130 bolts . 30. 93-94 fasteners . 30. 127. 12R rivet.... 30 horizontal deviation. and choice of eddy (;Ul'rcnt sensing elements . 36 bot rolled bar ... , eddy cune.nt tc~ting. 99 hybrid coil arrangemenLs, of eddy current sensing elements. 35·)6 hydrogen induced cra<,·k.i llg ac field measurcment indications. 125 hydrogcn sulfide cracking, 125

glass

I

a, non-conductor. 57 gold conductivity, 58mb/e, 59table relative cUlldu ctiv ity hy eddy current meter reading,

94 re sistivity. 59rable graph ite depth of penetration versus frequency_ 86 errtx:t rrl'lILu.:ncy on impedance-plaue diagnlm, 69

or

H H ~\tt

dTeCl sensors. 8. 39. 50-51

Clbove-ground storage lank !lOOT applications. 114 heal exchanger tubing applications. 109-110 installed heat exchanger/boiler tubing application. 11 3 hurdening eddy cmrent testing applicalions, l)X. 99 hardncss effect on conductivity. 59 BaSlelloy thickness loci on impedance- plane diagram. 7 1 Hustelloy X conductivity. 58table heat cxchanger rubes balnnce-of-plant, in electriL: power applicmions, 97

impedance, 7,18-20 conducti vit.y of te st object effe cts . 57-62 dimcnsional factors of te..,l objed. 62-6.1 discontinuity effeds of Ie'" object. 63-64 and fill factor. 89 and inducli vt:. reaclam:c. 20 and life-off. 89 permeability of t~st Object elli..'cts. tl2 readout mcchani~ms . 25 -27 and resistance . 20 impedance bridge circuit. ZJ-22 impedance-plant! diagram , 1~-19. 65

conduclivily and pemu;-ability. 74·79 conductivity locus. 65 -6H effect of freqllency, 68 -70 effect of frequc::ncy O il thicknc$$ mca ~lIrel1lents, 71-7'2 effect of mnterialthicknc.ss. 70-71 suppressi<Jn (If c(muuctivity variable , 73 ~ uppre ssion of lift-off "Variable. 72-73 impedance lesting systcms. 65 impedancc "CC lo r, 18-19 Inconei 60{) conductivity, 58table

Classruom T1'lIining Series: f:ieclromagneric TestiJiM

153

indium antinornide in Hall erred sensors, 50 induced currents. 3A induction. See elcctromagnetic induction induction coil. X, ~W inductive coil sensors. 49-51 inductive reactancc , 18 amI impedance. 20 industrial air conditioning: chillers. eddy current testing. 97 inuLlstry specific ations . 132-133 inilial magnetization. \U3 inner diameter coil. 32 inside probe. 32 inspection coils . 29 instrumentation eddy cnrrenl leSiing . 2 1-2+ for remote field 1 c:~ling . 115· 117 internal coils . 32-33. See also hohhill cojls In ternational Annealed Copp~r St~Uld<:lrd (lAC S) system . ,,7,128 iron. See al.w sta i nlcs~ steel: steel conductivity and resistivity of pure. 59wble as ferromagnet ic 1II:.1leria l, 44 penneability,62 iron . ingot conductivi ty and resist ivity. 591(1ble depth of pene tration vc rsus freq uency. 86

J jet engines eddy current testing lIpplicm ions. 95

L lamination eddy current testing application';. Sll laps cddy current testing applicati(}n~. 91 lead relative conduclivity by eddy CUITcnt meter reading . 94 thickness loci on impedance-plane diagram , 70. 71 rhic:kne.;;s lesting of coali ngs. 92 Leve ll personnel eertificmion fOT. 11 qualitication for. R Lc\'cl II per:;onnel ce rtification ror. II qualification for. H Lcvel III personnel certification foT. I I

154

Pers()} mel Training Publicatiolls

qualification rOT, 9 lift-off. 89 and depth of pcnerrarjon of eddy cu rren ts. R3 loci on impedan!.:e-planc diagram. 67-68. 78 . 79 suppression of lift-oft· variable on impedance-plane diagram. 72-73 li ft- off reference standards , 130- 131 limit frequency. H5 lines of force. 41 liquid crystal displays (LeDs) . for eddy current tes ting.

24.27 lodestone . 3

lVl magnesium conductivity. 58/ab/e

conductivity and resistivilY or pure , 59rable relative conductivity by eddy curren t merer reading:. 94 magnetic attraction. 3-6 magnetic domnin s. 44-45 and R and H curve. 40-42 magnelic ficld intensity. 41 magnerit' fIeld lines. 42 magnetic fields . 42 in eddy curren l lesling, 6-7 in flu x leakage tes ting. 7-8 right hand rule for. 43-44 magnetic flux densily. 41. 4:1 , 101 magnetic flux leakage lie1{b; . See fl ux leakage rields: nux leakage testing mag.netic flux l in~s , 42 mag.ne tic hysleresis , 44-45 mag.netic part ide testing. 7-8 , 39 mag.netizi ng current level, 54 magnetic pennt>ahility. 47-48.62 and depth of penetration of eddy currents . 83. 84 effecrs on coil impedance, 62 and impcdam·!.!-planc uiagram, 74-79 magnerie n:sonancc sensors . 54 magnelic rape system. 53 magnetism. 3lJ. 42 early observations of, 3-6 magne,[ite.3 magnetization degree or initial , 103 direcl l:urrenf. 106. 107-108 in nux leakage testing . 7-8 method select ion , 105-108 magneLi/.ing coils . 106-107

o

magnctodiodes. 51·53. 111

Oersted. Ilans Chri$lian , ~

lll:1gnelostutic energy, 40 m~H~rial soning alloys. 83 eddy current

te~ ti ng

ohm . 15 Ohm 's law, 15

<tpplicutions, 98

outer diameter co il. 31

sorting referelKc standards, 13 1 Maxwell. James Clerk. 3. 5·6

p

Maxwell's equations . 5. 7 metals conductivity of selected. 5Xtahlt? , 59wb/e primary melal s industries eddy cu rrent testing app licatioll ~.

99·100

relative conductivity of selected by eddy cum.'m meter readings . 94 reSi sti vi ty of selected. 5Yrab/e metal spacing. 92 metal thickness. lJ2 military srandards, J 33

pancake coil. 29 p.tper for 11ft-off referc:n(c st
13 and H curVe! . 40-42 permeability. See magnetic pemteahility personne l ce"rtilic<.ltion, 10-11

personnel qualification. 8-10 Persollnel Qualilication and Certitkation in NondeStructive Testing: Re.colllllltmded Prtlclice

MIL·STf)·]537A.133 MIL·STD·2032A. 1.13 rlHJdulation analy s i~ systems. 80-X2

Nu. SNT·TC·]II. 8. 9. 10 petroleum industry eddy current testing applicmions, 96 petroleum pipelines flux leakage leMing applications, 111 - 112

molybdenum conductivity and resistivity. 591able relative conductivity by eddy current meter retlding. 94 Monel conductivi ty.5Hlable

phase amplitude diagram. in remote lid<.llesting. 117. 119 phase analy~is "ystems, 65-79

phase angle.. 17 and impedance-plant: diagr
mooring w ire rope inspection, 111 multi-frequency systems, 86-H7

and inductive reactanc:e. 20

signal excitation. 23 multi-parameter SYSlCnlS, R6 narrow enc ircling coil. 31

and resistance. 20 di scri ntination , 24

pha~t:

N natural dis(ontinllit)' rdcrcm:e standards, 127, 129-1:'0

phast: lag. 17 phosphor hrollze (.;onductiviIY. 5~whh' rdati ve conductivity by eddy current meier reading , 9.+ pigs . for pipeline inspcclion.1l2 pipd ine inspection, 111 -112

ncodymium iron boron

in permanent magnets. 105

pits. See also corro:-.i on pining

niL'kcl

conductivity and resistivity, 59Table as poor conductor. 57 thi ek.ne~$ lc~ ting of comings, 91. 92

nickel copper hear exchanger tubing, 97 nickel zinc ferri te l:Ofcs , 74. 75 nOlH.:onductors, 14,57

ntJrrnalizing eddy .curre-n t l e~t in g. applkations. 99 nucl ear magne tic resonance magnctomc[~rs. 54

al1ificial discontinuity rcferenc~ standards. 13U eddy ('urrem le!sting applications. 91 , 93,96 plates ac field meaSllft!ment technique applic.:ation . 124 magnetic lape! !-ty~tcm application~. 5.1 polyethylene terephthalatc for lift-off rderencc standard, l30 primary field. 13 primary melals industries eddy current testing applications . 99-100

Classroom Traillillg Series: ElecfromagneTic Testing

155

principle of :;df induction. 17 prob~ coil. 29 procedure, 132 pythagor(!un theorem, 19

Q quadrature accuracy. and choice of eddy current sensing c leme nis. 37 quasistatic magnet ic fi~ld. 7

R rail head fatigue cracks. at fidd measuremcnt indit·alion s .

125 readoU[ mechanisms. for eddy Current testing. 25-27

Recommended Pructire No. SNT-TC-1A. 8. 9. 10 reference coil. 22 reference standards. 127. 12X- 131 for remote field testing, 12() for thickness tes ting, 92 remanence . 45 remote fie ld eddy current testing. 115 remote fic ld testing. IIS-121) instrumentation. 115-11 7 reference standards for. 120 res idua! field testing . 107. 108 res idual nux leakage ticl c.h. 101 res idua! Hmglletislll , 45-47.62 residu al stresses effect on conductivity. fiO resistance. 15 and impedance, 20 resi sti vi ty. 57-58 and depth of penetration of eddy currents, R4 sclecccd metals and alloys . 59tc1h!e reten tivity, 45 . 46 revcrse magnetization saturation point. 46 reverse polarity saturmion point. 47 reverse residual magnetism . 46 rhodium re1mive conduc tivity hy eddy curren l meter reading, 94 righl hand rule, for magnttic Iltlds. 43-44 rivet holes surface coi l appli<:atiom . 30 rods

eddy current lesting applications. 100 round bars flux leakage testing applic:a l io n ~. III

156

Persollnel Training PlIblicatiolls

s ~a1l1ariUJ11

cobalt

in permanent magnets. 105 sample rate . and choice of eddy CU!Tenr sensing elements, 37 scams eddy current testing applinttions. 91 selenium conductivity and resistivity, 59tabfe self-comparison technique. 34 self inductance. 17 se nsing elements eddy curren t testing. 29·37 flux leakage testing. 49-54 sigmll analysis. 7.24 remote field testing. 115 ,117- 120 signal demodulation. eddy current instrumentation. 24 signa l display. eddy current instrumentation. 24 sig nal excit<1tion. 2.1 and choict! of eddy current sensing elements, 3(') signal m()dlll ~lti()n, eddy current instrumentation . 23 signal preparation, eddy eurrenl ins{rumentmioll , 24 signal-to-noise ratio. cddy current instrumentati(ln. 24 si licon bront.t' relative conductivity by eddy curren Lmeter reading. 94 si lver conducti vity, 58roble conductiv ity and r~sistivi ty of tin solder, 59rable as good ('QlldllC'tor. 57 relative conductivity by eddy currenl meter reading. 94 sine wave. ac generators. J 6 single coil ilmlllgemenr. of eddy cunent sensi ng clements,

33 sin gle frequency sys tems. X6 signal excitation . 23 51 units, for clcerroJll~gne t ic lesting, 145mb/e. 146rable. 147/ab/e skin effect . 61 sman pigs . 112 sockel welds sllliace testing. 95 sortin g reference standards, 131 spccifications. 132-133 speed filter. 24 s pinnjng probe technique. 31 stainless steel depth o f penetraLion versus frequency. 86 heat exchanger tubing , 97 surface testing of pipes , 95 thi ckness Im:i on impedancc-plane diagram. 71

stainless steel 304. 59 conduc ti viTy.5Krable cffect of fre quency on impedance-plane diagrum, 69

subsUltaee discontinuities effects on coil impedance, 63 flux leakage fields . J[l2-103

stainless s t~el 316, 59

magnetic parlides attracted by, 40 surfacc coil s, 29-31 test coil arrangements, 33-36

stai nless slccl3 16L. 59 Slainless steel UNS 530400. 77 standards calibration standards, 35, 127-12R and procedures. 127-133 reference standards, 92, 120 . 127. 128-131

surface cracks flux leakage. 39 llanJral discontinuity reference ~taJldanls , 130 surf"1ce tests , 95

and spcc ifit:ations, 132- 133 Slellm generators

T

eddy (.'urrenl lcsting app licat ions . 96 steel. 7~. See also smin lcss S(c:e1

tangent. 20

l:oating thkkness reference standards . 129 hystcresis loop fm annealed low t:arbon. 47. 4~ hysteresis loop for hardent:d steel. 47 . 4X hysteresis loop for unmagner ized, 46 hystercsis k~o ps , 46 permeublli ty of hardened ferromagnctic, 62 phase angle t:hangt:s on impedance plant: caused by rrequency changes. 77 as poor conductor, 57 sorti ng reference standard. 131

or hot rods .Iud wires,

test rrequency selection. See freq uency se lection testing coi l. 22 thickness conductive coa ting thick ness, 92 effect on impedance measurement. 63 effect on imptdance-plane diagram. 70- 71 meta l thickness. 92 thidness te~ting aerospace applications . 9 1 comi ng thickness rererc n c~ standards, 129 effect of freque ncy 0 11 measurements , 7 1-72

rc:mote field testing, 117 t~stitlg

temperature dfect on conductivity. 60

J()()

and impedance-plane diagram . 78.79

testing or square billets. 99 thickness tesring of coa tings on. 91. 92 wire rope inspection . 110- 111

oprimu1l1 eddy curren t testing frequency. 8] refc:rcnce standards for. 92 through transmission aTTallg~me nt, of cddy current

st.eel . high-alloy

C(l1ldUelivity and resistiv ity. 5f.Jtable d~pth or pcnerrmion versus fr~q ll ell(;y, 86 slee l alloy UN5 043400 li ft-off and edge effect lc.x:i on impedance-plane diagram. 76 permeability, lift-off. and conductivity loci c.lIt impt:dance~p l alle diagrnm, 74 . 75 steel nlloy UNS 191422. 74. 75 ::aorage lank floor:; (above-ground)

flux leakage testing applications. 113-114 stress eon'osion cT<:!cking ae fidd measuremen t indicatioll!;. 124 , 125 aircraft stntctures. 95 hydrogen induced cracking. 125

se nsing t:lcments. 35-36 tin conductivity and rc:sistivity of foil , 59wble conductiv ity and re.'\istivity of pur~ , 59rahle

relative conductivi ty by eddy ellrrenlmeter reading. 94 titanium conductivity. 581l1hle. degree of harde ni ng de l~nnina t i()n. 98 depth of penetration ,'ersus frequC!ncy. 86 hc:at cxchanger tubing, 97

permeability. lift-off. and conductivity loci on impedanl.:e-plane diagram. 75 thickJleSS loci on impcdallce-planc diagram. 70. 71

stre~s ori~nted

titanium alloy UNS R5640 1, 77

strip-chart recorders for eddy currclll l~sting. 25-26 for m(lduimioll analysis. 8{) for r~mote field testing, 11 7, 118

ritaniull1 a.luminum vanadium a.lloy (Ti-6Al-4V) l'ondut:tivity. 58/(/bll: effect of frequency t)n impedance-planc diagram. 69 Ttlbing . See a/50 h~at eXChanger tubes <':il libnllioll standards. 127. 128

Clos.\'1'OO/11 Tra;nillg Series: EleCTromagnetic TestiUM

157

direct CllrrelH magnctiwtion, 107-108 encircling coil applications, 32 end effect. 62 flux leaknge testing applications . 111. 113 internal coil applicmions. 33 magnetodiode
surface testing. 95

u Unill'OTWCrsimlS, for elel'tromagnetic testing. 146rablc Ul1ites States milital)' standards. 133

unitS. ror eiectn,lrnagndic testing. 145wbfe. 146rabfe. 147whfe

v vertical deviation, and choice of eddy current scnsing clements . 36 virgin curve, 45 , 46 volt. 14 \'olra~e . 14 voltuge-plane diagram. 19 voltmeter. in eddy current test circuits, 21

w Wa... paloy

conductivity, 5?!abfe weheL 106 wclds ae field mcasuremcnt Icchni4 ue application, 121 ,124 smface testing of sockct. 95 \Ver alternating current tluorcscent te<.:hni4lles, 39 Wheatstone bridge. 21-22 wide encircling coil, 31 \vire rope inspection nux lcukagc tc.<;ting application~. 11 0-111 wires eddy current testing appl ic:nions , 100 \vood as non-conductor. 57

158

PerSO lJnel Training Publicatioll s

z I.111C

conductIvity and res istivity of commercial rolled ,

59table relative <.:onductlvity hy eddy CUlTentmeter reading:, 94

thidncss testing: of coatings, Y 1, 92 Zircaloy-2 conduct ivity,5Srable

zirconium comhu;tjvity, 581able

The American Society for Nondestructive Testing, Inc. 1711 Arlingate Lane

PO Box 28518 Columbus, OR 43228·0518 Catalog No.: 1643 ISIlN·10: 1·57117·122·3 ISBN·13: 978·1·57lJ7·l22·1