Welcome To Modal Testing And Analysis: From Brüel & Kjær

Welcome to Modal Testing and Analysis A Multimedia Presentation

from Brüel & Kjær Copyright© 2000 Brüel & Kjær Sound & Vibration Measurement A/S All Rights Reserved

Modal Testing and Analysis Main Menu Introduction: What, Why, How Quick Tour of PULSE Modal Test Consultant

Modal Hardware Next step to learn more PULSE Software - an Introductory Tour Company Presentation

About this CD-ROM

Additional Information

Copyright© 2000 Brüel & Kjær Sound & Vibration Measurement A/S All Rights Reserved

Introduction to Modal Testing and Analysis Introduction What is Modal Testing ?

Why do Modal Testing How to do Modal Testing

Copyright© 2000 Brüel & Kjær Sound & Vibration Measurement A/S All Rights Reserved

Modal Testing Solutions

Design Objectives/Problems Based on the forces and motion of a structure, the design problems can be divided into four classes

No motion

Motion

No force

Force

STYLING

STATICS

KINEMATICS

DYNAMICS

Styling With no motion and forces present (or they are ignored), the design problems are reduced to

STYLING Function  Size  Fashion 

Statics With steady-state forces, the designer may prove or optimize the structures ability to sustain or transmit these by

STATIC analysis

Building

Bridge

Kinematics With motion and negligible forces, the KINEMATIC problem can be considered as

“clearance design” disk drive

combustion engine

Dynamics

Dynamic forces  motion Structural Dynamics Analysis helps to prevent problems with: – noise – vibration – early wear – break down – etc.

Structural Dynamics Analysis | H(f) |

Frequency f1

f2

f3

f4 f5

Structural Weakness at Resonances 

Vibration problems are often due to excitation of resonances



Output = System · Response = System · Vibration = System · X=H·F

Input Excitation Force

Static vs Dynamic

Click picture to show movie

Internet links to the story about “Galloping Gertie”

Introduction to Modal Testing Static Testing and Analysis A science studied for over a century



F

Structural Dynamic Testing Term differentiating it from Static Testing and Analysis  In contemporary language it is Modal Analysis and Modal Testing Objectives of lecture: To give answers to – – – –

What is Modal Testing? Why do Modal Testing? How to do Modal Testing? What is the Brüel & Kjær Solution to Modal Testing

d



Present day demands   

Increasing speed in transportation Higher fuel economy More lightweight constructions

The demands achieved by reducing

the mass of structures Consequences  Structures become inherently weak  Resonances move down into the frequency region of excitation forces  Structures fail because of dynamic

loads

Finite Element Analysis (FE) vs Modal Testing

Finite Element Analysis on theoretical model

Modal Testing on real structure/prototype

What is Modal Testing

Copyright© 2000 Brüel & Kjær Sound & Vibration Measurement A/S All Rights Reserved

What is Modal Testing

Construction of a mathematical model of the inherent dynamic properties and behavior of a structure by experimental means

Measurement on Free-free Beam Roving hammer method:  Response measured at one point  Excitation of the structure at a number of points by hammer with force transducer 

FRF’s between excitation points and measurement point calculated



Modes of structure identified Amplitude

First Mode

Second Mode

Third Mode Beam Acceleration Force Force Force Force Force Force Force Force Force Force Force Force

Press anywhere to advance animation

Measurement on Free-free Beam Shaker method:  Excitation of the structure at one point by shaker with force transducer  Response measured at a number of points  FRF’s between excitation point and measurement points calculated 

Modes of structure identified Amplitude

First Mode

Second Mode Third Mode

Beam Acceleration

White noise excitation

 Force

Press anywhere to advance animation

Modal Testing On small simple structures

On individual parts On assemblies On trains

Click to animate model

On disk drives On aircrafts

On large complicated structures

On all types of structures Press Esc to stop animation

Operational Deflection Shape vs Modal Testing ODS

Goal:

Means:

Modal Testing

Determine the forced dynamic deflection at the

operating speed

Construction of a mathematical model of the inherent dynamic properties and behaviour of a structure

Measurement of Transmissibility between reference transducer and roving transducer

Measurement of the Frequency Response Function between force transducer at driving point and roving transducer

Xi

X(f)

T= Xref

Xi

Xref

F(f)

H(f) =

X(f) F(f)

Typical Application of ODS 

Case History from Super Tanker

3. Optimal modification by rod connection of engine top 1. First modification proposal: 10 tons of transverse girders for deck stiffening 2. Operational Deflection Shape at 2nd harmonic frequency