Ut P2

NDT Training & Certification

Ultrasonic Testing Part 2

mmz 2003

Ultrasonic Testing techniques 

Pulse Echo



Through Transmission



Transmission with Reflection

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Pulse Echo Technique Single probe sends and receives sound  Gives an indication of defect depth and dimensions  Not fail safe 

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Defect Position B

A

B

No indication from defect A (wrong orientation) mmz 2003

Through Transmission Technique Transmitting and receiving probes on opposite sides of the specimen

Tx

Rx

Presence of defect indicated by reduction in transmission signal No indication of defect location Fail safe method

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Through Transmission Technique Advantages  Less attenuation  No probe ringing  No dead zone  Orientation does not matter

Disadvantages  Defect not located  Defect can’t be identified  Vertical defects don’t show  Must be automated  Need access to both surfaces mmz 2003

Transmission with Reflection R

T

Also known as: Tandem Technique or Pitch and Catch Technique mmz 2003

A

Ultrasonic Pulse

short pulse of electricity is applied to a piezo-electric crystal  The crystal begins to vibration increases to maximum amplitude and then decays Maximum

10% of Maximum

Pulse length

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Ultrasonic Pulse • Is created by charging a capacitor in the circuitry then suddenly releasing this charge oC electrical energy, about IKv to 2Kv, Into the probe. • This electrical energy is‘ converted into a mechanical vibration by the piezo electric crystal in the probe. • The ultrasonic vibrations are formed by the collapse of the crystal after the electrical energy has been removed. • The behavior of the crystal, on collapse, can be likened to the behavior of a spring when it is stretched then released- The spring will return to its former shape then shorten then stretch, etc., until it finally comes to rest in its original shape. • This cycle of expansion and contraction is what forms the ultrasonic pulse. mmz 2003

Pulse Length  The

longer the pulse, the more penetrating the sound

 The

shorter the pulse the better the sensitivity and resolution

Short pulse, 1 or 2 cycles

Long pulse 12 cycles

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Ideal Pulse Length • To show separate, clear reflected signals on the CRT then the pulses of sound must be short and sharp. • To shorten the pulses the ultrasonic crystal must be damped with a backing medium which absorbs the sound energy

PULSE LENGTH controls RESOLUTION.

5 cycles for weld testing mmz 2003

RESOLUTION Resolution is the ability to separate on the time base two or more reflectors that are close together in terms of beam path length.

0

2

4

6

8

10

0

Longer Pulse

2

4

6

8

Shorter Pulse mmz 2003

10

PULSE REPETITION FREQUENCY (P.R.F.) or (P.R.R) 

The number of pulses of ultrasonic energy that leave the probe in a given time (usually per second).



Each pulse of energy that leaves the probe must return before the next pulse leaves otherwise they collide causing "ghost" or spurious echoes to appear on the CRT.



The time taken for the pulse to travel from die probe and return is known as the transit time.



The time between pulses leaving the probe is known as the clock interval. mmz 2003

The Sound Beam  Dead

Zone  Near Zone or Fresnel Zone  Far Zone or Fraunhofer Zone

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The Sound Beam NZ

FZ

Main Beam

Intensity varies Exponential Decay

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DEAD ZONE 

Seen on the CRT as an extension of the initial pulse,



the dead zone is the ringing time of the crystal and is minimized by the damping medium behind the crystal.



Flaws or the reflectors, that lying in the dead zone region of the beam will not be detected.



The dead zone can be seen at the start of the trace on a CRT displaying A-scan, but only with single crystal probes.



The dead zone increases when the probe frequency decreases.

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Initial Pulse

0

2

4

Dead Zone

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6

8

10

The side lobes has multi minute main beams Two identical defects may give different amplitudes of signals

Near Zone

Side Lobes

The main beam or the centre beam has the highest intensity of sound energy Main Lobe

Main Beam

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Any reflector hit by the main beam will reflect the high amount of energy

Sound Beam Near Zone  Thickness measurement  Detection of defects  Sizing of large defects only

Far Zone  Thickness measurement  Defect detection  Sizing of all defects

Near zone length as small as possible

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Near Zone (fresnel zone) 2

D Near Zone  4 V  f

2

D f Near Zone  4V mmz 2003

Near Zone  What

is the near zone length of a 5MHz compression probe with a crystal diameter of 10mm in steel? 2

D f Near Zone  4V 2 10  5,000,000  4  5,920,000  21.1mm mmz 2003

Near Zone 2

D Near Zone  4

2

D f  4V

 The

bigger the diameter the bigger the near zone  The higher the frequency the bigger the near zone  The lower the velocity the bigger the near zone Should large diameter crystal probes have a high or low frequency? mmz 2003

Which of the above probes has the longest Near Zone ? 1 M Hz 1 M Hz

5 M Hz

5 M Hz

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Near Zone 2

D Near Zone  4

2

D f  4V

 The

bigger the diameter the bigger the near zone  The higher the frequency the bigger the near zone  The lower the velocity the bigger the near zone Should large diameter crystal probes have a high or low frequency? mmz 2003

Beam Spread  In

the far zone (fraunhoffer) sound pulses spread out as they move away from the crystal

/2 

 K KV Sine  or 2 D Df mmz 2003

FAR ZONE (fraunhoffer zone)  Beyond

the near zone Far Zone

exists  The amount or beam divergence depends upon the crystal size and the wavelength

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Beam Spread K    Sine 2 D

KV or Df

Edge,K=1.22 20dB,K=1.08 6dB,K=0.56 Beam axis or Main Beam mmz 2003

Beam Spread

 K KV Sine  or 2 D Df  The

bigger the diameter the smaller the beam spread  The higher the frequency the smaller the beam spread Which has the larger beam spread, a compression or a shear wave probe? mmz 2003

Beam Spread  What

is the beam spread of a 10mm,5MHz compression wave probe in steel?



KV Sine  2 Df 1.08  5920  5000  10 o  0.1278  7.35 mmz 2003

Beam Spread  What

is the beam spread of a 10mm,4MHz compression wave probe is 3200m/sec Ans:4.96

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Which of the above probes has the Largest Beam Spread ? 1 M Hz 1 M Hz

5 M Hz

5 M Hz

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Beam Spread

 K KV Sine  or 2 D Df  The

bigger the diameter the smaller the beam spread  The higher the frequency the smaller the beam spread Which has the larger beam spread, a compression or a shear wave probe? mmz 2003

FAR ZONE The amplitudes of reflected sound from large and small reflectors follow different laws. LARGE REFLECTORS (larger than the width of the ultrasonic beam) follow the INVERSE LAW - The amplitude is inversely proportional to the distance/i.e... if the distance is doubled then the signal amplitude is halved (i.e.... reduced by 6dB). SMALL REFLECTORS (smaller than the width of the beam) follow the INVERSE SQUARE LAW - The amplitude is inversely proportional to the square of the distance, i.e. if the distance is doubled then the amplitude from the second reflector is one quarter of the amplitude of the nearer (12dB less). mmz 2003

SMALL REFLECTORS

LARGE REFLECTORS

(INVERSE SQUARE LAW)

(INVERSE LAW)

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Testing close to side walls

0

2

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4

6

8

10

Sound at an Interface  Sound

will be either transmitted across or reflected back Reflected

Interface

Transmitted

How much is reflected and transmitted depends upon the relative acoustic impedance of the 2 materials

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The Phenomenon of Sound REFLECTION REFRACTION DIFFRACTION

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The Phenomenon of Sound REFLECTION REFRACTION DIFFRACTION

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Law of Reflection  Angle

of Incidence = Angle of Reflection

60o

60o

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Inclined incidence(not at 90o ) Incident

Transmitted The sound is refracted due to differences in sound velocity in the 2 DIFFERENT materials mmz 2003

REFRACTION  Only

occurs when:

The incident angle is other than 0° 30° Water

Steel

Water

Steel

Steel

Steel Refracted

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REFRACTION  Only

occurs when:

The incident angle is other than 0° The Two Materials has different VELOCITIES 30°

30°

Steel

Water

Steel

Steel 65° 30°

No Refraction

Refracted mmz 2003

Snell’s Law Normal

Incident

I

Material 1

Material 2

R

Refracted

Sine I Vel in Material 1  Sine R Vel in Material 2 mmz 2003

Snell’s Law C

Sine I Vel in Material 1  Sine R Vel in Material 2

20

Perspex

Sine 20 2730  Sine 48.3 5960

Steel

0.4580  0.4580

48.3 C

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Snell’s Law C

Sine I Vel in Material 1  Sine R Vel in Material 2

15

Sine 15 2730  Sine R 5960

Perspex Steel 34.4 C

5960 SinR  Sin15 2730

SinR  0.565 R  34.4

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Snell’s Law C

20

Perspex Steel 48.3 24

C S mmz 2003

Snell’s Law C

C

When an incident beam of sound approaches an interface of two different materials: REFRACTION occurs

Perspex Steel

There may be more than one waveform transmitted into the second material, example: Compression and Shear

C C

SS mmz 2003

When a waveform changes into another waveform: MODE CHANGE

Snell’s Law If the angle of Incident is increased the angle of refraction also increases

C

C

Perspex

Up to a point where the Compression Wave is at 90° from the Normal

Steel

This happens at the

90°

FIRST CRITICAL ANGLE

CS

S

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SC

1st Critical Angle C 27.4

Compression wave refracted at 90 degrees

C

33 S mmz 2003

2nd Critical Angle C

C

57

S (Surface Wave) 90

Shear wave refracted at 90 degrees Shear wave becomes a surface wave mmz 2003

1st Critical Angle Calculation C 27.2

Sine I 2730  Sine 90 5960 Perspex

Sin90  1

C

Steel

2730 SinI  5960

SinI  0.458

S

I  27.26 mmz 2003

2nd Critical Angle Calculation C

Sine I 2730  Sine 90 3240

C 57.4 Perspex

Steel

Sin90  1

S

2730 SinI  3240

SinI  0.8425 I  57.4 mmz 2003

1st. C

2nd.

Before the 1st. Critical Angle: There are both Compression and Shear wave in the second material At the FIRST CRITICAL ANGLE Compression wave refracted at 90° Shear wave at 33 degrees in the material

90° Beyond the 2nd. Critical Angle: All waves are reflected out of the material. NO wave in the material.

Between the 1st. And 2nd. Critical Angle: Only SHEAR wave in the material. Compression is reflected out of the material.

At the 2nd. Critical Angle: Shear is refracted to 90° and become SURFACE wave

S C

33°

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Summary  Standard

angle probes between 1st and 2nd critical angles (45,60,70)  Stated angle is refracted angle in steel  No angle probe under 35, and more than 80: to avoid being 2 waves in the same material. C

S

C S

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One Defect Two Echoes

Snell’s Law  Calculate

the 1st critical angle for a perspex/copper interface  V Comp perspex : 2730m/sec  V Comp copper : 4700m/sec

2730 SinI   0.5808  35.5 4700

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