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Ultrasonic TestingUltrasonic TestingPart 2Part 2
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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
No indication from defect A (wrong orientation)
AB
B
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Through Transmission Technique
Transmitting and Transmitting and receiving probes receiving probes on opposite sides on opposite sides of the specimenof the specimen
Tx Rx
Presence of defect Presence of defect indicated by indicated by reduction in reduction in transmission signaltransmission signal
No indication of No indication of defect locationdefect 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
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Transmission with ReflectionRT
Also known as:Also known as:
Tandem TechniqueTandem Technique or or
Pitch and Catch TechniquePitch and Catch Technique
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Ultrasonic Pulse • A 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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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
5 cycles for weld testing
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The Sound Beam
• Dead Zone
• Near Zone or Fresnel Zone
• Far Zone or Fraunhofer Zone
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The Sound Beam
NZ FZ
Distance
Intensity varies
Exponential Decay
Main Beam
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Main Lobe
Side Lobes
Near Zone
Main Beam
The main beam or the centre beam has the highest intensity of sound energy
Any reflector hit by the main beam will reflect the high amount of energy
The side lobes has multi minute main beams
Two identical defects may give different amplitudes of signals
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Sound BeamNear 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
V
fD
f
V
D
4Near Zone
4Near Zone
2
2
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Near Zone
• What is the near zone length of a 5MHz compression probe with a crystal diameter of 10mm in steel?
mm
V
fD
1.21
000,920,54
000,000,510
4Near Zone
2
2
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Near Zone
• 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?
V
fDD
4
4Near Zone
22
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1 M Hz 5 M Hz
1 M Hz
5 M Hz
Which of the above probes has the longest Near Zone ?
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Near Zone
• 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?
V
fDD
4
4Near Zone
22
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Beam Spread• In the far zone sound pulses spread out
as they move away from the crystal
Df
KV
D
KSine or
2
/2
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Beam Spread
Df
KV
D
KSine or
2
Edge,K=1.2220dB,K=1.08
6dB,K=0.56
Beam axis or Main Beam
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Beam Spread
• The bigger the diameter the smaller the beam spread
• The higher the frequency the smaller the beam spread
Df
KV
D
KSine or
2
Which has the larger beam spread, a compression or a shear wave probe?
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Beam Spread• What is the beam spread of a 10mm,5MHz
compression wave probe in steel?
o
Df
KVSine
35.7 1278.0
105000
592008.1
2
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1 M Hz 5 M Hz
1 M Hz
5 M Hz
Which of the above probes has the Largest Beam Spread ?
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Beam Spread
• The bigger the diameter the smaller the beam spread
• The higher the frequency the smaller the beam spread
Df
KV
D
KSine or
2
Which has the larger beam spread, a compression or a shear wave probe?
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Testing close to side walls
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Sound at an Interface
• Sound will be either transmitted across or reflected back
Reflected
Transmitted
Interface How much is reflected and transmitted depends upon the relative acoustic impedance of the 2 materials
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The Phenomenon of Sound
REFLECTIONREFRACTION
DIFFRACTION
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The Phenomenon of Sound
REFLECTIONREFRACTION
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
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REFRACTION• Only occurs when:The incident angle is other than 0°
Water
Steel
Steel
Steel
Water
Steel
30°
Refracted
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REFRACTION• Only occurs when:The incident angle is other than 0°
Steel
Steel
Water
Steel
30°
Refracted
The Two Materials has different VELOCITIES
No Refraction
30°
30°
65°
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Snell’s Law
I
R
Material 1
Material 2
2 Materialin
1 Material
Vel
inVel
RSine
ISine
Incident
Refracted
Normal
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Snell’s Law
C
Perspex
Steel
C
20
48.3
2 Materialin
1 Material
Vel
inVel
RSine
ISine
5960
2730
48.3
20
Sine
Sine
4580.04580.0
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Snell’s Law
C
Perspex
Steel
C
15
34.4
2 Materialin
1 Material
Vel
inVel
RSine
ISine
5960
2730
R
15
Sine
Sine
2730
596015SinSinR
565.0SinR
4.34R
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Snell’s LawC
Perspex
Steel
C
20
S
48.3
24
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Snell’s Law
Perspex
Steel
S
CC
CC
S
When an incident beam of sound approaches an interface of two different materials:REFRACTION occurs
There may be more than one waveform transmitted into the second material, example: Compression and Shear
When a waveform changes into another waveform: MODE CHANGE
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Snell’s Law
Perspex
Steel
C
CS
C
SC
S
If the angle of Incident is increased the angle of refraction also increases
Up to a point where the Compression Wave is at 90° from the Normal
90° This happens at the
FIRST CRITICAL ANGLE
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1st Critical Angle
C
27.4
S
33
C Compression wave refracted at 90 degrees
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2nd Critical Angle
C
S (Surface Wave)
90
C
Shear wave refracted at 90 degrees
57
Shear wave becomes a surface wave
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1st Critical Angle Calculation
C
Perspex
SteelC
5960
2730
90
I
Sine
Sine
5960
2730SinI
458.0SinI
26.27I
S
190 Sin
27.2
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2nd Critical Angle Calculation
C
Perspex
Steel
C
3240
2730
90
I
Sine
Sine
3240
2730SinI
8425.0SinI
4.57I
S190 Sin
57.4
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1st.
2nd.
33°
90°
Before the 1st. Critical Angle: There are both Compression and Shear wave in the second material
S C
At the FIRST CRITICAL ANGLE Compression wave refracted at 90°
Shear wave at 33 degrees in the material
Between the 1st. And 2nd. Critical Angle: Only SHEAR wave in the material. Compression is reflected out of the material.
C
At the 2nd. Critical Angle: Shear is refracted to 90° and become SURFACE wave
Beyond the 2nd. Critical Angle: All waves are reflected out of the material. NO wave in the material.
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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
One Defect Two Echoes
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Snell’s Law• Calculate the 1st critical angle for a
perspex/copper interface
• V Comp perspex : 2730m/sec
• V Comp copper : 4700m/sec
5.355808.04700
2730SinI