Difficult to penetrate materials
square wave pulser
GE Inspection Technologies
Jim Costain
August 2009
Why do we have difficult-to-penetrate
materials ?
Ultrasound is subject to attenuation due to different effects
Attenuation
Absorption Diffusion
Divergence
Internal friction
X
Heat
X
Frequency
X
X
X
Grain size
X
Test Frequency is linked to attenuation
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The energy chain
The more energy we send into the material, the
better chance we have to go through it
• Where can we influence the energy sent to the
material ?
Probe
selection
Pulser type and
pulser setting
Material to probe
interface
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Material - Probe interface
We can improve energy conduction from the probe
to the material
• Optimization of probe surface
• Optimization of coupling quality
• Optimization of probe selection (acoustic
impedance of the contact face )
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Piezocomposite probes give you higher
electroacoustic efficiency
Piezocomposites materials provide
> A high pulse amplitude for good penetration through
the test object with a good signal to noise ratio
> A shorter acoustic pulse duration for a good axial
resolution
> A lower acoustic impedance (ideal for coupling
wedges and delay lines)
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• The lower the frequency,
the thicker the
piezoelectric
Volume
Piezoelectric volume in function of
frequency
• Low frequency probes
typically have larger
diameters
MHz
10
7.5
5
3.5
4
2
2.25
0.5
1
Low frequency probes
need more energy to
enter in oscillation
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Square Wave Pulsers - Spike Pulsers
The pulser’s setting is related
to the application
80
• Square wave pulsers
produce better results
when the probe frequency
is lower or equal at 2 MHz
75
70
65
60
55
50
0,5 MHz
Square wave pulser
have to be preferred
below 2MHz
1 MHz
2 MHz
4 MHz
10 MHz
Spike pulser (*)
Square Wave pulser (*)
(*) Comparisons realized with USD 15SX & USD 15SQ
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Spike excitation and the probe response
Low frequency probes
• They usually have big
diameters
• Thick crystals
→They need more energy
If the pulse length is too short,
the element doesn’t oscillate at
the maximum amplitude
Spike Pulser with a 2 MHz 1’ probe
→The excitation is too weak
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Square wave excitation and the probe
response
Up to
450
Volts
Up to 1000ns pulse width
Correct pulse length, The element is exactly excited
at resonance.
→This is the optimum setting
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Square wave excitation and the probe
response.
If the Pulse length is too
long, Element
oscillation is distorted
and extends in time
→Distortion.
→Double pulse.
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The instrument and its pulser
How to avoid wrong pulser adjustment ?
• For day to day applications, use the “Spike” mode of your flaw
detector
• Use Energy and damping to optimize penetration and
resolution
• For more difficult applications (highly attenuative materials),
optimize
• Voltage
• Pulse width
of the “square wave” mode of your flaw detector
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Optimum pulse width determination
• The pulse width
can be
calculated with
the following
formula
• It can also be
optimized
manually
Maximize amplitude of echo using the
pulse width parameter
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Conclusion
• The Square wave pulser is very useful for
thick and highly attenuative material using low
frequency transducers
• Can improve penetration and signal to noise
ratio on thick or attenuative materials
• Optimized usage at or below 2Mhz but can be
used with care at 4Mhz
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Thank you
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