AWS Certified Welding Inspector — FundamentalsNondestructive Examination (NDE)Medium
A CWI is reviewing an Ultrasonic Testing (UT) procedure for inspecting a butt weld in a 1-inch thick carbon steel plate. The procedure specifies the use of a 70-degree shear wave transducer. During calibration, the CWI observes that the amplitude of the reflected signal from a known reference reflector is significantly lower than expected. Which of the following is the most likely cause for this reduction in signal amplitude?
- AThe gain setting on the UT instrument is too high.
- BThe transducer frequency is too low.
- CThe surface of the test piece has excessive roughness.
- DThe couplant layer is too thin.
Show answer & explanationAnswer & explanation
Correct answer: C. The surface of the test piece has excessive roughness.
Excessive surface roughness on the test piece can cause significant scattering of the ultrasonic waves, preventing a clear and strong reflection from reaching the transducer, thus reducing the signal amplitude from the reference reflector. A thin couplant layer is generally better, low frequency increases penetration, and high gain would increase amplitude.
Why the other options are wrong
- A. A high gain setting would amplify the signal, making it appear stronger, not weaker.
- B. A lower frequency transducer generally provides better penetration and less scattering, not necessarily a reduction in amplitude from a reference reflector unless it's too low to detect the specific reflector size.
- D. A thin couplant layer is generally desirable for efficient sound transmission; a thick or uneven layer can cause issues, but 'too thin' is less likely to cause significant amplitude reduction than roughness.
UT Signal Attenuation Sources
Factors that reduce the intensity or amplitude of an ultrasonic signal as it travels through a material.
- Includes scattering, absorption, and beam divergence.
- Surface conditions, material properties, and transducer parameters affect attenuation.
- Leads to reduced signal-to-noise ratio and missed indications.
Memory trick: Rough surfaces scatter sound like a broken mirror scatters light.