NDT Ultrasonic Testing Principles 4 — Questions and Answers
Question 1: What is the primary advantage of phased array ultrasonic testing (PAUT) over conventional single-element UT?
- It uses higher frequencies unavailable to conventional transducers
- It electronically steers and focuses the beam without moving the probe (Correct answer)
- It eliminates the need for calibration reference standards
- It can detect flaws smaller than one wavelength in size
Correct answer: It electronically steers and focuses the beam without moving the probe
PAUT uses multiple elements with time-delayed firing sequences to electronically steer, focus, and sweep the beam, allowing volumetric inspection from a single probe position.
Question 2: In angle-beam shear wave testing of welds, the 45°, 60°, and 70° refracted angles refer to the angle of:
- The transducer wedge relative to the test surface
- The refracted shear wave in the test material relative to the normal (Correct answer)
- The incident longitudinal wave in the wedge material
- The beam spread half-angle in the far field
Correct answer: The refracted shear wave in the test material relative to the normal
In angle-beam testing, the quoted angle is the refracted shear wave angle measured from the normal (perpendicular) to the test piece surface, not the wedge or incident angle.
Question 3: What phenomenon occurs when the grain size of a metal approaches the wavelength of the ultrasonic wave used?
- The wave velocity increases proportionally
- Significant scattering occurs, increasing background noise and limiting penetration (Correct answer)
- The wave converts from longitudinal to shear mode automatically
- Resolution improves because the wave interacts with individual grains
Correct answer: Significant scattering occurs, increasing background noise and limiting penetration
When grain size approaches the wavelength, scattering from grain boundaries generates high background 'grass' noise and attenuates the beam, reducing the signal-to-noise ratio.
Question 4: The V1 (IIW) calibration block is primarily used to:
- Establish the DAC curve for a specific material and flaw geometry
- Calibrate the time base (range) and sensitivity of angle-beam and straight-beam probes (Correct answer)
- Determine the acoustic impedance of an unknown material
- Verify the couplant viscosity is appropriate for the test temperature
Correct answer: Calibrate the time base (range) and sensitivity of angle-beam and straight-beam probes
The IIW block provides reference features (25mm radius, 50mm radius, 1.5mm side-drilled hole) used to set instrument sweep range and sensitivity for both contact and angle-beam techniques.
Question 5: Time-of-flight diffraction (TOFD) technique measures:
- The amplitude of reflected signals from reference notches
- The travel time of diffracted waves from flaw tips to calculate flaw height (Correct answer)
- The surface wave velocity to determine material hardness
- The frequency shift of transmitted waves through a flaw
Correct answer: The travel time of diffracted waves from flaw tips to calculate flaw height
TOFD uses diffraction signals from flaw tips; by measuring the time difference between tip diffraction arrivals and comparing to the lateral wave, accurate flaw height sizing is achieved.
Question 6: Which factor does NOT directly affect ultrasonic wave attenuation in a material?
- Grain size of the material
- Test frequency
- Material density and elastic modulus
- The color or surface finish of the part (Correct answer)
Correct answer: The color or surface finish of the part
Attenuation is caused by absorption and scattering, which depend on material microstructure, grain size, and frequency; surface color or paint finish does not affect internal wave attenuation.
Question 7: In the through-transmission ultrasonic technique, what indicates the presence of a defect?
- An increase in the amplitude of the received signal
- A decrease or total loss of the transmitted signal amplitude (Correct answer)
- The appearance of additional echoes between the front and back wall signals
- A shift in the frequency of the transmitted wave
Correct answer: A decrease or total loss of the transmitted signal amplitude
In through-transmission, one transducer sends and another receives; a flaw blocks or scatters the beam, causing a reduction in received signal amplitude compared to the defect-free baseline.
NDT Ultrasonic Testing Principles 4
What is the primary advantage of phased array ultrasonic testing (PAUT) over conventional single-element UT?