NDT Ultrasonic Testing Principles 5 — Questions and Answers
Question 1: What is the significance of the second critical angle in ultrasonic testing?
- The angle at which maximum mode conversion from longitudinal to surface waves occurs
- The incident angle beyond which total internal reflection occurs and no energy enters the second medium (Correct answer)
- The angle producing maximum penetration depth in the test material
- The angle at which shear waves are completely absorbed at the interface
Correct answer: The incident angle beyond which total internal reflection occurs and no energy enters the second medium
The second critical angle is where the refracted shear wave reaches 90° and travels along the interface; beyond this angle, total internal reflection occurs and no bulk waves enter the material — only surface waves.
Question 2: What is 'dead zone' in pulse-echo ultrasonic testing?
- The region beyond the maximum test range of the instrument
- The zone immediately below the test surface where defects cannot be detected due to ring-down of the initial pulse (Correct answer)
- The area outside the beam spread where sensitivity drops to zero
- The depth range corresponding to the near field of the transducer
Correct answer: The zone immediately below the test surface where defects cannot be detected due to ring-down of the initial pulse
The dead zone is the region directly beneath the transducer face where the transmitted pulse has not yet fully decayed, masking any echoes from near-surface defects during that period.
Question 3: Mode conversion during ultrasonic testing refers to:
- Changing the frequency of the transducer during a scan
- The transformation of one wave type (e.g., longitudinal) into another (e.g., shear) at an interface or boundary (Correct answer)
- Switching from pulse-echo to through-transmission mode
- Converting analog A-scan signals to digital format for recording
Correct answer: The transformation of one wave type (e.g., longitudinal) into another (e.g., shear) at an interface or boundary
Mode conversion occurs when an ultrasonic wave strikes an interface at an angle, causing some energy to be transmitted or reflected as a different wave type (e.g., longitudinal converting to shear).
Question 4: Which display format provides the most complete volumetric representation of ultrasonic inspection data?
- A-scan (amplitude vs. time for a single point)
- B-scan (cross-sectional profile along a line)
- C-scan (plan view amplitude map of the inspected area) (Correct answer)
- S-scan (sector scan showing multiple angles simultaneously)
Correct answer: C-scan (plan view amplitude map of the inspected area)
The C-scan presents a top-view map showing amplitude or depth data across the entire inspected area, giving the most complete spatial picture of flaw distribution and coverage.
Question 5: In weld inspection using angle-beam UT, the 'skip distance' refers to:
- The distance between two successive transducer positions during raster scanning
- The surface distance from the probe to the point where the beam reflects off the back wall and returns to the surface (Correct answer)
- The minimum detectable flaw length along the weld axis
- The gap between the transducer wedge and the weld cap
Correct answer: The surface distance from the probe to the point where the beam reflects off the back wall and returns to the surface
The skip distance is the surface projection of the full beam path from probe exit point to the back-wall reflection point and back up to the scanning surface, equal to twice the half-skip distance.
Question 6: Electromagnetic acoustic transducers (EMATs) differ from piezoelectric transducers primarily because:
- They operate at much lower frequencies below 100 kHz only
- They generate ultrasonic waves directly in the part without requiring liquid couplant (Correct answer)
- They can only detect surface cracks and not sub-surface flaws
- They require the test material to be non-ferromagnetic to function
Correct answer: They generate ultrasonic waves directly in the part without requiring liquid couplant
EMATs generate ultrasonic waves through electromagnetic interaction with the part's surface, eliminating the need for liquid couplant — a major advantage for high-temperature or dry surface inspections.
Question 7: When calibrating an angle-beam transducer using the IIW block, what feature is used to set the beam exit point (index point)?
- The 1.5 mm diameter side-drilled hole at 10 mm depth
- The 50 mm radius curved surface of the block
- The 25 mm radius curved surface of the block (Correct answer)
- The notch at 0° on the top surface of the block
Correct answer: The 25 mm radius curved surface of the block
The 25 mm radius on the IIW block is used to find the beam exit point: the transducer is positioned to maximize the echo from the curved surface, and the index point aligns with the block's scribed center mark.
NDT Ultrasonic Testing Principles 5
What is the significance of the second critical angle in ultrasonic testing?