NDT Discontinuity Interpretation and Evaluation 4 — Questions and Answers
Question 1: A technician finds a UT signal that increases in amplitude as the probe is moved transversely across the weld but does not change when moved along the weld axis. This behavior is most consistent with:
- A transverse crack
- A planar discontinuity perpendicular to the beam (Correct answer)
- A volumetric inclusion
- Back-wall echo
Correct answer: A planar discontinuity perpendicular to the beam
A discontinuity that responds strongly to transverse movement but not axial movement is likely planar and oriented perpendicular to the probe's movement direction.
Question 2: The term 'relevant indication' in NDT means:
- Any indication seen during inspection
- An indication caused by a genuine discontinuity that must be evaluated against acceptance criteria (Correct answer)
- An indication that automatically requires rejection
- An indication larger than 1/16 inch in any direction
Correct answer: An indication caused by a genuine discontinuity that must be evaluated against acceptance criteria
A relevant indication is one caused by an actual discontinuity and must be compared to code acceptance criteria, unlike non-relevant or false indications.
Question 3: A linear porosity chain in a weld is more concerning than isolated pores of equal total area because:
- Linear chains are always larger in volume
- Aligned pores reduce effective cross-section and can act as a stress concentrator along a line (Correct answer)
- Chain porosity is always subsurface
- Isolated pores indicate better shielding gas coverage
Correct answer: Aligned pores reduce effective cross-section and can act as a stress concentrator along a line
Porosity chains reduce the load-carrying cross-section along a line and the interaction between pores elevates stress concentrations similar to a linear flaw.
Question 4: During liquid penetrant inspection, a rounded indication with a diameter approximately equal to its length is classified as:
- Linear
- Rounded (Correct answer)
- Relevant
- Non-relevant
Correct answer: Rounded
ASME and AWS codes classify an indication as rounded when its length is equal to or less than three times its width.
Question 5: When sizing a discontinuity by the 6 dB drop method in UT, the probe is moved until the signal amplitude drops to:
- 25% of peak
- 50% of peak (Correct answer)
- 70% of peak
- Zero
Correct answer: 50% of peak
The 6 dB drop method moves the probe until the signal falls to half (50%) of its peak amplitude, marking the edge of the reflector.
Question 6: A weld radiograph shows a dark line at the weld toe parallel to the weld edge but outside the weld metal. This is most characteristic of:
- Undercut (Correct answer)
- Overlap
- Incomplete penetration
- Crater crack
Correct answer: Undercut
Undercut appears as a dark line at the weld toe in the base metal, caused by melting away the base metal that is not refilled by weld metal.
Question 7: A discontinuity is found in a pressure vessel weld with high fracture toughness material. Compared to a brittle material with the same flaw size, the risk of fracture is:
- Higher because the flaw can propagate more easily
- Lower because more energy is required to propagate the crack (Correct answer)
- The same regardless of toughness
- Higher because tough materials hide cracks better
Correct answer: Lower because more energy is required to propagate the crack
Higher fracture toughness means more energy is required to propagate a crack, reducing fracture risk for a given flaw size.
NDT Discontinuity Interpretation and Evaluation 4
A technician finds a UT signal that increases in amplitude as the probe is moved transversely across the weld but does not change when moved along the weld axis.
This behavior is most consistent with: