NDT Discontinuity Interpretation and Evaluation 5 — Questions and Answers
Question 1: A technician observes a PT indication that appears only after extended dwell time and is very faint. This behavior is most consistent with:
- A tight, shallow crack (Correct answer)
- A wide open void
- A subsurface discontinuity
- A non-relevant mechanical mark
Correct answer: A tight, shallow crack
Tight cracks hold very little penetrant and produce faint, slow bleed-out indications that require longer dwell times to develop.
Question 2: In radiographic testing, scattered radiation primarily causes:
- Increased image sharpness
- Higher film density and reduced contrast (Correct answer)
- Bright white spots on the film
- Decreased overall film density
Correct answer: Higher film density and reduced contrast
Scatter radiation foggs the film uniformly, raising density overall and reducing the contrast needed to detect discontinuities.
Question 3: A stress corrosion crack detected in service differs from a fabrication crack primarily in that it:
- Is always larger
- Typically originates at the surface and is associated with a corrosive environment and tensile stress (Correct answer)
- Only occurs in welds
- Appears as a bright UT signal in all orientations
Correct answer: Typically originates at the surface and is associated with a corrosive environment and tensile stress
Stress corrosion cracking requires three simultaneous factors — susceptible material, tensile stress, and a corrosive environment — and typically initiates at the surface.
Question 4: During TOFD (Time of Flight Diffraction) inspection, a discontinuity is detected when:
- The lateral wave amplitude drops significantly
- Diffracted signals appear between the lateral wave and back-wall signal (Correct answer)
- The back-wall signal disappears completely
- The probe separates beyond the skip distance
Correct answer: Diffracted signals appear between the lateral wave and back-wall signal
TOFD detects discontinuities by capturing diffracted ultrasonic energy from flaw tips, which appears as additional signals between the lateral wave and back-wall echo.
Question 5: A magnetic particle indication is found on a shaft in an area known to have a geometric stress concentrator (keyway). The inspector should first determine:
- Whether the indication is relevant or non-relevant (Correct answer)
- The exact depth of the discontinuity using UT
- The hardness of the material at that location
- Whether the keyway must be redesigned
Correct answer: Whether the indication is relevant or non-relevant
Geometric features like keyways can produce non-relevant magnetic particle indications due to flux leakage at the geometry change, so relevance must be established first.
Question 6: When comparing a slag inclusion to a crack of the same length in a weld radiograph, the crack is generally more rejectable because:
- Cracks appear darker on film
- Cracks are planar with sharp tips that concentrate stress far more severely than volumetric inclusions (Correct answer)
- Slag is always below the surface
- Cracks indicate improper preheat only
Correct answer: Cracks are planar with sharp tips that concentrate stress far more severely than volumetric inclusions
Planar discontinuities like cracks have sharp tips that create extreme stress concentrations, making them structurally more dangerous than volumetric slag inclusions.
Question 7: A calibration reflector used in UT gives a signal at 80% full-screen height (FSH). A discontinuity signal from the component appears at 60% FSH at the same depth. The indication is:
- Rejectable because it exceeds 50% FSH
- Smaller than the calibration reflector and must be evaluated against the applicable acceptance criterion (Correct answer)
- Non-relevant and can be dismissed
- Equivalent to the calibration reflector in size
Correct answer: Smaller than the calibration reflector and must be evaluated against the applicable acceptance criterion
The 60% FSH signal is smaller than the 80% FSH calibration reflector; whether it is acceptable depends on the code's acceptance criterion relative to that reference.
NDT Discontinuity Interpretation and Evaluation 5
A technician observes a PT indication that appears only after extended dwell time and is very faint.
This behavior is most consistent with: