NDT Nondestructive Testing Methods & Techniques 3 — Questions and Answers
Question 1: What NDT method uses high-frequency sound waves transmitted through a couplant to detect internal flaws?
- Eddy current testing
- Acoustic emission testing
- Ultrasonic testing (Correct answer)
- Infrared thermography
Correct answer: Ultrasonic testing
Ultrasonic testing (UT) transmits high-frequency sound waves (typically 1–10 MHz) through a liquid couplant into the material to detect reflections from internal discontinuities.
Question 2: During a hydrostatic pressure test, what is the primary hazard compared to a pneumatic pressure test?
- Hydrostatic tests use a compressible fluid, storing more energy
- Pneumatic tests are more dangerous because compressed gas stores significantly more energy than liquid (Correct answer)
- Hydrostatic tests are more dangerous because water is heavier
- Both tests present identical hazards
Correct answer: Pneumatic tests are more dangerous because compressed gas stores significantly more energy than liquid
Pneumatic tests using compressed gas store far more elastic energy than hydrostatic tests using liquid; a catastrophic failure in a pneumatic test releases energy explosively, making it significantly more hazardous.
Question 3: In eddy current testing, what effect does a crack or void in a conductive material have on the eddy current flow?
- It increases the eddy current amplitude uniformly
- It forces eddy currents to flow around the discontinuity, changing the impedance signal (Correct answer)
- It eliminates all eddy currents in the test material
- It has no measurable effect on eddy current flow
Correct answer: It forces eddy currents to flow around the discontinuity, changing the impedance signal
Discontinuities interrupt the normal circular eddy current paths, forcing currents to reroute, which alters the coil impedance and produces a detectable signal change.
Question 4: Which document typically establishes the specific acceptance criteria that a boilermaker NDT inspector must apply when evaluating weld indications?
- The NDT equipment manufacturer's operating manual
- The applicable construction or inspection code (e.g., ASME Section VIII) (Correct answer)
- The local fire marshal's inspection report
- The OSHA 29 CFR 1910 general industry standard
Correct answer: The applicable construction or inspection code (e.g., ASME Section VIII)
Acceptance criteria for weld indications in pressure vessels are defined by the governing construction or inspection code, such as ASME Boiler and Pressure Vessel Code Section VIII.
Question 5: What is 'signal-to-noise ratio' in the context of ultrasonic testing?
- The ratio of the probe frequency to the material thickness
- The ratio of the flaw signal amplitude to the background noise level (Correct answer)
- The ratio of compression wave velocity to shear wave velocity
- The ratio of the instrument gain to the couplant viscosity
Correct answer: The ratio of the flaw signal amplitude to the background noise level
Signal-to-noise ratio (SNR) compares the amplitude of the relevant flaw indication to the background noise; a high SNR makes it easier to distinguish real defects from material noise.
Question 6: When performing liquid penetrant testing on a rough, as-welded surface, what precaution is most important?
- Use a shorter penetrant dwell time to avoid over-penetration
- Ensure thorough pre-cleaning to remove weld spatter and contaminants from the surface (Correct answer)
- Apply developer immediately without a dwell period
- Use only solvent-removable penetrant on rough surfaces
Correct answer: Ensure thorough pre-cleaning to remove weld spatter and contaminants from the surface
Rough as-welded surfaces trap contaminants that can mask discontinuities or cause false indications; thorough pre-cleaning is essential for reliable PT results.
Question 7: In magnetic particle testing, the 'circular magnetization' technique is best suited to detect:
- Longitudinal (lengthwise) discontinuities parallel to the current flow (Correct answer)
- Transverse (crosswise) discontinuities perpendicular to the length of the part
- Subsurface laminations deeper than 0.5 inches
- Corrosion pitting on internal surfaces
Correct answer: Longitudinal (lengthwise) discontinuities parallel to the current flow
Circular magnetization produces a magnetic field perpendicular to the current flow direction, making it most sensitive to longitudinal discontinuities that run parallel to the current path.
NDT Nondestructive Testing Methods & Techniques 3
What NDT method uses high-frequency sound waves transmitted through a couplant to detect internal flaws?