NDT Nondestructive Testing Methods & Techniques 5 — Questions and Answers
Question 1: What is 'TOFD' (Time-of-Flight Diffraction) primarily used for in boiler weld inspection?
- Measuring coating thickness on boiler tubes
- Detecting and sizing weld discontinuities using diffracted sound signals from defect tips (Correct answer)
- Performing surface roughness measurement before welding
- Verifying chemical composition of weld filler metal
Correct answer: Detecting and sizing weld discontinuities using diffracted sound signals from defect tips
TOFD uses the diffracted signals from the tips of planar weld defects to accurately detect and size through-wall height of discontinuities without requiring amplitude-based evaluation.
Question 2: A boilermaker discovers pitting corrosion on the inside of a boiler shell during a visual inspection. Which supplemental NDT method would best quantify remaining wall thickness at the pitted area?
- Liquid penetrant testing
- Magnetic particle testing
- Ultrasonic thickness measurement (Correct answer)
- Radiographic film technique
Correct answer: Ultrasonic thickness measurement
Ultrasonic thickness measurement uses pulse-echo or resonance techniques to precisely measure remaining wall thickness from one side, ideal for quantifying corrosion depth without draining the vessel.
Question 3: In acoustic emission testing (AET) of a pressure vessel during hydrostatic testing, what does a cluster of AE events from a localized area indicate?
- Normal background noise from fluid turbulence
- Active crack growth or significant structural activity at that location (Correct answer)
- Successful completion of the pressure hold period
- Proper calibration of the AE sensor array
Correct answer: Active crack growth or significant structural activity at that location
A cluster of acoustic emission events localized to one area during pressurization indicates active energy release — typically associated with crack growth, deformation, or structural discontinuity activity.
Question 4: What is the minimum required overlap of radiographic film in a panoramic exposure of a cylindrical pressure vessel?
- No overlap is required for panoramic exposures
- Overlap is determined by the applicable code, typically enough to ensure full coverage is documented (Correct answer)
- Exactly 2 inches of film overlap regardless of vessel diameter
- Overlap must equal 10% of the vessel circumference
Correct answer: Overlap is determined by the applicable code, typically enough to ensure full coverage is documented
The applicable construction code (e.g., ASME Section V) specifies minimum film overlap requirements to ensure that no weld area is missed between adjacent film placements.
Question 5: When using the dry powder magnetic particle method outdoors in windy conditions, what is the primary concern?
- The powder will become magnetized before application
- Wind disperses the powder, reducing particle concentration at the inspection area and causing false negatives (Correct answer)
- The powder will chemically react with moisture in the air
- Wind increases the sensitivity of the test by vibrating the particles
Correct answer: Wind disperses the powder, reducing particle concentration at the inspection area and causing false negatives
Wind scatters dry magnetic particles before they can be attracted to leakage fields over discontinuities, reducing particle density at the inspection area and potentially missing relevant indications.
Question 6: In ultrasonic testing, 'skip distance' refers to:
- The distance the probe must be moved between successive A-scan readings
- The surface distance traveled by an angle beam from entry point to the first point where it returns to the same surface (Correct answer)
- The depth at which the sound beam loses 6 dB of amplitude
- The minimum detectable flaw size for a given probe frequency
Correct answer: The surface distance traveled by an angle beam from entry point to the first point where it returns to the same surface
Skip distance is the surface distance from the probe entry point to where the angle beam, after reflecting off the back wall, returns to the scanning surface — essential for positioning the probe correctly to inspect a weld.
Question 7: Which NDT method is most appropriate for detecting delamination in a clad boiler vessel plate without removing the cladding?
- Liquid penetrant testing of the clad surface
- Straight-beam (normal incidence) ultrasonic testing (Correct answer)
- Magnetic particle testing with longitudinal magnetization
- Visual testing with a borescope
Correct answer: Straight-beam (normal incidence) ultrasonic testing
Straight-beam UT sends sound perpendicular to the plate surface; a delamination between the cladding and base metal reflects the beam early, revealing the interface separation as an intermediate echo.
NDT Nondestructive Testing Methods & Techniques 5
What is 'TOFD' (Time-of-Flight Diffraction) primarily used for in boiler weld inspection?