General Knowledge of NDT Methods and Their Applications
What the exam expects
You must be able to describe the physical principle behind each principal method, what it detects, and what it cannot detect. The classic set: visual (VT), penetrant (PT), magnetic particle (MT), eddy current (ET), ultrasonic (UT), radiographic (RT), leak testing (LT), acoustic emission (AE), and thermal/infrared (IRT). For each, know the energy form used, the material property exploited, and the discontinuity type it is suited to.
Core concepts to master
- Surface vs. volumetric: PT and MT are surface/near-surface only. UT and RT are volumetric. ET is surface and near-surface in conductive materials.
- Material restrictions: PT works on any nonporous solid; MT only on ferromagnetic materials; ET only on electrically conductive materials.
- Discontinuity orientation: UT is strongly orientation-dependent (reflector must be roughly perpendicular to the beam); RT is best for discontinuities with a through-thickness dimension (porosity, voids) and poor for tight planar flaws aligned with the beam.
- Access and geometry: RT requires access from both sides or a source/detector arrangement; UT requires single-side access.
Traps
Questions love the "which method for X" phrasing where one option is technically capable but impractical (e.g., RT for a tight lack of fusion in a thick weld). Watch for absolute words like "always" and "detects all." Also watch for confusion between detection and sizing capability.
Rule of thumb
If a flaw is surface-breaking and the material is ferromagnetic, MT generally beats PT for detecting tight, fine cracks because of the magnetic flux leakage mechanism. If the flaw is internal and planar, prefer UT over RT.
Worked example
A 25 mm carbon steel weld needs internal lack-of-fusion detection. RT may miss tight planar flaws oriented along the beam; UT with angle beam is the better choice. Answer: UT.