Non-Destructive Examination Methods
Know each NDE method's physics, what it detects, and its limits. Visual (VT) - surface only. Penetrant (PT) - surface-breaking discontinuities in nonporous materials; requires clean surface and dwell/development times. Magnetic particle (MT) - surface and near-surface flaws in ferromagnetic materials only; needs magnetization direction roughly perpendicular to the flaw. Ultrasonic (UT) - internal flaws; needs couplant, and sound travels poorly across certain interfaces; angle-beam finds planar flaws, straight-beam measures thickness. Radiography (RT) - internal flaws via density differences; best for volumetric flaws, poor for tight planar cracks unless oriented favorably; radiation safety required. Eddy current (ET) - conductive materials, surface/near-surface, good for tubing. Acoustic emission (AE) - detects active growth under load, not static flaws.
Traps abound: (1) Using PT on porous or painted surfaces (false indications/entrapment). (2) Using MT on austenitic stainless or aluminum (non-ferromagnetic). (3) Expecting RT to reliably catch laminar cracks parallel to the beam. (4) Forgetting that UT angle-beam sensitivity depends on flaw orientation. (5) Confusing sensitivity (smallest detectable) with resolution (separating close indications). (6) Ignoring surface finish effects on PT/MT/UT.
Rule of thumb: match the method to the flaw type and orientation - surface-breaking → PT/MT; volumetric internal → RT/UT; planar internal perpendicular to the beam → UT angle-beam. Worked example: a suspected surface crack in a carbon steel weld. MT or PT both work; choose MT for speed on ferromagnetic steel, but confirm magnetization direction is perpendicular to the expected crack to avoid a missed indication.