Fatigue Crack — Definition & NDT Use

In service, Fatigue Crack starts as a discontinuity that may or may not breach the acceptance criteria of the governing code; the NDT method's job is to detect it, characterise it, and size it so an engineer can decide whether to repair, monitor, or accept. Capillary action draws the penetrant into surface-breaking openings during the dwell; emulsifier or solvent removes the surface excess; the developer then provides a contrasting blotter that pulls the trapped penetrant back out, broadening the indication so it becomes visible to the inspector. As the alternating coil approaches the conductive surface it drives circulating eddy currents; any change in the part — a crack, a thickness change, a permeability shift — perturbs those currents and registers as a phase-and-amplitude shift on the impedance plane. Crack sizing is the high-stakes call: amplitude alone is not enough, so techniques such as TOFD, tip-diffraction, or 6dB drop are stacked to bound the height and length used in the engineering critical assessment. The fitness-for-service decision typically pairs the NDT call with material data and stress information; the inspector's job is to give the engineer a clean characterisation rather than to make the keep-or-reject call alone.

The decision tree around Fatigue Crack runs: detect, characterise, size, and refer to the acceptance table in the governing code; only the last step decides repair, accept-as-is, or fitness-for-service review. On welded fabrication it is most often paired with VT and one volumetric method (RT or UT) so surface and internal defects are both addressed. Whenever a crack is suspected the inspection plan upgrades from screening to characterisation — TOFD, MT, or tip-diffraction sizing — because the engineering critical assessment needs height and length, not just a yes/no.

Confusing fatigue crack with a generic "indication" is a recurring error; the term carries an engineering implication, and the report should distinguish the discontinuity (what was seen) from the disposition (what code says about it).