Crack — Definition & NDT Use
A linear or branching discontinuity in material caused by stress concentration, thermal cycling, hydrogen embrittlement, or fatigue loading. Cracks are one of the most critical defect types as they can propagate rapidly under loading. Crack detection is a primary focus of NDT inspections, particularly in welds and high-stress components. Cracks must be sized and characterized to determine their fitness-for-service and repair requirements.
In service, 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. 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 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.
- Etymology / Origin
- Old English cracian (to make a sharp sound); the metallurgical sense (planar discontinuity) is industrial-era usage.
- Formula
- Stress intensity K = σ × √(π × a) × Y; a = crack half-length, Y = geometry factor; fracture toughness K_IC governs the critical size.
- Units
- Crack length in mm; depth in mm; K in MPa·√m.
- Typical Range
- Detectable by MT/PT down to ~1 mm length; UT angle-beam reliable above ~3 mm height; PAUT/TOFD sizing accuracy ±1 mm in steel.
- Measured / Produced By
- MT (surface ferromagnetic), PT (surface non-magnetic), UT angle-beam (subsurface), PAUT/TOFD (sizing).
- Code References
- ASME Section XI IWB-3500 (crack sizing); API 579 Part 9 (crack-like flaw assessment); BS 7910 (engineering critical assessment)
- Worked Example
- A 4 mm × 25 mm crack in a 25 mm wall with K_IC = 100 MPa·√m and applied stress 200 MPa: K = 200 × √(π × 0.002) × 1.12 ≈ 17.8 MPa·√m — well below toughness, but trending requires re-inspection.
AWS D1.1
Structural Welding Code — Steel; defines visual and NDE acceptance for static and dynamically loaded welds.
ASME Section IX
Welding, brazing, and fusing qualifications referenced by every U.S. pressure-equipment code.
Confusing 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).
What does "Crack" mean in NDT?
A linear or branching discontinuity in material caused by stress concentration, thermal cycling, hydrogen embrittlement, or fatigue loading. Cracks are one of the most critical defect types as they can propagate rapidly under loading
Is crack always rejectable?
No. Whether a crack indication is rejectable depends on the acceptance criteria of the governing code (AWS D1.1, ASME Section VIII, API 1104, etc.), the size and orientation of the indication, and any fitness-for-service evaluation the engineer chooses to apply.
What other NDT concepts should I read alongside Crack?
The most directly related entries in this glossary are "discontinuity", "fatigue crack", "stress concentration"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.