Crack
A crack is a linear discontinuity in a material with negligible opening width, resulting from stress, fabrication, or in-service damage, and is the most safety-critical defect type in pressure equipment and structures.
Definition
A crack is a planar discontinuity that grows under tensile stress, fatigue cycling, or environmental attack. Because of its sharp tip, a crack acts as a stress concentrator and is the dominant flaw type in fatigue and fracture-mechanics analysis. ASME, API, and AWS codes universally reject cracks regardless of size in welded equipment, although FFS assessment may justify continued operation.
Detection
Surface cracks are detected by PT, MT, ET, and ACFM. Subsurface and embedded cracks are detected by UT, PAUT, TOFD, and RT.
Related terms
- Fatigue Crack — A fatigue crack is a crack that initiates and propagates under cyclic loading at stress levels below the static tensile strength of the material, producing characteristic beach marks visible on the fracture surface.
- Stress Corrosion Cracking (SCC) — Stress corrosion cracking is the brittle cracking of a normally ductile material under the combined action of tensile stress and a specific corrosive environment, propagating along grain boundaries (intergranular) or through grains (transgranular).
- Hydrogen-Induced Cracking (HIC) — Hydrogen-induced cracking is a stepwise internal cracking in carbon and low-alloy steel pipelines and vessels exposed to wet H2S service, in which atomic hydrogen accumulates at inclusions and recombines, generating high internal pressure that causes blister-l
- Lack of Fusion (LoF) — Lack of fusion is a weld defect in which the deposited weld metal fails to fuse with the base metal or with the previous weld pass, producing a planar discontinuity along the fusion boundary.
Further reading
surface breaking crack detection comprehensive methods · fatigue crack detection in cyclic loading
More defect terms
Lack of Penetration · Slag Inclusion · Porosity · Undercut · Overlap · Burn-Through · Weld Spatter · Lamination · Lamellar Tearing · Sulfide Stress Cracking
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.