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.
Definition
Fatigue cracks initiate at stress concentrators — weld toes, notches, corrosion pits — and propagate stably until a critical size is reached and fast fracture occurs. Detection is by surface MT/PT/ET and by UT/PAUT for embedded cracks. API 579 Part 9 provides fatigue assessment procedures.
Related terms
- 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.
- Creep Damage — Creep damage is the time-dependent permanent deformation and eventual cracking of materials subjected to constant stress at high temperature (typically above 0.4 of the melting point in Kelvin), occurring in fired heater tubes, reformers, and high-temperature
- 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).
Further reading
fatigue crack detection in cyclic loading
More defect terms
Lack of Fusion · Lack of Penetration · Slag Inclusion · Porosity · Undercut · Overlap · Burn-Through · Weld Spatter · Lamination · Lamellar Tearing
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.