Which takes up more of a fatigue life, starting the crack or growing it?
In most components, starting the crack takes up more of a fatigue life than growing it: the initiation phase consumes the bulk of the service life, and the growth phase is comparatively brief. A fatigue failure is two processes sharing one name, and fatigue life is conventionally divided into the cycles spent nucleating a crack and the cycles spent propagating it to final separation.
The split between fatigue crack initiation and fatigue crack propagation is not fixed. It depends on stress level, material and geometry, but for a component operating at modest nominal stress, the fatigue crack spends most of its existence very small. A part can be within a small fraction of its remaining life while carrying a crack that no routine inspection method would find.
This is why a fatigue fracture surface, dramatic as it looks, is often less informative than the few square millimeters at one end of it. The propagation record explains how the fatigue crack advanced; the origin explains why there was a crack at that location at all.
How do you find the origin of a fatigue crack?
The origin of a fatigue crack is found by convergence: radial marks, ratchet marks and the curvature of successive crack-front positions on the fracture surface all point back toward the initiation site. Low-magnification examination under raking light usually narrows the fatigue origin to a small area before any sectioning is contemplated.
Multiple ratchet marks along an edge of a fatigue fracture indicate several origins that started independently and later joined, which is itself a finding: it generally implies a high local stress or a broadly damaged surface rather than a single isolated defect.
What features are usually found at the origin of a fatigue crack?
The feature at the origin of a fatigue crack usually falls into a limited set of categories: machining and grinding marks transverse to the stress direction, forming laps and seams, non-metallic inclusions, porosity, corrosion pits, fretting scars, plating cracks, weld toes, and mechanical damage from handling, stamping or assembly. Each of these origin features implicates a different stage of the part’s life, which is why identifying the feature matters more to a dispute than confirming that fatigue occurred.
Distinguishing one fatigue origin feature from another requires magnification. At the scale that matters, a tool witness mark and a corrosion pit are not confusable, and an inclusion has a composition that energy-dispersive analysis will report directly. What is confusable is a fatigue origin that has been rubbed, oxidized or wiped before anyone looked at it.
Why does surface condition matter so much in fatigue?
Surface condition carries disproportionate weight in fatigue because the maximum stress in bending and torsion occurs at the free surface, and because a surface grain has fewer constraints on slip than an interior one. Fatigue is a surface phenomenon.
Roughness, grinding burn, decarburization and tensile residual stress from machining all shorten fatigue initiation life; shot peening, rolling and nitriding lengthen it by putting the surface into compression. In a fatigue failure, a surface treatment that was specified but not performed, or performed to the wrong parameters, is a recognizable and testable proposition rather than a matter of opinion.
Can a fatigue crack start below the surface?
Yes, not every fatigue origin is at the surface: in high-strength steels cycled at low amplitude, and in parts carrying an effective compressive surface layer, fatigue initiation can occur at a subsurface inclusion, producing a characteristic facet around the initiating particle. A subsurface fatigue origin shifts the question from surface processing toward melting and cleanliness practice.
Inclusion content can be rated on polished sections and compared against the material specification and against unused stock from the same lot, which is often the only way to establish whether the inclusion population was atypical.
Do stress concentrations or surface defects cause fatigue cracks on their own?
No. Stress concentrations do not create fatigue on their own, and neither does a surface defect on its own: fatigue initiation happens where the two coincide, such as a machining mark inside a fillet, a pit at a section change, or a fretting scar under a clamped joint. Geometry decides where, stress decides whether.
Where a fatigue origin sits at a geometric feature with no defect present, the geometry is the finding; where a fatigue origin sits at a defect on an otherwise unremarkable surface, the defect is the finding.
Which fatigue origin features were built into a part, and which were acquired in service?
Among fatigue origin features, inclusions, porosity, machining marks, decarburization and plating defects were present when the part entered service, while corrosion pits, fretting scars, impact damage and thermal cracking arrived later. The origin categories separate along that useful line between built in and acquired in service.
The distinction between built-in and service-acquired fatigue origin features frequently maps onto a dispute more directly than the failure mechanism does, and it is testable against exemplars: an unfailed sister part from the same batch will carry the built-in features and, if it saw comparable service, some of the acquired ones as well.
What destroys the evidence at a fatigue crack origin?
The initiation site of a fatigue crack is small, shallow and easily damaged: bringing the mating fracture faces back into contact grinds the origin, wire-brushing removes it, light rusting obscures it within days in humid storage, and solvents or preservatives leave deposits that interfere with elemental analysis. Sectioning through the fatigue origin before it has been documented intact removes the option of examining it whole.
To protect the origin of a fatigue fracture, separate the pieces, keep them dry, keep anything off the fracture faces, and photograph before anything else happens.
What can’t the origin of a fatigue crack tell you?
The origin of a fatigue crack does not establish the stress that was acting, the number of cycles applied, or whether the initiating feature was within specification; those come from the loading history, the drawings and the material records. Identifying the initiating feature at a fatigue origin establishes where the crack started and what gave it somewhere to start.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.