Metal Fatigue: S-N Curves, Crack Initiation and Prevention is the study of progressive, localised cracking in a component subjected to fluctuating stress at levels well below its ultimate tensile strength. A shaft, weld or bracket that would carry a static load indefinitely can still fail after enough load reversals, because each cycle advances a microscopic crack a little further. Fatigue is the dominant failure mode for rotating and reciprocating machinery, and understanding it is the difference between a planned overhaul and an unplanned catastrophic fracture.
Fatigue damage accumulates under cyclic loading even when the peak stress stays comfortably inside the elastic range. The key point is that the stress that causes failure over millions of cycles can be a fraction of the yield strength. Because the damage is localised at the surface and at geometric features, a part can look perfectly sound right up until it breaks. Load sources include rotating bending, torsional reversals, pressure pulsations, thermal cycling and vibration.
A fatigue fracture almost always develops in three distinct stages:
Reading these features on a broken part tells you the origin, the direction of growth and whether the load was high or low relative to the section.
The classic fatigue characterisation is the S-N curve, first mapped by August Wohler. It plots stress amplitude (S) against the number of cycles to failure (N) on a logarithmic scale. Higher stress means fewer cycles to failure. For many ferrous alloys the curve flattens into a horizontal asymptote: below a certain stress, the endurance limit (or fatigue limit), the part survives an effectively infinite number of cycles. Many non-ferrous alloys, notably aluminium, show no such plateau; their curve keeps sloping downward, so engineers quote a fatigue strength at a defined cycle count instead.
| Material class | True endurance limit? | Approx. fatigue ratio (Se/UTS) | Reference life |
|---|---|---|---|
| Wrought steels | Yes | ~0.5 (capped near 700 MPa) | 10^6 to 10^7 cycles |
| Cast iron | Yes | ~0.4 | 10^6 to 10^7 cycles |
| Titanium alloys | Yes | ~0.45 to 0.6 | 10^7 cycles |
| Aluminium alloys | No | ~0.3 to 0.4 | quoted at 5x10^8 cycles |
| Copper alloys | No | ~0.3 | quoted at 10^8 cycles |
The ratios above are engineering approximations for polished laboratory specimens; real components need correction factors.
Three effects push a real part below its textbook fatigue strength:
Fatigue splits into two regimes. High-cycle fatigue (roughly above 10^3 to 10^4 cycles) is stress-controlled, the material stays largely elastic, and the S-N curve governs design. Low-cycle fatigue (below that range) involves gross plastic strain each cycle, as in thermal transients or occasional heavy overloads; here the strain-life (Coffin-Manson) approach is used instead of stress-life. Knowing which regime a component lives in decides whether you design against stress amplitude or plastic strain range.
Because initiation dominates fatigue life, prevention concentrates on the surface and on geometry:
Surface-breaking fatigue cracks are found with non-destructive testing. Liquid penetrant testing reveals cracks on any non-porous material, while magnetic particle testing is faster on ferromagnetic parts and picks up slightly sub-surface indications. On a rotating asset, vibration trending often flags a growing crack before it breaks the surface. Feeding NDT results, inspection intervals and crack-growth history into a CMMS keeps the evidence in one place; teams using Fabrico schedule recurring inspections and attach findings to the asset record so a rising trend triggers action rather than a surprise failure. Book a Fabrico demo to see that inspection workflow.
Because fatigue is cumulative. Each load cycle advances a microscopic crack a tiny amount, so given enough cycles a stress well under the static strength can grow a crack to critical size and cause sudden fracture.
No. Most steels, cast irons and titanium alloys show a true endurance limit below which life is effectively infinite. Aluminium and many other non-ferrous alloys do not, so a fatigue strength is quoted at a stated cycle count instead.
Both are marks on the fracture surface left by crack propagation. Beach marks are macroscopic bands visible to the eye that reflect changes in loading. Striations are microscopic ridges, roughly one per cycle, seen only under an electron microscope.
It plastically deforms the surface layer, leaving it in residual compression. Since fatigue cracks start under tension at the surface, that compressive layer must be overcome first, delaying initiation and raising fatigue strength.
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