Materials
Fatigue Failure
Materials crack and break under repeated cyclic loading below their static strength
Fatigue failure is the progressive cracking of a material subjected to cyclic loading at stress levels well below its static strength. Cracks initiate at stress concentrations (notches, fillets, surface scratches), propagate stably under each load cycle, then catastrophically rupture when the remaining cross-section can no longer carry the peak load. Discovered after disastrous railway-axle failures in the 1840s. Characterized by S–N curves (stress vs. cycles to failure) and the Paris law for crack growth. Responsible for most service failures of metallic structures: aircraft, bridges, shafts, springs, pressure vessels.
- MechanismCrack initiation, propagation, fracture
- S-N curveStress amplitude vs cycles to failure
- Endurance limitStress below which steel runs forever
- Paris lawda/dN = C(ΔK)^m
- AluminumNo true endurance limit
- First explainedAugust Wöhler, 1860s railway axles
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why fatigue matters
- Aerospace. Pressurization cycles drive airframe inspection.
- Bridges. Truck loads cycle structural members.
- Wind turbines. Blades see millions of bending cycles.
- Engines. Crankshafts, valves, springs, blades.
- Medical implants. Hip stems, dental posts, stents flex billions of times.
- Pressure vessels. Pressurization cycles drive crack growth.
- Rail. Axles, rails, wheels — original fatigue motivator.
Common misconceptions
- Static strength is enough. Cyclic loads fail well below ultimate.
- Cracks visible warn early. Crack hides until just before final fracture.
- All metals have endurance limit. Aluminum and many alloys do not.
- Smooth parts are safe. Sub-microscopic defects suffice to nucleate.
- Compressive loads ignored. Mean stress matters; tensile mean far worse.
- Fatigue is fast. Most life is spent in slow stable propagation.
Frequently asked questions
How does fatigue work?
Three stages. (1) Crack initiation — repeated loading causes microscopic plastic strains at stress concentrations, eventually nucleating a crack. (2) Stable propagation — each cycle advances the crack tip by microscopic distance. (3) Final fracture — once the remaining cross-section can no longer carry peak load, a single overload tears the part apart. Initiation can consume 90% of fatigue life in low-stress applications.
What's an S-N curve?
Stress amplitude (S) plotted against number of cycles to failure (N), usually on log-log axes. Each test specimen runs at a constant stress until failure; many specimens build the curve. Lower stress → exponentially more cycles. Steels show a flat region (endurance limit) below which fatigue life is theoretically infinite. Aluminum shows continuously decreasing curve — no true endurance limit.
What's the endurance limit?
A stress threshold below which certain materials (most steels, some titaniums) survive infinite cycles. Typically 0.4–0.5 × ultimate tensile strength. Designers set steel structures below endurance limit for guaranteed lifetime. Surface finish, mean stress, environment, and size effects modify the practical endurance limit downward — Marin factors typically reduce predicted limits by 50%.
Why aluminum has no endurance limit?
Face-centered cubic metals lack the dislocation pinning mechanism that creates a true plateau in S-N curves. Aluminum, copper, and austenitic stainless steel all keep fatiguing at very low stresses given enough cycles (10⁹ or more). Designers specify a "fatigue strength at N cycles" instead — typically 10⁷ or 10⁸ cycles. Aircraft structures rely heavily on aluminum, so fatigue management drives airframe inspection schedules.
What's the Paris law?
An empirical relation for crack growth rate: da/dN = C(ΔK)^m, where da/dN is crack growth per cycle, ΔK is the stress intensity range, and C and m are material constants. Predicts how a known crack will grow under cyclic loading. Foundation of damage-tolerant design — assume initial flaws exist, predict their growth, schedule inspections before critical size is reached.
What causes fatigue cracks to initiate?
Stress concentrations. Sharp corners, fastener holes, surface scratches, weld defects, machining marks, corrosion pits — anywhere the local stress amplification factor (Kt) exceeds the average. Smooth fillet radii, polished surfaces, shot peening (compressive residual stresses), and careful manufacturing all delay initiation. Designers often state "no sharp internal corners" as a fatigue rule.
How is fatigue managed in practice?
Three philosophies. Safe-life: design so the part lasts longer than service life with a safety factor; retire on schedule (rotors, fans). Fail-safe: redundant load paths so any single crack is contained (multi-spar wings). Damage tolerance: accept cracks exist, monitor and inspect, replace when crack approaches critical size (most modern airframes). Choice depends on inspection access and consequence of failure.