Progressive structural damage and eventual fracture caused by repeated cyclic loading at stresses below the static ultimate strength.
Fatigue failure initiates at surface defects or stress concentrations, propagating a crack cycle-by-cycle. The S-N curve (Wöhler curve) plots applied stress amplitude vs. number of cycles to failure. Ferrous metals have an endurance limit (S_e) below which infinite life is expected; most polymers and non-ferrous metals do not: their S-N curve continues to decline. Typical fatigue limits for steel: S_e ≈ 0.5 × UTS.
Fatigue governs design of cyclically loaded 3D-printed parts: motor mounts, hinges, clips, and fan brackets. FDM layer interfaces and surface roughness act as pre-existing crack initiation sites, reducing fatigue life vs. injection-moulded equivalents by 30-70%. Smooth surfaces and rounded fillets dramatically improve fatigue life. Residual compressive stress (from post-processing) retards crack growth.
A part that survives static testing may still fail in fatigue after thousands of cycles. FDM surface roughness (Ra 5-30 µm) introduces stress concentrations that cut fatigue life severely. Fatigue of polymers is also influenced by viscoelastic heat generation: high-frequency cycling at thin sections can cause thermal failure without mechanical fracture.
Related terms: Tensile Strength, Stress, Residual Stress, Stress-Strain Curve
| Field | mechanics, engineering |
|---|---|
| Also called | Cyclic Fatigue, Fatigue Failure |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert