Understanding Fatigue Life and Endurance Limits

## What Is Fatigue? Fatigue is progressive structural damage under cyclic loading, occurring in three stages: crack initiation at stress concentrators, crack propagation with characteristic striations, and final sudden fracture. ## The S-N Curve ### Steel and Titanium: True Endurance Limit Ferrous metals and titanium exhibit an endurance limit: a stress below which infinite life is possible. For unnotched steels, approximately 0.5 x UTS (below 1400 MPa). The limit appears at 10^6 to 10^7 cycles. ### Aluminum, Copper: No Endurance Limit FCC non-ferrous metals have no true endurance limit. The S-N curve continues downward. Fatigue strength is reported at specific cycle counts: - 6061-T6: 97 MPa at 5 x 10^8 cycles - 7075-T6: 159 MPa at 5 x 10^8 cycles ## Factors Reducing Fatigue Life ### Surface Finish A machined surface retains 70-80% of polished fatigue strength. Hot-rolled or forged surfaces may retain only 40-60%. ### Stress Concentrators Notches, keyways, and holes create local stress concentrations (Kt). A keyway with Kt = 3.0 triples local stress. ### Mean Stress Tensile mean stress degrades fatigue life (Goodman relation: Sa/Se + Sm/Suts = 1). Shot peening introduces compressive residual stress, extending life by 3-10x. ### Corrosion Fatigue Seawater eliminates the endurance limit even in steel, reducing fatigue strength by 50-70%. ## Design Against Fatigue | Strategy | Effect | |----------|--------| | Increase fillet radii | Reduce Kt | | Shot peen surfaces | 3-10x life improvement | | Polish surfaces | Remove initiation sites | | Avoid weld toes in high-stress zones | Severe stress concentrators | | Control inclusion content | Reduce initiation sites |