Fatigue
The progressive structural weakening and microcrack failure of a material subjected to repeated cyclic loading stresses below its ultimate tensile strength.
Governing formula
Basquin's Relation: S_a = σ'_f (2 N_f)^b
SI unit
Cycles to Failure N_f
In depth
Cyclic loading causes microscopic slip band intrusions that initiate microcracks. Cracks propagate incrementally per stress cycle until remaining cross-sectional area cannot sustain the load, leading to sudden catastrophic brittle fracture.
Examples in the real world
Airplane fuselage pressure cycling or crankshaft rotation causing fatigue microcrack propagation and sudden component failure.