Explain the fatigue failure mechanism and factors affecting fatigue life.

Fatigue failure occurs under cyclic loading below ultimate strength through crack initiation, propagation, and sudden fracture. Factors affecting fatigue life include: stress amplitude, mean stress, surface finish, stress concentration, size effect, temperature, and environment. S-N curves characterize fatigue behavior. Design uses endurance limit (for steel) or fatigue strength at specified cycles with appropriate safety factors.