A component can fracture after many load cycles even when the peak stress in every cycle is below the static yield or ultimate strength. This failure mechanism is fatigue.

Crack initiation and growth

Repeated stress cycles can initiate small cracks at surfaces, defects or stress concentrations. Once a crack exists, cyclic loading can extend it a small amount each cycle until the remaining section can no longer carry the load.

S-N curves

An S-N curve relates cyclic stress amplitude $S$ to the number of cycles $N$ to failure for a specified test condition.

Higher stress amplitudes generally produce shorter fatigue lives. Some materials exhibit an endurance region in which failure life becomes very long below a characteristic stress level.

Stress range and mean stress

Fatigue depends not only on the maximum stress but on the cyclic range and mean value. A tensile mean stress can make a given alternating stress more damaging.

Statistical nature

Fatigue life has significant scatter because crack initiation depends on material, surface condition, environment, manufacturing defects and loading history.