The weakening of a material caused by repeated or alternating loads is called fatigue. A component can fail from fatigue even when each individual load is below the material’s single-use breaking strength.
Repeated loading creates tiny cracks, often at stress concentrations such as sharp corners, holes, threads, welds, or surface scratches. With each cycle, the crack can extend a little farther. Eventually, the remaining intact section becomes too small to carry the load, and sudden fracture follows. Fatigue is especially important in aircraft, bridges, vehicles, rotating machinery, and medical implants.
Engineers commonly study fatigue with an S–N curve, which compares applied stress with the number of cycles to failure. Some materials, including many steels, can show an endurance limit: below a certain stress range, they may survive very large numbers of cycles. Other materials, such as aluminum alloys, generally continue accumulating fatigue damage even at relatively low stresses.
Fatigue is not the same as ordinary wear, corrosion, or creep. Corrosion can accelerate fatigue by damaging the surface, while creep is time-dependent deformation under sustained load, especially at high temperature. Good design reduces stress concentrations and inspects parts before cracks become dangerous.