fatigue, in engineering, manifestation of progressive fracture in a solid under cyclic loading as in the case of a metal strip that ruptures after repeated bending back and forth. Fatigue fracture begins with one or several cracks on the surface that spread inward in the course of repeated application of forces until complete rupture suddenly occurs when the small unaffected portion is too weak to sustain the load. Structural and machine parts subject to vibrations and other cyclic loading must be designed to avoid fatigue fracture.