fatigue

fatigue, in engineering, manifestation of progressive fracture in a solid under cyclic loading, i.e., the repeated application of a load (stress) on a material or structure. For instance, fatigue can be observed in 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.

The study of fatigue is of considerable significance in engineering applications, particularly in cases of structures that are affected by cyclic loads, such as bridges or aircraft components. Therefore, structural and machine parts subject to vibrations and other cyclic loading must be effectively designed to avoid fatigue fracture.

Mechanism

The fatigue failure process occurs in three phases —the initiation of cracks, growth of cracks, and eventually, fracture. When a material is subjected to stress cycles (repeated loading and unloading), each cycle causes microscopic damage, which, with time, accumulates to form a minuscule crack. The gradual propagation of the crack occurs with the continued stress cycles, causing degradation of the structure. During the initial phases, the damage caused by fatigue is minimal and cannot be observed with the naked eye. However, as the crack grows and attains a considerable size, the unaffected section of the material fails to support the load, and rapid fracture abruptly occurs.

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materials testing: Fatigue