This phenomenon is called work hardening. The generation and movement of discontinuities actually strengthens the metal through plastic deformation. The initial development of a fatigue crack occurs at localized discontinuities in the metal’s crystal structure. Stage 1: Crack initiation - Fatigue cracks almost always initiate at a free surface and near a stress riser (e.g. One of the intriguing factors about fatigue development is that fatigue cracks can be initiated and propagated at stresses well below the yield strength of the material of construction (these stresses are usually thought to be related to elastic deformation, not plastic deformation. While applied stresses may be tensile, compressive or torsional, crack initiation and propagation are due to the tensile component. Fatigue is a failure mechanism that involves the cracking of materials and structural components due to cyclic (or fluctuating) stress.
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