Transcription of Fatigue Testing - ASM International
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Fatigue Testing - ASM International
Fatigue Testing Introduction Fatigue is the progressive, localized, perma-nent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fracture after a sufficient number of fluctuations. Fatigue fractures are caused by the simultaneous action of cyclic stress , tensile stress and plastic strain. If any one of these three is not present, Fatigue cracking will not initiate and propagate. The cyclic stress starts the crack; the tensile stress produces crack growth (propagation). Although compressive stress will not cause Fatigue , compression load may do so. The process of Fatigue consists of three stages: Initial Fatigue damage leading to crack nu-cleation and crack initiation Progressive cyclic growth of a crack (crack propagation) until the remaining uncracked cross section of a part becomes too weak to sustain the loads imposed Final, sudden fracture of the remaining cross section Fatigue cracking normally results from cyclic stresses that are well below the static yield strength of the material.
Prediction of Fatigue life The fatigue life of any specimen or structure is the number of stress (strain) cycles required to cause failure. This number is a function of many variables, including stress level, stress state, cy clic wave form, fatigue environment, and the metallurgical condition of the material. Small
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