Experiments have shown that low cycle fatigue is also crack growth. Historically, fatigue has been separated into regions of high cycle fatigue that require more than 10 4 cycles to failure where stress is low and primarily elastic and low cycle fatigue where there is significant plasticity. 4.1 Stress-life and strain-life methods.The stress values that cause fatigue damage are typically much less than the yield strength of the material. If the loads are above a certain threshold, microscopic cracks will begin to initiate at stress concentrations such as holes, persistent slip bands (PSBs), composite interfaces or grain boundaries in metals. Such as the reduced rate of growth that occurs for small loads near the threshold or after the application of an overload and the increased rate of crack growth associated with short cracks or after the application of an underload. However, there are also a number of special cases that need to be considered where the rate of crack growth is significantly different compared to that obtained from constant amplitude testing. To aid in predicting the fatigue life of a component, fatigue tests are carried out using coupons to measure the rate of crack growth by applying constant amplitude cyclic loading and averaging the measured growth of a crack over thousands of cycles. Most materials, such as composites, plastics and ceramics, seem to experience some sort of fatigue-related failure. In the nineteenth century, the sudden failing of metal railway axles was thought to be caused by the metal crystallising because of the brittle appearance of the fracture surface, but this has since been disproved. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.įatigue has traditionally been associated with the failure of metal components which led to the term metal fatigue.
Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading.
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