How to predict fatigue life Three methods of calculating total life, crack initiation, and crack growth Three methods of calculating total life, crack initiation, and crack growth Purely static loading is rarely observed in modern engineering components or structures. By far, the majority of structures involve parts subjected to fluctuating or cyclic loads, often resulting in fatigue-caused structural failure. In fact, 80% to 95% of all structural failures occur through a fatigue mechanism. For this reason, design analysts must address the implications of repeated loads, fluctuating loads, and rapidly applied loads. As a result, fatigue analysis has become an early driver in the product development processes of a growing number of companies. What is fatigue? Fatigue is failure under a repeated or varying load, never reaching a high enough level to cause failure in a single application. The fatigue process embraces two basic domains of cyclic stressing orstraining, differing distinctly in character. In each domain, failure occurs by different physical mechanisms: 1. Low-c ycle fat igue—where sign ific ant plast ic strain ing occur s. Low-cy cle fati gue involv es large cycles with significant amounts of plastic deformation and relatively short life. The analytical procedure used to address strain-controlled fatigue is commonly referred to as the Strain-Life, Crack-Initiation, or Critical Location approach. 2. High- cycl e fatigue —wher e stress es and strain s are largel y confin ed to the elasti c region . High- cycle fatigue is associated with low loads and long life. The Stress-Life (S-N) or Total Life method is widely used for high-cycle fatigue applications—here the applied stress is within the elastic range ofthe material and the number of cycles to failure is large. While low-cycle fatigue is typically associated with fatigue life between 10 to 100,000 cycles, high-cycle fatigue is associated with life greater than 100,000 cycles. Fatigue analysis refers to one of three methodologies: local strain or strain life, commonly referred to as the crack initiation method, which is concerned only with crack initiation (E-N, or sigma nominal); stress life, commonly referred to as total life (S-N, or nominal stress); and crack growth or damage tolerance analysis, which is concerned with the number of cycles until fracture. 1
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