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Fatigue is a process which causes premature failure or damage of a component subjected to repeated loading. It is a complicated metallurgical process which is difficult to accurately describe and model on teh microscopic level. Despite these complexities, fatigue damage assessment for design of components and structures must be made. Fatigue analysis methods have been developed that quantify fatigue damage.
There are three primary fatigue analysis methods: stress-life approach, strain-life approach, and fracture mechanics approach. These methods have their own region of application with some degree of overlap between them. The understanding of any one of these methods provides a technique which may be used to perform a fatigue analysis. However, it is the insights gained from the understanding of all three methods which allow the engineer to choose the method or methods that are most appropriate for the given problem.
Two over-riding considerations have promoted the development of fatigue analysis methods. The first has been the need to provide designers and engineers methods that are practical, easily implemented, and cost effective. The second consideration has been the need to reconcile these analytical approaches with physical observations. It has been through continued effort by many researchers that accepted design or analysis practices have been developed.
One of the most important physical observations is that the fatigue process can generally be broken into two distinct phases—initiation life and propagation life.