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Fractographic evaluations of a failed component’s fracture surface is an important step in establishing if fatigue was a mode of fracture. However, the three steps of a fatigue fracture are initiation of a crack, growth of the crack, and then uncontrolled growth which leads to the final fracture. The initiation mechanism, therefore, is very important in determining the cause of a fatigue fracture. Although fatigue can be the cause of an "initiated crack", the fracture "mode" can be fatigue, but the cause of the fracture can be a pre-existing crack that was formed by another mechanism.
Often the validity of a failure theory can be tested, and the application of the methods of fracture mechanics are described in, for example API-579, Fitness for Service.