When considering the response of metallic materials to cyclic loading, it is essential to distinguish between components such as machined parts, which are initially free of defects, and those such as castings and welded structures, which inevitably contain pre-existing defects. The fatigue behaviour of these two types of component is quite different. In the former case, the major part of the fatigue life is spent in initiating a crack; such fatigue is ‘initiation-controlled‘. In the second type of component, cracks are already present and all of the fatigue life is spent in crack propagation, such fatigue is ‘propagation-controlled’.
For a given material, the fatigue strength is quite different depending on whether the application is initiation- or propagation-controlled. Also the most appropriate material solution may be quite different depending on the application. For example with initiation-controlled fatigue, the fatigue strength increases with tensile strength and hence it is usually beneficial to utilise high strength materials. On the other hand, with propagation-controlled fatigue, the fatigue resistance may actually decrease if a higher strength material is employed.
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