Analysis of crack propagation
Fatigue cracks may propagate either from an existing flaw, such as an inclusion or a defect in a weld, or they may grow from damage nucleated as part of the early stages of fatigue itself. It is into this second category that fretting fatigue falls. The fretting process must first initiate an embryo crack and then propagate it in order to produce a failure. The life of a component suffering fretting fatigue may therefore be conveniently divided into initiation and propagation phases. In contrast to plain fatigue the initiation phase of fretting fatigue life is often, although not always, quite short. In reality the term fretting fatigue encompasses a range of conditions from mild contact tractions accompanying bulk stress amplitudes sufficient to cause failure in plain fatigue, to severe fretting in the presence of relatively low bulk stresses. The spectrum of loading conditions can be conveniently summarised in a diagram such as that shown in fig. 8.1. It is possible for fretting fatigue to affect significantly both crack initiation and crack propagation. Thus, a full understanding of fretting fatigue requires a consideration of the effect of fretting on both initiation and propagation phases of crack life. Initiation occurs at a microscopic scale and a detailed understanding can be achieved only by a micromechanics analysis, although some progress can be made by considering bulk properties of the contact. In contrast, once a crack has developed and is larger than several material grain sizes, it should be possible to explain its propagation by employing the same techniques of fracture mechanics as are used for other types of fatigue. Indeed, crack growth depends entirely on conditions at the crack tip and it is impossible for a crack to ‘distinguish’ whether the propelling stresses arise from a contact loading or from some other far field. Thus, the analysis of crack propagation would appear to be far more tractable than that of initiation and it will be addressed first. Crack initiation will be discussed further in Chapter 9.
KeywordsStress Intensity Factor Crack Length Crack Growth Rate Crack Arrest Stress Intensity Factor Range
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