By using the crack analogue model of rigid flat-ended contact, crack initiation in fretting fatigue is analyzed. The coefficient of friction at the edge of contact, which characterizes the asymptotic stress field, is considered as the primary controlling parameter in the process. Meanwhile, the maximum tangential stress criterion and the maximum shear stress criterion are used to predict opening-mode and shear-mode crack initiations, respectively. By examining the model prediction and comparing it with experimental observations, it is shown that the observed microcracks at the small angles to a fretting surface were nucleated in shear mode in the early stage of tests with a mooth initial surface, while the microcracks at the large angles were nucleated in opening model in the later stage with a rough worn surface. This understanding may help to establish the sequential damage mechanisms in the complex process of fretting fatigue.
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