Abstract
Fatigue in materials subjected to repeated cyclic loading can be defined as a progressive failure due to crack initiation (stage I), crack growth (stage II), and crack propagation (stage III) or instability stage. For instance, crack initiation of crack-free solids may be characterized by fatigue crack nuclei due to dislocation motion, which generates slip bands at the surface having slip steps in the order of 0. 1 μm in height [11, 80] or slip may occur at matrix-inclusion interfaces. These steps produce surface intrusions and extrusions as schematically indicated below for stages I and II. These intrusions caused by reversed slip due to load reversal are the source for crack initiation, which may consume most of the solid life before crack growth. This crack initiation may occur along the slip direction due to a local maximum shear stress. After the consumption of many cycles, the crack may change in direction when the maximum principal normal stress (in the vicinity of the crack tip) governs crack growth. In this stage II, some materials show striations and beach marks as common surface features of fatigue fracture.
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