| In recent years, crack closure phenomena during fatigue have been investigated for metallic materials and have been used to explain fatigue crack propagation behavior at relatively low rates. In particular, roughness-induced crack closure, in certain alloys, which is predominantly observed during the earlier stage of the fatigue test, has attracted continued interest among fracture and fatigue researchers. This interest is the result of the apparent correlations between the roughness dimension, or the crack surface microstructure, and the mechanical properties of a material. In this study, a 2-D model for the roughness-induced crack closure is quantitatively analyzed in terms of both the dimension of an asperity and its location from the crack tip. Based on an appropriate model, the roughness-induced crack closure is fractographically characterized. The finite element method is used to characterize the crack tip stress field in crack closure and to directly obtain the closure stress intensity factor (the effective stress intensity factor which acts as the driving force at the crack tip for a specified geometry). These results are then compared with analytical solutions. In addition, to investigate the effects of crack closure over a broad range of stress intensity, variations of the apparent closure stress intensity factor along the crack growth are predicted by using phenomenological approaches. In all analyses, the role of the roughness dimension on the crack surface as a microstructural parameter is thoroughly examined. |
