Crack initiation around holes in a unidirectional MMC under fatigue loading

Abstract

An investigation was undertaken to understand the fatigue process in unidirectional metal matrix composites (MMCs) with center holes. Currently, there is considerable confusion as to the true nature of initiation and growth of cracks from notches in MMCs. A key concern is whether surface cracks are through cracks. Furthermore, an explanation is required for the preferential initiation and growth of four major symmetric cracks at 65–72 degrees from the pole (loading axis). Clarification of these issues is critical for developing rational analysis procedures for damage tolerance. In this investigation, unidirectional eight-ply Ti 15-3/SCS-6 composites with a center hole were subjected to monotonic and tension-tension fatigue loading at room temperature. Optical and scanning electron microscopy were used to evaluate the initiation and growth of the cracks and to determine their angular orientation around the hole in the specimen. Under fatigue loading, it was found that the major cracks were continuous cracks through the depth of the specimen, and they were located at 65–72 degrees from the pole. The Hencky-Von Mises failure criterion, an octahedral shear stress criterion, was found to be successful in predicting the preferred initiation sites for the major fatigue cracks. Fatigue crack initiation appeared to be controlled primarily by shear stress induced reversed inelastic deformation of the matrix in the hole region. In contrast, under monotonic loading, fracture occurred at 90 degrees from the pole, similar to what is observed in homogeneous metals. Crack initiation in this case appeared to be controlled by fiber fracture. Thus, whereas failure was matrix dominated in fatigue, it was fiber dominated under monotonic loading.