Micro-Mechanical Aspects of Fiber Debonding and Frictional Sliding in Brittle Matrix Composites

It is now well established that the mechanical behavior of a fiber-reinforced brittle-matrix material largely depends on the properties of the fiber-matrix interface. In particular, to obtain enhanced toughness through fiber reinforcement, the interface must exhibit a relatively low fracture energy such that fiber debonding and crack bridging occur during matrix crack propagation. With further matrix crack extension, debond propagation and frictional fiber sliding occur, providing energy dissipation for enhanced toughness. Interfacial properties of importance include the fracture energy, frictional sliding characteristics and residual stress state. In addition, the morphology of the sliding interface has recently emerged as an important interfacial parameter which can greatly influence fiber debonding and sliding behavior. Experimental evidence of the role of fiber surface morphology during fiber sliding was first reported by Jero and Kerans. Recognition of the importance of the interfacial properties has prompted the development of a number of techniques to evaluate these parameters, including the single-fiber pull-out and push-down tests. These techniques are designed to directly determine composite interfacial properties, and therefore, assess the mechanical performance of fiber-reinforced brittle-matrix materials. Numerous fiber debonding and sliding experiments have been done using these two techniques. However, a number of factors make it difficult to determine fundamental interfacial properties from such tests. For instance, with the push-down technique, the sample geometry results in prominent end effects. In addition, the preparation of pushdown specimens may allow relief of residual stresses which are now known to play an important role in determining fiber debonding and sliding behavior. Furthermore, since the applied loads are compressive, the push-down technique does not properly evaluate the effects of locally sliding fiber strength, i.e. fiber fracture away from the matrix crack plane and the frictional pull-out of the fractured fibers. There are equivalent concerns with the standard single-fiber pull-out technique. The free length of fiber protruding from the edge of the specimen leads to low system stiffness, and accurate fiber displacement measurements are difficult to obtain. In this work, an improved single-fiber pull-out technique .has been developed and used to observe stable, progressive fiber debonding and sliding in two model composite systems. Fiber surface roughness is shown. to have a pronounced effect on the interfacial frictional sliding in these composite systems. Roughness-induced load fluctuations are observer during progressive debonding, with the fluctuation amplitude increasing with frictional sliding zone length