4.10 Residual Stresses in Metal Matrix Composites

Residual stresses are those stresses which exist in a material or component in the absence of any external loading. Because they are not immediately apparent, and add to those stresses externally applied, for monolithic materials they are often implicated in unforeseen failures. Further, it is often difficult to measure such stresses, or to predict their effect on behavior. These stresses can also bring up surprises during subsequent manufacturing steps, such as machining or joining. At one scale or another, residual stresses are almost always present in composite materials. In part this is because composites tend to derive their beneficial properties by bringing together what are often very different constituents, giving rise to all manner of misfits and hence residual stresses between the phases. There are numerous types of composites employing a metal as a matrix including continuous long-fiber/monofilaments, whiskers, short fibers platelets and particulates and these can be systematically arranged and aligned, or randomly dispersed or oriented depending on the properties desired. Accordingly they can be assembled by gaseous, liquid or solid state processing. These architectural and manufacturing issues alongside the choice of matrix and reinforcement strongly influence the magnitude and type of residual stress developed and hence their behavior. In order to simplify discussion of residual and internal stresses in metal matrix composites (MMCs), long fiber/monofilament composites will be referred to as continuous, whereas whisker and particulate containing systems will be referred to as discontinuous composites.