The effect of fatigue microcracks on rapid catastrophic failure in AlSiC composites

The fatigue resistance of discontinuous reinforced aluminum metal matrix composites is greatly influenced by the rapid nucleation and growth of a very large number of microcracks. These microcracks, which result from an abundance of potential crack initiation sites, are often retarded in growth by the presence of reinforcement through crack trapping. With continued fatigue, microcracking becomes so extensive that it induces widespread coalescence leading to increasingly larger microcracks. Inevitably, some of these large microcracks link together to form the fatal crack and instability takes place very shortly afterwards at unusually small critical crack sizes. The present study examines the form of catastrophic failure in smooth specimens of a 2124 aluminum alloy reinforced with silicon carbide whiskers. Experimental observations of microcrack initiation and stage-by-stage growth through to final failure are reported. Effort is directed at characterizing the distribution and orientation of microcracks present, particularly when linkage(s) result in the formation of the fatal crack, and developing a geometric probability method for predicting coalescence. Results show that microcracks initiate and grow preferentially in arrays which maximize the energy release rate. Near the end of life, the interaction of some microcracks brings about large increases in their stress intensity factor leading to coalescence and fatal crack formation.