Impact of fatigue microcracks on the application of aluminum-silicon carbide composites in unified life cycle engineering

Advances in the reliability of engineering materials are dependent upon the ability to predict and detect the presence of critically-sized flaws prior to the next inspection opportunity. It is the role of fracture mechanics and nondestructive evaluation (NDE) to provide these capabilities. In advanced materials such as Al-SiC composites, however, fatigue failure under high stress is governed by the nucleation and growth of distributed "microcracks". These microcracks have growth behaviors not well-described using linear elastic fracture mechanics (LEFM) methods such as the Paris equation and are often much smaller than the detection threshold of conventional NDE techniques. The "fatal" crack in Al-SiC composites can form when several microcracks link up near the end of the fatigue life. Coupled with the reduced ductility of these materials, it spends as little as 5 percent of the total life as a detectable crack. The difficulties that arise in applying unified life cycle engineering (ULCE) to Al-SiC composites, therefore, are that for most of the fatigue life microcracks grow in manners not easily characterized by LEFM and are too small to detect by NDE. By the time growth can be described using LEFM and a crack can be detected, failure is imminent. To circumvent these obstacles to the safe use of Al-SiC composites, this study presents a reliability strategy based on tracking the evolution of entire microcrack distributions rather than just single cracks. Experimental data on the growth behavior of microcracks within the distributions are obtained from surface replicas of smooth specimens of 2xxx-type Al-SiC composites. The data serves as the foundation for a Monte Carlo simulation developed to reconstruct the microcrack length distributions. Such information allows reliability inspections to be scheduled when critically-sized cracks are expected to emerge from the population of microcracks. Works in NDE are also pursued to detect the effects of microcrack populations, as opposed to individual ones, on eddy current probe impedance signals. The reliability philosophy presented here is shown to encompass not only Al-SiC composites, but a very different metallic material (304 stainless steel) in which distributed microcracking plays a role in fatigue failure.