Micromechanisms of fatigue crack propagation in particulate-reinforced metal matrix composites

Consequences of the interaction of cracks with SiC particles are examined with emphasis on micromechanisms influencing fatigue crack propagation in high strength aluminum alloy matrix composites. Fatigue crack propagation is found to show three distinct regimes; each accompanied by growth mechanisms reflecting different roles of SiC particles. At near-threshold levels, SiC particles impede fatigue crack growth by deflecting the crack to promote roughness-induced crack closure and by acting as crack traps along the crack front. A two-dimensional crack trapping analysis based on the interaction of a finite crack with a SiC particle indicates that a limiting criterion for fatigue crack growth in SiC{dollar}sb{lcub}rm p{rcub}{dollar}/Al composites can be established, which requires that the maximum plastic-zone size exceed the effective mean particle size or that the tensile stress in the matrix beyond the particle on the crack front exceed the yield strength of the material. Implications of crack closure and crack trapping to near-threshold crack growth, including load-ratio and particle-size dependence of fatigue thresholds, are discussed in terms of contributions from each mechanism. At higher stress intensities, limited fracture of SiC particles ahead of the crack tip leads to the development of uncracked ligaments along the crack length, resulting in a reduced crack-tip stress intensity from ligament bridging. Micromechanical models are developed for such bridges induced by both overlapping cracks and co-planar ligaments, based on the notion of a limiting crack opening displacement or limiting strain in the ligament. The predicted reduction in crack tip stress intensity is shown to be consistent with experimental observations.