On the particle-size dependence of fatigue-crack propagation thresholds in SiC-particulate-reinforced aluminum-alloy composites: Role of crack closure and crack trapping

A study has been made of ambient-temperature fatigue-crack propagation behavior in P/M Al-Zn-Mg-Cu metal-matrix composites reinforced with either 15 or 20 vol.% silicon-carbide particulate, with specific emphasis on the role of SiC-particle size on the fatigue-crack growth threshold condition. It is found that measured threshold stress-intensity levels ΔK TH , are a function of both SiC-particle size and volume fraction; however, whereas coarse-particle distribution results in higher ΔK TH values at low load ratios, fine particles give higher threshold at high load ratios. Such behavior is analyzed in terms of the interaction of SiC particles with the crack path, both in terms of the promotion of (roughness-induced) crack closure at low load ratios and by crack trapping by particles. Consideration of the latter mechanism yields a limiting requirement for the intrinsic threshold condition in these materials that the maximum plastic-zone size must exceed the effective mean particle size; this implies that for near-threshold crack advance, the tensile stress in the matrix must exceed the yield strength of the material beyond the particle.