Fatigue crack growth resistance in SiC particulate and whisker reinforced P/M 2124 aluminum matrix composites

Fatigue crack growth is examined in P/M 2124 aluminum alloys reinforced with SiC particles (SiCp) and whiskers (SiCw) over a wide spectrum of growth rates from 10(-12) to 10(-4) m per cycle. Effects of aging treatment, orientation of crack growth direction with respect to the rolling direction, mean stress (or stress ratio), and reinforcement volume percent on the fatigue crack growth threshold are investigated in terms of crack tip shielding mechanisms. Comparison to fatigue crack growth in the unreinforced alloy indicates that crack growth resistance in the composites is superior to the monolithic alloy at low stress intensity ranges, Delta K. Specifically, at the lower growth rates the superior crack growth resistance of the composites is due to the formation of tortuous crack paths and a consequently enhanced roughness induce crack closure. Fatigue response near threshold is found to be relatively insensitive to changes in aging treatment with observed variations reflecting the associated changes in plastic behavior. In whisker reinforced composites, the orientation of the crack with respect to the rolling direction had a significant effect on fatigue crack growth rate while in the particulate reinforced composites orientational effects were not significant. Increasing the volume fraction resulted in higher crack growth resistance at low growth rates in the particulate reinforced materials. At high stress ratios, resistance in both whisker and particulate reinforced composites was lowered, and the measured fatigue crack propagation threshold, Delta K-th, was found to be independent of the reinforcement morphology and volume percent. (C) 1997 Elsevier Science S.A.