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 reinforce d with SiC particles (SiCp) and whiskers (SiCw) over a wide spectrum o f growth rates from 10(-12) to 10(-4) m per cycle. Effects of aging tr eatment, orientation of crack growth direction with respect to the rol ling direction, mean stress (or stress ratio), and reinforcement volum e percent on the fatigue crack growth threshold are investigated in te rms of crack tip shielding mechanisms. Comparison to fatigue crack gro wth in the unreinforced alloy indicates that crack growth resistance i n the composites is superior to the monolithic alloy at low stress int ensity ranges, Delta K. Specifically, at the lower growth rates the su perior crack growth resistance of the composites is due to the formati on of tortuous crack paths and a consequently enhanced roughness induc e crack closure. Fatigue response near threshold is found to be relati vely insensitive to changes in aging treatment with observed variation s reflecting the associated changes in plastic behavior. In whisker re inforced composites, the orientation of the crack with respect to the rolling direction had a significant effect on fatigue crack growth rat e while in the particulate reinforced composites orientational effects were not significant. Increasing the volume fraction resulted in high er crack growth resistance at low growth rates in the particulate rein forced materials. At high stress ratios, resistance in both whisker an d particulate reinforced composites was lowered, and the measured fati gue crack propagation threshold, Delta K-th, was found to be independe nt of the reinforcement morphology and volume percent. (C) 1997 Elsevi er Science S.A.