Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of -1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 mu m particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R = -1, the fatigue strength of the SiCp/Al composite was the same as that of the unreinforced alloy, but at R = 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiCp/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10(7) cycles in the unreinforced alloy, but significantly unconservative estimation in the SiCp/Al composite. At R = 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R = -1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiCp/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.