Effect of stress ratio on fatigue crack propagation in a functionally graded metal matrix composite

A functionally graded metal matrix composite, gradually distributed SiC particulate reinforced Al matrix composite, was fabricated by powder metallurgy processing. Fatigue crack growth tests of the composite were conducted in crack growth direction of from 5% to 30% SiC volume fraction layers under sinusoidal wave-form with stress ratios of 0.1, 0.3, 0.5 and 0.7, respectively. The fatigue crack growth rates increase with an increase in stress ratio. This is interpreted by crack closure mechanism. The retardation of fatigue crack growth was found when crack propagated from low-volume fraction of SiC layer to high-volume fraction of SiC layer. The crack deflection and branching at interfaces were observed, which decreased crack growth rates.