Fatigue of a Particle-Reinforced Cast Aluminum Matrix Composite at Room and Elevated Temperatures

Constant amplitude low-cycle fatigue, fatigue crack growth, and variable amplitude fatigue tests were performed on a 20% volume fraction SiC particle-reinforced A356 cast aluminum matrix composite at room temperature at 150°C. Data obtained were analyzed and compared with those of the unreinforced matrix material. Variable amplitude loading fatigue life calculations were performed using commercially available software.

The low-cycle fatigue results revealed that the composite material had somewhat lower fatigue resistance than the unreinforced material at room temperature and that the composite fatigue resistance decreased with increasing temperature. Room temperature fatigue crack growth rates were higher, and ΔK_th values were lower in the composite compared to those of the unreinforced material. There was not much difference between the composite fatigue crack growth rates at the two temperatures. Positive R ratio increased fatigue crack growth rates of the composite, but had more effect at room temperature than at 150°C. SEM fractography revealed that pores were preferred fatigue crack initiation sites. For small ΔK, fatigue cracks grew predominantly through the matrix, while with increasing ΔK more and more particles were fractured. Micromechanisms of crack growth were of the ductile type in the matrix, while particles were cleaved. Particle/ matrix interface debonding was not observed. Both reasonable and unreasonable fatigue life predictions were obtained with this metal matrix composite using the common low-cycle fatigue and fatigue crack growth approaches.