Fatigue Damage Evolution in a Short Fiber Reinforced Metal Matrix Composite

The micromechanisms of fatigue failure of a short, alumina-silicate fiber reinforced cast aluminum alloy (A356) are investigated in this study. The nature of damage evolution is studied by three complementary perspectives — i) monitoring of the mechanical response, ii) microscopy on the gage length and fracture surface, and iii) probing of the microstructural changes in the bulk nondestructively using acoustic emission. The damage evolution in the composite is driven by strain or fatigue cycles imposed on the specimen and is manifested as three distinct mechanisms: a) cracking at hollow shot particles early in the life, b) microcracking in the form of fracture of fibers oriented in the direction of the loading and splitting or decohesion at fiber/matrix interface of transversely oriented fibers, and c) void formation at fiber ends and other stress concentrations. The interaction among the different modes, which defines the evolution of microstructural damage, is described. A flow chart for the progression of damage is presented and the most important steps in the damage evolution are identified. Suggestions are made for improving fatigue performance by tailoring the microstructure of the composite.