Effects of oxidation and damage on the mechanical response of metal matrix composites
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abstract
Metal matrix composites (MMC's) develop surface oxide layers when exposed to elevated temperatures, and due to their low toughness these brittle scales often become sites of crack initiation under applied thermomechanical loading. Microcracks forming at the surface facilitate further oxidation which eventually leads to oxidation of the fibers and premature failure of the composite. Recent oxidation experiments on SiC/Ti-15-3 composites indicate that oxide layers may grow to large thicknesses, locally inducing large eigenstrains and altering the material constitution by replacing the ductile metal with a brittle oxide. Experiments on SiC/Ti -21S show that even though the rate of surface oxidation is very small, diffusion of oxygen embrittles the metal ahead of the oxide. As a result, the critical energy for crack growth substantially decreases which results in crack growth that opens paths for oxygen to reach the fiber-matrix interface. In this paper, the modeling of the effective thermomechanical response of a unidirectional four-ply Ti-15-3 matrix and Ti -21S MMC's in an oxidizing environment are presented in two stages. First, the growth of the oxide layer is evaluated by solving the oxidation problem. Assuming a slow oxidation time scale, compared with the time scale of the applied mechanical loading, the oxidation problem is solved independently from the mechanical problem. In a second stage, the time dependent properties of the oxidized composite are taken into account to evaluate the damage evolution due to the growth of surface cracks and interface cracks at the fiber-matrix interface.
