An Investigation into Thermomechanical Fatigue of Metal Matrix Composites
by
Metin Karayaka
Abstract
Experimental and theoretical approaches are used to characterize the thermomechanical deformation behavior of metal matrix composites. Experiments on unreinforced and SiC particulate reinforced L1 2xxx T4 have been conducted under several mechanical strain-temperature phasing conditions. Based on stress range, substantial improvements in fatigue life have been observed. However, based on strain range, the effect of reinforcement on fatigue lives differs depending on the mechanical strain-temperature phasing, temperature, and strain rate. Several deformation mechanisms of unreinforced and reinforced A1 2xxx-T4 have been identified, including void formation, crack initiation, intergranular/transgranular crack growth, oxide penetration at the crack tips, crack deflection due to particle interference, and mean stress effects.
Theoretical approaches include the development of a general micromechanistic constitutive equation, based on Eshelby’s equivalent inclusion theory, and a life prediction methodology for metal matrix composites. Synergistic effects of particulate reinforcement on high temperature thermomechanical behavior are studied. The constitutive model provides insight into the internal stress-strain behavior, including effective and hydrostatic stresses, of both the matrix and the reinforcement developed during cyclic loading conditions. The deformation behavior of the constituents is used to develop an experimentally based micromechanistic life prediction model. The damage caused by internal stresses, oxidation, creep, and fatigue mechanisms as a function of reinforcement volume fraction is quantified for wide range of loading conditions.
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