Prediction of thermomechanical fatigue lives in metal matrix composites

To identify the role of silicon carbide participate reinforcement on high-temperature thermomechanical fatigue behavior of Al 2xxx-T4, experiments have been conducted under thermomechanical out-of-phase and in-phase loading conditions. A general constitutive representation, based on Eshelby's inclusion theory, is used for the determination of volumetric average stresses and strains under cyclic loading of the metal matrix composite. This constitutive representation is used with a life prediction model, based on the matrix stress-strain behavior, which predicts contributions of fatigue, creep, and environmental damages to failure under both isothermal and thermomechanical fatigue loading. In isothermal fatigue experiments at 200 °C and 300 °C, pure fatigue damage and creep damage are the dominant damage mechanisms in the short-life regime. In the long-life regime, however, the stress levels are too low to induce considerable creep damage; so, oxidation damage becomes dominant. When fatigue damage is dominant, the model predicts a decrease in life, based on strain range, with increasing volume fraction of reinforcement. Based on stress range, improved fatigue lives are predicted with increasing volume fraction of reinforcement. The reinforced alloy exhibits longer lives when compressive hydrostatic stresses in the matrix at the high-temperature end of the cycle reduce the creep damage.