Abstract
This study develops a microscale constitutive model to characterize the mechanical and creep strain response of fiber-reinforced ceramic matrix composites (CMCs) under stepwise loading at elevated temperatures, explicitly accounting for oxidation of both the interphase and reinforcing fibers. The model incorporates four key creep-induced damage mechanisms: stress-dependent stochastic matrix cracking, time-dependent interface oxidation, stress–oxidation–coupled interface debonding, and stochastic fiber failure. By systematically integrating the interactions between interface slip and matrix cracking, six distinct damage configurations—termed Mode I through VI—are defined based on matrix crack spacing (long, mediate, short) and interface degradation state (partial vs full debonding, with or without oxidation). These configurations serve as the physical basis for deriving the microscale creep constitutive relations. The model is applied to predict the time-dependent mechanical and creep strains of a 2.5D SiC/SiC composite under constant and stepwise elevated-temperature loading, with results differentiated across all six damage modes. Parametric analyses quantify the effects of stepwise load magnitude and dwell time, as well as creep temperature, on global strain evolution and internal damage progression—including fiber fracture, interface oxidation kinetics, and debonding extent. Model predictions are validated against experimental strain data for the same 2.5D SiC/SiC composite tested under stepwise loading at 1000°C, 1100°C, and 1300°C. Finally, quantitative relationships linking cumulative creep strain to stepwise loading history and progressive damage accumulation are established.
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