Titanium matrix composites (TMC’s) are known to undergo significant environmental degradation at elevated temperatures. As a part of this research effort the TMC SCS-6/Ti-ß21S 4 has been fatigue tested at 482°C and 650°C. Additional specimens have been oxidized at 700°C and then fatigued at 482°C to failure. The life limiting physical mechanisms identified from the experiments are material inelasticity, surface embrittlement and subsequent surface cracking, fiber/matrix debonding, fiber-bridging, and eventual fiber failure.
A model incorporating all of these physical phenomena has been developed in an ongoing effort by the authors. This model will be briefly presented herein. The model utilizes the finite element method coupled with models for material inelasticity, surface embrittlement. and crack propagation. Material inelasticity is predicted using Bodner’s unified viscoplastic model. Crack propagation is modeled via the inclusion of cohesive zone elements. Surface embrittlement is accounted for through the degradation of material properties. Both monotonic and fatigue loadings have been modeled at 482°C and 650°C for oxidized and unoxidized specimens. Results indicate that surface crack propagation rates are significantly slower when matrix viscoplasticity is included in the model instead of elasticity. Furthermore, surface cracking in degraded specimens enhances fiber stresses compared to undegraded specimens. This difference apparently causes premature failure of the degraded composite.