A model for predicting the damage and environmental degradation dependent life of SCS-6/Timetal 21S [0]_4 metal matrix composite

摘要

A method is developed herein for predicting the life of a continuous fiber titanium metal matrix composite. As a part of the research effort, the titanium metal matrix composite, SCS-6/Timetal 21S [0]_4, has been fatigue tested at 482℃ and 650℃. Additional specimens have been environmentally degraded at 700℃ and then fatigued at 482℃ to failure. The research focuses on initial oxygen dissolution and its effect on the life of the material. The life--limiting physical mechanisms identified from the experiments are material inelasticity, surface embrittlement, and subsequent surface cracking, fiber/ma- trix debonding, fiber--bridging, and eventual fiber failure. A model incorporating all of these physical phenomena has been developed 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 modelled via the inclusion of cohesive zones. Surface embrittlement is accounted for by degrading material properties. Both monotonic and fatigue loadings have been modelled at 482℃ and 650℃ for degraded and undegraded 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 environmentally degraded specimens enhances fiber stresses compared to undegraded specimens. This difference apparently leads to the premature failure of the degraded composite.