A MODEL FOR PREDICTING THE DAMAGE AND ENVIRONMENTAL DEGRADATION DEPENDENT LIFE OF SCS-6 TIMETAL(R)21S[0](4) METAL-MATRIX COMPOSITE/

A method is developed herein for predicting the life of a continuous f iber titanium metal matrix composite. As a part of the research effort , the titanium metal matrix composite, SCS-6/Timetal(R)21S [0](4), has been fatigue tested at 482 degrees C and 650 degrees C. Additional sp ecimens have been environmentally degraded at 700 degrees C and then f atigued at 482 degrees C to failure. The research focuses on initial o xygen dissolution and its effect on the life of the material. The life -limiting physical mechanisms identified from the experiments are mate rial inelasticity, surface embrittlement, and subsequent surface crack ing, fiber/matrix debonding, fiber-bridging, and eventual fiber failur e. A model incorporating all of these physical phenomena has been deve loped herein. The model utilizes the finite element method coupled wit h models for material inelasticity, surface embrittlement, and crack p ropagation. 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 m aterial properties. Both monotonic and fatigue loadings have been mode lled at 482 degrees C and 650 degrees C for degraded and undegraded sp ecimens. Results indicate that surface crack propagation rates are sig nificantly slower when matrix viscoplasticity is included in the model instead of elasticity. Furthermore, surface cracking in environmental ly degraded specimens enhances fiber stresses compared to undegraded s pecimens. This difference apparently leads to the premature failure of the degraded composite. (C) 1998 Elsevier Science Ltd. All rights res erved.