A Model for Predicting the Effect of Environmental Degradation on Damage Evolution of Metal-Matrix Composites

Abstract

A life prediction model is being developed by the authors for application to continuous fiber metal-matrix composites (MMCs). The specific systems considered in this study are silicon-carbide fibers imbedded in titanium matrix. Due to multiple nonlinearities, the model utilizes a computationally based framework derived from thermodynamics and continuum mechanics. Matrix inelasticity, damage evolution, and environmental degradation due to oxidation-related effects are also included within the model. To computationally implement the model, the finite element method is used with an evolutionary analysis of a unit cell accomplished via a time-stepping algorithm. Matrix inelasticity is modeled with the Bodner anisotropic hardening viscoplastic model. Damage growth such as fiber-matrix debonding, surface cracking, and matrix cracking is modeled via the inclusion of cohesive zone elements in the unit cell. The locations of these elements are chosen to correspond with experimentally observed damage. As environmental degradation varies in form, depending on the specific system, it is accounted for by including either an outer surface layer that is embrittled due to oxidation or degraded material properties that result from oxygen-induced changes in microstructure. The current paper outlines the formulation utilized by the authors to solve this problem, and recent results are discussed. Specifically, results are given for a four-ply unidirectional composite subjected to monotonic and fatigue loadings. In both cases, environmental degradation influences the initiation and evolution of damage.

A Fibre Stress‐based Parameter for Thermomechanical Fatigue Life Predictions of Titanium Matrix Composites

“…However, the titanium alloy used here is remarkably resistant to the formation of oxides. Studies have shown that the Timetal 21S still forms the layer of brittle oxide, but cracks initiate in the matrix beneath the oxide [6 ]. This suggests that the environmental damage arises from a phase transformation, ( β → α ), which embrittles the matrix, as seen in SCS‐6/Ti–15–3 [7 ].…”