Abstract
A method for predicting the life of an embrittled metal matrix composite using the finite element method coupled with models for material inelasticity, surface embrittlement, and crack propagation was developed herein. The titanium metal matrix composite, SCS-6/Ti[]21 S [0]4 was fatigue tested at 482[] C and 650[] C. The material was also oxidized at 700 C and then fatigued at 482[] C to failure. Oxidation studies were performed on Ti-[]21S to correlate microstructural changes with a loss in mechanical properties. The research focuses on initial oxygen dissolution and its effect on the life of the material. The life limiting physical mechanisms identified were material inelasticity, surface embrittlement and subsequent surface cracking, fiber/matrix debonding, fiber-bridging, and eventual fiber failure. Material inelasticity was predicted using Bodner's unified viscoplastic model. Crack propagation was modeled via the inclusion of cohesive zone elements. Surface embrittlement was accounted for through a degradation in mechanical properties. Both monotonic and fatigue loadings were modeled at 482[]C and 650[]C for oxidized and unoxidized specimens. Considering monotonic data, surface cracks propagated sooner and farther in an elastic analysis than in a viscoplastic analysis. Considering cyclic fatigue, the effects of matrix viscoplasticity and surface cracking in oxidized specimens shed load to the fiber. The strength of the fiber, and in turn, composite is directly related to a critical fiber stress. Due to the enormous computational time required, only 14 cycles were predicted. However, the fiber stress in the oxidized composite was significantly higher than in the unoxidized composite. This difference apparently leads to the premature failure of the oxidized composite.
Foulk, James Wesley (1997). A model for predicting the damage and oxidation dependent life of SCS-6/Ti-B21S [0]4 metal matrix composite. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1997 -THESIS -F68.