Published: Jan 1996
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Parameters and models to correlate the cycles to failure of a unidirectional metal matrix composite (SCS-6/Timetal 21S) undergoing thermal and mechanical loading are examined. Three different cycle types are considered: out-of-phase thermomechanical fatigue (TMF), in-phase TMF, and isothermal fatigue. A single parameter based on either the fiber or matrix behavior is shown not to correlate the cycles to failure of all the data. Two prediction methods are presented that assume that life may be dependent on at least two fatigue damage mechanisms and therefore consist of two terms. The first method, the linear life fraction model, shows that by using the response of the constituents, the life of these different cycle types are better correlated using two simple empirical relationships: one describing the fatigue damage in the matrix and the other fiber-dominated damage. The second method, the dominant damage model, is more complex but additionally brings in the effect of the environment. This latter method improves the predictions of the effects of the maximum temperature, temperature range, and frequency, especially under out-of-phase TMF and isothermal fatigue. The steady-state response of the constituents is determined using a 1-D micromechanics model with viscoplasticity. The residual stresses due to the CTE mismatch between the fiber and matrix during processing are included in the analysis.
metal matrix composites, titanium matrix, silicon carbide fibers, thermomechanical, fatigue, elevated temperature, micromechanics
Assistant professor, George W. Woodruft School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Ga
Senior scientist, Wright Laboratory Materials Directorate, Wright-Patterson AFB, OH