A MECHANISTIC-BASED THERMOMECHANICAL FATIGUE LIFE PREDICTION MODEL FOR METAL-MATRIX COMPOSITES

The framework for developing a mechanistic-based life prediction model for metal matrix composites is described. For a composite consisting of unidirectional silicon carbide fibers in a titanium aluminide matri x, SCS-6/Ti-24Al-11Nb (at%) [0]8, three dominant damage mechanisms wer e identified: (1) matrix fatigue damage, (2) surface-initiated environ mental damage, and (3) fiber-dominated damage. Damage expressions were developed for each mechanism along with a method for determining the constants. The damage is summed to obtain the total life. The model is capable of making predictions for a wide range of histories, includin g isothermal fatigue at different frequencies and stress-ratios, therm omechanical fatigue (TMF) under in-phase and out-of-phase cycling cond itions, thermal cycling at constant stress, and stress holds at either maximum or minimum stress. Considering the wide range of cyclic condi tions, the predictions compare favorably with experiments. In addition , the controlling damage mechanism for each history is predicted.