LIFE PREDICTION OF CROSS-PLY METAL-MATRIX COMPOSITES UNDERGOING THERMOMECHANICAL FATIGUE

A life prediction model that was originally developed for the axial lo ading of unidirectional metal matrix composites (MMCs) undergoing comb ined thermal and mechanical loading is extended to the axial loading o f cross-ply MMCs by adding an internally initiated matrix fatigue dama ge term. This new term accounts for the growth of cracks that initiate at the location where fibre-matrix separation occurs in the transvers ely-oriented plies. A comparison of the model predictions to experimen tal data on SCS-6/Timetal 21S shows that the model reasonably accounts for the dependence of applied stress, temperature and environment, as well as cyclic frequency. The dominant damage accumulation process fo r cross-ply MMCs with weak fibre-matrix bonds is described by this int ernally initiated matrix fatigue damage process for most stress-temper ature cycle combinations. However, the fibre-dominated damage accumula tion process operates under in-phase TMF when both stress and temperat ure are high. Environment-enhanced matrix fatigue is the dominant dama ge accumulation process under isothermal fatigue when stress is low an d temperature is high.