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.