Time-Dependent Behavior of Continuous-Fiber-Reinforced Metal Matrix Composites: Modeling and Applications

A time-dependent approach employing a four-phase concentric cylinder model has been developed to predict the response of metal matrix composites (MMCs) subjected to thermomechanical loadings in which both plastic and creep responses of the composites are considered. The progressive development of plasticity in the matrix phase is determined using the deformation theory of plasticity while the creep deformation of this phase is estimated using the Bailey-Norton equation with an Arrhenius-type expression for the time-dependent creep coefficient. The model is applied to SCS-6/Ti-β21S composite to study the evolution of the stress and strain states in the constituents of the composite during initial cool-down and subsequent thermal cycles. The model is then employed to examine the influence of several critical parameters on the composite internal stress and strain states. These parameters include the thickness of the equivalent composite media, the type of fiber coating material, the thickness of the reaction zone, cooling rate during initial cool-down, and the kinetics of creep process during thermal cyclic loading. Results of these applications indicated that the process-induced thermal stresses in the matrix phase can be relaxed due to creep following initial cool-down from fabrication. This stress reduction is enhanced at a slower cooling rate. Comparison of different fiber coating materials shows that the use of carbon coating induces compressive stress state in the brittle interfacial region. TiB2-coated fibers, however, are found to be less affected by the growing interphase thickness in preserving the compressive radial stress component in the matrix and the interphase zone. Furthermore, it is found that the matrix activation energy for creep, Q, is history-dependent and can be correlated with the level of creep strain accumulated in the matrix phase. In addition, the residual thermal stresses induced in the matrix phase during initial cool-down can be relaxed by the application of subsequent thermal cycles.