TIME-DEPENDENT BEHAVIOR OF CONTINUOUS-FIBER-REINFORCED METAL-MATRIX COMPOSITES - MODELING AND APPLICATIONS

A time-dependent approach employing a four-phase concentric cylinder m odel has been developed to predict the response of metal matrix compos ites (MMCs) subjected to thermomechanical loadings in which both plast ic and creep responses of the composites are considered. The progressi ve development of plasticity in the matrix phase is determined using t he deformation theory of plasticity while the creep deformation of thi s phase is estimated using the Bailey-Norton equation with an Arrheniu s-type expression for the time-dependent creep coefficient. The model is applied to SCS-6/Ti-beta 21S composite to study the evolution of th e stress and strain states in the constituents of the composite during initial cool-down and subsequent thermal cycles. The model is then em ployed 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 coa ting material, the thickness of the reaction zone, cooling rate during initial cool-down, and the kinetics of creep process during thermal c yclic loading. Results of these applications indicated that the proces s-induced thermal stresses in the matrix phase can be relaxed due to c reep following initial cool-down from fabrication. This stress reducti on is enhanced at a slower cooling rate. Comparison of different fiber coating materials shows that the use of carbon coating induces compre ssive stress state in the brittle interfacial region. TiB2-coated fibe rs, however, are found to be less affected by the growing interphase t hickness in preserving the compressive radial stress component in the matrix and the interphase zone. Furthermore, it is found that the matr ix activation energy for creep, Q, is history-dependent and can be cor related with the level of creep strain accumulated in the matrix phase . In addition, the residual thermal stresses induced in the matrix pha se during initial cool-down can be relaxed by the application of subse quent thermal cycles.