THERMO-INELASTIC RESPONSE OF FUNCTIONALLY GRADED COMPOSITES

A recently developed micromechanical theory for the thermo-elastic response of functionally graded composites is further extended to include the inelastic and temperature-dependent response of the constituent phases. In contrast to currently employed micromechanical approaches applied to this newly emerging class of materials, which decouple the local and global effects by assuming the existence of a representative volume element at every point within the composite, the new theory explicitly couples the local and global effects. Previous thermo-elastic analysis has demonstrated that such coupling is necessary when: the temperature gradient is large with respect to the dimension of the inclusion phase; the characteristic dimension of the inclusion phase is large relative to the global dimensions of the composite; and the number of inclusions is small. In these circumstances, the concept of the representative volume element is no longer applicable and the standard micromechanical analyses based on this concept produce questionable results. Examples of composite materials that fall into this category include large-diameter fiber composites such as SiC/Ti and B/Al. Herein, we extend this new approach to include the inelastic and temperature-dependent response of the constituent phases in order to be able to realistically model functionally graded metal matrix composites in the presence of large temperature gradients. The inelastic behavior of the matrix phase is modeled using two inelastic models, namely the Bodner-Partom unified viscoplasticity theory and the classical incremental plasticity theory. Results are presented that illustrate the differences between elastic and inelastic analyses, defining under what circumstances the inclusion of inelastic effects is important. Application of the theory to composites with thermal barrier coatings demonstrates the utility of the concept of internal temperature management through functional grading of the microstructure using differently-distributed particulate inclusions.