THERMO-INELASTIC RESPONSE OF FUNCTIONALLY GRADED COMPOSITES

A recently developed micromechanical theory for the thermo-elastic res ponse of functionally graded composites is further extended to include the inelastic and temperature-dependent response of the constituent p hases. In contrast to currently employed micromechanical approaches ap plied to this newly emerging class of materials, which decouple the lo cal and global effects by assuming the existence of a representative v olume element at every point within the composite, the new theory expl icitly couples the local and global effects. Previous thermo-elastic a nalysis has demonstrated that such coupling is necessary when: the tem perature gradient is large with respect to the dimension of the inclus ion phase; the characteristic dimension of the inclusion phase is larg e relative to the global dimensions of the composite; and the number o f inclusions is small. In these circumstances, the concept of the repr esentative volume element is no longer applicable and the standard mic romechanical analyses based on this concept produce questionable resul ts. Examples of composite materials that fall into this category inclu de large-diameter fiber composites such as SiC/Ti and B/Al. Herein, we extend this new approach to include the inelastic and temperature-dep endent response of the constituent phases in order to be able to reali stically model functionally graded metal matrix composites in the pres ence of large temperature gradients. The inelastic behavior of the mat rix phase is modeled using two inelastic models, namely the Bodner-Par tom unified viscoplasticity theory and the classical incremental plast icity theory. Results are presented that illustrate the differences be tween elastic and inelastic analyses, defining under what circumstance s the inclusion of inelastic effects is important. Application of the theory to composites with thermal barrier coatings demonstrates the ut ility of the concept of internal temperature management through functi onal grading of the microstructure using differently-distributed parti culate inclusions.