The effect of interface on local stress and displacement fields and thermo-mechanical properties of composites is studied. The inclusions are assumed to be uniformly but non periodically distributed in the matrix. The interface is varied theoretically by considering two models. The first one is the flexible interface model, in which the continuity of tractions at the interfaces is maintained but there exist jumps in the displacements, such that the jumps in the tangential and normal displacements are proportional to shear tractions and normal tractions, respectively. Two parameters are introduced to describe the degree of adhesion between inclusion and matrix. Specific interface condition can be simulated by proper selection of the two parameters. The second model describes the interface as a layer between the inclusion and the matrix. This layer, called interphase, has a given thickness and the thermo-mechanical properties different from those of the matrix and the inclusions. The elastic properties of the layer are assumed uniform or variable. The perfect bond is assumed at both the matrix-interphase and interphase-inclusion interfaces.; For both of these interfacial representations, a unified approximate approach to evaluate the effective thermo-mechanical properties is used. Initially, the boundary value problem of the isolated inclusion embedded in the matrix is solved. Then, stress disturbance in the inclusion due to the presence of other inclusions is accounted for by using a successive iteration method (Mori and Wakashima, 1990) based on Mori-Tanaka theory (Mori and Tanaka, 1973). The successive iteration yields solutions that converge into closed forms under a certain condition. The analytical forms of the local stress and displacement fields and the effective properties are obtained. The latter are predicted by using the concept of the average strain in the composite. The approach is simple and can be easily extended to other boundary conditions and is valid for any shape. In the numerical results presented, the inclusions are assumed to be cylindrical or spherical in shape for simplicity. The influence of various mechanisms at the interface is studied and the results are compared with the perfect bonding case, bounds, as well as the other analytical results. It is shown that imperfect interface may have a significant effect on the local fields and the effective properties of composites.
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