A micromechanical characterization of residual thermal stresses in carbon fiber/epoxy composites containing a non-uniform interphase region

There is growing speculation that the interphase in polymer composites is often a region of nonuniform material properties. This is significant given the critical role of the interphase in determining overall composite behavior. The present investigation utilizes a micromechanical model based on the three-phase method of cells to examine how spatial variations in the interphase elastic properties are predicted to influence the residual thermal stresses in carbon-fiber-reinforced epoxy. This is the first such study of its kind based on a true three-phase version of the method of cells. A total of sixteen different composite configurations are considered in which the interphase Young's modulus and/or the interphase thermal expansion coefficient may vary as a function of the radial coordinate. The interphases are specified such that their Young's modulus and thermal expansion coefficient may be above or below that of the epoxy matrix. The residual thermal stresses, as well as the effective composite properties, are evaluated as a function of the fiber volume fraction, the interphase thickness and the spatial nonuniformity of the interphase properties. The results indicate that the introduction of interphase property gradients is predicted to primarily influence the state of stress within the interphase. Depending upon how the interphase properties are specified to vary, the residual stresses within the interphase may either be compressive or tensile.