Microstress distribution in graphite fibre/epoxy composites containing an elastomeric interphase: response to uniaxial and biaxial loading conditions

Abstract

A micromechanical model based upon the method of cells is introduced to characterize three-phase composites that contain a distinct and homogeneous interphase region. Initially, the performance characteristics of the model are shown to be quite consistent with those of a concentric cylinder model formulation. Subsequently, a parametric study is performed that examines the mechanical response of model graphite/epoxy composites as a function of selected interphase properties. The micromechanical model is utilized to establish an interdependence among the interphase Young's modulus, the interphase thickness and the average stresses within the fibre, interphase and matrix resulting from two external loading conditions: uniaxial longitudinal tension and biaxial transverse shear. Material combinations are considered wherein the interphase Young's modulus is systematically varied above and below the matrix Young's modulus. The simulation indicates that the selected interphase properties significantly influence the stress state within each of the three composite constituents. The manner in which the stress states are modified proves to be non-intuitive in many of the cases considered. In particular, there are material domains encountered where the model predicts that certain stress components in a constituent will exhibit (1) a maximum with respect to variations in the interphase Young's modulus and/or (2) a minimum with respect to variations in the interphase thickness.