Effect of Fiber-Matrix Interphase Defects on Microlevel Stress States at Neighboring Fibers

A fiber with inferior interphase properties generates higher stresses at the fiber matrix interfaces of neighboring fibers. In this paper detailed calculations are presented for this effect. The mechanical behavior of the interphase between fibers and matrix is modelled by continuity of tractions and a linear relation between displacement differences across the interphase and the conjugate tractions. The proportionality constants characterize the stiffness and the strength of the interphase. For a cell of the composite which contains a cluster of fibers, calculations have been carried out by the use of the boundary element method. Four cases have been considered: (1) perfect composite. (2) a single fiber in the composite with lower interphase stiffness, (3) a single fiber in the composite with interphase cracks and lower interphase stiffness, and (4) a missing fiber in the composite. The interface stresses for neighboring fibers have been calculated. Also, for the case of the fiber with an interphase crack the strain energy density near the crack tip has been calculated and the condition for crack propagation has been discussed.