Matrix cracking in transversely loaded fiber composites with compliant interphases

Some preliminary results are presented from a study on the effect of interphases on the transverse strength of unidirectional fiber-reinforced composites. It is assumed that a thin reaction zone (intermolecular bonding at the fiber/matrix interface) defines the bond between the fibers and the matrix. A variationally coupled finite element - boundary element method is employed to obtain the mechanical response of a hexagonal unit cell which contains a center fiber and parts of surrounding fibers. For a given interfacial stiffness, the unit cell is loaded until a critical value of the circumferential stress component in the matrix just outside the interface is reached. Under the same applied load, the problem is then solved with a radial matrix crack of a given length at the critical location. The mixed mode stress intensity factors are extracted by evaluating interaction integrals over a finite element domain surrounding the crack tip, and the direction of crack propagation is predicted by a maximum principal stress criterion. The circumferential stress distributions in the matrix just outside the interfaces are then examined to determine future crack nucleation sites.