Macroscopic properties of fiber reinforced composites are dependent on the micromechanics of the filament-matrix interphase. We present here some preliminary results of our studies aimed at understanding the behavior of the interphase when the composite is subjected to cyclic tensile loading. We have used carbon/polycarbonate mono-filament composites as a model system for studying the effects of loading direction (axial and transverse), frequency, and amplitude. The interphase was varied by etching the filament surface. Damage was characterized by fiber matrix debonding, reduction in interphase stress transfer efficiency, and changes in the locus of failure as determined by SEM fractography.

The effect of fiber surface treatments on the relationship between the tensile strength of a filament and the shear strength of its interphase is one of the central issues facing composite materials technologists today. We demonstrate here that analysis of fragmentation phenomena in monofilament composites can simultaneously yield information about these two parameters. Characterization of shear stress transfer zones in non-critical fragments has led us to the determination of interphase strength.

A phenomenological treatment that highlights the role of the matrix in the fragmentation process is presented here. This analysis considers issues such as the strain energy exchange between a failing fiber and the matrix, as well as interphase relaxation due to the viscoelastic nature of the matrix. Our observations of the fragmentation phenomena in AU4/polycarbonate monofilament composites indicate that the fiber/matrix interaction in this system is governed by micromechanical locking.