Interfacial shear strength studies using the single‐filament‐composite test. I: Experiments on graphite fibers in epoxy

Abstract

Abstract The failure of the interface in a carbon fiber‐epoxy system was studied for six different epoxy blends using the single‐filament‐composite technique. The blends were formulated to yield a wide range of stiffnesses, and their effect on interfacial failure was examined. Specimens were made from Hercules IM6‐G carbon fiber and the different blends of epoxy, and then strained to obtain a distribution of fiber fragment lengths. Birefringence patterns near the fiber breaks were observed and recorded. Some of the specimens were strained until they failed and the resulting fracture surfaces were observed under a scanning electron microscope to determine fracture patterns and the existence of debonding. The fragment length distributions were interpreted using a Monte‐Carlo simulation of a Poisson/Weibull model for fiber strength and flaw occurrence. The results were used to calculate an effective interfacial shear strength. From this analysis we conclude that one cannot accurately predict the interfacial properties of a composite based solely upon conventional single fiber and bulk matrix properties. Local matrix properties and fiber/matrix interactions, on a microscale, play a key role in composite strength.