Fibre/matrix stress transfer through a discrete interphase. Part 1: single-fibre model composites

This paper reports a study of the effect of an interphase on strain development in fibre-fragments. In order to form an interphase, an epoxy resin with known properties was applied to the surface of unsized reinforcing fibres and cured. These were then embedded in a matrix resin coupon, prior to fragmentation testing. The study included an examination of the effect of interphase thickness, by applying multiple coats of one of the resins, and the effect of the interphase properties, by varying the coating resins. It was found that the average fragment lengths at saturation were difficult to distinguish, as a result of the scatter introduced by the statistical distribution of fibre strengths. However, the strain interval between onset of fragmentation and saturation was found to be more sensitive to variations in the interphase properties.A finite element model was used to examine the strain development in the fragments in more detail. The mechanical properties of the fibre, interphase and matrix were accurately incorporated into the model, providing a realistic representation of the state of strain in the experimental samples. The predicted deformations around the fibre-break provided an explanation for the experimental observations.