Room vs. temperature studies of model composites: modes of failure of carbon fibre/epoxy interfaces

Abstract

Laser Raman spectroscopy (LRS), transmission polarised optical microscopy (TPOM) and scanning electron microscopy (SEM) have been used to characterise the interface of model single-fibre composites. The composites consisted of single-carbon fibres embedded in epoxy resin. Local stress measurements as a function of applied strain were performed using LRS at both room temperature (RT) and 60 °C. Consecutively, the coupons were strained to failure and field emission SEM was used to study the fracture surfaces. In a parallel study, identical systems were subjected to incremental tension and fracture events were recorded as a function of applied strain. At RT, TPOM was used to provide additional insight in the local stress transfer. The stress transfer was found to depend on the combined effect of interfacial chemistry and thermal stresses. Thus, in the case of sized fibres, there is a distinct change in the interfacial failure mode at high temperature, whereas in the case of unsized fibres, the stress transfer is dominated by thermal stresses: at high temperature it is weak, due to the relief of the thermal stress field.