Fiber–matrix adhesion from the single-fiber composite test: nucleation of interfacial debonding

Abstract

The level of fiber–matrix interfacial adhesion in composites is traditionally evaluated by means of a stress-based parameter. Recently, it was suggested that an interfacial energy parameter might constitute a valid alternative. From an overview of the literature regarding the single-fiber composite fragmentation test, it appears that energy-based approaches have already been proposed in the past, but were either not successful, or not fully developed. Our recent energy balance scheme, proposed for the analysis of the initial interface debonding which occurs at fiber breaks during a fragmentation test, is presented and expanded here. The effects of thermal residual stress in the fiber, and of friction in the debonded area, are now incorporated in the energy balance model. We use a different shear-lag parameter proposed by Nayfeh, rather than the commonly used Cox parameter. New, extensive single-fiber fragmentation data regarding the interface crack initiation regime is presented, using sized and unsized E-glass fibers embedded in UV-curable or epoxy polymers. Some data for unsized carbon in epoxy is also presented. Fiber fragmentation is forced to take place entirely in the linear elastic region of the stress–strain curve, by means of pre-stressed single fibers. The importance of this procedure is discussed. Future work will focus on the interface crack propagation regime.