Abstract
The plane strain fracture toughness of seven different particle-filled epoxies has been measured using compact tension specimens. These toughened epoxies were based on 828 epoxy resin filled with three types of phenolic beads and four types of carbon beads. Significant increases in toughness were observed (up to about 50% with 30% volume fraction of bead) and the mechanisms of toughening have been studied using scanning electron microscopy. The suggested major toughening mechanisms are crack pinning, localized plastic deformation associated with particle-matrix debonding and transparticle fracture. The shape of the load-displacement records obtained during the compact tension tests have been correlated with the failure mechanisms and compared with other studies in the literature. Based on the results obtained from the fracture toughness testing of bead filled epoxies, a carbon bead filled epoxy was selected as the matrix material for a hybrid composite. A method of preparing glass fibre laminates using bead filled epoxy as matrix has been developed which results in the beads concentrating at the inter-ply regions. The interlaminar fracture behaviour of the hybrid composite has been investigated using DCB (double cantilever beam) and ENF (end notch flexure) specimens for Mode 1 and Mode 2 tests respectively. The hybrid composite shows an increase in both GIC initiation and GIIC values as compared to a GFRP laminate with pure epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15 and 20 %. However, the pure epoxy glass fibre composite shows a higher GIC propagation value than that of the hybrid composites due to fibre bridging which is less pronounced in the hybrids as the presence of the beads results in a matrix rich interply region. The relationship between the Mode 1 interlaminar fracture mechanics parameters (i.e. GIC and KIC values), obtained from DCB specimens, has been reviewed using orthotropic fracture mechanics. It has been shown that KIC values calculated using an isotropic analysis of the DCB specimen are an overestimate and that they can be corrected by a factor derived from orthotropic fracture mechanics.