The transfer of matrix toughness to composite mode I interlaminar fracture toughness in glass-fibre/vinyl ester composites

The transfer of matrix toughness to composite mode I interlaminar fracture toughness (G_Ic) has been investigated in unidirectional glass-fibre reinforced composites with brittle and rubber-toughened vinyl ester matrices. Single-edge-notch bend (SENB) and double cantilever beam (DCB) specimens were used for matrix and composite G_Ic characterisation, respectively. The initial crack opening displacement rate was used as the parameter for comparison of G_Ic results. Matrix G_Ic was completely transferred to composite G_Ic for crack initiation (G_Ic-init) in the brittle-matrix composites, but in the toughened composites transfer was only partial due to the presence of fibres. The conclusion is that the maximum contribution to energy absorption by the matrix is more accurately reflected by G_Ic-init, and should be used for further assessment of the enhancing effect of fibre bridging during steady-state crack propagation, instead of matrix G_Ic. A plot of composite G_Ic for steady-state crack propagation, G_Ic-prop versus G_Ic-init indicates that the enhancing effect of fibre bridging is greater in the toughened composites. This enhancement is related to a larger deformation zone size in the toughened matrices.