Fracture toughness of CFRP with modified epoxy resin matrices

The fracture toughness of unidirectional continuous carbon fibre composites (CFRP) containing modified epoxy resin matrices has been studied in transverse impact fracture and Mode I delamination fracture tests. It is shown that the transverse impact fracture toughness values for composites with modified matrices are generally similar to those without modifiers in the temperature range 0°C to 80°C. At sub-zero temperatures the composites containing rigid Al₂O₃ powders in the matrix have much larger toughness values than the control. Potential energy release rates and stress intensity factors in Mode I delamination were measured to construct the crack growth resistance curve (R curve) by including the residual displacement effect. The R curves for rubber modified matrix composites (both with and without Al₂O₃ fibres or powders) are much superior to that of the control. However, addition of these rigid fillers without the presence of rubber did not significantly alter the R curve behaviour. Toughening mechanisms are discussed in relation to the plasticising effect of the rubber and the embrittling effect of the rigid fillers on the non-linear deformation of the modified matrices. The present results on carbon fibre composites with a low fibre volume fraction (V_f=0·20) follow reasonably well the already established relationship between resin toughness and the composite delamination fracture toughness for similar composites with V_f over 0·55.