Effect of nonwoven interlayer melting temperature and matrix toughness on mode I and mode II fracture toughness of carbon fiber/benzoxazine composites

Abstract

Two toughening approaches were investigated in this study to improve the interlaminar fracture toughness (ILFTs) of carbon fiber/benzoxazine composites. A matrix modification as well as an incorporation of thermoplastic polyamide (PA) veils were utilized to investigate the change in the ILFT properties. Two PA veils with different melting temperatures were analyzed and the ILFT properties of the resultant laminates were determined by the Mode-I and Mode-II critical strain energy release rates (GIC and GIIC). The incorporation of both veils was found to increase both GIC and GIIC performances. The veil with a lower melting temperature than the curing temperature of the resin resisted a Mode-I crack propagation better than the veil with a high-melting temperature. A significant increase in the GIC measurements could therefore be achieved with the use of the toughened matrix system. In the Mode-II ILFT testing, the high-melting temperature veil outperformed the low-melting temperature veil in resisting both crack initiation and propagation. No significant toughening improvement was observed with the use of a toughened matrix system in the Mode-II experiment. A combination between matrix toughening and interleaving with the high-melting temperature veil led to an enhancement in Mode-II properties. The fracture surfaces of the tested specimens were then examined for a determination of toughening effects of the interleaved veils on the composites.