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.

Investigation of interlayer toughening of carbon fiber composites using non-woven polyamide veils under different curing pressures

“…These results corresponded with our previous study on the influence of the PA veils on the enhanced fracture toughness mechanism of the carbon-fiber composites. 36 In this study, the increase in the curing pressure resulted in a decrease in the G IC values of the interleaved specimens, while this significant change in the G IC values of the non-interleaved samples was not observed. This observation was confirmed by the statistical t test method with p-values of 2.5%, 1%, and 0.5% for the differences in NPC G IC, ini , PC G IC, ini , and G IC, prop, respectively, of samples cured at 1-atm and 7-atm.…”

“…In this study, non-woven PA veils were integrated to enhance the Mode-I ILFT of the composite laminates, as previously identified and discussed in our previous research. 36 This enhancement could be achieved only when there is sufficient bonding between the veil and matrix. The reduction of the G IC values at a high curing pressure could be due to the external pressure that squeezed out a major amount of resin from the panel, and the remaining resin was not sufficient to create a uniform bonding between the veil, matrix, and fiber throughout the entire panel.…”

“…23,24 In the context of UD fabrics, the phenomenon of fiber bridging emerges as a prevalent occurrence within toughened interlayers, yielding a notable enhancement in Mode-I ILFT. Our preceding research 36 delved into the integration of thermoplastic PA (polyamide) veils possessing distinct melting temperatures (126°C and 178°C) between each carbon fiber ply. The G IC values of laminates incorporating interleaved structures, irrespective of the veil's melting temperature, exhibited an approximate 50% upsurge compared to the non-interleaved counterpart.…”