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

Carbon fiber-reinforced composites have become increasingly favored in the aerospace and automotive sectors for their remarkable performance attributes. Nonetheless, their broader adoption is hindered by the concern of delamination. To address this limitation, our study delved into the influence of incorporating a thermoplastic toughening agent on the interlaminar fracture toughness (ILFT) of woven carbon fiber composites. Our investigation focused on the impact of diverse curing pressures on these composites, analyzing configurations both with and without an integrated non-woven polyamide (PA) veil. The PA veil was meticulously positioned within the central layer of a composite, combining benzoxazine and carbon fiber constituents. Our primary objective entailed optimizing curing conditions to enhance ILFT across both Mode-I and Mode-II fracture modes. Our findings unveiled that elevating curing pressure had a limited effect on ILFT in non-interleaved composites while interleaved samples exhibited substantial ILFT reductions under heightened curing pressure. Raising pressure from 1 atm to 3.5 atm caused approximately a 27% drop in the Mode-I strain energy release rate (GIC) value, and this reduction deepened to 42% at 7 atm. This highlighted the inverse relationship between increased curing pressure and GIC values in interleaved specimens, while non-interleaved counterparts experienced less pronounced changes. A parallel trend manifested in Mode-II ILFT, where escalated curing pressure correlated with diminished Mode-II strain energy release rate (GIIC) values. Interleaved samples suffered a notably more significant decline, attributed to resin content reduction caused by heightened pressure. This, in turn, compromised the bond between the PA veil and the matrix, facilitating the propagation of cracks within the composite. This ultimately culminated in significantly reduced GIC and GIIC values. Our study contributes valuable insights into optimizing the interlaminar fracture toughness of carbon fiber composites, shedding light on the complex interplay of curing conditions and material configurations.