The effect interleaving has on thin-ply non-crimp fabric laminate impact response: X-ray tomography investigation

Abstract

To improve the damage resistance and tolerance of thin laminates manufactured with thin-ply non-crimp-fabrics, we interleaved non-woven veils (two different types of co-polyamide veil were studied) into the interlaminar regions. We devised an impact, compression after impact (CAI) and quasi-static indentation experimental campaign, where X-ray micro-computed tomography illustrated how: (a) matrix cracking, delamination and fibre failure interact during out-of-plane loading and (b) interleaving affects the thickness of the interface. One type of thermoplastic interlayer avoided resin accumulation, reduced the initiation of delamination and improved CAI strength by up to 28%.

Effects of post-infusion dwell on vacuum infusion of thermoset composites toughened by thermoplastic interlaminar veils

“…During infusion, resin flows more rapidly through the highly permeable veils than through the reinforcement fabrics and accrues in interlaminar regions, creating resin-rich interlaminar regions and increasing inter-ply thickness. 2,8 Moreover, local variations in preform permeability induce distorted flow fronts, resulting in air entrapment and dry spot formation, particularly within fiber tows. 13,[50][51][52] The formed intra-tow voids tend to migrate to resin-rich regions such as veil interfaces, as formation of voids in resin-rich areas is energetically more favorable than formation of voids within fiber bundles.…”

“…Previous studies have demonstrated that insertion of non-woven thermoplastic veils increases impact strength of thermoset composites, [1][2][3][4][5][6][7] yet can also cause increased porosity because of local variations in preform permeability created by the highly permeable veils. 2,8 One of these reports speculated that modifying the manufacturing process to provide more time for resin to saturate dry regions may reduce laminate void content, 2 although no such studies have been reported. To promote impregnation of partially saturated interlaminar regions, a lowtemperature post-infusion dwell was introduced to allow greater time for low-viscosity resin to equilibrate pressure and redistribute during the post-filling stage.…”

“…11,[17][18][19][20] However, because only atmospheric pressure is applied from vacuum-only consolidation, VI requires use of low-viscosity thermoset resins, which in general are brittle and thus susceptible to impact damage. 1,8,20,21 Introducing thermoplastic content into thermoset composites through bulk resin modification or interlaminar toughening can mitigate the inherently low toughness of thermosets. 1,3 Bulk matrix toughening generally is achieved by mixing rubber/thermoplastic liquid or particles into the thermoset resin.…”

“…2,3,[5][6][7] Exhibiting greater permeability than reinforcement fabrics, the veils also can serve as interlaminar flow distribution media, through which resin can flow more rapidly during the infusion process. 2,8 However, the local variations in preform permeability can induce non-uniform flow fronts and high porosity. 2,8 In the VI process, local compaction pressure exerted on the preform varies as the resin flow front progresses during the infusion (from vacuum to atmospheric pressure), and the top flexible vacuum bag surface allows preform thickness to adjust accordingly.…”

“…2,8 However, the local variations in preform permeability can induce non-uniform flow fronts and high porosity. 2,8 In the VI process, local compaction pressure exerted on the preform varies as the resin flow front progresses during the infusion (from vacuum to atmospheric pressure), and the top flexible vacuum bag surface allows preform thickness to adjust accordingly. 13,22 The continuously evolving preform deformation and pressure gradient result in a unique processing stage in VI, called "post-filling" stage, which refers to the period between the completion of infusion and resin gelation.…”