Improving the performance of out-of-autoclave composite laminates using an interlaminar toughening technique

The increasing demand for composite materials has meant that there is a need to develop out-of-autoclave (OOA) manufacturing processes that can produce cost-effective high-quality components. Autoclaves cannot meet the demand for high-volume production or large components; furthermore they represent an expensive and energy-consuming process. OOA manufacturing processes, however, are not currently capable of producing laminates with the low level of porosity or high mechanical properties that can be achieved using autoclaves. In addition to this, OOA techniques are often limited to the use of relatively brittle thermosetting resin systems due to the requirement for low viscosity; hence it is necessary to develop low-cost toughening techniques that are compatible with OOA manufacture. Consequently, the use of two recently developed Benzoxazine resin systems (BZ9120, BZ9130) and a thermoplastic interlaminar toughening technique (non-woven polyamide (PA) fibre veils) to manufacture low-cost, high-quality composite laminates via Vacuum Assisted Resin Transfer Moulding (VARTM) was investigated. In order to determine the suitability of the Benzoxazine resin systems to these ends, the Mode-I and -II interlaminar fracture toughness (ILFT), flexural stiffness and glass transition temperature (Tg) of carbon/BZ9120 and carbon/BZ9130 laminates with and without veils were assessed using double cantilever beam (DCB), end-notched flexure (ENF), three-point bend and dynamic mechanical thermal analysis (DMTA) test methods, respectively. Composites are highly sensitive to environmental conditions; hence the effect of hot/wet conditioning on Mode-I and -II ILFT, flexural stiffness and Tg was investigated. The ability to reverse the changes in Mode-I and -II ILFT broughtabout by hot/wet conditioning was also characterised. The overall resistance to Mode-I and Mode-II crack growth was improved by up to 100% due to the inclusion of the interlaminar veils, and this was further increased due to hot/wet conditioning. However, it was found that neither the Mode-I or Mode-II ILFT can be restored fully once the saturation point has been reached due to the formation of intra-laminar matrix cracks. Overall, PA-toughened specimens were found to outperform baseline specimens, while the BZ9120 resin system outperformed the BZ9130 system in relation to ILFT and hot/wet conditioning; hence, subsequent investigations on the manufacture and damage tolerance of carbon/BZ9120 and carbon/BZ9120/PA laminates only were conducted. Damage tolerance was evaluated by subjecting specimens to a 30 J drop-weight impact, and thereafter conducting compression-after-impact (CAI) tests. The open-hole compression (OHC) performance was also evaluated. Damage areas were examined using X-ray imaging and micro-computed tomography (μCT). The OHC response of carbon/BZ9120 laminates was found to compare well with commercially available carbon/epoxy systems qualified for use in aircraft structures. The inclusion of PA veils decreased the resultant damage area by up to 36% for a 30 J impact, but was found to cause the formation of long, cylindrical voids within the carbon-fibre tows. The formation of voids due to the presence of the veils was addressed by altering the manufacturing process to include a dwell period prior to curing to facilitate post-filling of the intra-tow channels. The dwell period resulted in a significant improvement in laminate quality, which was manifested in large increases (up to 35%) in compressive strength and ILSS. Additionally, the duration of the curing reaction, measured using DMTA curing, was reduced by almost 20 minutes in the presence of PA veils, which could be of significant benefit to the end users.