Warp-knit laminates for stampable sandwich preforms

Thermoforming of doubly-curved sandwich components will offer new design possibilities and cost-effective manufacturing routes. However, the occurrence of folds and wrinkles in face sheets is directly linked to the fabric architecture and constitutes a process-limiting factor. Drapability tests have shown that knitted fabrics are less susceptible to wrinkling than woven preforms because their loop structure can more easily accommodate large deformations. In the experiments described here, a novel glass warp-knit fabric was impregnated with polyetherimide (PEI). The characterisation at ambient and forming temperatures of PEI eight-harness satin reinforced laminates and PEI laminates reinforced with the new warp-knit structure is presented. Mechanical tests at room and forming temperatures show that warp-knit reinforced composites have non-linear tensile behaviour. However at forming temperatures they exhibit quasi-isotropic behaviour and potential for high global and local strains, which is desirable for thermoforming of complex shapes. The influence of the fabric's compressive response on deconsolidation was also investigated. Deconsolidation onset was determined by thermo-mechanical and microscopic analysis. Thermoforming experiments were performed on woven and knitted PEI reinforced laminates. The potential of laminates with knitted reinforcement for the processing of complex shaped parts has been demonstrated. Consequently, these laminates will be used as new face sheets for stampable thermoplastic sandwiches. Finally, a thermoforming map representing the areal draw ratio as a function of the laminate processability was deduced from the high temperature tensile tests.