Warp-knit laminates for stampable sandwich preforms

Thermoforming of doubly-curved sandwich components will offer new design po ssibilities and cost-effective manufacturing routes. However, the occurrenc e of folds and wrinkles in face sheets is directly linked to the fabric arc hitecture and constitutes a process-limiting factor. Drapability tests have shown that knitted fabrics are less susceptible to wrinkling than woven pr eforms because their loop structure can more easily accommodate large defor mations. 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 an d PEI laminates reinforced with the new warp-knit structure is presented. M echanical tests at room and forming temperatures show that warp-knit reinfo rced composites have non-linear tensile behaviour. However at forming tempe ratures they exhibit quasi-isotropic behaviour and potential for high globa l 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-mechanic al and microscopic analysis. Thermoforming experiments were performed on wo ven and knitted PET reinforced laminates. The potential of laminates with k nitted reinforcement for the processing of complex shaped parts has been de monstrated. Consequently, these laminates will be used as new face sheets f or stampable thermoplastic sandwiches. Finally, a thermoforming map represe nting the areal draw ratio as a function of the laminate processability was deduced from the high temperature tensile tests. (C) 2000 Elsevier Science Ltd. All rights reserved.