Process-induced shape distortions in aerospace thermoplastic composites

Thermoplastic composite materials are of great interest in aerospace structures due to their potential for shorter manufacturing cycle times, high production rates, and their ability to be re-heated and shaped multiple times. Thermoplastic resins offer many new possibilities in their ease of repair, recycling, and welding capabilities. Aerospace-grade thermoplastic composites such as carbon fibre-reinforced polyether-ether-ketone (PEEK) are processed well above their melting point at temperatures as high as 390ºC to allow proper forming and consolidation of the material to take place. During subsequent cool-down from the process temperature, residual stresses develop due to effects of material anisotropy, part geometry, and tool-part interactions that eventually lead to undesired shape distortions in the final part geometry. As observed with thermoset composites, common distortions include spring-in of corner angles and warpage of flat sections. The tight dimensional tolerances required for aerospace parts demand that process-induced shape distortions be well understood in order to eliminate scrap parts and fitting problems during the assembly stage of the components. In this project, L-shape flanges with a corner designed at 90° are manufactured from aerospace-grade AS4/PEEK thermoplastic composite in a hot press using a matched-die tooling configuration. A thermoforming technique is employed that involves heating previously-manufactured flat panels of the material to the processing temperature prior to transfer and consolidation within a relatively cold tool held at constant load and temperature. L-shape flanges consisting of a quasi-isotropic layup of unidirectional plies as well as short randomly-oriented strands of AS4/PEEK are thermoformed at 105°C, 215°C, and 290°C. Spring-in angles of the manufactured parts are quantified using a coordinate measuring machine and the results are compared with predictions from the Nelson-Cairns expression based on material thermal expansion anisotropy. The spring-in angles are also evaluated against measurements of change in part corner angle as a function of temperature due to thermo-elastic effects during heat-up from ambient temperature in a quasi-isotropic and ROS part. The parts are further assessed in terms of thickness measurements, surface quality observations, cross-section optical microscopy, and mechanical strength testing.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat