Biaxial characterisation of poly(ether-ether-ketone) for thermoforming: A comparison between bulge and in – plane biaxial testing

The biaxial response of extruded PEEK films at conditions relevant to thermoforming has been investigated extensively using a combination of load controlled (bulge test) and displacement controlled (biaxial stretcher) experiments. Results from bulge testing yielded average and maximum strain rate ranges of 2.5 – 5 s-1 and 5 – 18 s-1 respectively, across the forming temperature range. In-plane biaxial characterisation highlighted the anisotropic non-linear viscoelastic behaviour of the films with strong dependence on the yield and strain hardening behaviour on the temperature and strain history at conditions equivalent to the forming process. The combined approach to material characterisation highlights the pros and cons of each test method, the complementary nature of the data generated and the need to use both methods to have a complete data set for developing accurate material models and validated numerical simulations of thermoforming.