Shear Characterisation of UD Thermoplastic Composites

Intra-ply shear is one of the main mechanisms to accumulate deformations in thermoplastic composites forming processes. In this paper a shear characterisation method for uni-directionally (UD) reinforced thermoplastics in their molten configuration is presented. Bar-like specimens made from UD carbon PEEK material are subjected to oscillating torsional loads. By utilising linear visco-elastic theory, the small strain oscillatory responses are translated to storage and loss shear moduli. The frequency dependent moduli are subsequently translated to the transient domain and show a close-to elastic response for the considered instantaneous strain rates. The application of such characterisation data is demonstrated by modelling the stamp forming process of a complex shaped product with the aid of the finite element method. The process considered deals with an initially flat quasi-isotropic laminate consisting of eight plies, which is formed at high temperatures. Predicted intra-ply shear strains are compared with those measured by photogrammetry in a real formed part. Predicted and measured shear strain distributions and their magnitudes were in good agreement. Moreover, critical regions in which small wrinkles develop were indeed indicated by the forming predictions.