Experimental characterisation and constitutive modelling of the intra-ply tensile and shear properties of unidirectional fibre reinforced thermoplastics (UD FRTPs) under solid-state stamp forming conditions

To enable the success of solid-state stamp forming of unidirectional fibre reinforced thermoplastics (UD FRTPs), it is essential to accurately characterise and model the material deformation under desired conditions. This paper comprehensively investigates the intra-ply tensile and shear properties of unidirectional carbon fibre reinforced polyamide 6 (UD CF/PA6), which is a type of commonly used UD FRTP. To accomplish this, tensile and V-Notched Rail (VNR) shear tests are conducted for characterising the intra-ply transverse tensile and longitudinal shear properties, respectively. The temperature effects (180 – 220 ℃, at 0.01 /s for the transverse tensile deformation and at 0.04 /s for longitudinal shear deformation) and strain-rate effects (0.001 – 0.25 /s for transverse tensile deformation and 0.004 – 0.4 /s for the longitudinal shear deformation, both are at 200 ℃) are studied. It is found that temperature has significant effects on the intra-ply deformation properties, while the strain-rate effects are marginal. This paper also proposes a new physically based constitutive model considering all the deformable constituents, i.e., the polymer constituent reinforced by fibres (PrF) and the polymer-fibre interface (P-F). This model not only shows good prediction of the thermomechanical properties of UD CF/PA6 under intra-ply deformations, but also gives insights into the deformation mechanisms. The new physically based constitutive model is successfully embedded into Finite Element Analysis (FEA) software and validated through accurate prediction of intra-ply deformation of a CF/PA6 specimen under bias-extension. The methodologies and model developed here offer an effective tool for predicting the intra-ply deformation behaviours and guiding the solid-state stamp forming process of UD FRTPs.