Bending of fibre-reinforced thermoplastic sheets

When forming continuous fibre-reinforced thermoplastic (CFRT) sheets into three-dimensional components, interply shearing may be necessary in order to accommodate the out-of-plane bending because the fibres severely constrain the deformation along the fibre directions within their planes. Furthermore, thermoforming takes place at elevated temperatures so that the molten matrix polymer becomes fluid. These two factors are of prime importance in analysing any forming process with thermoplastic composite materials. This paper examines the process of forming unidirectional Plytron® (a glass fibre-reinforced polypropylene composite, originally developed by ICI, UK) sheets into V-bends at a constant elevated temperature, and compares the experimental results with those predicted by an analytical model for plane strain bending of an incompressible Newtonian fluid reinforced with a single family of inextensible fibres. The shape of a strip as it is formed, the effects of temperature and forming speed on the forming loads are also investigated. A major conclusion from this study is that Plytron sheets demonstrate a viscoelastic response when formed within their melting range and the degree of elasticity is increased by reducing the temperature, which, in turn, can reduce the fibre instability. The theoretical model provides useful results for evaluating the effective longitudinal viscosity of the composite sheet, the effects of forming speed and punch geometry on the bending stresses and also highlights the limitations of a Newtonian fluid model in comparison with the actual material response.