Effect of preform shape on buckling of quasi-isotropic thermoplastic composite laminates during sheet forming

An important issue in sheet forming of continuous fibre-reinforced thermoplastic composites is the tendency of the laminates to buckle out of plane under rapid forming conditions. This paper outlines a method used to predict the stress patterns responsible for buckling, and presents experimental results that correspond with the predictions. A finite element formulation for ideal fibre-reinforced Newtonian fluids, featuring the twin kinematic constraints of material incompressibility and fibre inextensibility, is used. A mixed penalty finite element approach is adopted, with independent interpolation of tension and velocity solution fields. An analysis model consistent with an assumption of plane stress is used. For multi-ply lay-ups, each ply is analysed individually, and average stress predictions for the laminate are obtained on this basis. A detailed comparison between numerical stress predictions and experimental buckling patterns is presented for central indentation of circular unidirectional, cross-ply and quasi-isotropic preforms. Parameters influencing the magnitude and location of peak tangential stresses include tangential fibre lengths and diaphragm/composite viscosity ratios. The effect of sheet width and shape on the instability patterns is investigated for quasi-isotropic laminates of different shapes, using both numerical and experimental techniques.