Forming of stacked unidirectional prepreg materials

Abstract: To reduce cost of structural composites, the development of more efficient manufacturing methods is of great interest. An automatic tape layer (ATL) can be used to perform the layup in an efficient way for flat sheets and the second step is by pressure forming the prepreg stack onto a given mould. Sheet forming of thermoset prepreg, also known as hot drape forming, is a promising manufacturing method when combined with automatically stacked laminates. To reach the desired shape, without flaws such as wrinkles and severe fibre angle deviation, knowledge about the forming behaviour of stacked material is important. A simulation of the forming can add to the understanding of mechanisms causing defects and also how to avoid them.A continuum mechanical finite element approach is used to model the forming of stacked unidirectional prepreg. This with the aim of finding mechanisms causing defects, such as out-of-plane wrinkling and in-plane waviness. Data from experimental characterisation of the materials are required for the constitutive material models. An experimental approach is used to investigate the intraply (within the layer) and interply (between the layers) deformation. The intraply deformation behaviour is investigated by a bias extension test, where resistance to deform and fibre rotation are registered. The interlaminar friction characteristics in the prepreg/prepreg interface are retrieved from tests performed with a specially designed rig. Measurements on the bending stiffness of the prepreg, both transverse to and in the fibre direction, are used in the out-of-plane model.Different aerospace-grade thermoset prepreg materials are tested and major differences in behaviour are found. Since epoxy is brittle, the resin is toughened by thermoplastics, for some materials in liquid phase and for others by solid particles. These particles seem to influence both the intraply deformation modes and increase the level of friction between layers.The experimental data from shear, friction and bending tests are used to calibrate the chosen material models. In-plane forming simulations confirm the affect stacking sequence have on the forming behaviour. Full-scale forming simulations on a joggles beam are performed with one of the tested material systems. Two different stacking sequences, with the same amount of fibre in the main directions, are analysed and compared. Both numerical and experimental forming shows that the coupling between layers, due to the interply friction, will affect the forming behaviour and in some cases create wrinkles. Wrinkles can develop both in the plane and out-of-plane, where only the in-plane waviness is present in the performed forming simulations.