Using pre-stacked material that is formed as a first step in the manufacturing process offers reduced process cycle time for production of complex structural components. The forming is achieved by forcing layers to deform by e.g. intraply deformation and interply slippage, where the latter is the scope of this study. The prepreg-prepreg friction is experimentally determined for four unidirectional carbon/epoxy prepreg systems. The materials differ considering volume fraction of fibres, fibre stiffness and phase of thermoplastic toughener (solved or particles). The study shows large individual differences between the tested materials, where the material systems with particle tougheners seem to obey a boundary lubrication friction, while the other materials show hydrodynamically dominated friction. A large difference between the high and low friction materials, almost a factor of 10, correlates to trends seen in the herein performed surface roughness measurements. Vacuum as well as autoclave consolidated materials are tested.

PostprintThis is the accepted version of a paper published in Journal of composite materials. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination. AbstractThe work presented herein aims to investigate the in-plane properties of cross-plied unidirectional prepreg using the bias extension method. The study is focusing on different generations of carbon fibre/epoxy material systems and testing is performed at elevated temperatures to enhance formability. Using digital image correlation (DIC), the specimen deformation and fibre rotation is captured during tests. The study shows that most cross-plied unidirectional material deforms at different characteristic length scales as deformation continues: first seemingly continuous and later in form of bands.Further, the different type of prepreg materials investigated behaves differently; for one type the pin-jointed net theory fit well, enabling simple estimation of resulting fibre angle, while for the another two not. Different loading speed and temperatures are investigated in order to pin-point its influence on the deformability.

Sheet forming of unidirectional prepregs is gaining increased interest as a cost efficient alternative manufacturing method. Its potential lies within the use of automatically and efficiently stacked flat prepregs, which in a second step can be formed. A successful forming requires understanding of the properties of the uncured material. Here, the in-plane deformation behaviour of two different unidirectional thermoset prepregs is investigated. Experimental measurements are performed, showing the importance of stacking sequence and its effect on the forming behaviour of stacked prepreg. Finite element models are developed, using material models calibrated from bias extension tests and interlaminar friction tests. The method developed can be used to predict the reaction force and fibre reorientation during in-plane forming of thermoset prepreg, for one of the considered material systems. Further, it enables prediction of the effect of stacking sequence, which is promising for future full-scale forming simulations.