Material characterisation and modelling of pre-sheared 2D woven CFRP

Carbon or glass fibre woven textiles are often the reinforcement of choice when a part with geometric complexity is required to be formed to shape, due to their good deformation characteristics. In these cases, in-plane shear is the dominant deformation mechanism as the 2D woven textile conforms to the 3D shape [1][2]. As the fabric is formed, the tows rotate away from their default orthogonal axes and become locked in the cured composite component, making the composite stiffer in the direction where the tows come together, and more compliant in the direction where the tows rotate away. These changes are part-specific and can affect its performance, potentially causing the composite part to fail earlier than expected. Therefore, an extensive modelling and experimental campaign was carried out, focusing on the change in material properties due to in-plane shear and representative volume elements (RVE) rotations.
Pre-sheared composite plates with various shear angles were made from twill woven prepreg, where each ply was sheared individually using a picture frame and the assembled stack was then cured in a hot press. Test coupons were cut in multiple directions from the cured plates, and then subjected to tensile tests, where their in-plane strain was recorded using stereo digital image correlation (DIC). Stress-strain graphs were obtained from the tensile tests, from which Young's modulus was extracted. Coupon quality, the achieved shear angles, and loading angles were analysed.
Finite element (FE) models at the RVE level with various shear angles were created based on the realistic fabric geometry from an in-house multi-chain digital element software SimTex [3]. The RVE uses an adaptive grid-based voxel mesh with periodic boundary constraints and is subject to different loads. The voxel elements belonging to a tow have their orientation and material assigned, accounting for tow path and volume fraction of each tow section. Stress and strain were extracted and used to obtain the homogenised material properties in terms of engineering constants, presented in a 360° polar plot.
The results from both the experiment and the FE model are compared and evaluated based on different shear angles and loading directions. The modelling framework provides a satisfactory estimation of the homogenised RVE linear properties and provides the user with a good understanding of 2D woven composite material behaviour accounting for in-plane shear during forming.