A finite element forming model for bi-axial fabrics considering material bending stiffness

A macroscale finite element (FE) forming model was developed to capture the effect of fabric wrinkling, by incorporating the effects of bending stiffness. The fibre-orientation dependency for bending stiffness was addressed using a non-orthogonal constitutive framework previously developed for tracking fibre orientations of bi-axial fabric materials. A method was developed to calculate the variation of bending stiffness with curvature along the fibre direction. The bending behaviour of a non-crimp fabric (NCF) with pillar stitches was characterised by a cantilever bend test, providing input data for the material model. Simulations were performed to replicate the bias-extension behaviour of the NCF material, showing good agreement with experimental data. Wrinkles were initiated within the central area of the specimen where pure shear was assumed, at an extension of 7.5mm. Once stitches started to fail, the amplitude of the wrinkle decreased due to the release of constraints. Results indicate that the stitch-imposed constraints significantly affect the formation of out-of-plane wrinkling defects in NCF. This suggests that the critical shear angle (locking angle) can be used as a criterion for the evaluation of macro wrinkling defects, and also implies that the local removal of stitches could be a potential way to eliminate forming-induced wrinkling defects for NCF.