Finite element forming simulation for non-crimp fabrics using a non-orthogonal constitutive equation

The forming behaviour of non-crimp fabric (NCF) was simulated using finite element (FE) analysis incorporating a non-orthogonal constitutive model. NCFs feature asymmetric shear behaviour caused by the stitching used to hold the tows together. This asymmetric shear property causes an asymmetric draping pattern of NCF, even when formed over a symmetrical hemispherical forming tool. Current work focuses on the feasibility of a continuum mechanics model to simulate the asymmetric forming behaviour of NCF. The constitutive equation consists of two parts: the tensile contribution from fibre reinforcement and the shear stiffness. For the fibre directional properties, a non-orthogonal equation originally developed for woven fabric was adopted. The shear stiffness was modelled through a constitutive equation incorporating picture-frame shear data. Both a picture-frame shear test and forming of NCF over a hemisphere tool were simulated by commercial finite element software with the current constitutive model implemented within a user material subroutine. The virtual picture-frame test confirmed the validity of the constitutive equation in simulating planar deformation behaviour of NCF. Furthermore, the numerical analysis of hemispherical forming suggests that increasing blank-holder force decreases the asymmetry of the draped pattern.