Shear and drape behavior of non-crimp fabrics based on stitching geometry

Non-crimp fabrics (NCFs) enable relatively high mechanical properties by keeping the reinforcement tows straight. Handling of those fabrics is improved by stitching the reinforcement tows together. While the stitching has little structural influence on the final part, it highly affects the NCFs capability to shear and drape over a mold during preforming. High tensile strain in the stitching yarn has been correlated to the NCF resistance to shearing and even adverse drape defects such as tow undulations. Furthermore, stitching causes the shear behavior of the fabric to be anisotropic, with different behavior in positive and negative shear. In the current study, a model based on the stitching geometry and reinforcement tow directions has been created to find the tensile strain in the stitching yarn as the NCF is being sheared. It was found that the stitch angle was the main driving stitching parameter, and that this angle could be selected so that shearing in both positive and negative directions produces no tensile strain in the stitching yarn. The model showed good correlation with uniaxial bias-extension and drape tests for NCFs with different stitching parameters. Finally, design charts are displayed which can be used to select the stitching parameters of standard industrial NCF-machines which results in NCFs with near-symmetric shear behavior and thus good shear and draping performance.