Numerical Characterization of the In-Plane Shear Behaviour of Non-Crimp Fabric Composites

Experimental off-axis tensile tests aimed at characterising the in-plane shear behaviour of biaxial non-crimp fabric (NCF) composite laminates obtained from different directions within the same panel (warp and weft) reveal noteworthy variations. The in-plane shear modulus, shear strength, and shear strain at failure are significantly higher in the weft direction compared to the warp direction. To gain insights into the underlying reasons for these discrepancies, a parametric study is performed. This numerical study utilises mesoscopic 3D finite element (FE) models, representing the unit cell of a [+45,-45]2S NCF laminate. The analysis indicates that neither the presence of stitching yarns nor the out-of-plane fibre crimp induced by the yarns account for the substantial differences observed in the experimental findings. The maximum initial tangent in-plane shear modulus (Gxy), due to out-of-plane fibre crimp is only 2%. However, this difference increases to approximately 6.1% when stitching yarns are also included in the FE model. Moreover, it is found that fibre volume fraction of the tows and the non-linear behaviour negligibly impact the in-plane shear performance of the NCF laminate in both directions. In contrast, the study reveals that the primary factor contributing to the differences between the warp and weft directions is related to misalignment between the nominal 45° tows within the NCF composite laminate panels. The misalignment observed in the experimentally tested specimens is approximately 6°, leading to an 8.5% decrease in the Gxy of SP laminate (warp direction) compared to ST laminate (weft direction).