In-plane shear behaviour of non-crimp fabric laminates by means of 3D finite element analysis

Abstract

In order to characterize the in-plane shear behaviour of NCF laminates, experimental off-axis tensile tests of [+45,-45]2S laminates in different loading directions (parallel to the stitching yarns: warp-SP and normal to the stitching yarns: weft-ST) were carried out. Since the specimens were cut from the same panel, identical results were expected for both directions. However, the experimental tests showed that the shear modulus, the shear strength and the shear strain at failure are higher in the weft direction (ST) than in the warp direction (SP). To identify the cause(s) of such differences, a parametric study has been performed using a mesoscopic scale 3D FE model of the representative unit cell of a [+45,- 45]s NCF laminate. The unit cell is composed by four laminas that have been stacked with the correspondent +45º and -45º orientations. Each lamina contains two half rectangular crosssection tows and resin pockets between them and in the top left and bottom right corners. The stitching yarns and the waviness induced by them in the tows have been added to the unit cell in order to evaluate their impact in the in-plane shear behaviour. The fibre waviness appears in the thickness direction across the length of the unit cell and since it is not normal to the tows it has been modelled according to a new approach. That is, the element’s coordinate system of the finite elements placed in the crimped part of the tow are rotated to take into account for the waviness of the fibres. The study revealed that neither the stitching of the tows nor the out-ofplane waviness of the fibres is responsible for the significant differences found experimentally. On the other hand, the parametric study revealed that a misalignment between the nominal 0º and 90º tows of the NCF panels is what contributes the most for the differences found in the inplane shear performance between SP and ST laminates.