Prediction of out-of-plane behavior of stitched sandwich structure

Abstract This article provides an improvement of the technology of through-the-thickness reinforcement of thermosetting composite sandwiches by the insertion of stitching yarns. The aim of this work is to propose a model that would lead to the representation of the tri-dimensional elastic and ultimate performances of these 3D structures under out-of-plane shearing and compressive stress. To reach that goal, an analytical approach is proposed without the contribution of the numerical finite element technique. From experimental observations, this complex structure can be considered as a simple lattice made up of the stitches treated as bars composed of the UD impregnated yarns. The transverse compression and the out-of-plane shear module can be determined from the geometrical parameters of the structure. Moreover, the ultimate stress can be approximated once the cinematic of the failure of the structure has been experimentally identified for each stress case. Finally, the buckling of stitches ends up in the ruin of the material in each case. Thus, a classical theory of buckling can be used and a particular work has been done on the representation of the anchorage point of the stitches on the skin. The final model which is endowed with a good sensitivity enables us to predict the ultimate values in both loading cases. Finally, the variation of stitching angle allows to virtually optimize the material used according to the external loading.