Elastic constants estimation of stitched NCF CFRP laminates based on a finite element unit-cell model

Usually polymer matrix laminates are made of individual plies with unidirectional fiber reinforcement laid upon each other with varying fiber orientations with respect to the thickness coordinate. Due to missing fiber reinforcement in the thickness direction, this results in poor through-thickness (out-of-plane) properties of composite laminates. One method to enhance the out-of-plane properties is the stitching of dry semi-finished fiber products in the thickness direction prior to the infusion of the resin. On the other hand, the insertion of stitching yarns in the thickness direction may affect the mechanical properties in the laminate plane such as stiffness and strength.To quantify the effect of structural through-thickness reinforcement in non-crimp fabric carbon fiber/epoxy laminates under uniaxial tension and compression loading an experimental study was carried out which included a systematic variation of the yarn diameter, the stitching pattern and direction as well as the load direction. The results showed a significant effect of the stitching on the in-plane Young’s modulus which was attributed to local displacements of the in-plane fibers and changes in the fiber volume fraction.A finite element based unit-cell model was developed to estimate the elastic constants of structurally stitched non-crimp fabric laminates taking into consideration the yarn diameter, the stitching pattern and direction as well as the load direction. Depending on these parameters, local changes of the fiber volume fraction as well as regions with undisturbed and disturbed fiber orientations within the laminate plies are taken into account.