Influence of 3-D periodic stitching patterns on the strain distributions in polymer matrix composites

This study investigates the effects of periodic stitching patterns on the strain distribution of three-dimensional (3-D) stitched carbon composite structures. Quasi-isotropic composite specimens with three different periodic stitching patterns (0 degrees, 45 degrees, and 90 degrees) were fabricated and tested under uniaxial loading. Surface strain fields were obtained using digital image correlation (DIC). Different strain distributions developed between the front and back of the stitch seams in the stitched specimens. Higher strain concentrations developed in a direction perpendicular to the in-plane stitching except in the case of the 45 degrees stitched specimens, where strain concentrations developed in both the longitudinal and transverse directions. The stitch seam geometry on the front side of the stitched specimens produced a significantly lower Poisson's ratio compared to the back side. The load-displacement behavior and the tensile modulus were altered due to stitching and were found to increase with the stitch angle. This behavior indicates that stitching can give rise to localized zones of altered modulus of elasticity, which can be beneficial in tailoring the mechanical properties of polymer matrix composite structures.