Improving the structural integrity of foam-core sandwich composites using continuous carbon fiber stitching

This study aims to improve the structural integrity of foam-core sandwich structures by embedding carbon-fiber/epoxy stitched reinforcements with fiber-volume fraction (Vf) as high as 55%. Sandwich panels with 1.5 mm thick carbon fiber skins and 30 mm thick PVC foam-core were stitched with 0, 12k and 24k carbon fiber yarns. X-ray computed tomography (CT) was utilized to determine the "as-manufactured" quality of the stitched panels. Flatwise-, edgewise-compression and 3-point bending tests were conducted with in-situ 3D Digital Image Correlation (DIC) to record the deformation and analyze the structure's mechanical response. It was observed that compared to non-stitched samples - under flatwise compression loading, the reinforced samples with a stitch Vf of 0.34 and 0.55 showed an increase in stiffness and strength by 130 % and 163%, and 29% and 53 %, respectively. For edgewise-compression and 3-point bending loading cases, stiffness and strength of stitched samples showed an increase of ∼20 % over the benchmark. Using DIC, X-ray CT and a fractography analysis, the external and internal damage as well as failure mechanisms were evaluated. Analytical solution was employed to understand the parametric influence of the stitch Vf and stitch density per unit area in pursuit of an optimized sandwich structure.