Influence of stitching on the fracture of stitched sandwich composites

Abstract Sandwich composites consist of two rigid outer facesheets that are bonded to a lightweight internal core. Although they have a high stiffness-to-weight ratio, these structures have low interlaminar strengths, which may lead to core–to–facesheet separation when subjected to out-of-plane stresses. In this study, sandwich composites with carbon/epoxy facesheets and a 110 kg/m3 foam core, were manufactured with through-the-thickness reinforcements, or stitches, using a vacuum-assisted resin transfer molding process. A design of experiments approach was used to investigate the influence of stitch parameters (stitch density, linear thread density, and facesheet thickness) on fracture energy. Single cantilevered beam tests were performed to obtain the mode I dominant fracture energy for each treatment combination. This study reveals and discusses the unique fracture surface morphologies associated with stitch processing parameters for a sandwich composite. For increased fracture toughness, the optimum stitch parameters for different linear thread densities are captured in the response surface model.