Response Surface Model for Mechanical Properties of Robotically Stitched Composites

Composite structures are extensively used in several industries such as aerospace, automotive, sports, and construction due to their many advantages, including tailorable mechanical properties, high strength-to-weight ratios, and high specific stiffness. However, due to their low interlaminar tensile and shear strength, composites are prone to delaminations, which can degrade the overall mechanical performance of the structure. Through-thickness stitching provides a third-direction reinforcement to enhance the interlaminar tensile and shear strengths. In this study, quasi-isotropic composite test specimens were manufactured with a novel through-thickness robotic chain stitching with different patterns and tested under uniaxial tensile and three-point bend loadings. A design of experiments (DoE) approach was used to investigate the influence of stitch parameters (stitch density, stitch angle, and linear thread density) on the tensile strength, tensile modulus, and flexural strength of stitched composites. Experimental results are then used to develop a statistically informed response surface model (RSM) to find optimal stitching parameters based on a maximum predicted tensile strength, tensile modulus, and flexural strength. This study reveals and discusses the optimum selection of stitch processing parameters to improve the in-plane and out-of-plane mechanical properties.