Effects of stitching parameters on the flexural properties of stitched foam core sandwich composites

One of the primary challenges faced by sandwich composites is facesheet-core debonding, which can be mitigated through various techniques such as z-pinning and stitching through the thickness. This study investigates the impact of stitching on the bending behavior of sandwich composites comprising E-glass composite facesheets and a polyurethane foam core, employing experimental, numerical, and analytical methods. Specimens were stitched at three stitch spacings of 0.5, 1, and 2 cm, with a stitch pitch of 0.8 cm and stitch seam angles of 0 degrees, 90 degrees, 0/90 degrees, +/- 45 degrees, 45 degrees/90 degrees, and +/- 60 degrees. Analysis of facesheet bending stress, core shear stress, and bending rigidity of stitched specimens was conducted through three-point bending tests and compared with unstitched specimens. Results indicate that reducing stitch spacing, thereby increasing stitch density, improves bending strength, and the best bending behavior observed at +/- 45 degrees stitch seam angles. Damage assessment revealed fractures and depression of the foam, wrinkles on the upper facesheet, and buckling failure of resin columns. Additionally, a theoretical model predicted bending rigidity, showing good agreement (4%-15%) with experimental data. Finite element analysis using the ABAQUS program validated the experimental results, suggesting numerical modeling as a viable method for predicting flexural properties of stitched foam core sandwich composites.