Effective Stitch Stiffness in Delaminated Composites

The effective stiffness of through-the-thickness Kevlar stitches in delaminated carbon/epoxy composites was determined under out-of-plane tensile and interlaminar shear loading. Stitched composite panels were fabricated with Teflon sheets serving as delaminations. Test methods were developed to produce out-of-plane tension as well as interlaminar shear loading on the stitched delaminations. Test results indicate that the effective tensile and shear stiffness of the Kevlar stitches decrease with increasing load. These stiffness decreases are believed to be associated with localized debonding of the Kevlar stitches in the vicinity of the delaminated interfaces. Examination of failed specimens suggests that the Kevlar stitches remain bonded to the surrounding carbon/epoxy composite throughout a majority of their length.