Performance of stitched/unstitched woven carbon/epoxy composites under high velocity impact loading

In this paper, response of stitched/unstitched woven fabric carbon/epoxy composite laminates subjected to high velocity impact loading is discussed. Aerospace grade plain and satin weave carbon fabrics were used to manufacture the laminate using a toughened SC-15 epoxy resin system with an affordable vacuum assisted resin infusion molding process. For fabrication of stitched laminates, a 3-cord Kevlar thread was used to stitch the fabric preform in lock stitch fashion in an orthogonal grid of size 12.7 and 25.4 mm with 6 mm stitch spacing. Unstitched laminates used in the study were made of 7, 17 and 37 layers whereas the stitched laminates were made of 7 and 17 layers. Four laminates of each type were subjected to high velocity impact loading at different velocity to determine the ballistic limit. The ensuing damage was characterized through ultrasonic NDE. Results of the study indicate that the damage was well contained within the stitch grid incase of stitched laminates. However, ballistic limit was higher for the unstitched laminates. Ballistic limit increased with the increase in the thickness of the laminate. Further, satin weave laminates exhibited higher ballistic limits in most of the cases.