Impact failure modes of thin graphite epoxy composites embedded with superelastic nitinol

Energy absorption during complete penetration of thin graphite/epoxy composites is experimentally shown to be significantly improved by low volume fractions of embedded superelastic shape memory alloy (SMA) fibers. Graphite/Bismaleimide laminates were embedded with 3% and 6% volume fractions of superelastic nitinol fibers. Quasi-static tests as well as low velocity (13.9 ft/s) impact tests, at an impact energy of 31.5 ft-lbs, resulting in complete penetration were performed on wide clamped-clamped beams. Interaction between the base composite and the SMA creates an increase in absorbed energy over that of the base composite of as much as 41%. C-scans of the bi-directional SMA hybrids show a 22% larger delamination area compared to plain graphite epoxy. Although damage initiation and peak loads do not seem to be affected by the nitinol fibers, the energy absorption after peak loads is greatly increased. It is believed that the cause of the increase in absorbed energy is the SMA distributing the impact load to a greater volume of graphite. These results indicate the potential of SMA to improve the ballistic protection currently offered in tough composite materials used in personal armor.