Impact failure modes of graphite epoxy composites with embedded superelastic nitinol
Energy absorption during complete penetration of thin graphite 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 were performed on wide clamped-clamped beams to identify progressive damage mechanisms. Low velocity (13.9 ft/s) impact tests, at an impact energy of 31.5 ft-lbs, resulting in complete penetration were also performed on wide clamped-clamped beams. These tests show that only after peak load is there a contribution made by the SMA to the load deflection behavior of the composite. Owing to the SMA's high strength and high strain to failure it remains undamaged after failure of the base composite. The interaction between the base composite and the SMA creates an increase in absorbed energy over the base composite of as much as 41 % in a Graphite/Bismaleimide laminate embedded with a 6% volume fraction of nitinol fibers. C-scans of the hybrids embedded with bi-directional nitinol fibers show a 22% larger delamination areas compared to plain graphite epoxy. The larger delaminations are a result of the nitinol fibers distributing the impact energy to a larger area of the base composite. This interaction between the nitinol and the graphite is one of the reasons for the increases in absorbed energy. Fiber pull-out and strain energy of the nitinol fibers also adds to the increase in absorbed energy.
Although damage initiation and peak loads do not seem to be affected by the embedded nitinol fibers, the energy absorption after peak loads is greatly improved. This improvement is a result of increased energy distribution through the SMA to the graphite. The large improvements in energy absorbing capabilities offered by SMA fibers give SMA hybrid material systems promise in applications where penetration resistance is imperative.