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Increased impact damage resistance of shape memory alloy composites

It is well known that composites, although strong and lightweight, can suffer badly when impacted. This can have catastrophic consequences to a structure. By embedding superelastic shape memory alloys into a composite structure, it is possible to reduce impact damage quite significantly. Superelastic shape memory alloy (SMA) wires absorb a lot of the energy during the impact due to their 'elastic' and hysteretic behavior. The mechanism behind superelasticity is the reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and recoverable strains of up to 8% can be achieved. This paper will report on the results, in which superelastic shape memory alloys were pre-strained to 1.5% and 3% and then embedded into glass fiber/epoxy composite plates. These plates were then impact tested. The effect of embedding wires at different depths of the specimen, different types of wires (martensitic NiTi and stainless steel) and also different volume fractions of wires was also investigated. The results of the impact tests were examined by ultrasonic C-scan to determine the size of the delamination area. The energy absorbed and the maximum impact force were also determined.