Effect of Transformation Toughening in Shape Memory Alloy Reinforced Composites

Shape memory alloy (SMA) reinforced composites are being increasingly investigated by researchers due to their crack closure capabilities and enhanced toughness when compared to monolithic materials.  SMAs are capable of recovering their original shape after an “apparent” plastic deformation due to a reversible thermoelastic martensitic phase transformation.  Therefore, the purpose of this study is to characterize the effect of the SMA reinforcement phase transformation properties on the composite toughening behavior.  J-integral fracture toughness experiments were performed on nickel-titanium (NiTi) SMA embedded epoxy samples in the as-received and heat treated conditions.  Heat treating the NiTi produced a decreased martensite transformation stress.  Additionally, non-transforming aluminum alloy 1100 reinforced and monolithic epoxy samples were examined.  The experiments were performed using a 3-point bend set-up according to ASTM standard D 6068-96.  The results showed that the heat treated NiTi reinforced samples produced the highest fracture toughness when compared to the as-received NiTi reinforced, aluminum reinforced, and monolithic samples.  The fracture toughness behavior suggests that the SMA martensitic phase transformation, in addition to the SMA elastic properties, is responsible for the increased toughness.  The results will lead to a deeper understanding of fracture and deformation mechanics driving toughening behaviors in composites with phase transforming reinforcements.