Experimental Study on the Superelastic Behaviour of NiTi Auxetic Plates

In this paper, the superelastic behavior of Shape Memory Alloys (SMA) has been experimentally investigated for solid and auxetic geometric plates. Shape Memory Alloys are a category of smart alloys that can recover their original shape and position due to thermal stimuli or mechanical unloading. It happens due to reversible phase transformations between austenite and martensite. Among all SMAs, the Nickel-Titanium (NiTi) alloy exhibits exceptional superelastic property in which it can recover up to 6-8% of its original strain at temperatures higher than its austenite finish temperature (A_f) without any permanent deformation. Auxetic structures are geometries with a negative Poisson's ratio, which enhances stress distribution more evenly.

This work focuses on understanding the combined effect of superelasticity and auxetic geometries on the stress-strain responses. Two auxetic geometries named bow-tie (BT) and longitudinal-grid (LG) are fabricated from NiTi sheets using a laser cutting machine. A universal testing machine (UTM) has been used to perform cyclic tensile experiments under a range of varying strain levels, i.e., (4%, 5%, and 6%) at various strain rates to evaluate the rate-dependent superelastic response. The material's austenite finish temperature (A_f) was determined to be 21°C by the differential scanning calorimetry (DSC), indicating its entirely austenitic behaviour at room temperature. Hence, the tests have been performed at 30°C, 60°C, and 90°C to assess the temperature-dependent variations in transformation stress and recovery behavior of NiTi auxetic structures compared to solid NiTi plates.