Study of thermomechanical behaviour of nitinol under uniaxial tensile testing

Uniaxial tensile testing is done on Nitinol specimens of three different geometries namely tube, plate and rod. The testing was done at 27 degrees C for plate samples (Af= 18 degrees C), 37 degrees C (Af = 58 degrees C) for rod samples and 0 degrees C, 27 degrees C and 37 degrees C for tube samples (AF = 18 degrees C) using MTS tensile testing machine. Samples were cyclically loaded at varying strain values. Thermo-mechanical properties including primary modulus, secondary modulus, phase transformation stresses, strain recovery, plastic modulus and failure stresses were determined for all the samples and corresponding to all loading histories. National Instruments' Data Acquisition system was used for acquiring data. A special program was designed in LabVIEW to facilitate test monitoring and data logging. Effect of cyclic loading was studied on all the samples. Small strain hardening effect was observed in martensitic samples Similar loading on austenitic samples showed cyclic softening effect. Both strain hardening and cyclic softening were prominent in the martensite transformation region (plateau region).;Influence of testing temperatures on the tube samples was studied. It was observed that stress transformation of martensite became more difficult with the increase in testing temperature above AL Higher values of critical transformation stresses, austenite modulus and plateau module were observed. Little effect was observed in the stress induced martensite region of the stress strain curves. A correlation between maximum cyclic loading strain and corresponding residual strain was studied for austenitic and martensitic tubes. The results obtained from the tests were analyzed and compared to various scholarly studies.