Non-Symmetric Tension-Compression Behaviour of NiTi Alloy

Abstract

Development of tensorial constitutive equations suitable to model the thermomechanical behaviour of shape memory alloys (SMA) would greatly help engineering design of sophisticated components. While a number of studies have focused on mechanical behaviour under tensile loading, only a few have been done to characterise properties under other stress states. The aim of our study is to report some new experimental results obtained on a equiatomic NiTi. The experimental study was carried out using both uniaxial tension and compression tests which were performed with the same form for the specimens (sheet samples of gauge length 40 mm and cross section 5.6 mmx2.7 mm) in order to avoid any geometrical effect. This choice allows to submit the material to the same prior thermomechanical treatment, i.e. a cold rolling leading to a thickness reduction of 18% followed by an annealing at 430°C for 1/2 hour. The temperature of the experiments were achieved using a silicon oil bath. A special device was designed in order to avoid buckling during compression. Preliminary tests were performed in order to study the effect of this device which was concluded as negligeable. The results show that tension and compression behaviours are not symmetric, especially for superelastic behaviour. Moreover, the amplitude of the one-way memory effect and the slope of the linear variation of the transformation stress with the temperature are compared for these two stress states.