Experimental study of torsion behaviour of NiTi alloy

The goal of this paper is to have a better understanding of the parameters which have an influence on torsional response of NiTi shape memory alloys (SMAs). The SMA investigated in this study was a titanium rich NiTi characterized by high transformation temperature (A(s) of similar to 95 degreesC in fully annealed conditions). Resulting from post-forming annealing, two metallurgical conditions were achieved : restored conditions (470 degreesC-1 h) and recrystallised conditions (600 degreesC-1 h). The phase transformation temperatures and heat of transformation were measured for both conditions by differential scanning calorimetry. A first set of torsional tests were conducted in isothermal conditions for three different mechanical loading paths. The torque against the angular displacement was measured for each tested temperature. As a comparison, isothermal stress-strain curves obtained in tension and compression were reported which showing the influence of the solicitation mode on mechanical parameters (shear and young moduli, yield strength, critical strain and stress). The second set of tests is designed to measure the amplitude of shape memory effect and the recovery torque during heating. Thermomechanical experiments include the torsion response for thermal cycles under constant applied torque and for specimens deformed in martensitic phase and heated above A(f) while their displacement was kept constant. The thermo-mechanical cycles were repeated at least three times for each specimen and for one of them the cycling effect (up to 50 cycles) was investigated. We found that the highest recovery torque and the largest level of strain recovery were obtained for restored conditions.