Study On The Effect Of Processing Methods On The Recovery Stress Of NiTi Shape Memory Alloy

The ring pre-stress concrete structure of Shape Memory Alloy was put forward on the basis of the shape memory effect, according to the principle of axial compressive strength of concrete increases with the increase of other biaxial stresses.In this paper, the recovery stress of NiTi SMA wires'has been tested on the BZ2216 Digital Force Measurement Apparatus after refit, and its'change law with the increase and decrease of temperature has been studied, after being prestrained on the Hydraulic Universal Testing Machine, using the self-made fixture on the principle of interference coordinating. Meanwhile, the effect of temperature, incentive modes and incentive times, as well as prestrain level and tensile test ways on NiTi SMA wires'have been discussed.It is shown that, the recovery stress of NiTi SMA increases with the increase of temperature, when over the softening point, the alloy becomes soft and the stress decreases; The higher the prestrain level, the greater the recovery stress, but when over 8%, it decreases instead; Compared with being incentived with high current intensity, under low current intensity, the amount of austenite became from martensite is much more, the stress is greater; After being incentived many times, there is still recovery stress; Under the same prestrain level, the recovery stress of prestrained NiTi SMA via cold drawing is slightly greater than that via cold drawn. After about 80℃, the latter is much greater than the former.Within the temperature of 100℃, the maximum recovery stress was got by 6% prestrained NiTi SMAs via cold drawn, which may reach about 350MPa. Thus 6% prestrained NiTi SMAs can be the most suitable used in the ring pre-stress concrete structure. As SMAs'recovery stress decreases after the electricity powered off, there is only some residual stress when it is cooling to room temperature, sometimes it is even none residual stress left, finally in the special structure, the SMAs can only play the role as ordinary hoops. To make the structure realize the ring pre-stress, we need to incentive the SMA wires continuously or incentive it again and again.