Research On The Mechanical Behavior Of NiTi Sma Over A Wide Range Of Strain Rates

Shape-memory alloys (SMA) that exhibit unique properties such as shape memory effect and superelasticity, are now being used in variety of commercial applications including coupling, actuator, and medical devices. These new applications require accurate knowledge of the response of shape-memory alloys under various working conditions. These requirements lead to a better understanding of alloy properties as a function of stress state, strain rate and temperature, and establishing of one dimension and three dimension constitutive relations. The Ni-Ti alloys are considered the most favorable shape-memory alloy materials for various applications, and peoper have paied more attention to this material. The mechanical behavior of Ni-Ti shape-memory alloys in the (σ, ε, ε, T)-space under complex working conditions continue to be studied.In order to understand the mechanical behavior of NiTi shape-memory alloy (SMA) at various temperatures, stress states and strain rates, a series tests are performed. The dynamic tensile and compressive response of a NiTi shape-memory alloy are investigated at various strain rates, using a modified split Hopkinson bar. The quasi-static uniaxial compression on SMA samples are performed, using an Instron servohydraulic testing machine. Strain rates from 10-2/s to 104/s are achieved at initial temperatures from the range of 291K to 573K. The dynamic and static deformation behavior of Ni-Ti shape-memory alloys, in relation to different substructures, is discussed for different stress states and temperatures and a few noteworthy conclusions are as follows:(1) The resulting temperature rise of deformation at a strain rate of 10-2/s can reach about 6.5 °C, so the rate effect on mechanical behavior of SMA at an adiabatic loading of higher strain-rate actually should take account as combination effects of the rate and temperature;(2) The phase yield limit and dislocation yield limit change with the strain rate, but the stress-plateau is disappear when strain rate reach 104/s;(3) The phase yield limit and dislocation yield limit of compression are higher than that of tention, and the phase work-harding rate is also higher than that of tension;(4) The phase yield limit increases with the test temperature increasing;(5) The reverse transformation doesn't occur during high strain rate deformation even though the temperature exceeds the As and Af of the undeformed specimen.