Experimental study of pseudoelastic NiTi alloy under cyclic loading

Abstract

The phase transformation temperatures of pseudoelastic NiTi alloy were defined by differential scanning calorimetry. The effect of the stress range on the functional properties of the NiTi alloy under uniaxial tension in ice water at a temperature of 0 0 С was studied. The cylindrical specimens with 4 mm in diameter and a gage length of 12.5 mm were tested under static and cyclic loading with frequency of 0.5 Hz. All cyclic tests were performed under the crosshead displacement controlled condition on the STM100 machine. At the temperature above Af the effect of the cyclic loading on the maximum stress in general could be characterized by several stages: strengthening, softening, stabilization and rapid decrease of the maximum stress, which is caused by the initiation and macrocrack growth. With the increase of the maximal stress in the first cycle from 509 MPa to 605 MPa, the strain range also increases. Statement of the problem. Shape memory alloys (SMA) belong to functional materials characterized by the shape memory effect and pseudoelasticity. Their application depends on the temperature of phase transformations, mechanical and functional properties, the type of load (static, cyclic and thermomechanical). Due to the high ability to dissipation energy the SMA with the effect of pseudoelasticity are increasingly used in parts of machines, implants [1], damping devices [2 − 4] or other structuralelements [5]. Since they are subjected to intense cyclic loadings during operation, it is important to ensure their reliability and durability for low-cycle fatigue. Therefore, in order to construct the structural elements and devices made of SMA it is necessary to study the regularities of changing stress and strain based parameters, which characterize functional properties.. Analysis of available investigation results. It is known that with the increase of load cycles number, the SMA functional properties (hyperelasticity effect) which can be characterized by the strain range, residual strain [6] get worse. The effect of maximum stress, mean stress and stress amplitude on SMA cyclic behavior is investigated. It is determined that the maximum strain depends in general on the stress level and load type. Particularly, residual and transformation hardening increase with the growth of loading cycles number and strain rates from 3,3 10-4 s-1 до 3,3 10-2 s-1 in super-elastic NiTi SMA micro-tube (50.32% Ni) specimens in 2.5 mm in diameter during strain-controlled testing [7].