No abstract

The mechanical loading frequency affects the functional properties of shape memory alloys (SMA). Thus, it is necessary to study the effect of frequency in order to use successfully these materials in real structures. Based on the pseudoelastic cyclic behavior, the experimental methodology that allows testing of NiTi wires in stress controlled mode is proposed. Cyclic tensile tests are carried out using universal testing machine STM-100 at room temperature with loading frequencies of 0.1 Hz and 10 Hz. The functional dependencies are determined based on the experimentally obtained hysteresis loops. These functional dependencies comprise dissipated energy and damping factor. It iis found that the increase of loading frequency results in the worsening of functional properties, namely, to the decrease of dissipated energy and damping factor. This is caused by the fact that the regions of austenitic and martensitic transformation under the high loading frequency are absent. That is, the transformation of austenite into martensite does not occur, that stands for the pseudoelasticity effect. Nevertheless, it should be noted that the increase of loading frequency in 100 times augments the lifetime of pseudoelastic wire made of NiTi alloy roughly by 30%. It is determined that the increase of loading frequency results in the decrease of maximum strain in two times in the first loading cycle, and practically in 5 times after 200 cycles of loading.

Shape memory alloys are functional materials characterized by the effect of shape memory and superelasticity. Due to these properties, they are widely used, particularly, in bioengineering, aeronautics, robotics and civil engineering. The temperatures of phase transformations and the influence of external temperature and strain rate on the functional and mechanical characteristics of Ni55.75Ti44.15 shape memory alloy are investigated in this paper. The temperature of alloy phase transformations is obtained by differential scanning calorimetry (DSC) in the temperature range from -70°C to 70°C. Diagrams of differential scanning calorimeters at different heating and cooling rates of Ni55.75Ti44.15 alloy is constructed and analyzed. Samples for mechanical tests are made of round rod 8 mm in diameter. The samples working area is 12.5 mm in length and 4 mm in diameter. Mechanical tests are carried out at temperatures close to the maximum value of the completion temperature of martensitic-austenitic transformation Af = 14.7°C. Diagrams of deformation under uniaxial tension are constructed and stresses of phase transformations, Young's modulus and relative elongations of transformation areas at different loading speeds and exterior temperatures are determined. Using Clausius-Clapeyron formula, it is shown that with simultaneous changes in temperature and strain rate, the stresses of phase transformations are largely due to changes in temperature rather than load rates. The coefficients of Clausius-Clapeyron equation for superelastic Ni55.75Ti44.15 alloy with shape memory, which are consistent with those known in the literature, are determined.