Issue 50

V. Iasnii et alii, Frattura ed Integrità Strutturale, 50 (2019) 310-318; DOI: 10.3221/IGF-ESIS.50.26 316 dis W W f W A B N    (13) The parameters (Tab. 2) of the Eqn. (13) were determined by the approximation of the experimental data using the least squares method. Therefore (Fig. 5), the total dissipation energy is not constant, but increases with the increasing number of loading cycles to specimen failure. Figure 5 : Dependence of the total dissipation energy on the number of loading cycles in ice water at 0°С and at 20°С in the air. The fatigue life of NiTi alloy increases with the decrease in test temperature when using the strain range (Fig. 2), as well as dissipated energy (Fig. 4). C ONCLUSIONS 1. Low-cycle fatigue of pseudoelastic Ni55.8Ti44.2 shape memory alloy was studied under the uniaxial tensile deformation at temperature of 0°С and 20°С, which is above the austenite finish temperature ( A f = – 38.7°С). Fatigue tests were carried out on cylindrical specimens under displacement–controlled mode at 0°С and under stress–controlled mode at 20°С. The fatigue life was described by stress, strain and energy failure criteria. 2. The fatigue life of NiTi alloy increases with the decrease of test temperature from 20°С to 0°С in the case of presenting the results depending on the strain range and dissipated energy. Nevertheless, in the case of employing the stress range, the Odqvist’s parameter or the total dissipation energy, and the lifetime of NiTi alloy under the low temperature in the medium of distilled ice water is less comparing with the room temperature. 3. Regardless the test temperature, with the increase of number of cycles to failure, the stress and strain ranges, as well as the dissipation energy decrease, and the total dissipation energy and Odqvist’s parameter increase. 4. Therefore, as in the case of traditional structural elements, the total dissipation energy of the low-cycle fatigue is not constant and increases proportionally to the increase of cycles number to failure. It is obvious, that if the energy of fatigue failure is constant, then the dissipation energy is wasted not only for the formation of fatigue damage, but also on the heating of specimen, as well as on the forward and reverse phase transformations during the cyclic loading. 5. The slope of the fatigue lifetime curves of NiTi alloy to the OX axis is greater at the temperature of 0°С in comparison with the 20°С in the case of employing the stress range, strain range, Odqvist’s parameter and total dissipation energy as the failure criteria, and is less while employing the dissipation energy as the failure criterion.