Issue 50

V. Iasnii et alii, Frattura ed Integrità Strutturale, 50 (2019) 310-318; DOI: 10.3221/IGF-ESIS.50.26 313 approximately the same (parameter β in Tab. 1). Such effect of temperature on low-cycle fatigue lifetime are similar for NiTi wire of 0.5 mm in diameter and NiTi tube with outer diameter of 0.9 mm and inner 0.7 mm under rotating-bending fatigue test at temperatures 20°C - 80°C [21]. Fatigue lifetime increase with the decrease in temperature from 383 K to 293 K for 3 kinds of Ti–Ni base shape memory alloy wires with the compositions of Ti– 50.0 at%Ni, Ti–50.5 at%Ni and 50.85 at%Ni, respectively under rotary bending fatigue tests [19]. This was observed for wires with diameter of 1.0 mm. Figure 1 : Dependence of the stress range on the number of loading cycles in ice water at 0°С and at 20°С in the air. The fatigue life was estimated using the Odqvist’s parameter, which characterizes the accumulated plastic strain  p , and under uniaxial cyclic loading is determined by formula [34] 2 p N      (7) where N is the numbers of loading cycles. T, °С   R 2     R 2 A B R 2 Eq. (1) Eq. (6) Eq. (9) 0 8.754 ±1.339 0.14 ±0.026 0.868 943.7 ±53.84 0.0814 ±0.00898 0.933 2.472 0.0581 0.941 20 5.379 ±1.634 0.198 ±0.05 0.944 788.6 ±10.56 0.0396 ±0.00195 0.997 3.018 0.0205 0.999 Table 1 : Equations parameters for Ni55.8Ti44.2 alloy Replacing N in Eqn. (7) on N f and taking into account that for the SMA the plastic strain range can be replaced by the expansion of the elastic deformation Δε, the formula (7) can be rewritten as follows: 2 N      (8) In the Eqn. (8), the strain range  was determined in the same way (at N =0.5 N f ) as in the previous cases. According to the Fig. 3, the Odqvist's parameter increases linearly proportional to the number of loading cycles before the failure of the specimen and is well described by the dependence f f A B N     (9)