Issue 50

V. Iasnii et alii, Frattura ed Integrità Strutturale, 50 (2019) 310-318; DOI: 10.3221/IGF-ESIS.50.26 312 1 1 / f N N i i i i D W W      (5) where W i is the dissipation energy at i- th loading cycle. So it is important to study the influence of temperature, above the austenite finish temperature, on structural fatigue of pseudoelastic NiTi alloy. E XPERIMENTAL SETUP AND MATERIAL he influence of temperature (at 0°C and 20°C) on structural fatigue was studied on pseudoelastic Ni55.8Ti44.2 alloy. Characteristics of thermal transitions during SMA phase transformations were investigated using Differential Scanning Calorimetry (DSC) by DSC Q1000 TAI [24]. Austenite finish temperature is A f = – 38.7°С. Material has the following mechanical properties at 0°С and 20°С: yield strength,  0.2 = 447 MPa and 523 MPa, ultimate tensile strength,  UTS = 869 MPa and 780 MPa [24, 25]. The chemical composition of the alloy according to the delivered certificate is as follows: 55.78% Ni;0.005% Co; 0.005% Cu; 0.005% Cr; 0.012% Fe; 0.005% Nb; 0.032% C; 0.001% H; 0.04% O; 0.001% N and 44.12% Тi. Cylindrical specimens with a diameter of 4 mm and gage length of 12.5 mm, machined from rod 8 mm in diameter, were tested under uniaxial cyclic loading at temperature 0°С and 20°С at stress ratio R =  min /  max = 0 (here  min and  max are the minimum and maximum stresses) on the servo-hydraulic machine STM-10 [26] with automated control and data acquisition system under sinusoidal load with a frequency of 0.5 Hz. Fatigue tests were carried out under displacement–controlled mode at 0°С. In this case, the maximum stress, except for the first twenty loading cycles, remains constant [24]. Therefore, it can be assumed that the stress range was constant during the testing (the stress range was changed less than 3%). Fatigue tests were carried out under stress–controlled mode at 20°С. Longitudinal strain was measured by Bi-06-308 extensometer produced by Bangalore Integrated System Solutions (BISS), maximum error did not exceed 0.1%. The crosshead displacement was determined by inductive Bi-02-313 sensor with an error not more than 0.1%. The tests at 0°С were carried out in the chamber filled with ice and ice water. This provided the constant temperature of 0°C measured by chromel–alumel thermocouple mounted on the sample with an error not more than 0.5°C. A literature review [24] shows that water have not significant influence on fatigue behaviour of NiTi alloys [21, 27–29] . R ESULTS AND DISCUSSION he dependences of the stress range Δσ on the number of cycles to failure N f for NiTi alloy in ice water at 0°С and at 20°С in the air are shown in Fig. 1. Stress range was determined at the number of half-cycles to failure. Experimental data under low-cycle fatigue presented on Fig. 1, are plotted according to the failure criterion of the specimen, and could be well-enough described by power function f N         (6) The parameters   and   in Eqn. (6), that were determined by fitting of experimental data (Fig. 1), are given in Tab. 1. The increase of testing temperature from 0 to 20°С increases the fatigue lifetime under low-cycle fatigue at N f > 1000 cycles and decreases the angle of relationship between lgΔσ and lg N f . A similar effect of testing temperature (323K, 333K) on the fatigue lifetime was found for Ti 50.7at%Ni alloy [30]. Fig. 2 shows experimental fatigue curves in coordinates strain range versus number of cycles to failure of the specimen. The strain range values were determined at the number of half-cycles to failure, in the same way as stress range. The experimental data were fitted by means of Eqn. (1) with the determined parameters, which are given in Tab. 1. The linear behavior of the dependence of the strain amplitude on the number of cycles to failure under low-cycle fatigue at different ratios between the test temperature and the austenite finish temperature, is confirmed by the results obtained by the authors [31–33]. In contrast to the data presented in Fig. 1, using strain range as a criterion of fatigue failure, fatigue lifetime of pseudoelastic Ni 55.8 Ti 44.2 alloy at 20°С is significantly lower than at 0°С. Moreover, the slope angle of both curves in logarithmic scales is T T