PSI - Issue 68

N. Bykiv et al. / Procedia Structural Integrity 68 (2025) 405–408

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N. Bykiv et al. / Structural Integrity Procedia 00 (2025) 000–000

Increasing the frequency of negative values of the stress ratio R σ leads to an increase in the sample temperature (Zhang et al., 2017), which results in rapid fracture (Chernenko et al. 2013; Wang et al. 2022; Stachiv et al. 2021). In the range of positive R σ values, SMAs exhibit better fatigue properties due to more uniform strain distribution (Pyndus et al., 2018; Yasnii et al., 2021), reduced microcrack propagation, and lower internal material heating (López 2021; Stachiv et al. 2021; Wang et al. 2022). As most studies focus on the functional properties of SMA under the strain-controlled mode, this paper, in contrast, is devoted to the stress-controlled mode. Cylindrical specimens with a diameter of 4 mm (Fig. 1), made from an 8 mm diameter nickel-titanium rod, were used in the experimental work. Tests were performed on an STM-100 universal testing machine at room temperature, which is higher than the finish temperature of the martensite-austenite transformation A ƒ = 14 °C (Bykiv et al., 2021).

Fig. 1. Drawing of the SMA specimen

The longitudinal deformations and displacements of cylindrical specimens during the test were measured using an extensometer model Bi-06-308 and an inductive sensor Bi-02-313, respectively. The error of no more than 0.1%. For each test, the extensometer was calibrated, and the base was adjusted. The diameter of the working section of the specimen was also measured at least three times. The tests were carried out with stress ratios of 0.1 and 0.5. In addition, the specimens were divided into groups according to the maximum stress σ max = [450-550] MPa (Table ).

Table 1. Detailed testing parameters No. of specimen f , Hz

σ max , MPa

F , kN

R σ

1 2 3 4 5 6

450 500 550 450 500 550

5.51 5.94 6.37 5.51 6.16 6.74

0.1

0.5

0.5

During the test, the Test Builder software recorded force F , displacement S and longitudinal deformations Δ. According to the obtained data the dissipated energy W dis was calculated using formula (1): !"# " = !"# "%& + 12 ( " − "%& )( " + "%& ) (1) where W dis i , W dis i-1 is the dissipated energy of the i th and previous cycles, MJ/m 3 ; ε і , ε і-1 is the relative strain of the i th and previous cycles, mm/mm; σ і , σ і-1 is the stress of the i th and previous cycles, MPa. After 50-100 loading cycles, the hysteresis loops decreased in area for both stress ratios (Fig. 2). Only under the maximum stress range of 550 MPa for both stress ratios (Fig. 2e, f), the shape of the hysteresis loops remains almost constant until the fracture of the specimens. This may be due to exceeding the stress of austenitic-martensitic transformation by 28%. This stress for the studied material was estimated as 428 MPa and has been presented in our previous paper (Bykiv et al., 2021).