PSI - Issue 16

Volodymyr Iasnii et al. / Procedia Structural Integrity 16 (2019) 67–72 Volodymyr Iasnii, Petro Yasniy / Structural Integrity Procedia 00 (2019) 000 – 000

69 3

3. Results and discussion Typical hysteresis loops for loading cycles ( N = 1, 10, 20 cycles) at the same maximum stress and different values of the stress range are presented at Fig. 2. Obtained results show a significant reduction of the hysteresis loop area after first ten cycles and its stabilization after twenty cycles for both stress ratios (Fig. 2).

Fig. 1. Dependences of the stress range on the number of cycles. Δσ 1 = 509 MPa (16), 530 MPa (13), 605 MPa (10), 748 MPa (12) at stress ratio R = 0 (Iasnii et al. (2018)). The functional properties of the pseudoelastic SMA can be characterized by residual strain. The residual strain is regarded as related to some oriented martensite, which is not transformed back into austenite during the reverse phase (Auricchio et al. (2003)). With the increasing of loading cycles, the residual strain grows (Fig. 3a). The increase of the initial stress range from 509 MPa to 605 MPa augments the residual strain that leads to degradation of pseudoelasticity. However, with further increase of the initial stress range to 740 MPa, the dependence of residual strain on the number of loading cycles shifts down to the same dependence for Δσ 1 = 605 MPa. The indicated inversion from the general law is due to the fact that the initial stress range is 8.7% at Δσ 1 = 740 MPa in the first cycle that exceeds the maximum deformation under which the super-elastic effect is still visible.

Fig. 2. Stress – strain curves: maximum stress σ max = 538 MPa and stress ratio R = 0 (left), σ max = 530 MPa and stress ratio R = 0.5 (right).