PSI - Issue 28

V. Iasnii et al. / Procedia Structural Integrity 28 (2020) 1551–1558 Author name / Structural Integrity Procedia 00 (2019) 000–000

1557

7

The comparative dependencies of the total strain energy density Σ W t , the total elastic strain energy Σ W e and the total dissipation energy Σ W dis of NiTi alloy upon the experimental lifetime at R = 0 and 0.5 at temperature 0 °C are presented on Fig. 8. In general, with the increase in the number of loading cycles the relative contribution of dissipation energy to the total strain energy density decrease. In addition, the variation of the experimentally determined total dissipation energy Σ W d far exceeds the variation of the total elastic strain energy Σ W e . It should be also noted that the dependencies of the total strain energy density Σ W t and the total elastic strain energy density Σ W e almost coincide. From the mentioned above fact, it can be concluded that the fatigue failure is controlled by the total elastic strain energy density of Σ W e and to a lesser extent by the energy dissipated density Σ W d .

1,00E+05

1,00E+04

Σ W t , Σ W e, Σ W dis

1,00E+03

1,00E+02

ΣWd, ΣWe, ΣWt,

1,00E+01

Power law (ΣWd) Power law (ΣWe) Power law (ΣWt)

1,00E+00

10

100

1000

10000

N f , cycles

Fig. 8. Dependence of the dissipated energy density on the number of loading cycles at R = 0 and R =0.5. As it can be seen from Fig. 8, the total elastic deformation energy Σ W e can be described by power law function that depends on N f at various stress ratios

q

W N

 



 

e

f

W

e

(10)

The parameters �� � and q of equation (10): q = - 0.9341; �� � = 11.693. The influence of cyclic loading and strain range on the austenite Young’s modulus is mentioned in the papers (Nayan et al. 2008; Phillips, Wheeler, and Lagoudas 2018; Predki et al. 2006). In particular, with the increase of maximum tangential stress at fully reverse torsion of hollow rod made of NiTi with 50.8% of Ni, austenite Young’s modulus is decreasing at the comparative number of loading cycles (Predki et al. 2006). There was obtained the similar regularity of the stress range effect (Δσ = 257; 315; 405 and 450 MPa) on the cross–section austenite Young’s modulus at uniaxial tension for the NiTi alloy of 55.88%Ni at test temperature 22°С ( A f =21.4 °C) (Nayan et al. 2008). The variation of mechanical properties, particularly austenite Young’s modulus, determined on the different specimens can be explained by the differences of texture, transformation temperature and precipitations that can be the result of insignificant changes of chemical composition or/and the thermal treatment technology (Mahtabi et al. 2018). Therefore, the change of austenite Young’s modulus will reflect also on the total elastic strain energy density, that controls the fatigue behaviour of SMA. That is, the Σ W e criterion, to the larger extent, takes into account the individual mechanical properties of specimens The number of cycles to failure, that were calculated by the criterion of the total elastic strain energy density, are presented in Table 2. The error between the calculated ( N f ,p ) lifetime of NiTi shape memory alloy by the total elastic strain energy density Σ W e and total strain energy density Σ W t criterions does not exceed 10%.