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

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In general, the difference of strain range at a different stress range level decreases with the increase of the number of loading cycles. Dependences of dissipation energy on the loading cycle number are given in Fig. 4. A sharp decrease in dissipation energy is observed that passes into the stabilization region both for values of stress ratio and the initial stress range, as well as in the case of strain range during the first 10 – 20 load cycles. As the stress range increases from 492 MPa to 727 MPa, the value of the dissipation energy increases at the initial deformation stage up to 20 cycles. The experimental values of the dissipation energy on the loading cycles number for full unloading and part unloading at N > 20 cycles are in the narrower rangeand does not exceed the martensite finish stress level. Dependences of the dissipated energy on the stress range for both stress ratios could be fitted by power function (Fig. 5): m dis W      (1)

where α = 4  10 -8 and m = 2.703 are the parameters of the material.

Fig. 5. Dependences of the dissipated energy on the stress range at the half-cycles to failure for stress ratio R = 0 and R = 0.5.

Values of dissipated energy and stress range correspond to the half-cycles to failure. The dependence (1) could be used for estimating the dissipated energy for other stress ratio and stress range at low cycle fatigue. 4. Conclusions The results show that the functional fatigue of the NiTi alloy under the stress-controlled cyclic loading is dependent on the stress range and stress ratio. Energy dissipation and strain range are invariant to the loading cycles number for both stress ratio ( R = 0 and 0.5) at N > 20. When stress ratio increases from 0 to 0.5 at the same stress range, the strain range and energy dissipation are decreasing. With the change of the stress ratio from 0 to 0.5 the residual strain in the first and next cycles increases significantly even at lower values of stress range. Regardless of stress ratio, dependences of the dissipated energy on the stress range could be fitted by power function and could be used for estimating the dissipated energy for other stress ratio and stress range under low cycle fatigue.