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Functional behavior of pseudoelastic niti alloy under variable amplitude loading

Iasnii Volodymyr, Yasniy Petro, Lapusta Yuri, Yasniy Oleg, Dyvdyk Oleksandr

Acta Mechanica et Automatica

2020

Vol. 14, no. 3

154--160

Cyclic loading of superelastic NiTi shape memory alloy (SMA) causes forward and reverse austenite–martensіte transfor-mations, and also increases the volume of stabilized martensite. This appears in the change of stress-strain curve form, the decrease of dissipation energy, and increase of residual strain, that is, named transformation ratcheting. In real structures, the SMA components in most cases are under the action of variable amplitude loading. Therefore, it is obvious that the loading history will influence the functional fatigue. In the present work, the effect of stress ratio on the functional properties of superelastic NiTi shape memory alloy under variable amplitude loading sequence with two blocks was investigated. The studies were carried out under the uniaxial tension of cy-lindrical specimens under load-full unload and load-part unload. The change of residual strain, strain range, dissipation, and cumulative dissipation energy density of NiTi alloy related to load sequences are discussed. Under both stress ratios, the residual strain in NiTi alloy is increased depending on the number of loading cycles on the high loading block that is similar to the tests at constant stress or strain amplitude. An unusual effect of NiTi alloy residual strain reduction with the number cycles is found at a lower block loading. There was revealed the effect of residual strain reduction of NiTi alloy on the number of loading cycles on the lower amplitude block. The amount of decrement of the residual strain during a low loading block is approximately equal to the reversible part of the residual strain due to the stabilized martensite. The decrease of the residual strain during the low loading block is approximately equal to the reversible part of residual strain due to the stabilized martensite. A good correlation of the effective Young’s modulus for both load blocks with residual strain, which is a measure of the volume of irreversible martensite, is observed.

Degradation of functional properties of pseudoelastic niti alloy under cyclic loading: an experimental study

Iasnii Volodymyr, Yasniy Petro

Acta Mechanica et Automatica

2019

Vol. 13, no. 2

95--100

The influence of the cyclic loading on the functional properties of NiTi was studied. Cylindrical specimens with a diameter of 4 mm and a gage length of 12.5 mm were tested under uniaxial cyclic loading with control crosshead displacement at a temperature of 0°С. The dependences of the stress and strain range as well as dissipation energy on the number of loading cycles at different initial stress range were analysed. During the first 10 loading cycles, a rapid decrease in the strain range and energy dissipation was observed. Dissipation energy was invariant to the loading cycles’ number at N > 20 cycles and to the stress range that did not exceed the martensite finish stress level, was within the same scatter band and can be described by the single dependence. With the stress range growth at N < 20 cycles from 509 to 740 MPa, the value of dissipation energy increases and that of relative dissipation energy decreases. Loss coefficient, which characterises material damping ability, significantly decreases during the first 10 loading cycles and remains practically unchanged up to the failure of the specimens. At the stabilisation area, the loss coefficient is almost non-sensitive towards the stress range.