Page 9 - УДК

9

exceed the stress levels at which the martensite transformation completes. With the

stress range growth at N < 20 cycles, the dissipation energy and residual strain increase.

The fatigue life of NiTi alloy increases with the decrease of test temperature

from 20 °С to 0 °С in the case of presenting the results depending on the strain range

and dissipated energy. Nevertheless, in the case of employing the stress range,

Odqvist’s parameter or the total dissipation energy, and the lifetime of NiTi alloy under

the low temperature are less comparing with the room temperature.

It was found that increasing the stress ratio from 0 to 0.5 significantly reduces

the fatigue life of NiTi alloy when used to describe the stress range, strain range and

dissipation energy density and increases when using Odqvist’s parameter. There was

revealed a weak correlation of the NiTi alloy fatigue life at different stress ratio with

the parameter W t in the form of the sum of the dissipation energy density W d and the

elastic energy density W e.

It was found that the transition from high (I) to low block (II) partially recovers

the functional properties of NiTi alloy, which is caused by the inverse transformation

of the residual martensite due to partial reduction of residual stresses.

There was proposed low-cycle fatigue failure criterion (total elastic energy

density) of pseudoelastic nitinol under constant amplitude loading taking into account

the stress ratio and the variable amplitude loading. It was shown that dissipation energy
does not affect the fatigue damage formation and fatigue lifetime of pseudoelastic SMA

in contrast to traditional structural materials,.
There was elaborated the methodology for the lifetime prediction of

pseudoelastic SMA under low-cycle fatigue taking into account the stress ratio and
variable amplitude, which is based on the criterion of fatigue failure (total elastic

energy density) determined under constant amplitude. There was proposed the method

for quick determination of fatigue failure model parameters based on the quasi-static

uniaxial tensile test results under constant amplitude loading.

On the example of stress ratio effect, it was shown that the fatigue crack growth

driving force of nitinol alloy is not stress intensity factor range ΔK, but the maximum

stress intensity factor K max. The obtained phenomenon was associated with the fatigue