Iasnii V. Development of methods for predicting the fatigue life of pseudoelastic shape memory alloys

To solve an important scientific and technical problem, which consists in increase the reliability of structural elements operation devices made of shape memory alloys, results was formulated and substantiated. The results include fatigue failure criteria and methods for predicting their durability, taking into account the influence of stress ratio and variable amplitude loading. The significant decrease in stress of austenitic-martensitic transformation σAM (up to 14%) and the fracture stress (up to 24%) of the SMA NiTi alloy after electrolytic hydrogenation compared to the material in virgin state was observed. Increasing the intensity and duration of hydrogenation can lead to absence of plastic deformation region on the stress-strain curve due to a combination of processes caused by hydrogenation of intensive martensitic transformation and hydrogen embrittlement of martensitic structure. There is a tendency to reorientation of the macrofracture surface from perpendicular, to the axis of specimens, to the screw dislocation line, which is obviously due to the intensification of martensitic transformation in the plane of maximum shear stresses. The basic regularities of the influence of the parameters under variable amplitude loading sequence with two blocks on the functional properties and structural fatigue of the pseudoelastic SMA are established. Residual strain significantly increases with the increase of loading cycles number at stress ratio R= 0,09 – 0,13 due to its proportional increasing on the block I and reaches almost 7% before failure. The transition from high (I) to low (II) block partially restores the functional properties of the NiTi alloy, the residual strain decreases within the block II, which can be caused by a decrease in residual martensite. With the increase of loading cycles’number, the dissipation energy increases at the start and end of block II, and decreases at the start and end of block I at the stress ratio Rσ = 0.09 – 0.13. In contrast to this, the dissipation energy is proportional to the number of loading cycles at the start and end of block I at stress ratio Rσ = 0.33–0.51. The elastic strain energy density, as a criterion of pseudoelastic nitinol fatigue failure under low-cycle loading is substantiated. It is shown that, unlike traditional structural materials, the dissipated energy does not affect the fatigue life of pseudoelastic SMA. A method for predicting the durability of a pseudoelastic SMA under low-cycle fatigue with constant amplitude taking into account the stress ratio and the variable amplitude is developed. This method is based on the fatigue fracture criterion, total elastic energy density determined under constant amplitude. The relative error between calculated and experimental data does not exceed 30.1% and all calculated durability values are within the 2.5 predicted range. The method for quick determination of parameters in the fatigue fracture model based on the criterion of total elastic energy density according to the test results under quasi-static uniaxial tensile loading at constant amplitude.