Large scale simulation of NiTi helical spring actuators under repeated thermomechanical cycles

TLDR: In this article, two different geometric configurations of four and two-coil helical springs subjected to axial end-forces are investigated under the effect of a large number of thermal cycles leading to the saturated deformation state of the coils.

Abstract: As typically utilized in applications, a shape memory alloy (SMA) actuator operates under a large number of thermomechanical cycles, hence the importance of accounting for the cyclic behavior characteristics in modeling and numerical simulation of these actuators. To this end, the present work is focused on the characterization of the cyclic, evolutionary behavior of binary 55NiTi using a newly developed, multi-axial, material-modeling framework and its finite element analysis (FEA) implementation for use in the simulations of SMA actuators. In particular, two different geometric configurations of four- and two-coil helical springs subjected to axial end-forces are investigated under the effect of a large number of thermal cycles leading to the saturated deformation state of the coils. In addition, two different boundary conditions were examined, corresponding to: (a) the loading end cross section assumed to be free-to-twist, and (b) the loading end cross section assumed to be restrained against twist rotation. The study has led to the following five important conclusions: (i) the states of stresses and strains in the coils exhibited marked spatial non-homogeneities, both along the length as well as the cross section of the wires; (ii) the cyclic deformation response of the coils exhibits a similar evolutionary character to that of the 55NiTi material when tested under simple isobaric tensile stress conditions; (iii) the end boundary conditions affect the evolution of the deformation response; (iv) the magnitudes of the evolving nonlinear deformation states (i.e., axial displacements on the martensite and austenite sides, as well as the actuation displacement) were found to be proportional to the number of coils in an essentially linear manner, and (v) the change in coil diameter, while maintaining the pitch height, wire diameter and the number of coils fixed, has a significant effect on the response of the helical spring, both with regard to the resulting stress state and the evolutionary axial displacement behavior during the thermal cycles.