Experimental and computational study on phase transformations in superelastic NiTi snake-like spring

Abstract

One of the major drawbacks of mechanically loaded superelastic NiTi structures is their short fatigue life, which is believed to be related to cyclic variation of relatively large strains due to martensitic phase transformation. Thus, determination of spatial and temporal evolution of martensite within such structures may help to avoid critical loading scenarios in practical applications. However, required data are often difficult to obtain and interpret. In this study, we present a possible approach to this issue and illustrate it on a comprehensive study on isothermal stretching of a planar superelastic NiTi snake-like spring, a prospective versatile superelastic actuator element. Specifically, we complement mechanical, electrical resistivity and x-ray diffraction methods with finite element simulations employing a progressive macroscopic constitutive model and reconstruct the spatial distribution in the most strained parts of the spring. We reveal that martensite tends to form in a localized manner there, which affects distribution of phases as well as strains along the spring. Results also exemplify the substantial influence of tension-compression asymmetry of the superelastic response to distribution of phases within cross-section of a bent structure.