Effect of loading history on phase transition and martensitic detwinning in shape memory alloys: Limitations of current approaches and development of a 1D constitutive model

Numerous constitutive models have been proposed so far to investigate the unique properties of shape memory alloys. In these models, start conditions for transformation as well as martensitic detwinning are assumed to be independent of loading history. In this work, experiments are carried out on SMA wires and tubes to show how loading history affects the transformation and martensitic detwinning start conditions. Based on the experimental findings, enhanced kinetic laws are proposed to consider the effect of loading history on transformation as well as detwinning start conditions. It is shown that these kinetic laws satisfy the principles of thermodynamics as well. The main feature of the proposed kinetic laws is its simplicity in calculating the amount of martensitic volume fractions. The conventional phase diagrams are further generalized to transformation surfaces in order to directly capture the effects of loading history. For a specified loading history, these surfaces in conjunction with geometrical approaches determine the conditions for the beginning of transformations and detwinning. It is shown that the newly-proposed model can successfully reproduce the experimental findings for various incomplete loading-unloading cycles on SMA wires and tubes.