Energetics of macroscopic helical domain in different tube geometries and loading

Superelastic NiTi polycrystalline shape memory alloy tubes, when subject to slow quasistatic stretching, transform to a high strain phase by the formation and growth of a macroscopic helix-shaped domain as deformation progresses. This paper performed an experimental study on the effects of the external applied nominal strain and the tube geometry (tube radius R, wall-thickness h and length L) on the helical domains in isothermal stretching of the tubes. The evolution of the macroscopic domains with the applied strain in different tube geometries are quantified by in-situ optical measurement. We demonstrate that the equilibrium shape of the macroscopic helical domain and its evolution are governed by the competition between the domain front energy and the elastic-misfit bending strain energy of the tube system. The former favors a short helical domain, while the latter favors a long slim helical domain. The experimental results provided basic physical and experimental foundations for further modelling and quantification of the macroscopic domain morphology evolution in tube geometries