Evolution of phase transformation fronts and associated thermal effects in a NiTi tube under a biaxial stress state

This letter presents results from an experiment involving a pseudoelastic NiTi tube loaded under combined tension and internal pressure. Upon reaching a critical state of stress, the initially austenitic phase (A) transforms to the martensitic phase (M) resulting in macroscopic deformation of the material. The transformation-induced deformation is inhomogeneous, in this case in the form of localized helical bands of M that propagate, eventually consuming the whole specimen. On unloading, the material transforms back to A again in an inhomogeneous manner. The evolution of the transformations is captured using stereo digital image correlation. Because the rate of loading was somewhat higher than optimal, the latent heats of transformation caused a small amount of heating during transformation to M and cooling when the material reverted back to A. Infrared thermography showed heating and cooling fronts that develop to follow the transformation fronts. The local rise in temperature during loading resulted in a nearly linear increase in the transformation stresses and a similar decrease during unloading. The changes in stresses were shown to correlate quite well with the temperature dependence of the transformation stress established in independent experiments. The results illustrate that the effects of latent heats can distort the recorded stress-deformation responses under biaxial stress states.