Rate induced thermomechanical interactions in NiTi tensile tests on strips

The paper uses tensile experiments on NiTi strips at different displacement rates to establish and simulate the thermomechanical interactions caused by the latent heat of the reversible transformation between the austenitic and martensitic phases. The evolution of deformation in the specimen is synchronously monitored with digital image correlation, and the temperature field through infrared imaging, essential for structural modeling. Transformation leads to localized deformation that propagates through the specimen, while the latent heat released/absorbed at the propagating fronts locally heats/cools the specimen. The sensitivity of the transformation stress to temperature results in a complex interaction between the heat transfer conditions and the nucleation and evolution of transformation in the specimen. At low rates of loading, the alternate phase propagates nearly isothermally with a small number of fronts producing relatively flat stress plateaus. Higher rates lead to significant heating/cooling that results in progressive nucleation of multiple fronts and apparent "hardening" responses. The experiments are simulated in a three-dimensional static displacement transient temperature finite element analysis, using a new fully coupled thermomechanical constitutive model. Transformation strain and entropy are its internal variables whose evolution is governed by the motion in the stress -temperature space of a single transformation surface governing both transformations. The prevailing localization is captured by the introduction of softening over the unstable branches of the recorded isothermal material response. The results demonstrate how the important role of the thermal interaction between the specimen and the environment can be addressed. This, together with appropriate calibration of the constitutive and structural model, enable the analysis to reproduce the effect of rate on the recorded response, the evolution of localization patterns, and the associated thermal fields. The results can guide the development of constitutive and structural models of phase transforming materials with strong thermomechanical interactions.