Session: 12-09-01: Instabilities in Solids and Structures

Paper Number: 150662

150662 - Effect of Rate on the Tensile Response and Localized Transformation Patterns in Niti Tubes 

Shape memory alloys undergo phase transformation at room temperature levels by the application of stress. The phase change induces strain of the order of 7% that is fully recoverable upon unloading. The transformation leads to inhomogeneous deformation patterns representing the coexisting “low” and “high” strain phases. The latent heats associated with the reversible transformations locally heat/cool the structure increasing/lowering the transformation stress. 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, which thermodynamically couples the problem [1, 2]. In NiTi tubes, the deformation patterns are helical bands of the alternate phase that propagate along the length and broaden until the whole specimen is transformed. When a tube is stretched at low rates, the heat exchange with the environment keeps the specimen essentially at the ambient temperature, the stress traces a plateau, and the transformation is via a single helical band. By contrast, it will be shown that, at an average strain rate of 10^-2 s^-1 monitored by high speed digital image correlation and infrared imaging, three helical bands nucleate simultaneously at one end. They propagate along the length staying parallel and, on reaching the other end, broaden. At this rate, heat exchange with the environment is limited; the temperature continually increases causing an increasing stress and an apparent “hardening” response (contrast this with the strip behavior in [3]). The presentation will show how a recently developed fully coupled thermomechanical constitutive model, implemented in a 3-D static-displacement transient-temperature finite element analysis, can be used to successfully simulate such higher rate experiments. Unique features of the constitutive model include: modeling the reversible phase transformations of NiTi through a single surface in the deviatoric stress-temperature space with the transformation strain and entropy as the internal variables; and use of softening to model the inhomogeneous deformation associated with the transformations exhibited in tension. The results demonstrate the challenges that such thermomechanical analyses must overcome.

References

[1] Tsimpoukis, S., Kyriakides, S. (2024b). Thermomechanics of phase transformation induced localization in NiTi tubes. Part I experiments. International Journal of Solids and Structures Vol. 300 112905. https://doi.org/10.1016/j.ijsolstr.2024.112905

[2] Tsimpoukis, S., Kyriakides, S., Landis, C.M. (2024). Thermomechanics of phase transformation induced localization in NiTi tubes. Part II constitutive modeling and simulations. International Journal of Solids and Structures Vol. 300, 112906. https://doi.org/10.1016/j.ijsolstr.2024.112906

[3] Tsimpoukis, S., Kyriakides, S. (2024a). Rate Induced Thermomechanical interactions in NiTi tensile tests on strips. Journal of the Mechanics and Physics of Solids Vol. 184, 105530.https://doi.org/10.1016/j.jmps.2023.105530

 

Presenting Author: Solon Tsimpoukis The Universsity of Texas at Austin

Presenting Author Biography: PhD in Engineering Mechanics from the University of Texas at Austin, 2004. Experienced in thermomechanics of Shape Memory Alloys, Experiments and modeling

Authors:

Solon Tsimpoukis The Universsity of Texas at Austin
Stelios Kyriakides The University of Texas at Austin