Effect of tension/compression asymmetry and anisotropy on the response of pseudoelastic NiTi tubes under equibiaxial stress

The stress-strain response and inhomogeneous deformation of pseudoelastic NiTi tubes loaded under combined axial force and internal pressure are governed not only by the well-known tension/compression asymmetry but also anisotropy (Bechle and Kyriakides, 2016). This behavior was simulated successfully in Kazinakis et al. (2022) using a finite element analysis coupled to a recently developed constitutive model that incorporates these key material characteristics. Interestingly, equibiaxial tension induces monotonic stress-strain hystereses and nearly homogeneous deformations, but with the hoop transformation strain being approximately double the axial one. This letter uses analysis to highlight how the tension/compression asymmetry and anisotropy individually influence the predictions of this experiment. When both asymmetry and anisotropy are included, the analysis captures the level of transformations stresses and the extent of axial strain, overpredicting the hoop strain by a small amount. The hardening is somewhat reduced compared to the experiment allowing weak and diffuse inhomogeneous deformation to develop. In the absence of anisotropy, the hoop and axial hystereses coincide with much higher stresses, and strains that lie between the experimental values, while the tube deforms uniformly. If the model is assumed symmetric with the properties of the hardening uniaxial compression, the behavior is the same as that of the previous case. When the model is calibrated instead to the partially unstable uniaxial tensile response, the analysis predicts again coincident stress-average strain hystereses with lower stress plateaus than measured and strongly inhomogeneous deformation. (c) 2022 Elsevier Ltd. All rights reserved.