Buckling and collapse of pseudoelastic NiTi tubes under bending

Abstract

Pseudoelastic NiTi structural components have been shown to exhibit tension/compression asymmetry. Under tension, phase transformation induces localized strains of nearly 7% that spread under nearly constant stress, while under compression the transformation leads to hardening at significantly higher stress, the strain is smaller and grows homogeneously. Bending of tubes ovalizes their cross-section, a nonlinearity that reduces the bending rigidity and precipitates instabilities that can lead to collapse. This paper uses experiment and analysis to examine the interaction of these geometric nonlinearities with the complex material behavior of pseudoelastic NiTi tubes of moderate diameter-to-thickness ratios under bending. An experiment on a tube with a diameter-to-thickness ratio of 18.7 shows that transformation causes the nucleation of several dispersed high strain banded patterns of martensite on the tensioned side. The zone where the bands coalesce into intersecting diamonds undergoes excessive local ovalization causing buckling and collapse at a relatively low overall tube curvature. The bending experiment is simulated with a finite element model coupled with a custom constitutive model of pseudoelastic NiTi that allows for plastic deformation of the martensitic phase. The analysis reproduces the moment-end rotation response, and the nucleation and evolution of the high strain diamond shaped patterns. Zones that develop such patterns exhibit accelerated growth of ovalization that degrades the local bending rigidity. In the presence of small geometric imperfections this degradation evolves into a buckle in the form of diffuse local ovalization that precipitates collapse that can negate the recoverable property of NiTi. Parametric studies show that as the tube D/t increases, the interaction between the material and geometric nonlinearities becomes stronger making buckling more imperfection sensitive.