Instabilities during Tension-Torsion of Superelastic NiTi Tubes
John Shaw (University of Michigan), Benjamin Reedlunn (Sandia National Laboratories), Samantha Daly (University of Michigan)
Instability in Solids and Structures
Mon 9:00 - 10:30
Barus-Holley 190
The tensile response of as-received, cold-drawn superelastic NiTi is known to exhibit transformation-induced instabilities that lead to localization and propagation of phase transformation fronts, phenomena that have been studied extensively in wires. As shown by Q. P. Sun and co-workers, the morphology of phase transformation fronts in NiTi tubes under tension is more complex, where strain localization occurs initially as a narrow, one to two-turn helix, which under continued stretching may lengthen and/or broaden, but eventually tends to evolve to nominally ring-like fronts having fine angled fingers. Here, we performed axial-twist experiments on superelastic NiTi tubes with stereo digital image correlation (DIC) to characterize transformation morphologies during more general multi-axial loading. Mechanical responses were studied for various controlled axial stretch-twist histories, including radial paths and non-proportional paths, spanning pure tension to simple torsion to pure compression. Interestingly, strain localization is observed in all tension-twist experiments, even in the simple twist experiment, but not in any experiments that involved compression. As the amount of twist/axial extension was increased, the nominal front angle progressively changed from nearly circumferential (ring-like in pure extension) to nearly longitudinal bands (in simple twist). Additionally, we characterized the transformation surfaces across the axial-twist space, quantifying both the onset and saturation of stress-induced transformation for the first time. Each transformation surface was well captured by a smooth (three-parameter) elliptical surface in both strain and stress space, which should provide useful data for constitutive modeling.