Experimental Study of the Biaxial Cyclic Behavior of Thin-Wall Tubes of NiTi Shape Memory Alloys
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INTRODUCTION
NEAR equiatomic NiTi shape memory alloys (SMAs) have been widely used in the medical field because of their excellent superelasticity and biocompatibility. The self-expanding NiTi endovascular stent is one of the most important practical applications of SMAs. Stents are often made from thin-wall round tubes by the laser cutting method. The thickness of the stent strut is usually extremely small (around 0.1 to 0.2 mm), while the diameter of the stent is often several millimeters.[1] Commercially available vascular stents generally have thickness/radius ratios as low as around 1:20.[2–4] Although there have been extensive studies on the superelastic behavior of NiTi,[5] most of the experimental research was carried out under uniaxial loading.[6–12] A small number of studies have focused on the biaxial mechanical behavior of thin-wall tubes. Lim and McDowell[13] found that the phase transformation of SMAs does not follow the von Mises criteria under nonproportional loading conditions. Helm and Haupt[14] reported strong interaction between the different components of the stress and strain state. McNaney et al.[15] detected significantly different mechanical characteristics in tension and torsion loading directions. They found that the phase transformation stress was higher in torsion and the reduction in the elastic modulus before phase
transformation only appeared in tension. Grabe and Bruhns[16] concluded that superelastic properties are greatly influenced by the loading paths and also detected the insufficiency of the von Mises equivalence for describing the multidimensional material behavior. In our previous work, we found that the special phase transformation exhibition totally disappears in the equivalent stress-strain curves under large-angle out-of-phase nonproportional loading.[17] In the aforementioned references,[13–17] the thickness/ radius ratios of the experiment tubes are around 1:5, except for that of 1:10 in Reference 15, which is much larger than that of the medical stent. In practice, the stent is implemented into the human body and its performance is related to the curing of human diseases. It is worthwhile to investigate its precise mechanical response. Specimens should have dimensions near those of commercial stents in order to understand this complex mechanical response. Therefore, very thin-wall tubes were used in this study to examine the mechanical behavior of NiTi under combined torsion-tension loading. Pure torsion and pure tension experiments were performed for comparison. II.
MATERIAL AND EXPERIMENTAL PROCEDURE
A. Material and Specimen X.M. WANG, Associate Professor, Q.T. ZHOU and H. LIU, Graduate Students, and Z.F. YUE, Professor, are with the School of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, Xi’an, 710129, People’s Republic of China. Contact e-mail: [email protected] C.H. DENG, Senior Engineer, is with the Xi’an Aerospace Propulsion Institute, Xi’an, 710100, People’s Republic of China. Manuscript submitted October 20, 2011. Ar
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