Experimental Study of NiTi Thin-Walled Tubes Under Uniaxial Tension, Torsion, Proportional and Non-Proportional Loadings

The superelastic response of NiTi shape memory alloys under various loading conditions is experimentally investigated using thin-walled tube specimens. The isotropic relation between the elastic and shear moduli of austenite and martensite as well as the von Mises stress and strain equivalency are also studied in uniaxial tension and pure torsion. In addition to proportional loading, several non-proportional loading experiments based on both force and displacement control modes are conducted. Axial and shear stress-strain behaviors as well as effective stress-strain curves during proportional and non-proportional loadings are investigated. Experimental results in proportional loading based on displacement (strain) control indicate that a smooth hardening in transformation regions occurs, while a relatively sharp hardening takes place in force (stress) control. This phenomenon shows that in the stress control, a small stress increment causes a large strain increment while, in the strain control, a small strain increment corresponds to a small stress increment during the transformation. Experimental results in multi-axial loading show that the loading path and the loading control mode affect the mechanical response of NiTi tube. Furthermore, several combined tension-torsion experiments are conducted, and the mechanical coupling between tension and torsion in thin-walled tubes is investigated.