Changes in Length During Free End Torsion of Nitinol Rods

Nitinol, a nickel-titanium shape memory alloy (SMA), is emerging as an important and useful alloy. Nitinol possesses unique thermomechanical properties which may manifest in either superelastic or shape memory behavior, depending on the alloy’s composition. In addition to high strain recovery, the material is MRI and bio-compatible and possesses excellent kink resistance. These properties have led to an increasing number of applications in biomedical, aerospace and other industries.

The behavior of Nitinol rods in tension has been well studied. However, there has been a lack of work done regarding the torsional behavior. In this research, the torsional behavior of Nitinol is investigated under different boundary conditions. Torsion deformation is fundamentally more complex than tension deformation because the amount of shear strain that develops over the cross-section of a rod is inhomogeneous. This shear strain gradient causes phase transformation in Nitinol to evolve inhomogeneously through the cross section.

In the case of equiatomic Nitinol, the parent phase, austenite, is an ordered BCC superlattice β-phase (CsCl structure). The product phase, martensite, is a monoclinic distortion of a B19 lattice. During the transformation, there is no diffusion of atoms; only shear lattice distortion takes place. At high temperature, transformation from austenite to martensite can be stress-induced at constant temperature. This is referred to as superelastic behavior.

Due to crystallographic texture evolution, some materials exhibit axial lengthening during free-end torsion. This effect is known as the Swift effect. It is shown experimentally that during free-end, torsion-induced transformation from austenite to martensite, Nitinol undergoes axial lengthening. Outside the transformation region, axial lengthening does not occur during torsion. In fixed end torsion, the Swift effect during transformation creates a biaxial loading condition with torsion and effective compression. The deformation, microstructure and texture development subject to the shear strain are studied.