Valentina Imbeni, R.O. Ritchie

Shape-memory alloys exhibit strongly nonlinear thermomechanical response associated with stress or temperature-induced transformations of their crystalline structure. These reversible transformations lead to the special properties of superelasticityand shape memory.
Nitinol, a nearly equiatomic NiTi alloy is one of very few alloys that are both superelastic and biocompatible; moreover, the temperature range within which Nitinol superelasticity is exhibited includes human body temperature (Duerig, T.W., Tolomeo, D.E., Wholey, M., 2000. An overview of superelastic stent design. Minimally Invasive Ther. Allied Tech. 9, 235–246).
As a result, Nitinol is now widely used in biomedical devices such as endovascular stents, vena cava .filters, dental .files, archwires and guidewires for non invasive surgery, etc.
NiTi has been widely adopted in the medical community by the drive to use less and less invasive procedures in order to reduce unnecessary patient trauma. These procedures require instruments and devices that can pass through very small openings and then 'elastically' spring back into the desired shapes.
Although the use of Nitinol for such prosthetic devices has been generally very successful, there have been distinct problems with certain components due to the difficulty of being able to accurately compute stresses, particularly in the presence of complex in vivo loading (e.g., tension plus torsion) patterns. The root of the problem is that there are no reliable constitutive laws available for such phase-transforming materials which take into account loadings other than simple tension and compression.
Most of the mechanical testing on polycrystalline Nitinol found in the literature has been performed on wires and is thus one-dimensional. As a result, most phenomenological constitutive models are based on uniaxial data, oftentimes extended to three-dimensions in an ad hoc fashion. It is our goal to study and understand the macroscopic and microscopic aspects of the transformation brought about by in vivo multiaxial loading which will aid in the design of biomedical devices with improved reliability and safety
Our present work focuses on the biaxial testing (tension-torsion) of thin-walled tubes chosen to minimize the gradient in the torsional strain along the radial direction.
In order to monitor the martensitic phase transformation associated with superelasticity, we have performed in situ multiaxial (tension-torsion) loading experiments at the Stanford Synchrotron Radiation Laboratory (http://www-ssrl.slac.stanford.edu/) to monitor the material’s state of transformation, both from the perspective of identifying the nature and volume fraction of phases present as a function of stress and stress state.
Material orientation (texture) has a profound effect on the mechanical response of the Nitinol tubes therefore texture studies have also been conducted on NiTi tubes and plates.