Torsional Behavior of Shape Memory Alloy Tubes for Biomedical Applications

Shape memory alloys have been implemented in various applications throughout aerospace, biomedical, and various other fields. Some currently investigated applications of shape memory alloys include biomedical stents, rotary blade actuators, and pedicle screws for securing osteoporotic vertebrae. There have been many models developed by previous authors that discuss the reactions of this alloy in tension, compression, and one dimensional bending. It was the goal of this investigation to expand upon previous works to model the behavior of a shape memory alloy tube in pure torsion. The motivation for this investigation was the biomedical applications of this alloy. Because of its distinct properties of superelasticity and shape memory effect, the alloy is ideal for the design and implementation of medical devices that are smaller and more efficient than previous methods. The goal of this investigation was to model the properties of an SMA tube in torsion for use in an active orthoses for the treatment neuromuscular disorders and various other biomedical devices which involve the implementation of nitinol tubes in torsion. To this end, a model using COMSOL Multiphysics was developed which, in turn, will aid in predicting the behavior of an SMA tube under a torsional load and allow for calculation of an optimal torque tube for our applications. The outputs gained from this model were the angle of deformation under an applied torque, the resultant torque upon unloading of the tube, and the stress-strain relationships. From this information, different tube geometries will be experimentally tested to determine the best design for implementation. Once the analytical forces were determined from the SMA tubes using the COMSOL model, these results were then compared to experimental values from previous works to evaluate the accuracy of the model for the desired conditions.