SHAPE MEMORY EFFECT BEHAVIOR OF NITI TORQUE TUBES IN TORSION

In order to simulate the torsional behavior of NiTi torque tubes, two different 3D thermo-mechanical constitutive models are utilized. Firstly, an available incremental constitutive model is used in which a return mapping algorithm is implemented to numerically calculate the strains for any applied stresses. Secondly, Microplane theory is employed based on which 1D constitutive laws are considered for associated stress and strain components on any arbitrary plane passing through a material point followed by a homogenization process to generalize the 1D equations to a 3D macroscopic model. Both of the constitutive models are implemented in ABAQUS by developing UMAT. In order to compare the two approaches, torque-angle of rotation and shear stress-shear strain responses for torsion of thin-walled Nitinol torque tubes with different thicknesses are studied. The numerical results of these two approaches show to be in a good agreement indicating the capability of Microplane theory in constitutive modeling of shape memory alloys. This theory provides explicit relationships to calculate strains in terms of stresses, and this makes it very beneficial in obtaining the SMA responses in a fast and easy manner.