Thermo-Mechanical Experiments and Modeling of Shape Memory Alloy Structural Elements

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Intellectual ImpactThe proposed research consists of experimental and modeling work to characterize the thermomechanical behavior of shape memory alloy (SMA) structural elements. NiTi SMAs possess unique thermomechanical properties and are often deployed in the form of thin wires or tubes (for example, orthodontic wire and arterial stents) that are loaded in bending, torsion, or some combination. However, the number of such experiments available in the literature is relatively few. The PI's recent experimental work on NiTi SMAs, using optical and infrared imaging and a new temperature control scheme, has helped to clarify issues arising from self-heating/cooling, mechanical instabilities, strain and temperature localization, and phase front propagation. These techniques will be extended in new experimental setups to probe the structural behavior of SMA elements under bending/torsion and column buckling. Special attention will be paid material-level and structural-level instabilities and to interactions between deformation and temperature fields arising from phase transformations. These experiments will be used to develop a thermomechanical model and simulation tool for SMA space rods that capture the important thermomechanical coupling and instabilities in a computationally tractable way. The PI's existing thermodynamic constitutive model will be adapted for a new finite element that is endowed with axial, bending, and torsional degrees of freedom. This will be implemented in a finite element framework where the mechanical equilibrium and heat equations are solved simultaneously, coupled with kinetic laws for internal field variables.Broader ImpactsThe hope is that the outcome of the proposed research will enable better use of SMA materials in commercial, engineering, and biomedical applications. The experimental data and simulation tools will be widely disseminated by conference presentations and journal publications. A better understanding of the material behavior and its subtleties and availability of simulation tools should help engineers determine which applications can make appropriate use of the material's remarkable behavior and to assess their performance. The intent of the educational portion of this proposal is to increase both undergraduate and graduate student's awareness of these non-classical materials. Interesting research projects for adaptive aerospace structures that use the unique capabilities of SMAs will be offered to foster undergraduate involvement. Much of the proposed research will be conducted with individually supervised graduate students. In addition, theoretical and modeling issues associated with such materials will be addressed by introducing a new advanced graduate course in solid mechanics entitled, "Constitutive Modeling of Complex Materials".