Instabilities in Shape Memory Alloys and Structures

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One of the most unique properties of shape memory alloys, like NiTi, is pseudoelasticity, which is the ability of the material to be deformed at room temperature to strains of a few percent and return to its unstrained configuration when unloaded. This unique characteristic is derived from a diffusionless transformation between two solid-state phases, which can be induced by changes in either stress or temperature. These characteristics are being exploited for novel structural applications in diverse fields; however, the design of shape memory structures is complicated by the: inhomogeneous deformations associated with the coexistence of the two phases during transformation, thermomechanical interactions, and significant tension-compression asymmetry. This project will use an integrated experimental-computational approach using material and structural samples to unravel these complications to enable the design and wider use of this unique class of materials. This project will progress the science of smart materials; advance the national health, prosperity, and welfare; and secure the national defense by enabling shape memory structural applications, like biomedical devices, aerospace components, damping and energy absorption, microelectromechanical systems, and morphing structures. In addition to the training of MS and PhD graduate students, the project includes undergraduate participation to attract them to research and graduate school. Outreach to K-12 students through lectures and laboratory demonstrations will be used to expand their interest in STEM subjects.Pure bending experiments on NiTi tubes have revealed complex interactions between the prevalent material nonlinearities with structural ones that lead to localization, collapse and failure. The proposed study has three main components: (i) Experiments on NiTi tubes loaded under bending, tension, compression, pressure, and torsion are planned in order to explore the results of these interactions and the associated limit states and provide a basis for developing the necessary analytical tools. (ii) A recently developed constitutive model framework that captures the tension-compression asymmetry and the inhomogeneous deformations associated with tensile stress states will be expanded to include thermal effects. The model will be calibrated using a spectrum of uniaxial, biaxial, and thermomechanical experiments. (iii) The constitutive model will be implemented in finite element models of the structures studied, testing their ability to reproduce the measured structural behaviors and the associated limit states. This multi-pronged approach to this class of problems involving interacting material and structural instabilities should provide the necessary tools for designing shape memory structures. The fundamental nature of this investigation is expected to also impact other materials that exhibit propagating instabilities like L?ders banding in steels, foams, and wood.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.