Instabilities in Shape Memory Alloys and Structures

形状记忆合金(如NiTi)最独特的特性之一是伪弹性，即材料在室温下变形到几个百分点的应变，然后在卸载时恢复到无应变状态的能力。这一独特的特性源于两个固态相之间的无扩散转变，这种转变可以由应力或温度的变化引起。这些特性正被用于不同领域的新型结构应用；然而，形状记忆结构的设计是复杂的：与相变过程中两相共存相关的不均匀变形、热机械相互作用以及显著的拉伸-压缩不对称性。这个项目将使用一种综合的实验-计算方法，使用材料和结构样本来解开这些复杂的问题，以便能够设计和更广泛地使用这类独特的材料。该项目将促进智能材料科学的发展，促进国家健康、繁荣和福利，并通过使形状记忆结构应用于生物医学设备、航空航天部件、减振和能量吸收、微电子机械系统和变形结构来保障国防安全。除了培养硕士和博士研究生外，该项目还包括本科生的参与，以吸引他们进入研究和研究生院。将通过讲座和实验室演示向K-12学生推广，以扩大他们对STEM科目的兴趣。NiTi管的纯弯曲实验揭示了普遍存在的材料非线性和结构非线性之间的复杂相互作用，这些非线性会导致局部化、坍塌和失效。拟议的研究包括三个主要部分：(I)计划在弯曲、拉伸、压缩、压力和扭转载荷下进行NiTi管的实验，以探索这些相互作用的结果和相关的极限状态，并为开发必要的分析工具提供基础。(2)最近开发的本构模型框架将扩展到包括热效应在内，该模型框架捕获了与拉应力状态相关的拉-压不对称和不均匀变形。该模型将使用一系列单轴、双轴和热机械实验进行校准。(Iii)本构模型将在所研究结构的有限元模型中实现，以测试它们再现测量的结构行为和相关极限状态的能力。这种多管齐下的方法解决这类涉及相互作用的材料和结构不稳定性的问题，应该为设计形状记忆结构提供必要的工具。这项调查的基本性质预计也将影响到其他表现出传播不稳定性的材料，如L？钢、泡沫和木材中的带状材料。这一裁决反映了美国国家科学基金会的法定使命，并已通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.