Towards a Scientific Understanding of Fatigue Damage Tolerance in Shape Memory Materials

形状记忆合金可以“记住”其原始形状，并在加热时恢复到其变形前的形状。它们主要用于生物医学应用。它们在温度或应力的作用下发生结构转变。在反复变形或循环载荷下，形状记忆合金会发生机械疲劳。疲劳是金属在循环载荷下的逐渐劣化，导致由于开裂而导致的损坏并最终失效。这在许多应用中会对致动性能产生负面影响。因此，本项目的目的是通过开发一种基本方法来估计疲劳阈值应力强度作为关键材料参数的函数，从而推进此类合金中疲劳进展的科学知识。 本文将讨论材料变量如何影响宏观疲劳参数，如控制裂纹扩展。 这项工作的影响是消除与形状记忆材料中的疲劳阈值确定相关的非线性。建议的建模将揭示理解，可以提前预测疲劳损伤。这项研究将研究最重要的形状记忆材料NiTi，这是不太清楚，然后考虑新的类别的有前途的合金，目前仍然未经测试。拟议中的研究认识到，有必要改革这一领域的教育，努力向高中生推广。提出了一种新的方案，建立一个小型的疲劳试验机使用快速成型方法。学生将在夏令营期间学习如何建造机器，并观察合金丝在疲劳下的失效。形状记忆合金（SMA）的疲劳损伤容限的基本认识尚未建立，尽管这些合金的重要性。缺乏对疲劳的理解阻碍了SMA的广泛使用，并将SMA的使用限制在非常少的组合物中。本计画致力于从原子尺度到微机械尺度的模拟，以发展对形状记忆合金疲劳门槛值的上级了解。这种基于科学的疲劳损伤预测方法是形状记忆社区面临的挑战之一，特别是在血管内支架和潜在应用（如SMA可能经历数千次循环的弹性热量冷却）中的不同应力下。这项工作需要深入了解几个领域-物理疲劳，原子模型和形状记忆行为。该提案将通过从头计算和分子动力学在原子尺度上进行建模，以了解：（i）弹性各向异性（包括立方，四方和单斜晶格）的作用，（ii）位错介导的滑移导致不可逆性，残留马氏体和残留马氏体的作用，以及它们在产生裂纹闭合力中的作用。要考虑的合金，如NiTi和CoNiAl表现出不同的晶格，有序和相变应力作为组成和热处理的函数。为了验证模型在不同的长度尺度，电子显微镜在细观尺度的疲劳实验将被利用。只有这些技术的汇合可以提供所需的洞察力，以推进目前的知识SMA的机械疲劳。本文将通过实验对两种形状记忆合金系统（NiTi和Co-Ni-Al）进行评价。建议的实验考虑非常精细的测量位移裂纹尖端允许确定裂纹前进。同时，重点将是理论发展，以计算裂纹闭合力，由于转换应变，这将占弹性模量张量，各向异性的内部牵引力，和转换和塑性区。该提案计划预测三种不同材料条件下的疲劳裂纹进展：（i）整体奥氏体状态（ii）马氏体状态（去孪晶和自适应情况）和（iii）应力诱导转变（奥氏体到马氏体）条件。对这三种材料条件的研究将提供对SMA疲劳行为的完整理解。提出了一个新的项目，为学生建立一个小型的疲劳试验机，使用快速成型方法和测试SMA线在高速旋转。导丝的曲率决定应变幅度，导丝的旋转产生在拉伸和压缩之间交替的循环载荷，最终导致疲劳失效。学生将在夏令营期间学习如何建造机器，并观察SMA线在疲劳下的失效。

Non-Technical AbstractShape memory alloys can "remember" their original shape and return to their pre-deformed shape when heated. They are utilized mainly in biomedical applications. They undergo a structural transformation under the application of temperature or stress. Under repeated transformations or cyclic loadings, shape memory alloys can undergo mechanical fatigue. Fatigue is the progressive deterioration of metals under cyclic loading, resulting in damage due to cracking and ultimately failure. This can negatively affect the actuation performance in many applications. Therefore, the purpose of this project is to advance the science-based knowledge of fatigue progression in such alloys by developing a fundamental approach to estimate the fatigue threshold stress intensity as a function of key material parameters. The work will address how the material variables affect the macroscopic fatigue parameters such controlling crack advance. The impact of this work is to remove the empiricism associated with fatigue threshold determination in shape memory materials. The proposed modeling will reveal understanding that can advance prediction of fatigue damage. This research will examine the most important shape memory material NiTi which is not well understood, and then consider new classes of promising alloys that currently remain untested. The proposed research recognizes the need for revamping education in this field with outreach efforts to high school students. A novel project is proposed to build a small fatigue machine using rapid prototyping methodology. The students will learn how to build the machine during the summer camp and observe the failure of alloy wires under fatigue. Technical AbstractFundamental understanding of shape memory alloy (SMA) fatigue damage tolerance has not been established despite the significance of these alloys. The lack of understanding of fatigue hinders the widespread use of SMAs and has confined SMA utilization to very few compositions. This project is an effort on modeling from atomistic to micro- mechanical scales to develop a superior understanding of fatigue thresholds in shape memory alloys (SMAs). This science-based methodology of fatigue-damage prediction is one of the challenges that the shape memory community faces especially under varying stress in endovascular stents and potential applications such as elastocaloric cooling where the SMAs could see thousands of cycles. The work requires deep knowledge of several fields- physics of fatigue, atomistic models and shape memory behavior. The proposal will utilize modeling at atomic scale via ab-initio calculations as well as molecular dynamics to understand: (i) the role of elastic anisotropy (including cubic, tetragonal and monoclinic lattices), (ii) the role of dislocation-mediated slip resulting in irreversibility, residual martensite, and residual austenites, and their role in producing crack closure forces. The alloys to be considered such as NiTi and CoNiAl exhibit different crystal lattices, ordering and transformation stresses as a function of composition and heat treatment. To verify the models at various length scales, electron microscopy in meso-scale fatigue experiments will be utilized. Only the confluence of these techniques can provide the needed insight to advance the current knowledge of mechanical fatigue of SMAs. This proposal will evaluate two shape memory alloy systems with experiments (NiTi and Co-Ni-Al with varying compositions). The proposed experiments consider very fine measurements of displacements at crack tips allowing determination of crack advance. Concurrently, the focus will be on theoretical developments to compute the crack closure forces due to transformation strains which will account for the elastic moduli tensors, anisotropy of internal tractions, and transformation and plasticity zones. The proposal plans to predict the fatigue crack advance under three different material conditions: (i) monolithic austenitic state (ii) martensitic state (detwinned and self-accommodated cases) and (iii) stress- induced transformation (austenite to martensite) conditions. The study of these three material conditions will provide a complete spectrum of understanding of SMA fatigue behavior. A novel project is proposed for students to build a small fatigue machine using rapid prototyping methodology and test SMA wires under high-speed rotation. The curvature of the wire dictates the strain amplitude and the rotation of the wire generates cyclic loading that alternates between tension and compression and ultimately results in fatigue failure. The students will learn how to build the machine during the summer camp and observe the failure of SMA wires under fatigue.