Fatigue Initiation Resistance in Shape Memory Alloys-Theory and Experiments

了解如何防止在航空航天、汽车和国防应用中使用的材料在原子尺度上形成裂纹（称为疲劳），对于避免可能导致人命损失的灾难性结构故障至关重要。材料的疲劳在重复的物理压力（负载）下以原子尺度开始，并导致小裂纹的成核（诞生）。裂纹在可变载荷下会扩展，并产生部件和结构故障。新的实验工具，如高分辨率显微镜，允许可视化的原子运动，负责疲劳现象，这些测量提供了前所未有的洞察过程，导致微观裂纹形成的开始。这些实验还可以对旨在预测疲劳何时开始的模型进行关键检查。有了更好的理解，人们可以开发出耐疲劳的新材料。这项工作的重点是一类特殊的金属，称为形状记忆材料，在负载时改变其形状，并在移除负载时恢复其原始形状。这种现象类似于橡胶在释放所施加的力时如何拉伸并恢复其原始形状。 与传统的钢和铝合金相比，这些形状记忆材料可以潜在地表现出更高的抗疲劳性。拟议的工作将促进对成核机制的理解，提高疲劳寿命，量化与底层微观结构相关的随机性（结果的可变性），最终提高组件和结构的安全性和可靠性。为了加强这一领域的教育，将引入涉及建造疲劳试验机的高级设计项目。一个新的教科书疲劳，包括目前的方法测量疲劳和疲劳的不同模型也将produced.Technical SummaryThe智力这项工作的目的中心更好地了解疲劳起始行为从基本原子到代表性的微观力学尺度，使确定的疲劳抗力和寿命的可预测性增强。为了验证各种长度尺度下的模型并减轻预测程序中任何不必要的伪影的风险，拟议的实验包括单晶力学测试和沿沿着多个明智选择的区域轴的高分辨率透射电子显微镜。所获得的信息将具有3D性质，并将使用模板匹配（特马）和几何相位分析（GPA）方法进行分析，进一步开发先进的算法。建模工作将结合Frank-Bilby的缺陷演化概念，沿着与分子静力学，以研究在往复（循环）载荷下滑动的能量障碍和各向异性弹性理论，因为它已经确定，通过控制较低长度尺度下的这些特性，可以实现材料抗疲劳性能的增强。因此，利用这种独特的技术组合提供了更深入的了解负责疲劳下的结构演变的过程，并允许直接观察的基础损伤过程。因此，该提案将通过创建一套新的工具，在改善高性能形状记忆合金的疲劳设计方面产生更广泛的影响。早期的研究还没有阐明的实验和理论解决过程中的长度尺度是相关的疲劳成核。这项研究将有能力检查目前尚未测试但有相当大优势的潜在新材料。该奖项反映了NSF的法定使命，通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical SummaryUnderstanding of how to prevent the formation of cracks at the atomic scale of materials, known as fatigue, that are used in aerospace, automotive, and defense applications is paramount to avoid catastrophic structural failure that could result in loss of human life. Fatigue of a material begins at the atomic scale under repeated physical pressure (loading) and result in nucleation (birth) of small cracks. The cracks can grow under variable loads and produce component and structural failures. New experimental tools, such as high resolution microscopy, allows visualization of the atomic motions that are responsible for fatigue phenomenon, and these measurements provide unprecedented insight into processes that result in the beginning of microscopic crack formation. These experiments can also provide a critical check on the models aimed at predicting when fatigue starts. With better understanding, one can develop new materials that withstand fatigue. The focus of the work is on a special class of metals, called shape memory materials, that change their shape upon load and recover their original shape upon removal of the load. This phenomenon is similar to how rubber stretches and returns to its original shape upon release of the applied force. These shape memory materials can potentially exhibit higher fatigue resistance compared to conventional steels and aluminum alloys. The proposed work will advance understanding of the mechanism of nucleation and improve fatigue lifetimes, quantifying stochasticity (variability in the results) linked to the underlying microstructure, ultimately improving the safety and reliability of components and structures. To enhance education in this field, senior design projects that involves building a fatigue test machine will be introduced. A new textbook on fatigue that covers current methods of measuring fatigue and the different models of fatigue will also be produced.Technical SummaryThe intellectual aims of this work centers on a better understanding of fatigue initiation behavior from fundamental atomistic to representative micro-mechanical scales to enable determination of fatigue resistance and enhanced predictability of lifetime. To verify the models at various length scales and mitigate risks of any unwarranted artefacts in the predictive procedures, the proposed experiments include single-crystal mechanical tests and High Resolution Transmission Electron Microscopy along multiple judiciously chosen zone axes. Information obtained will be of a 3D nature and will be analyzed using Template Matching (TeMA) and Geometric Phase Analysis (GPA) methods, further developing advanced algorithms for the same. The modeling efforts will incorporate Frank-Bilby concepts for defect evolution along with Molecular Statics to study the energy barriers and Anisotropic Elasticity Theory for slip under to- and fro-(cyclic) loading, as it is established that material performance enhancement for fatigue resistance can be achieved by controlling such characteristics at lower length scales. Thus, utilizing such a unique combination of techniques provides greater insight into the processes responsible for the structural evolution under fatigue and allows for the direct observation of the underlying damage processes. Therefore, the proposal will create significant broader impacts in terms of improving fatigue design for high performance shape memory alloys by creating a new set of tools. Early studies have not elucidated the experiments and theory addressing processes at length scales that are relevant to fatigue nucleation. This research will have the capability to examine potentially new materials that currently remain untested but promise considerable advantages. The outreach efforts include preparation of a textbook on fatigue combining theory and experiments, and a design project for fatigue initiation experiments under rotary-bending of wires subject to different mean strainsThis 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.