Fatigue Initiation Resistance in Shape Memory Alloys-Theory and Experiments

了解如何防止材料在原子尺度上形成裂纹，即疲劳，用于航空航天、汽车和国防应用，对于避免可能导致人员生命损失的灾难性结构失效至关重要。材料的疲劳开始于原子尺度，在重复的物理压力（加载）下，导致小裂纹的成核（产生）。裂缝可以在可变载荷下生长，并产生构件和结构的破坏。新的实验工具，如高分辨率显微镜，可以看到导致疲劳现象的原子运动，这些测量结果为微观裂纹形成的过程提供了前所未有的洞察力。这些实验还可以为预测疲劳何时开始的模型提供关键检查。有了更好的理解，人们就可以开发出抗疲劳的新材料。这项工作的重点是一种特殊的金属，称为形状记忆材料，它在载荷下改变形状，在载荷解除后恢复原来的形状。这种现象类似于橡胶在外力释放后拉伸并恢复到原来的形状。与传统的钢和铝合金相比，这些形状记忆材料可能具有更高的抗疲劳性。提出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），最终提高部件和结构的安全性和可靠性。为了加强这一领域的教育，将引入高级设计项目，包括建造疲劳试验机。还将编写一本新的疲劳教科书，涵盖当前测量疲劳的方法和不同的疲劳模型。本工作的智力目标集中在更好地理解疲劳启动行为，从基本的原子尺度到具有代表性的微机械尺度，以确定疲劳抗力和增强寿命的可预测性。为了在不同的长度尺度上验证模型，并降低预测过程中任何不合理的伪影的风险，建议的实验包括沿多个明智选择的区域轴进行单晶力学测试和高分辨率透射电子显微镜。获得的信息将具有3D性质，并将使用模板匹配（TeMA）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缺陷演化的概念以及分子静力学来研究来回（循环）载荷下滑移的能量势垒和各向异性弹性理论，因为已经确定通过在较低长度尺度上控制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可以更深入地了解疲劳下结构演变的过程，并允许直接观察潜在的损伤过程。因此，该提案将通过创建一套新的工具，在改进高性能形状记忆合金的疲劳设计方面产生重大而广泛的影响。早期的研究还没有阐明在长度尺度上处理与疲劳成核有关的过程的实验和理论。这项研究将有能力检测目前尚未测试但具有相当优势的潜在新材料。外联工作包括编写一本疲劳结合理论和实验的教科书，以及一个不同平均应变下钢丝旋转弯曲疲劳启动实验的设计项目。该奖项反映了美国国家科学基金会的法定使命，并通过基金会的智力价值和更广泛的影响审查标准进行了评估，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.