CAREER: Understanding the Interaction between Mechanical Twinning and Fatigue Crack Growth

该学院早期职业发展(CAREAGE)项目将研究亚微米微结构特征--机械孪晶对提高金属疲劳裂纹扩展抗力的影响。循环机械载荷导致裂纹的形成，称为疲劳，仍然是在役结构金属的常见危险。机械孪生是一种有效的本质强化和提高金属塑性的方法，但由于在典型的低载荷高周疲劳过程中没有形成孪晶，其对裂纹扩展的影响尚不清楚。从这项研究中获得的见解将促进先进的抗疲劳材料的设计，支持积极的设计策略，以提高能源和材料效率。与新材料开发所需的成本和时间投资相比，智能地修改已建立的合格金属的微观结构将加速增强型抗断裂材料的部署。除了所取得的技术进步，该奖项还将为K-12、本科生和研究生提供教育经验。研究生和本科生将密切参与综合材料科学和机械工程研究。将把教育单元纳入K-12课堂和女孩夏令营的外展活动中，以提高学生的科学自我效能。此外，还将为几乎没有社会资本的学生举办研讨会，如第一代学生和代表性不足的学生，帮助他们导航并申请奖学金和研究机会。研究表明，退火孪晶界提高了滑移的可逆性，特别是在纳米晶合金中，但尚不清楚机械孪晶束如何影响疲劳裂纹扩展。由于在高周疲劳过程中施加的局部化性质和低于屈服应力，通常不满足形核孪晶的临界孪生应力。这一职业项目的研究将量化第一阶段高周疲劳裂纹扩展过程中预填充形变孪晶界滑移不可逆性的大小，以加强对基于微观组织的裂纹扩展阻力的理解。裂纹-微结构相互作用的实验量化将通过高分辨率数字图像相关和电子显微镜进行测量。关于孪晶界、孪晶密度和位错密度作用的知识将有助于将原子模拟收集的信息连接到已建立的中尺度框架，以支持抗疲劳材料设计。该项目由土木、机械和制造创新部门(CMMI)和已建立的刺激竞争研究计划(EPSCoR)联合资助。该奖项反映了NSF的法定使命，并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。

This Faculty Early Career Development (CAREER) project will investigate the influence of a submicron microstructural feature called mechanical twin on enhancing the fatigue crack growth resistance in metals. Cyclic mechanical loading causing crack formation, known as fatigue, remains a common danger to structural metals in service. Mechanical twinning is an efficient means for intrinsically strengthening and increasing ductility in metals, but its effect on crack growth remains unclear due to its lack of formation during the typical low loads of high cycle fatigue. Insight gained from this study will promote the design of advanced, fatigue resistant materials supporting aggressive design strategies to improve energy and material efficiency. Intelligently modifying the microstructure of established, qualified metals will lead to accelerated deployment of enhanced fracture resistant materials in comparison to the cost and time investment required for new material development. In addition to the technical advances made, this award will also provide educational experiences for K-12, undergraduate, and graduate students. Graduate and undergraduate students will be intimately involved in integrated material science and mechanical engineering research. Educational modules will be incorporated into outreach for K-12 classrooms and girls’ summer camps to increase students’ science self-efficacy. Additionally, workshops will be held for students with little social capital, such as first generation and underrepresented students, to help them navigate and apply for fellowship and research opportunities.Fatigue crack growth, particularly in Stage I, is highly dependent on dislocation-microstructure interactions. Research has shown that annealing twin boundaries enhance the slip reversibility, especially in nanocrystalline alloys, but it is unclear how bundles of mechanical twins affect fatigue crack growth. Due to the localized nature and lower than yield stresses applied during high cycle fatigue, the critical resolved twinning stress to nucleate twins is typically not met. The research in this CAREER project will quantify the magnitude of slip irreversibility at pre-populated deformation twin boundaries during Stage I, high cycle fatigue crack growth to enhance the understanding of microstructure-based crack growth resistance. Experimental quantification of crack-microstructure interactions will be measured through high resolution digital image correlation and electron microscopy. Knowledge generated about the role of twin boundaries, twin density, and dislocation density will serve to connect information gathered from atomistic simulations to established mesoscale frameworks to support fatigue resistant material design.This project is jointly funded by the Division of Civil, Mechanical and Manufacturing Innovation (CMMI) and the Established Program to Stimulate Competitive Research (EPSCoR).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.