GOALI/Collaborative Research: Understanding Multiscale Mechanics of Cyclic Bending under Tension to Improve Elongation-to-Fracture of Hexagonal Metals

交通运输业减少化石燃料消耗的各种战略的核心是减少结构重量，通常称为“轻量化”。某些金属，如钛和镁，具有被称为六方闭包（HCP）的晶体结构，这有助于提高强度-重量比。然而，HCP金属通常不具备在室温下形成所需形状所需的延展性。与加热材料不同的是，该学术联络研究项目（GOALI）将实施一种名为“连续拉伸弯曲”（CBT）的新型增量成形工艺，并对其进行表征和建模。该项目的目标是将HCP金属在室温下的成形性提高一倍。通过与GOALI合作伙伴波音公司合作，该团队将解决对工业具有直接价值的成型问题，同时实现航空航天结构的轻量化。此外，建模和材料表征工具将被封装在开源软件中，供整个科学界免费使用。参与研究的学生将通过实习机会获得对行业挑战的知识和理解。该项目的一个重要组成部分将是发起一个名为“顶点连接”（Capstone Connect）的外展计划。将专门为高中学生创建一个在线论坛，以便他们在处理最后一年的顶点项目时与学术和行业专家联系。学生不仅会对工程设计项目有更深入的了解，而且这些互动将启发他们未来的STEM职业生涯。例如，虽然已经证明了通过CBT提高钢材的失效延伸率（ETF）的能力，但在HCP金属中的应用仍然有限。此外，为了优化CBT工艺条件，并将潜在的想法转化为实际的成形操作，需要更深入地了解延展性提高背后的机制。本项目将利用高分辨率数字图像相关（HRDIC）和高分辨率电子背散射衍射（HREBSD）来观察局部滑移活动、应变梯度、位错重排、子结构发育和相关背应力，这些因素在CBT期间ETF的显著增加中发挥作用。实验活动将有助于在关键机制长度尺度上为新的非局部晶体塑性有限元（CPFE）模型提供信息和验证，使人们能够理解CBT中的力学，从而提高HCP金属的ETF。这种结合实验和建模的努力将为CBT提供前所未有的见解，并且项目的实际成功将通过与波音公司形成领先的钛组件来证明。该奖项反映了美国国家科学基金会的法定使命，并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。

At the core of various strategies to reduce consumption of fossil fuels in the transportation industry is the goal to reduce structural weight, generally termed ‘lightweighting’. Certain metals, such as titanium and magnesium, have crystal structures known as hexagonal closed-packed (HCP), which contribute to superior strength-to-weight ratios. However, HCP metals often do not have the required ductility to form them into the desired shapes at room temperature. Instead of heating the material, with the accompanying expense, this Grant Opportunities for Academic Liaison with Industry (GOALI) research project will implement, characterize, and model a novel incremental forming process called ‘continuous bending under tension’ (CBT). The goal of the project is to double the formability of HCP metals at room temperature. By working with GOALI partner Boeing, the team will solve forming problems that are of immediate value to industry while enabling the lightweighting of aerospace structures. Furthermore, the modeling and materials characterization tools will be encapsulated in open-source software for free access to the entire scientific community. The students involved in the research will gain knowledge and understanding of industrial challenges through internship opportunities. An essential part of the project will be the instigation of an outreach program called Capstone Connect. An online forum will be created specifically for senior high-school students to connect with academic and industrial specialists as they tackle their final year Capstone projects. Not only will students gain deeper insights into engineering design projects, but the interactions will enlighten them concerning future STEM careers. While the ability to increase elongation-to-failure (ETF) in steels, for example, via CBT has been demonstrated, application to HCP metals has been limited. Furthermore, a deeper understanding of the mechanics behind the improved ductility is required to both optimize CBT process conditions and to transfer the underlying ideas into practical forming operations. This project will utilize high resolution digital image correlation (HRDIC) and high-resolution electron backscatter diffraction (HREBSD) to observe local slip activity, strain gradients, dislocation rearrangement, substructure development and associated back stresses that play a role in the remarkable increase in ETF during CBT. The experimental campaign will serve to inform and validate a novel non-local crystal plasticity finite element (CPFE) model at the critical mechanism length-scale, enabling understanding of mechanics in CBT to improve ETF of HCP metals. This combined experimental and modeling effort will provide unprecedented insights into CBT, and the practical success of the project will be demonstrated via the forming of a leading-edge titanium component with Boeing.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.