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

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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.