High-Entropy Materials, Ultra-Strong Molecules, and Nanoelectronics Emerging Capabilities and Research Objectives: Proceedings of a Workshop (2019) / Chapter Skim
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1 Ultra-Strong Molecules
1 Ultra-Strong Molecules
Pages 3-20
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SOURCE: Vikram Deshpande, University of Cambridge, presentation to the workshop.
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Deshpande discussed how ultra-high molecular weight polyethylene fibers and composites perform in a ballistics setting. He noted that ballistic protection is currently one of the largest applications of ultra-high molecular weight polyethylene composites because these materials usually have more than twice the ballistic limit of similarly weighted steel.
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straining, but at high strain rates there is no time for molecular relaxations, and the fibers exhibit a strain rate sensitivity. Deshpande introduced the 1996 hypothesis of Phil Cunniff, Army Research Laboratory, regarding the material metrics that determine the ballistic performance of fibrous systems.
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However, the tapes made by early fabrication processes have a lower ballistic limit than the fibers made from the same molecular weight polyethylene because of their poor molecular alignment. If the fibers with already aligned molecules are rolled to make tapes instead, performance improves.
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Karin Dahmen, University of Illinois, Urbana-Champaign, noted that histograms of the size of the wiggles might provide interesting information. Deshpande agreed that this would be a worthy exercise to gather information about FIGURE 1.4 Penetration pressure increases with decreasing ply thickness.
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Deshpande reiterated two key assertions about the improvement of the bal listic performance of ultra-high molecular weight polyethylene and other high performance fiber composites: (1) the properties of the parent materials need to be enhanced (e.g., create better fibers with higher molecular weights and better alignments or explore other two-dimensional [2D]
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He noted that although carbon fibers used to be made from cellulose, most carbon fibers in use today are made from polyacrylonitrile, which is a synthetic, semicrystalline organic polymer resin that balances tensile strength, compressive strength, and tensile modulus. Kumar noted, however, that polyacrylonitrile-based carbon fiber does not have the highest electrical or thermal conductivity.
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Kumar returned to his discussion on defects with an emphasis on eliminating entanglements to double the strength of the fiber. Polyethylene fibers produced using the gel spinning process have a solution with 5 to 15 percent concentration.
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A workshop participant asked if there are ways to reduce the high costs and the complicated process associated with making carbon fibers, while still achieving the same strength. Kumar remarked that there are three things that contribute nearly equally to the cost of carbon fiber: the cost of raw material, the cost of energy, and the cost of the infrastructure.
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high-stress concentration at the center and low-stress concentration on the edge upon breaking shows that the van der Waals adhesion is enough to avoid using additional clamps on the graphene membrane, thereby eliminating stress at the clamp sites. This is significant given that mechanical testing on a carbon nanotube, another 1D system, traditionally fails at the clamp sites, he explained.
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The next step after measuring the breaking load was to measure the strength from the breaking force. Hone clarified that although it is possible to find linear elasticity formulas for this, such formulas do not usually give good answers: the deduced strength is too high, and the strain limit indicated by finite element modeling is greater than the linear regime.
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14 100X 14 FIGURE 1.6 Strength of materials in relation to density. SOURCES: James Hone, Columbia University, presentation to the workshop; chart from CES EduPack, Granta Design Limited, Cambridge, UK, 2019.
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Current issues of interest for Hone are how strength will scale at the macroscopic level and how statistical modeling of failure at different scales can be useful. His objective is to put grain boundary information from both experiments and models into finite element simulations using the cohesive zone model.
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SCALABLE METAMATERIALS Xiaoyu "Rayne" Zheng, Professor of Mechanical Engineering and Director, Advanced Manufacturing and Metamaterials, Virginia Polytechnic Institute and State University Xiaoyu Zheng introduced his presentation with an overview of the manu facturing capabilities of scalable metamaterials. He noted that the production of 3D architected metamaterials is expanding because of additive manufacturing at both the macro-micro-length and nano-length scales.
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Zheng's group also used fabrication technology to study tensile behavior of bend-stretch lattices, discovering that thicker walls experience brittle failure, while thinner walls combined with multiscale architectures are more ductile and highly
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Cui, J Ye, et al., 2016, Multiscale metallic metamaterials, Nature Materials 15(10)
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Cui, J Ye, et al., 2016, Multiscale metallic metamaterials, Nature Materials 15(10)
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deposit the HEAs into a scaffold and then etch off the polymer to create the hollow tube. Another workshop participant asked if it is possible to mix solid and hollow tubes to allow for different compression and tension characteristics.
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