Collaborative Research: Emergent Mechanics of Randomly Packed Elastic Filaments

当红衣主教建造她标志性的杯巢时，她用自己的身体作为模板，将细树枝、草束和树皮条塑造成一个结构，尽管它很柔软，但它能可靠地保持形状，免受各种机械干扰。这种自然选择的工程解决方案是几何形状，弹性和摩擦之间微妙相互作用的结果，尽管其在建筑，包装，自我修复，减震和材料可重复使用性方面的潜力尚未被表征或建模。“鸟巢”，如果定义为一个随机包装的细长，弹性元素，是一种不寻常的材料：它是没有吸引力的相互作用的凝聚力;它是集体软和塑料，而其元素是硬和弹性。 通过协调的物理和计算实验，这个合作项目将通过将理想化的“嵌套系统”的整体力学性能与组分的性质和几何形状的变化相关联来推进软颗粒材料的科学。结果将产生新的知识，在颗粒物理学，并将呼吁新兴的偶然的建筑和工程范例。事实上，通过非永久性接触和设计具有规定机械性能的轻质材料来建造的能力贯穿了许多当前非常重要的领域：土木工程和建筑（可靠，廉价，可重复使用和自我修复的材料），运输（轻质复合材料，减震器），先进制造。这符合提高产业竞争力的国家需要，从而促进国家健康，繁荣和福利;并确保国防。该项目还承诺通过在相关的主角（鸟类）和通常无法进入的物理学和工程学领域之间建立一座不寻常的桥梁来捕捉广大观众的想象力。此外，将在个别机构开展STEM外联活动，以分别吸引中学生和女生学习科学和工程。本科生也将被提供在任何一组的职位，接触到先进的工程研究。随着纵横比的增加，无序颗粒填料的力学行为的变化。当施加的应力分布在球体的一维接触链中时，细长体引入弯矩和长程相互作用。非永久性摩擦接触使该系统与半柔性聚合物网络和其他非织造材料不同，后者从永久性交联中获得机械响应。从无序的，随机包装，弹性纤维或长丝为基础的结构，如鸟巢，实验证据表明，这些材料系统表现出频率依赖性的弹塑性行为，有限的拉伸响应，和增强的比强度。 在缺乏理论框架和严格适用的统计力学原则的情况下，本项目将开发一个用于这些材料的基准测试和物理表征的实验平台。 这些将由高保真度的计算对应物来补充，以指导对新的机械状态和转变的搜索。该项目将深入了解鸟巢状系统的宏观和微观力学之间的关系，为新型材料的规范设计铺平道路。 此外，该奖项还将激发颗粒物理理论的新方向，并从自然选择的工程结构中解释功能机制。该奖项反映了NSF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。

When a cardinal builds her iconic cup-nest, she uses her own body as template and molds thin twigs, grass strands, and bark strips into a structure that, despite its softness, reliably holds its shape against various mechanical perturbations. This naturally-selected engineering solution is the result of a subtle interplay between geometry, elasticity, and friction that has not yet been characterized or modeled despite its potential in building, packaging, self-repairing, shock-absorption, and material reusability. The 'bird nest', if defined as a random packing of slender, elastic elements, is an unusual material: it is cohesive without attractive interactions; it is collectively soft and plastic while its elements are hard and elastic. Through coordinated physical and computational experiments, this collaborative project will advance the science of soft granular materials by relating bulk mechanical properties of idealized 'nest systems' with variations in constituents' properties and geometry. Results will generate new knowledge in granular physics, and will appeal to emerging aleatory architecture and engineering paradigms. Indeed, the ability to build through impermanent contacts and design lightweight materials with prescriptive mechanical properties cuts through many areas of high current importance: civil engineering and architectures (reliable, inexpensive, reusable and self-repairing materials), transportation (lightweight composites, shock absorbers), advanced manufacturing. This is in line with the national need of increasing industry competitiveness, which advances the national health, prosperity, and welfare; and secures the national defense. The project also promises to capture the imagination of a broad audience by creating an unusual bridge between relatable protagonists (birds) and often inaccessible fields of physics and engineering. Additionally, STEM outreach activities will be conducted at individual institutions to attract middle school students and female students, respectively, towards science and engineering. Undergraduate students will also be offered positions in either group for exposure to advance engineering research.With increasing aspect ratio, the mechanical behavior of disordered granular packings changes. Where applied stresses distribute in chains of 1D contacts for spheroids, slenderness introduces bending moments and long-range interaction. Impermanent frictional contacts set the system apart from semi-flexible polymer networks and other non-woven materials which derive mechanical response from permanent crosslinks. Experimental evidence from disordered, randomly packed, elastic fibers or filaments based structures, such as the bird nests, suggest that these material systems exhibit frequency-dependent elastoplastic behavior, finite tensile response, and enhanced specific strength. In the absence of a theoretical framework and strictly applicable principles of statistical mechanics, an experimental platform for the benchmarking and physical characterization of these materials will be developed in this project. These will be complemented by a high fidelity computational counterpart to direct a search for novel mechanical states and transitions. The project will provide insights into the relationship between macroscopic and microscopic mechanics of bird nest-like systems, paving the way towards prescriptive design of novel materials. Moreover, it will spur new directions in granular physics theory and explain a functional mechanism from a naturally-selected engineered structure.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.