Collaborative Research: A Unified Theory of Crack Nucleation and Growth for Materials Subjected to Repetitive Surface Acoustic Waves and Dynamic Impacts

表面声波（saw）普遍存在于许多自然发生的破坏性现象中，如地震和海啸。此外，长期以来，人们一直推测saw会对船舶螺旋桨产生空化、对风力涡轮机叶片的高速冲击以及射流对岩石的破碎造成表面损伤。在新兴的医学应用中，如纳米脉冲碎石术，saw也被认为在确保患者肾结石无创崩解方面发挥着至关重要的作用。尽管如此，saw触发材料表面断裂的基本机制在很大程度上仍然未知。这不是一个孤立的现象，因为无论加载类型如何，裂纹的成核和扩展已经是一个困扰了几十年的问题。在这种情况下，该奖项支持基础研究，以解释如何重复应用saw和其他动态载荷可以引起脆性材料表面裂纹的成核并影响其随后的生长。从这个项目中获得的见解将极大地有利于科学家和工程师寻求理解和预测一个仍然难以捉摸的基本现象：脆性材料在机械载荷作用下的损伤。提议的研究是跨学科的，将工程师和计算科学家聚集在一起，充分探索这类问题。重要的是，它还包括旨在鼓励高中和本科阶段STEM领域代表性不足的少数民族学生从事材料力学研究的外展活动。尽管近年来在断裂领域取得了理论进展，但目前对裂纹成核及其在固体中向扩展转变的科学理解仍然不完整，而且尚未得到充分探索。本研究的重点是实验、理论和计算研究脆性材料在各种条件下反复动态加载下的裂纹形核和扩展。将研究两种原型系统：浸没并遭受多次冲击载荷的材料也会导致saw，以及遭受反复冲击载荷的干燥材料。新的实验将在玻璃和Begostone上进行，Begostone是一种工程材料，其弹性、强度和韧性可以在很大范围内方便地变化。实验将与基于新的连续统理论的模拟结合进行，该理论将惯性效应和低周疲劳纳入裂纹成核和扩展的统一模型。这些研究的结果将揭示一个基本但长期未解决的问题，即低周疲劳和惯性载荷如何降低材料系统的强度和韧性，并最终导致其在任意机械载荷下的断裂和失效。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。

Surface acoustic waves (SAWs) are prevalent in many naturally occurring destructive phenomena, such as earthquakes and tsunamis. Further, SAWs have long been speculated to contribute to surface damage produced by cavitation on ship propellers, high-speed impact on wind turbine blades, and fragmentation of rocks by jetting streams. In emerging medical applications, such as Nano-Pulse Lithotripsy, SAWs have also been postulated to play a vital role in ensuring the success of noninvasive disintegration of kidney stones in patients. Despite this, the fundamental mechanisms by which SAWs trigger fracture on the surface of materials remain largely unknown. This is not an isolated phenomenon as the nucleation and propagation of cracks regardless of the loading type has been a vexing problem for decades. In this context, this award supports fundamental research to explain how the repeated application of SAWs and other dynamic loadings can give rise to the nucleation of cracks on the surface of brittle materials and affect their subsequent growth. Insights from this project will significantly benefit scientists and engineers seeking to understand and predict a fundamental phenomenon that has remained elusive: the onset of damage in brittle materials in response to mechanical loads at large. The proposed research is interdisciplinary, bringing together engineers and computational scientists to fully explore this class of problems. Importantly, it also includes outreach activities designed to encourage under-represented minority students in STEM fields at the high school and undergraduate levels to pursue research in mechanics of materials. Despite recent theoretical progress in the field of fracture, the current scientific understanding of crack nucleation and its transition to growth in solids remains incomplete and under-explored. This research is focused on the experimental, theoretical, and computational study of crack nucleation and growth in brittle materials in response to repeated, dynamic loadings under a wide range of conditions. Two prototypical systems will be studied: materials that are submerged and subjected to multiple shock loadings that also cause SAWs, and dry materials that are subjected to repeated impact loads. New experiments will be conducted on both glass and Begostone, an engineered material whose elasticity, strength, and toughness properties can be conveniently varied over a substantial range. The experiments will be carried out in conjunction with simulations based on a new continuum theory that will incorporate inertial effects and low-cycle fatigue into a unified model of crack nucleation and growth. The results of these studies will shed light on the fundamental yet long-unresolved question of how low cycle fatigue and inertial loads can degrade the strength and toughness of material systems and ultimately result in their fracture and failure in response to arbitrary mechanical loads.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.