Time Effects in Sand: Delayed Micro-Cracking, Contact Fatigue, and Aging

沙子中的时间效应：延迟微裂纹、接触疲劳和老化

• 批准号: 1537222
• 负责人: Radoslaw Michalowski
• 金额: $ 42.36万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-09-01 至 2019-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1537222&HistoricalAwards=false
• 关键词: Time; Effects; Sand; Delayed; Micro

The strength of soils can change over time, and therefore affect the performance of structures resting on them. Silica sand, after a disturbance caused by natural events such as earthquakes, or after compaction by vibratory means, can exhibit a weakening-like behavior. However, a gradual improvement of strength is observed in time following the disturbance. There is no consensus among researchers as to what the key causes of such behavior are, and this research addresses some of the fundamental questions regarding time effects. The immediate impact of this research will be on the broad area of physical infrastructure; it will allow for better planning of construction processes and more educated predictions of civil infrastructure behavior, and thus will benefit society at large. This research will also have a broader impact, for instance in chemical and pharmaceutical industries,where operations involve powders and granular materials. Finally, material aging is an important process in extraterrestrial environments. Aging and contact fatigue caused by thermal cycling is believed to be responsible for the elevated density of the near-surface layer of lunar soil (regolith). The knowledge of this phenomenon will be important in planning for exploration of the Moon and Mars environments. This research is to address the fundamental issue: what causes time-dependent behavior in silica sand? A hypothesis is explored that identifies fracturing of microscopic textural features on grain surfaces at inter-granular contacts as the key cause of this behavior. This fracturing does not stop at the end of the loading process, but continues at constant load, with a decaying rate. Experimental evidence will be gathered from tests on individual contacts. Custom-designed testing equipment will be constructed. Sand grain surfaces will be characterized using atomic force microscopy and scanning electron microscopy. The process of delayed fracturing finds its justification in the rate process concept, and it is expected to answer some of the fundamental questions regarding factors affecting crack propagation and healing. Mathematical description of the contact fatigue process will be sought through constructing a model with individual grains comprised of sub-particles fused together with bonds capable of carrying both forces and moments. Cracking in the model will be simulated by the stress corrosion process causing debonding of sub-particles within the contact regions between grains. The model will mimic the physical behavior of contacts in silica sand.

土壤的强度会随着时间的推移而变化，因此会影响其上结构的性能。 硅砂在受到地震等自然事件的扰动后，或在通过振动手段压实后，可能会表现出类似弱化的行为。 然而，在扰动之后，观察到强度随时间逐渐改善。 对于这种行为的关键原因是什么，研究人员之间没有达成共识，这项研究解决了一些关于时间效应的基本问题。 这项研究的直接影响将是对物质基础设施的广泛领域;它将允许更好地规划施工过程和更有教育意义的预测民用基础设施的行为，从而使整个社会受益。 这项研究也将产生更广泛的影响，例如在化学和制药行业，其中操作涉及粉末和颗粒材料。 最后，材料老化是地外环境中的一个重要过程。由热循环引起的老化和接触疲劳被认为是造成月球近表层土壤（风化层）密度升高的原因。对这一现象的了解将对规划月球和火星环境的探索非常重要。 这项研究是为了解决一个根本问题：是什么原因导致硅砂的时间依赖性行为？ 一个假设是探索，确定断裂的微观纹理特征的晶粒表面在粒间接触的关键原因，这种行为。这种破裂不会在加载过程结束时停止，而是在恒定载荷下以衰减速率继续。 将从对个别接触者的测试中收集实验证据。 将建造定制设计的测试设备。 将使用原子力显微镜和扫描电子显微镜表征砂粒表面。 延迟断裂的过程中发现其理由的速率过程的概念，它有望回答一些基本问题的影响裂纹扩展和愈合的因素。接触疲劳过程的数学描述将寻求通过构建一个模型与单个晶粒组成的子颗粒融合在一起，能够承载力和力矩的债券。 模型中的裂纹将通过应力腐蚀过程模拟，应力腐蚀过程导致晶粒之间接触区域内的子颗粒脱粘。该模型将模拟硅砂中接触的物理行为。