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

土的强度会随着时间的推移而变化，因此会影响其上的结构的性能。在地震等自然事件引起的扰动后，或通过振动方式压实后，硅砂可以表现出类似于减弱的行为。然而，在扰动之后，及时观察到强度的逐渐提高。对于这种行为的关键原因是什么，研究人员之间并没有达成共识，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建设过程，并对民用基础设施行为进行更有根据的预测，从而使整个社会受益。这项研究还将产生更广泛的影响，例如在化工和制药行业，这些行业的运营涉及粉末和颗粒材料。最后，材料老化是外星环境中的一个重要过程。热循环引起的老化和接触疲劳被认为是月球土壤(风化层)近表层密度增加的原因。对这一现象的了解将在计划探索月球和火星环境时发挥重要作用。这项研究是为了解决一个根本的问题：是什么导致了硅砂中的时间相关行为？探索了一种假说，认为颗粒间接触处颗粒表面微观纹理特征的断裂是这种行为的关键原因。这种破裂不会在加载过程结束时停止，而是以衰减的速度在恒定加载时继续进行。实验证据将从对个人接触者的测试中收集。将建造定制设计的测试设备。砂粒表面将用原子力显微镜和扫描电子显微镜进行表征。延迟压裂过程在速率过程概念中找到了它的合理性，它有望回答一些关于影响裂纹扩展和愈合的因素的基本问题。接触疲劳过程的数学描述将通过构建一个模型来寻求，该模型由子颗粒组成的单个颗粒融合在一起，并能够传送力和力矩。模型中的裂纹将由应力腐蚀过程模拟，该过程导致颗粒之间接触区内的亚颗粒脱粘。该模型将模拟硅砂中接触的物理行为。

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.