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

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

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.