Stress Corrosion Micro-Cracking or Static Fatigue: The Principal Cause of Rate Effects and "Aging" in Sand

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Time and rate effects in sands affect both the short- and long-term behavior of earth structures, yet understanding of the causes of that behavior is lacking. A hypothesis is suggested that makes use of the process of micro-fracturing of the microscopic morphological features on grain surfaces to explain the nature of time effects in sand. The new approach this research is taking makes it potentially transformative. Experimental observation techniques based on Scanning Electron Microscopy and Atomic Force Microscopy will be employed to produce visual evidence for time-delayed micro-fracturing of asperities on grain surfaces (static fatigue), whereas micro- and macro-loading tests will be used to gain quantitative evidence. The scientific merit of this research is in its creative and original concept exploiting the multi-scale approach to explain the puzzling time-related effects in sand. The engineering relevance of this research is that the time-dependent behavior of soils can have important consequences on design and long-term behavior of earth structures and on soil-structure interaction (for instance, the load on retaining structures can increase in time as a result of the time-dependent nature of sand behavior). The impact of this research will affect areas beyond geotechnical engineering, e.g., space exploration missions, which require predictions of surface mobility, planning of drilling operations, and processing of the Lunar/Martian regolith. Due to the absence of water and atmosphere, Lunar regolith grains have a rich surface morphology, a characteristic central to the hypothesis in this research. This research promotes a new concept, advances discovery, and promotes learning.