Contact fatigue in silica sand—Observations and modeling

•Texture of silica sand grains is rich in asperities prone to fracture when loaded.•Delayed fracturing of grain surface features at contacts causes contact fatigue.•An air-dry silica sand contact loaded with 2.4 N remains active for about 18 days.•Distinct element model mimics time-dependent increase in the number of contact points within an individual inter-granular contact.

Aging and rate effects in sand have been a subject of studies for nearly three decades, yet there is no consensus as to what the direct cause of time effects in silica sand is. A hypothesis is advocated in this paper that identifies delayed micro-fracturing of grain surface textural features at contacts as the key contributor to time effects in silica sand.Grain-scale studies were carried out focusing on observations of time-dependent fracturing of the micro-morphological features of grain surfaces in contact, and quantitative measurements of the relative position of a grain in contact with a stainless steel plate. An apparatus was constructed that allows monitoring the time-dependent relative displacement of two grains in contact (convergence) or a grain in contact with a solid plate. Potentiometers were used to measure displacements, while a calibrated spring in one of the potentiometers was used for loading the grains with a required force. The results reveal that the rate of convergence is the largest during the period immediately after the load is applied, and the convergence continues at constant load with a decaying characteristic for about 18 days.Simulations of grain-to-solid plate and grain-to-grain interactions were attempted using the distinct element method. The sand grain was simulated as an assembly of sub-particles fused together with time- and load-dependent parallel bonds. Simulation results are consistent with the hypothesis that contact fatigue (or stress corrosion micro-cracking) causes a time-dependent increase in stiffness of contacts. Consequently, an increase in the macroscopic small-strain stiffness takes place, which contributes to what is often referred to as sand aging.