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.