Contact testing and simulation of the time-dependent interaction between sand particles

An apparatus was constructed for testing the response of contacts between silica sand grains subjected to sustained loads. Testing results clearly indicate the delayed convergence of grains, likely caused by fracturing of asperities at the contact. The resolution of measurements was about 150 nm, and the difficulties in testing will be discussed. With the development of a constitutive law for time-dependent behavior of an individual contact as a future goal, the distinct element method (DEM) was adopted as a simulation tool. An individual grain was simulated as an assembly of bonded sub-particles. The true texture of a silica sand grain surface was scanned using atomic force microscopy, and the contact region was replicated in simulations by "carving" the surface from high-resolution assembly of sub-particles. Simulations indicate that the near-contact regions of grains are subjected to time-dependent micro-fracturing, leading to an increase in the number of interaction points within an individual inter-granular contact area. This, in turn, leads to an increase of contact stiffness. Increase in the small-strain stiffness of sand is then a consequence that has been observed in the field.