Characterization of contact evolution on sand grain surfaces and their time-dependent contact properties based on Greenwood and Williamson model

This paper focuses on the microscopic roughness of silica sand grain, and its morphological evolution (contact maturing) under a constant load. The surface topology with rich texture is obtained by Atomic Force Microscopy and various roughness parameters are investigated by statistical approaches. The surface profiles in the same contact region before and after being subjected to sustained loading are both measured to identify time-dependent morphological changes on the contact surface. The two congruent surface topologies, initial and mature surfaces, not only show the delayed fracturing of contact asperities, but also present quantitative differences in the spatial roughness parameters over time. These differences are intensified with consideration of upper profiles only, indicating that asperities with higher elevations are susceptible to the contact maturing process. With the geometric information obtained from the same region at two different measurement times, the Greenwood and Williamson asperity model of rough surface is used to estimate contact proximity, real area of the load-bearing contacts, and the number of contact spots. The model results of the post-loaded surface incorporated with true surface profiles show an increased number of contact points as well as the true contact area under constant loading. It is expected that subcritical fracturing of contact asperities leads to closer proximity of contact entities when loaded by prolonged loading, resulting in firmer and stiffer intergranular contact. This may very well be the reason for time effects in sand assemblies.