Influence of morphology on the micro-mechanical behavior of soil grain contacts

The development of the discrete element method (DEM) has provided important insights into the micro-scale behavior of geological materials with major applications in geotechnical, geological and petroleum engineering. Mechanical response of grain contacts including normal and tangential behavior are given as input quantities in various DEM applications. However, it was only achieved in recent years to properly capture the real soil grain behavior in the laboratory and provide useful input parameters to be further utilized in DEM analyses. In this paper, an experimental study was conducted investigating the normal and tangential contact behavior of soil grains composed of limestone sand (denoted as LS), which is a material of biogenic origin and it may find applications, for example, in offshore engineering. In their previous work (Senetakis et al. in Tribol Int 111:1–8, 2017), the authors reported on the dynamic inter-particle friction of this material and concluded that due to the smooth surface of the brand of grains they tested, the inter-particle friction was found lower in comparison to quartz grains of Leighton Buzzard sand (LBS). That study quantified friction at relatively large displacements of shearing, of the order of 80 μm or above, where a steady state sliding was observed. In the present work, the mechanical behavior of LS sand grains was enhanced with additional test data to further explore normal and tangential behavior. The focus of the study was to investigate the differences in inter-particle friction at different reference displacements, termed as small displacement range, where the behavior exhibited a peak, and large displacement range, where a fairly steady state sliding was occurred as well as to stress the differences in the observed responses between LS and LBS grains in terms of tribological and contact mechanics behavior. It is shown in the study that very high friction angles may be observed at displacements of about 10–30 μm for LS grain contacts when sheared under low normal forces, with a substantial drop of the mobilized friction leading to a steady state at much greater sliding displacements. This drop of friction with shearing displacement was not observed when the grains were sheared at higher normal forces. The morphological effects on friction were more evident at low normal forces for LS grains. In the normal loading tests, the LS sand grains exhibited significant plastic behavior. After the conduction of a number of normal loading–unloading cycles, the observed plastic deformations continued to increase, even though this observed trend was dominant at the first cycle. The observed apparent contact Young’s moduli of LS grains were found to be much lower than those of LBS grains. These observations should not be ignored by DEM modelers when the numerically created sample targets to represent a granular material such as crushed LS.