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We investigated experimentally the normal coefficient of restitution (COR) of impactors colliding on the surfaces of two different blocks; a soft‐porous block composed of plaster (dry impacts) and a hard‐crystalline rock composed of granite (dry and wet impacts). The experiments were performed in a range of low‐impact velocities and various particle types were used including perfectly spherical smooth glass beads and ceramic balls, rough glass beads as well as natural rough sand grains. Image processing was carried out to quantify the formed contours of craters caused by the surface damage of the soft blocks due to the impact. The results indicated very low COR values on the plaster block compared with the granite block as the energy was dissipated by means of surface plastic deformations on the plaster, however the contour crater images showed that the collision mechanisms depended on the roughness of the impactor. The behavior of impactor‐fluid‐block systems was dependent on both surface roughness and global morphology of the impactors. Discrete‐based (DEM) numerical simulations were performed to provide further insights into the behavior of the impactor‐block systems subjected to collision using the COR values from the experiments as the micro‐scale parameters and data from the literature as the macro‐scale parameters for the model calibration. The numerical output was used to observe the development of compression and tension force chain networks and how these involved during and after impact on different base blocks. [ABSTRACT FROM AUTHOR]

We examined with micromechanical‐based experiments the influence of shearing rate on the tangential contact behavior of smooth (very low surface roughness) flat quartz surfaces with montmorillonite and crushed aggregates as gouges. On decreasing the shearing rate by one order of magnitude, significant changes in the stick‐slip instability were observed; increase in force‐drop, recurrence interval and slip velocity. The initial tangential stiffness increased on decreasing the shearing rate. The type of gouge particles also influenced the behavior of the interfaces; for example, higher friction and stable sliding were observed for angular particles (non‐plastic gouge) compared to flaky particles (plastic gouge). The partition of the tangential force‐displacement relationship based on the shape of the curve was also dependent on the shearing rate. On a qualitative standpoint, the results presented in this micromechanical‐based study were in agreement with macro‐scale test results published in the literature, although different scale tests were also performed in the present work, from grain‐level to direct shear test size. The changes in the behavior of stick and slip components of the tangential force and the contact area with shearing rate were analytically studied, and the corresponding variations in the normal load‐displacement relationship were also discussed. This experimental micromechanical‐based study with additional analytical support, provides a preliminary understanding of the frictional behavior of different gouges under shearing rates of one folder difference and also provides insights into the stick‐slip instability behavior at a microscopic (grain‐scale) standpoint. [ABSTRACT FROM AUTHOR]