Experimental study on the coefficient of restitution of grain against block interfaces for natural and engineered materials

Abstract

The coefficient of restitution (COR) is an important input parameter in the numerical simulation of granular flows, as it governs the travel distance, the lateral spreading and the design of barriers. In this study, a new custom-built micro-mechanical impact loading apparatus is presented along with impact experiments on engineered and natural materials. The COR and energy loss of various grains and base block combinations are studied, including fairly regular shaped Leighton Buzzard sand (LBS) grains as a natural soil and granite/rubber as base blocks, apart from the use of engineered materials for the grains (chrome steel balls, glass balls) and blocks (stainless steel, brass). The repeatability of the new micro-mechanical impact loading apparatus was checked by impacting chrome steel balls on stainless steel block. In all the test combinations, the higher and lower values of the COR are found for granite block (ranging between 0.75-0.95) and rubber block (ranging between 0.37-0.44) combinations, respectively. For the tested grain-block combinations, lower values of COR were observed for impacts between materials of low values of composite Young’s modulus.

Effect of Grain Size and Surface Roughness on the Normal Coefficient of Restitution of Single Grains

“…A recent study by Marinack et al [32] showed that the effect of air drag on the grain velocity (for the range of velocities used in this study) is negligible and the COR values only differ by less than 1%. Repeatability of the tests and data acquisition using the impact loading apparatus used in the present study was presented by Sandeep et al [22].…”

“…A micromechanical impact apparatus was used to measure the COR of grain and base block impacts for different material combinations. This apparatus was developed inhouse and is described Materials 2020, 13, 814 3 of 14 in a recent study by Sandeep et al [22], but for completeness, it is detailed here as well. An image of the impact loading apparatus is shown in Figure 1.…”

“…The range of the observed scatter is very low in magnitude for glass balls due to their spherical shape and consistent morphological and elastic characteristics. For natural quartz sand grains, Sandeep et al [22] observed higher values of the standard deviation which they attributed to irregular shape and defects in the grains compared with engineered materials such as glass and chrome steel balls. Contrary to the impacts on the metal surfaces, i.e., brass blocks in this study, the impacts on granite and rubber blocks showed an increasing trend of COR values with increasing grain size.…”

“…Recent experimental studies [18][19][20] obtained the values of inter-particle friction (µ) at the grain-scale for various materials including glass balls (with an average µ ≈ 0.1). The normal stiffness between two bodies can be obtained using quasi-static grain-scale experiments [18,21] or through COR using dynamic impact loading tests [22]. In numerical simulations such as DEM, the normal contact force (F N ) during collisions is calculated as (after Lommen et al [23], Tang et al [24]): (1) where U N is the normal displacement, K N is the normal contact stiffness, C N is the damping coefficient of the normal force which depends on the COR.…”

“…For 3 mm steel balls impacting on conglomerate rock surface, Imre et al [37] reported an average COR value of 0.87. A recent study by Sandeep et al [22] reported the impact loading behavior of glass balls, steel balls and quartz sand grains (with a focus on 2 mm size balls) on brass, granite and rubber base blocks. However, compared with metal-metal impacts, a very limited amount of literature is available on the effect of grain size on brittle materials (such as rocks) and rubber blocks.…”