Abstract
We study the shear behaviour of various granular materials by conducting novel perpetual simple shear experiments over four orders of magnitude of relatively low shear rates. The newly developed experimental apparatus employed is called “3D Stadium Shear Device” which is an extended version of the 2D Stadium Shear Device [1]. This device is able to provide a non-radial dependent perpetual shear flow and a nearly linear velocity profile between two oppositely moving shear walls. Using this device, we are able to test a large variety of granular materials. Here, we demonstrate the applicability of the device on glass beads (diameter 1 mm, 3 mm, and 14 mm) and rice. We particularly focus on studying these materials at very low inertial number I ranging from 10−6 to 10−2 . We find that, within this range of I , the friction coefficient μ of glass beads has no shear rate dependence. A particularly appealing observation comes from testing rice, where the attainment of critical state develops under much longer duration than in other materials. Initially during shear we find a value of μ similar to that found for glass beads, but with time this value decreases gradually towards the asymptotic critical state value. The reason, we believe, lies in the fact that rice grains are strongly elongated; hence the time to achieve the stable μ is primarily controlled by the time for particles to align themselves with respect to the shear walls. Furthermore, the initial packing conditions of samples also plays a role in the evolution of μ when the shear strain is small, but that impact will eventually be erased after sufficient shear strain.
Highlights
The mechanics of granular flow have been widely studied experimentally
The granular system was sheared to the same shear strain under different shear rates, and the normal and shear stress were measured only after the system achieved the critical state
By comparing the results from 14 mm glass beads with other smaller glass beads, one can find that the error bars are relatively large due to the smaller w/d ratio (w/d equal to 8.7, 40.7 and 122 for 14 mm, 3 mm and 1 mm glass beads respectively)
Summary
The mechanics of granular flow have been widely studied experimentally. Three flow regimes of granular materials include the quasi-static, inertial and collisional regimes [2], which are classified using the dimensionless Inertial number I [3]. It is defined as the ratio of the confinement timescale and the typical time of deformation [4, 5]:. We employed a novel experimental apparatus called the “Stadium Shear Device” (SSD) that enables testing granular materials under perpetual shear conditions, here for the first time in three dimensions (i.e. 3D SSD). We conducted perpetual shear tests on a large variety of materials, and present results examining the shear rate dependency of friction coefficient μ and the evolution of μ with shear strain
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