Abstract
Granular friction behaviors are crucial for understanding the ubiquitous packing and flow phenomena in nature and industrial production. In this study, a customized experimental apparatus that can simultaneously measure the time history of normal and tangential forces on the inside-shearing unit is employed to investigate the granular friction behaviors during a linear reciprocating sliding process. It is observed that the evolution behaviors of two normal forces distributed separately on the shearing unit can qualitatively reflect the effects of the force chain network. During the half-loop of the reciprocating sliding, the total normal force, which indicates the load-bearing capacity of the granular system, experiences the following typical stages: decreases abruptly and stabilizes momentarily, further decreases significantly to the minimum, gradually increases to the maximum, and then remains stable. These stages are associated closely with the relaxation, collapse, reconstruction, and stabilization of the force chain, respectively. Interestingly, the coefficient of friction (COF) can reach a stable value rapidly within the initial sliding stage and subsequently remain constant. The average COF within stable ranges decreases significantly with the external load G in the power-function form, G−0.5. Meanwhile, the COF increases slightly with the sliding velocity. Finally, a complete illustration of the dependences of the granular COF on the external load and sliding velocity is provided. Our study contributes to granular friction research by providing an innovative experimental approach for directly measuring the COF and implicitly correlating the evolution of the force chain network.
Highlights
Granular friction is vital to the ubiquitous packing and flow phenomena in nature and industrial production, such as geographical faults [1, 2], powder mixing and densification [3], and granular flow lubrication [4, 5]
Granular friction research is associated with two experimental schemes, as follows: (1) the surface-sliding scheme, where the shearing unit is sliding against the surface of the granular system [10,11,12,13,14,15]; (2) the inside-sliding scheme, where the shearing unit is sliding inside the granular system [16,17,18]
During the half-loop of the reciprocating sliding, the normal force acting on the shearing unit underwent four typical stages: decreased abruptly and stabilized momentarily, further decreased significantly to the minimum, gradually increased to the maximum, and remained unchanged
Summary
Granular friction is vital to the ubiquitous packing and flow phenomena in nature and industrial production, such as geographical faults [1, 2], powder mixing and densification [3], and granular flow lubrication [4, 5]. Because the normal force applied to the shear surface is typically fixed in this scheme, the COF can be directly measured. Compared with the fixed normal load in the surface-sliding scheme, the normal force acting on the inside-sliding shearing unit is typically dynamic, owing to the evolution of the force chain network [28]. Shearing motion can cause the continuous collapse and reconstruction of the force chain, resulting in dynamic changes in the force transmission paths and load capacity [32,33,34,35]. A customized experimental apparatus that overcomes the shortcomings of the current inside-sliding scheme was employed to investigate the friction characteristics of granular matter. The experimental apparatus can simultaneously measure the normal load and tangential friction forces acting on the shearing unit, thereby providing the time history of the. The effects of the external load and sliding velocity on granular friction behaviors are discussed
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