Abstract
In the discrete element simulation of granular materials, the modelling of contacts is crucial for the prediction of the macroscopic material behaviour. From the tribological point of view, friction at contacts needs to be modelled carefully, as it depends on several factors, e.g. contact normal load or temperature to name only two. In discrete element method (DEM) simulations the usage of Coulomb’s law of friction is state of the art in modelling particle–particle contacts. Usually in Coulomb’s law, for all contacts only one constant coefficient of friction is used, which needs to reflect all tribological effects. Thus, whenever one of the influence factors of friction varies over a wide range, it can be expected that the usage of only one constant coefficient of friction in Coulomb’s law is an oversimplification of reality. For certain materials, e.g. steel, it is known that a dependency of the coefficient of friction on the contact normal load exists. A more tribological tangential contact law is implemented in DEM, where the interparticle friction coefficient depends on the averaged normal stress in the contact. Simulations of direct shear tests are conducted, using steel spheres of different size distributions. The strong influence of interparticle friction on the bulk friction is shown via a variation of the constant interparticle friction coefficient. Simulations with constant and stress-dependent interparticle friction are compared. For the stress-dependent interparticle friction, a normal stress dependency of the bulk friction is seen. In the literature, measurements of different granular materials and small normal loads also show a stress dependency of the bulk friction coefficient. With increasing applied normal stress, the bulk friction coefficient reduces both in the experiments and in the simulations.
Highlights
Solid-like granular materials generally comprise a high number of particle–particle and particle–environment contacts with high variations in normal loading
Motivated by the above experimental findings on granular media and results obtained on the wheel–rail contact for steel, the authors will use a non-constant coefficient of friction in discrete element method (DEM) simulations of direct shear tests on steel spheres
It is known from tribology that the coefficient of friction is not constant but depends on several factors, such as contact normal load, relative motion, surface roughness, contact temperature and contact conditions, etc
Summary
Solid-like granular materials generally comprise a high number of particle–particle and particle–environment contacts with high variations in normal loading. [3,7] or [11], who simulated direct shear tests and compared the results to experiments It is frequently found in the literature that the bulk/peak friction angle of a granular material is constant, i.e. independent of the normal stress. The stress dependency of the bulk friction angle, found in the experimental results, could not be reproduced in the DEM simulations, which used a constant interparticle friction coefficient for all load cases. Motivated by the above experimental findings on granular media and results obtained on the wheel–rail contact for steel, the authors will use a non-constant coefficient of friction in DEM simulations of direct shear tests on steel spheres (particle–particle contact).
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