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We performed a series of numerical vertical compression tests on assembliesof 2D granular material using a Discrete Element code and studied the resultswith regard to the grain shape. The samples consist of 5,000 grains made fromeither 3 overlapping discs (clumps - grains with concavities) or six-edgedpolygons (convex grains). These two grain type have similar external envelopes,which is a function of a geometrical parameter $\alpha$.  In this paper, the numerical procedure applied is briefly presented followedby the description of the granular model used. Observations and mechanicalanalysis of dense and loose granular assemblies under isotropic loading aremade. The mechanical response of our numerical granular samples is studied inthe framework of the classical vertical compression test with constant lateralstress (biaxial test). The comparison of macroscopic responses of dense andloose samples with various grain shapes shows that when $\alpha$ is considereda concavity parameter, it is therefore a relevant variable for increasingmechanical performances of dense samples. When $\alpha$ is considered anenvelope deviation from perfect sphericity, it can control mechanicalperformances for large strains. Finally, we present some remarks concerning thekinematics of the deformed samples: while some polygon samples subjected to avertical compression present large damage zones (any polygon shape), densesamples made of clumps always exhibit thin reflecting shear bands. This paperwas written as part of a CEGEO research project www.granuloscience.com

Polygons vs. clumps of discs: a numerical study of the influence of grain shape on the mechanical behaviour of granular materials