We study the effect of both particle size and shear testers on the failure (yielding) and the steady state shear strength of granular materials. Physical experiments are carried out on four fine limestone powders using a geotechnical direct shear tester and the standard Schulze ring shear tester to explore both material behaviors and device influence. The direct shear tester is validated by comparing the yield locus measurements with the Schulze ring shear tester. A good agreement is found between these two testers and this unveils the possibility of using different testers across different scientific communities/regimes. The termination locus is then studied in direct shear tester for the four limestone powders. A pronounced particle size effect is evident on both the angle of internal friction and the steady state angle of internal friction, which highlight the effect of the inter-particle cohesion

We study the effect of particle cohesion on the steady state shear strength of a granular material. For cohesive powders, the steady state shear loci (termination loci) from DEM simulations are nonlinear with a peculiar pressure dependence due to the non-linear increase of contact adhesion with pressure in the contact model. Physical experiments are carried out on fine limestone powder in the direct shear box to validate the interesting non-linearity in the material behavior, as detected by the simulations and to provide a basis for calibration of DEM. In order to enhance the reproducibility, the standard test procedure from Jenike shear tester is ameliorated for the direct shear tester. Finally, the difference between the yield (transition from static to flow) and the steady state shear stress (required to maintain shear motion) will be addressed.