Grain rotation versus continuum rotation during shear deformation of granular assembly

The importance of grain rotation during shear deformation has been widely recognized in the mechanics of granular materials, which has led to extensive use of the Cosserat continuum theory in localization problems. Strain gradient theory, which relates the macro deformation gradient to higher-order stresses, is another possibility to overcome the ill-posedness of governing equations. This paper attempts to show an experimental basis for applying strain gradient theory to granular media. LAT (Laser-Aided Tomography), a technique to visualize the interior of 3-D granular assembly, is used to detect the grain rotation as well as the continuum rotation. A Discrete Element simulation is also conducted to reinforce the experimental data. It is concluded that the average grain rotation is roughly identical to the continuum rotation, which supports the applicability of rotational gradient theory, a particular case of strain gradient theory from the micro-mechanical point of view.