(115f) Visualization of Shear Motions of Powders in the True Biaxial Shear Tester

The true biaxial shear tester is used successfully since many years to investigate the flow properties of cohesive powders. In this tester, pure strain and mixed stress-strain controlled experiments are possible for investigating steady state flow. This state is defined as a permanent deformation of the material without volume change while the stresses at the specimen's boundaries remain constant. Recent investigations have shown that this state, especially for cohesive powders, is not always as steady as it should be by definition. In this paper a recent extension of the true biaxial shear tester, which allows for getting a view inside the shear motion of the bricked shaped powder specimen, is introduced. By applying a dark colored powder pattern onto a light powder sample, the movement of the powder can be captured using a CCD-camera. Development of shear bands and inhomogeneities inside the sample can be visualized. Experiments with cohesive and non cohesive powders, with purely strain controlled, volume preserving shear cycles, as well as mixed stress-strain controlled experiments are presented. Comparisons of the recorded images as well as stress-strain data to discrete elements simulations are performed.