| We study stress characteristics of static and quasi-static granular assemblies in three experiments. In conical and wedge-shaped piles, we examine the normal stress distribution beneath the pile using two different construction techniques. A local minimum in the stress profile is found near the pile center for conical piles made with a localized source procedure. No central minimum is found for similar conical piles constructed using a more uniform "raining procedure." Three-dimensional wedge-shaped piles also show similar behavior, but the central minimum is not as well pronounced as in the conical case. The second experiment allows us to examine force distributions in the bulk of two-dimensional piles through the use of photoelastic disks. A two-dimensional localized source procedure creates an internal pile structure with preferred angles, while an analog to the raining procedure creates a more uniform packing. The structure of the stress chains, consisting of about one third of the total grains, exhibits similar construction-dependent angle behavior. Finally, a two-dimensional shearing experiment is performed to examine stress characteristics in photoelastic disks. This method allows us to probe many quasi-static configurations of grains. A most interesting result of the shearing experiment is the discovery of a transition with global packing fraction, gamma. As this density approaches a critical value, gammac, from above, the following occur: (1) The mean stress, sigma, decreases dramatically; (2) there is a slowing down of dynamic processes; (3) qualitative changes in the stress distributions and statistics occur; (4) the spatial network of stress chains changes, with long uninterrupted nearly radial chains occurring intermittently in space and time near gammac and a more complex network occurring throughout the system for larger packing fraction. |
