Stress Fluctuations for Continuously Sheared Granular Materials

Experiments on continuously sheared granular materials (glass spheres) with diameters 1.0≤d≤5mm show large fluctuations in the normal stress, σ(t). Experiments are carried out in an annular Couette geometry for rotation rates 0.003< ̇ \.θ<1rad/s. Power spectra from σ(t), P(ω), show rate invariance in the fluctuations: ̇ \.θP(ω) is a function only of ω/ ̇ \.θ, independent of ̇ \.θ. ̇ \.θP(ω) depends relatively weakly on d. The distributions of stresses ρ_σ are similar to recent predictions for static arrays, but the width of ρ_σ varies only weakly with d, suggesting stronger spatial correlation effects than expected from theory.