Stress Fluctuations for Continuously Sheared Granular Materials

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