Slow dynamic nonlinear elasticity in novel systems

Slow dynamic nonlinear elastic recovery is found to be universal amongst geological materials and other materials with complex microstructures, such as concrete and cracked glass blocks. The slow dynamic behavior is characterized by a log(time) recovery after an initial drop of stiffness, induced by strain events as small as a microstrain. Slow dynamics has also been observed on the seismological scale with the recovery occurring over years. Insomuch as materials that exhibit slow dynamics form fault gouge, it is thought that slow dynamics could play a role in dynamic earthquake triggering. Here we present studies of slow dynamics in simplified systems—unconsolidated bead packs and a single bead system—to inform theory. A consensus theoretical understanding of the universality of slow dynamics and, in particular, the log(time) recovery is still needed. Our results imply that some previously proposed mechanisms, i.e., force chains, glassy microstructures, and cracking, cannot play essential roles as they are presumably absent in one or more of the studied systems.