Slow dynamics and the role of moisture

The elastic behavior of rocks and other composite materials cannot be entirely captured by the traditional theory of nonlinear elasticity, where the stress field is related to powers of the strain field. Rather, these materials display non-classical nonlinear elastic behavior, such as hysteresis, end-point memory, fast dynamics, and slow dynamics. Slow dynamics (SD) is characterized by a drop in material stiffness due to a minor mechanical conditioning, followed by a slow recovery back to the original macroscopic elastic state. SD has drawn particular attention because the recovery, often logarithmic in time, has been observed in a wide variety of materials, on lengths scales from the laboratory to the seismic, and on time scales from milliseconds to years. The universal character suggests a simple, fundamental mechanism for the SD recovery. This talk will present recent experiments that seek to test proposed SD mechanisms. In particular, the role of moisture will be investigated. The main experimental venue is the simplified structure introduced in previous work—a single bead confined between two slabs of a similar material. The main benefit of this structure over more commonly studied SD materials (sandstones and concrete) is the ability to control the environment around the contact points.