Nonlinear slow dynamic elasticity in metal systems

Abstract

Nonlinear “slow dynamic elasticity” is found to be universal amongst geological materials and other materials with complex microstructures, such as cement 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 dynamic behavior is also reported in unconsolidated aggregates of glass beads under pressure and in single glass beads confined between glass plates, systems that are probed by diffuse ultrasound and monitored for stiffness changes using coda-wave interferometry. At present, there is no consensus as to the microphysical basis for the behavior, though hypotheses include glassy dynamics at grain-grain interfaces, microcracking, thermal activation of a distribution of atomic-scale barriers, and moisture. Here we extend the materials known to exhibit slow dynamics to include metal structures: an aluminum bead pack under pressure, and individual aluminum and steel beads confined between aluminum and steel plates, respectively. We find that the strength of the slow dynamics is largely the same as it is for glass beads, suggesting that neither cracking nor glassy micro-structures are essential.