Slow Dynamic Nonlinear Elasticity in Unconsolidated Granular Systems

John Yoritomo

Research Physicist
Acoustic Division
U.S. Naval Research Laboratory

Wed, November 3, 2021 - 4:00 PM
Karl Herzfeld Auditorium of Hannan Hall - Rm 108

Slow dynamic nonlinear elasticity is found to be universal in brittle materials with complex microstructures, such as sandstones, concrete and cracked glass blocks. The slow dynamic behavior is characterized by a log(time) recovery after an initial drop of stiffness, induced by condiioning  strains as small as a microstrain. Slow dynamics has also been observed on the seismological scale with the recovery occurring over years. In this talk, I will present experimental studies of slow dynamics in simplified systems—unconsolidated bead packs and a
single bead system—to inform theory. Particular care is used here in the experimental design to overcome the difficulties inherent in bead pack studies. This includes the design of the bead pack support, the use of low-frequency conditioning, and the use of ultrasonic waves as a probe with coda wave interferometry to assess changes. 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.

 

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