Slow dynamics in a single glass bead

TL;DR

This study investigates slow nonlinear recovery of stiffness in a simplified system of a single glass bead, revealing that force chains are not essential for slow dynamics in granular materials.

Contribution

It introduces a minimal experimental setup with a single glass bead to study slow dynamics, isolating effects from force chains and microstructure.

Findings

Slow dynamic recoveries observed with different conditioning methods

Force chains are not necessary for slow dynamics in granular media

Ultrasonic coda wave interferometry effectively probes slow recovery

Abstract

Slow dynamic nonlinearity is ubiquitous amongst brittle materials, such as rocks and concrete, with cracked microstructures. A defining feature of the behavior is the logarithmic-in-time recovery of stiffness after a mechanical conditioning. Materials observed to exhibit slow dynamics are sufficiently different in microstructure, chemical composition, and scale (ranging from the laboratory to the seismological) to suggest some kind of universality. There lacks a consensus theoretical understanding of the universality in general and the log(time) recovery in particular. Seminal studies were focused on sandstones and other natural rocks, but in recent years other experimental venues have been introduced with which to inform theory. One such system is unconsolidated glass bead packs. However, bead packs still contain many contact points. The force distribution amongst the contacts is unknown. Here, we present slow dynamics measurements on a yet simpler system - a single glass bead confined between two large glass plates. The system is designed with a view towards rapid control of the contact zone environment. Ultrasonic waves are used as a probe of the system, and changes are assessed with coda wave interferometry. Three different methods of low-frequency conditioning are applied; all lead to slow dynamic recoveries. Results imply that force chains do not play an essential role in granular media slow dynamics, as they are absent in our system.