Granular packings and fault zones

Area of Science:

• Physics of granular materials
• Computational mechanics
• Geophysics

Background:

• Understanding the failure mechanisms of granular materials is crucial in various fields, including geotechnical engineering and materials science.
• Previous studies have explored granular packing and failure, but the specific role of localized deformation structures like shear bands and their internal dynamics remain areas of active research.

Purpose of the Study:

• To analyze the failure mechanisms of two-dimensional (2D) packing of elastic grains using a numerical model.
• To investigate the formation and characteristics of shear bands and localized deformation states within the granular packing during failure.
• To compare the observed phenomena, such as seismic activity distribution and stress patterns, with natural geological processes.

Main Methods:

• A numerical model was employed to simulate the behavior of 2D elastic grain packing under stress.
• The model tracked the evolution of grain arrangements and stress distributions leading to failure.
• Analysis focused on identifying shear band formation, characterizing internal structures within shear bands (e.g., rotating bearings), and examining the statistical distribution of localized events.

Main Results:

• Packing failure occurs through the formation of shear bands or faults.
• During failure, the system separates into two distinct grain-packing states.
• Within shear bands, local "rotating bearings" spontaneously form, characterized by low stiffness against shearing.
• The distribution of "seismic activity" in the packing mirrors that of earthquake distributions in tectonic faults.
• The principal stress directions observed in the bearing state resemble those found at the San Andreas Fault.

Conclusions:

• The formation of shear bands with rotating bearing structures is a key mechanism in the failure of 2D elastic grain packing.
• The analogy between granular packing failure and tectonic earthquakes is supported by the similar statistical distributions of seismic activity.
• The study provides insights into the micro-mechanical processes governing macroscopic failure in granular systems and highlights potential parallels with geological faulting.