Force Quantification Methods in Cohesive Photoelastic Granular Experiments

Holly Hilbrant, Iowa State University
Olivia Marcelli, Iowa State University
Jacqueline Reber, Iowa State University
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Abstract

Granular systems are everywhere; grain silos, cereal boxes, and in natural settings such as sand dunes or fault gouge. Granular systems are composed of grains: macroscopic solid particles that interact with each other via contact forces. Within granular systems, force distributes through individual contacts, creating a set of grains jammed together forming a so called force chain. Force chains arrange anisotropically throughout a granular system and support most external force. Because granular systems are widespread in geologic settings, understanding their behavior under different conditions is critical for geologic hazard assessments. Here, we study how partial cementation and fluid saturation influence force chain patterns.

We model granular systems using polyurethane discs. In experiments, we change the fraction of cemented grains, the orientations of the cemented grain clusters, and the presence of a fluid phase. Polyurethane has photoelastic material properties that lead to stress-induced birefringence. Under force, photoelastic discs display alternating dark and light fringes that become more pronounced as force increases. We view these fringes using a circularly polarized light setup called a polariscope. We take photos at regular intervals while the experiment is being deformed. Strips of cemented grains do not separate during deformation, allowing them to not only support compressional forces but also extensional and shear forces. This leads to more complex fringe patterns that cannot be analyzed by the typical method of force quantification in photoelastic granular systems. As an alternative method, we quantify forces through the gradient of light intensity in each grain. We find that both cementing grains and fluid saturation increase the force across the experiment compared to uncemented dry experiments. In addition, a fluid phase allows for forces to be more evenly distributed across the system.

In addition to the magnitude of force, we also seek to quantify the orientation of these force chains. We map the direction of force-bearing contacts using network analysis. We represent individual grains as nodes and their contacts as edges; weighing the edges by the magnitude of force distributed between the two grains. Using the magnitude of the edges in the network, we compare the orientation of force chains between experiments. Looking forward, we will further quantify how force chains are distributed spatially in the experiments to determine how cementation and fluid saturation impact the force distribution and consequently the stability of the granular system.

Session

Experiments of all sorts