Photoelastic force measurements in granular materials

Abstract

Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local of stress. In this review paper, we summarize past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus.

Particle scale force sensor based on intensity gradient method in granular photoelastic experiments

“…The challenge is how to experimentally extract particle scale stress information non-invasively. Currently, there are several experimental techniques up to this task, including X refractive ray/computed tomography scan [4][5][6], reflectively index matching [7], and photoelastic imaging [8][9][10].…”

“…The force resolving method is based on a nonlinear fitting algorithm that can reveal the contact vector forces in quasi-2D granular materials composed of discs with different sizes [11]. Details of this nonlinear fitting algorithm have been well described in a recent review paper [10]. However, the implementation of this technique depends on the accuracy of the initial guess for the contact forces before fitting [9], and is highly computationally consuming [10].…”

“…Details of this nonlinear fitting algorithm have been well described in a recent review paper [10]. However, the implementation of this technique depends on the accuracy of the initial guess for the contact forces before fitting [9], and is highly computationally consuming [10]. By contrast, while unable to solve the contact vector forces of each particle, the G 2 method can well reveal the particle scale pressure/stress in a reasonable stress range, which is first reported by Howell et al [14].…”

Preface: Focus on imaging methods in granular physics

“…A common technique is to perform quasi-two-dimensional (Q2D) experiments, allowing the complete tracking of particle positions 22,23 as well as the particleparticle interactions. 24 Another option is to restrict the data collection to the surface of the granular system, with the disadvantage that bulk properties can differ significantly from the behavior at the surface. [25][26][27][28][29][30] If the volume fraction of the particles is sufficiently low, stereo-camera or volumetric methods can be used to capture three-dimensional (3D) dynamics.…”

An experimental investigation of the force network ensemble

“…A model of the force modulated light intensity can then be fitted to camera images of the particles [13,14]. Figure 2.…”