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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 stress. In this review paper, we summarize the 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. [ABSTRACT FROM AUTHOR]

Powders and grains exhibit unpredictable jamming-to-flow transitions that manifest themselves on geophysical scales in catastrophic slip events such as landslides and earthquakes, and on laboratory/industrial scales in profound processing difficulties. Over the past few years, insight into these transitions has been provided by new evidence that slip events may be accompanied, or even preceded, by electrical effects. In the present work, we quantify the correlation between slip and the separation of electrical charges, using an archetypal granular material: photoelastic polymers. We measure a strong correlation between material displacement, acoustic emissions, and voltage. We find that the generation of voltage is associated with surface, rather than bulk properties of the granular materials. While voltage precursors are only occasionally observed in this system, there is some asymmetry in the cross-correlation between the slip and voltage signals that indicates differences between the pre- and post-slip dynamics. [ABSTRACT FROM AUTHOR]

Although jammed granular systems are athermal, several thermodynamiclike descriptions have been proposed which make quantitative predictions about the distribution of volume and stress within a system and provide a corresponding temperaturelike variable. We perform experiments with an apparatus designed to generate a large number of independent, jammed, two-dimensional configurations. Each configuration consists of a single layer of photoelastic disks supported by a gentle layer of air. New configurations are generated by cyclically dilating, mixing, and then recompacting the system through a series of boundary displacements. Within each configuration, a bath of particles surrounds a smaller subsystem of particles with a different interparticle friction coefficient than the bath. The use of photoelastic particles permits us to find all particle positions as well as the vector forces at each interparticle contact. By comparing the temperaturelike quantities in both systems, we find compactivity (conjugate to the volume) does not equilibrate between the systems, while the angoricity (conjugate to the stress) does. Both independent components of the angoricity are linearly dependent on the hydrostatic pressure, in agreement with predictions of the stress ensemble. [ABSTRACT FROM AUTHOR]

It is a long-standing question whether granular fault material such as gouge plays a major role in controlling fault dynamics such as seismicity and slip-periodicity. In both natural and experimental faults, granular materials resist shear and accommodate strain via interparticle friction, fracture toughness, fluid pressure, dilation, and interparticle rearrangements. Here, we isolate the effects of particle rearrangements on granular deformation through laboratory experiments. Within a sheared photoelastic granular aggregate at constant volume, we simultaneously visualize both particle-scale kinematics and interparticle forces, the latter taking the form of force-chains. We observe stick-slip deformation and associated force drops during an overall strengthening of the shear zone. This strengthening regime provides insight into granular rheology and conditions of stick-slip periodicity, and may be qualitatively analogous to slip that accompanies longer term interseismic strengthening of natural faults. Of particular note is the observation that increasing the packing density increases the stiffness of the granular aggregate and decreases the damping (increases time-scales) during slip events. At relatively loose packing density, the slip displacements during the events follow an approximately power-law distribution, as opposed to an exponential distribution at higher packing density. The system exhibits switching between quasi-periodic and aperiodic slip behavior at all packing densities. Higher packing densities favor quasi-periodic behavior, with a longer time interval between aperiodic events than between quasi-periodic events. This difference in the time-scale of aperiodic stick-slip deformation is reflected in both the kinematics of interparticle slip and the force-chain dynamics: all major force-chain reorganizations are associated with aperiodic events. Our experiments conceptually link observations of natural fault dynamics with current models for granular stick-slip dynamics. We find that the stick-slip dynamics are consistent with a driven harmonic oscillator model with damping provided by an effective viscosity, and that shear-transformation-zone, jamming, and crackling noise theories provide insight into the effective stiffness and patterns of shear localization during deformation. [ABSTRACT FROM AUTHOR]

A photoelastic material will reveal its internal stresses when observed through polarizing filters. This eye-catching property has enlightened our understanding of granular materials for over half a century, whether in the service of art, education, or scientific research. In this review article in honor of Robert Behringer, we highlight both his pioneering use of the method in physics research, and its reach into the public sphere through museum exhibits and outreach programs. We aim to provide clear protocols for artists, exhibit-designers, educators, and scientists to use in their own endeavors. It is our hope that this will build awareness about the ubiquitous presence of granular matter in our lives, enlighten its puzzling behavior, and promote conversations about its importance in environmental and industrial contexts. To aid in this endeavor, this paper also serves as a front door to a detailed wiki containing open, community-curated guidance on putting these methods into practice (Abed-Zadeh et al. in Photoelastic methods wiki https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/home, 2019). [ABSTRACT FROM AUTHOR]

Studies the process through which a meteor comes to rest and forms a crater. Visualization of the forces generated in the impact bed through the use of photoelastic particles; Usability of the bidisperse mixture in suppressing crystallization; Snapshots of force chains during a meteor collision.

Stress visualization within 3-dimensional particles could provide significant insights for understanding of the behaviors of complex particles. However traditional photoelastic methods are only able to produce simple particle shapes. Recently, 3D-printing has created new possibilities for enhancing the scope of stress analysis of physically representative particles. We report the results of X-ray computed tomography and 3D-printing, combined with traditional photoelastic analysis, to visualize strain for particles ranging from simple 2D discs to complex 3D-printed coffee beans, including internal voids. The relative orientation of the print layers and the loading force can affect the optical response of the discs without affecting the mechanical properties. Furthermore, a semi-quantitative measurement of the generated stresses within 3D-printed complex particle is presented. Potential limitations and areas of future interest for stress visualization of 3-dimensional particles are also outlined. [Display omitted] • 3D printing photoelastic UV-curable resin enables stress analysis of complex particles. • Ray tracing confirms shadow at multiple faces of complex particles is from refraction. • Print orientation affects optical properties of 3D printed discs with applied load. • Semi-quantitative stress measurement can be obtained from complex 3D printed particles. [ABSTRACT FROM AUTHOR]