Photoporomechanics: An Experimental Method to Visualize the Effective Stress Field in Fluid-Filled Granular Media

Abstract

Effective stress governs the mechanical behavior of porous media. In this study, we use photoelasticimetry to visualize the evolving effective stress field in fluid-filled granular media in processes that couple fluid flow and mechanical deformation. We refer to this experimental method as photoporomechanics. We develop a fabrication process to produce millimeter-scale residual-stress-free photoelastic spheres with high geometric accuracy. We use color to quantify the forces acting on the particles over a wide range of forces, while using light intensity for a small range of forces. We then provide an application of photoporomechanics to illustrate the evolution of effective stress during one-dimensional consolidation: a process by which the stresses caused by a sudden load are gradually transmitted through a fluid-filled granular pack as the fluid drains and excess pore pressures dissipate. Our technique provides a powerful experimental model system to study the grain-scale underpinning of coupled solid-fluid processes in granular media.

Crossover from viscous fingering to fracturing in cohesive wet granular media: a photoporomechanics study

“…In summary, we have studied the morphology and rheology of injection-induced fracturing in cohesive wet granular packs via a recently developed experimental technique, photoporomechanics, which extends photoelasticity to granular systems with a fluid-filled connected pore space. 32 Experiments of water injection into cohesive photoelastic granular packs with different tensile strength, injection flow rate, and defending fluid viscosity have led us to uncover two invasion regimes: viscous fingering, and fracturing with leak-off of the injection fluid. Contrary to the observed effective stress shadow for cohesionless granular packs, 31 here we discover a hoop effective stress region behind the water invasion front.…”

“…The photoelastic response offers a unique opportunity to gain additional understanding of the coupled pore-scale flow and particle mechanics during fluid-induced fracturing of the cohesive granular pack. To interpret the photoelastic response, we rely on the results of calibration experiments, 32 which have shown that, for the range of interparticle forces expected in our fracturing experiments, the relation between light intensity and force is monotonically increasing and approximately linear.…”

“…17,18 While fracturing during gas invasion in fluid-saturated media has been studied extensively in experiments 12,[15][16][17][18][19][20][21][22] and simulations, 13,[23][24][25][26][27][28][29][30] the underlying grain-scale mechanisms behind the morphodynamics and rheologies exhibited by deformable granular media remain poorly understood. To tackle this challenge, Meng et al 31 adopt a recently developed experimental technique, photoporomechanics, 32 to directly visualize the evolving effective stress field in a fluid-filled cohesionless granular medium during fluid-induced fracturing. The effective stress field exhibits a surprising and heretofore unrecognized phenomenon: behind the propagating fracture tips, an effective stress shadow, where the intergranular stress is low and the granular pack exhibits undrained behavior, emerges and evolves as fractures propagate.…”

Photoelastic stress response of complex 3D-printed particle shapes

“…To date, quantitative studies [6,7] have largely focused on 2D particle systems, primarily flat discs [8], but also non-spherical shapes such as ellipses [9] and pentagons [10]. While experiments on monolayers of photoelastic spheres can be found in the literature [11][12][13], interpretations of the resulting fringe patterns remains challenging, even when the system is index-matched. In response to these experimental limitations, there have been advances in computational modelling by using the Finite Element Analysis (FEA) method.…”

Photoelastic Stress Response of Complex 3D-Printed Particle Shapes

“…To date, quantitative studies [6,7] have largely focused on 2D particle systems, primarily flat discs [8], but also non-spherical shapes such as ellipses [9] and pentagons [10]. While there have been recent experiments on monolayers of photoelastic spheres [11][12][13], it remains a challenge to interpret the resulting fringe patterns, even when the system is index-matched.…”