Stress evolution within a granular system undergoing subcritical failure: insights from photo-elastic imaging of a 2D glass disc pack

Compacting granular systems composed of brittle materials not only deform but can also fracture. In these systems, stress transmits across grain contacts and forms force chain networks. A fracture can occur when the heterogeneously distributed stress becomes locally high and critical. Here, we use photoelasticity to visualize the evolution of stress distributions within a uniaxially loaded, homogeneous 2D array of discs made of soda-lime glass, a transparent, isotropic non-crystalline material. We run experiments in water-saturated and nominally dry conditions, through loading, holding and unloading periods. We optically and acoustically (via acoustic emissions) monitor the evolution of stress field, bulk deformation, and crack propagation. Photoelasticity data are analyzed by image recognition and processed to map stress distributions. 

Preliminary results show five characteristics that set the fracturing of granular matter apart from continuum solids. First, we can extract the stress transmissions, which, despite the macroscopic homogeneity, show force chains and an inhomogeneous stress field. Second, these stress concentrations lead to a local excess of strength and disc fractures. The birefringence patterns in individual discs are altered by fractures but still carry load. During unloading, the fractures can slip or frictionally lock and the stress acting on them don’t fully relax. Third, unloading and reloading cause cracking before reaching the previous target. Fourth, cracking continues during holding periods in a time-dependent manner; perhaps subcritical crack growth redistributes stresses and thus leads to cascades or spurts of acoustic emission events. Finally, homogeneously highly stressed subdomains of discs develop that confine grains and thus suppress localization and fracturing.