Experimental Investigation of the Origins of Brittle Fracture Roughness

Will Steinhardt, & Shmuel Rubinstein

Published August 12, 2019, SCEC Contribution #9423, 2019 SCEC Annual Meeting Poster #158

As a crack front moves through a material, it leaves in its wake a fracture surface that preserves the time history of the crack front’s motion. For geological materials, this surface is rough, a remnant of the crack’s tortuous path. Rough crack faces contain more surface area than smooth, flat ones, and thus require more energy to produce. As a result, roughness plays a crucial role in determining the evolution of crack fronts. In addition, faults begin as fractures and then become frictional systems, where the roughness of the fracture can play an important role in determining the size of earthquakes (Zeilke et al. 2017).

Brittle fracture surfaces of both natural and manmade materials exhibit a myriad of features that create macro-scale roughness (Hull, 1999). Among the most fundamental crack surface features are long, step-like discontinuities known as step lines (sometimes called river lines or fracture lances) and are associated with the formation of en echelon cracks. We have developed a system to study these features by observing hydraulic fractures in brittle hydrogels. Heavily cross-linked hydrogels have been shown to behave like classically brittle materials (Livne et al. (2004)), but with the benefits of highly tunable rheology, being optically clear, and having breakdown pressures 2-3 orders of magnitude lower than rocks. Studying hydraulic fractures allows us to match the index of refraction of the interior of the fracture to the bulk material so that we can use a combination of high-speed photography and scanning laser sheet illumination to observe the fracture dynamics in three dimensions at more than 1000 volumes per second. We observe that macro-scale roughness comes in the form of step-like perturbations of the fracture front, resulting from material heterogeneity, which leave in their wake a curved linear scar known as a step line. Our dynamic three-dimensional observations of these steps and their interactions allow us to understand the rules that govern their growth and interaction.

Citation
Steinhardt, W., & Rubinstein, S. (2019, 08). Experimental Investigation of the Origins of Brittle Fracture Roughness. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)