Microstructural Controls on Mixed Mode Dynamic Fracture Propagation in Crystalline and Porous Sedimentary Rocks

Michael J. Braunagel, & William A. Griffith

Published August 16, 2021, SCEC Contribution #11532, 2021 SCEC Annual Meeting Poster #160

The growth and interaction of fractures in fault damage zones plays an important role in earthquake rupture propagation. However, crack propagation in rocks is a complex process due to grain-scale heterogeneity and rapidly evolving stress states under dynamic loading conditions, and, as a result, damage zone structure is fundamentally different in crystalline vs. porous sedimentary rocks. In this work, we use digital image correlation and theoretical linear elastic fracture mechanics to make instantaneous measurements of the opening (mode I) and in plane shear (mode II) components of the stress intensity field during dynamic mixed mode crack initiation and propagation in crystalline and porous sedimentary rocks. Both rock types display similar properties in fracture parameters as observed in engineered materials, including rate dependent fracture initiation toughness and the direct relationship between propagation toughness and crack velocity; however, measured propagation toughness are above quasi-static values at crack velocities well below the branching velocity. Additionally, due to grain scale controls on the fracture process, we observe mixed mode crack propagation is fundamentally different between these two rock types. Mixed mode propagation is energetically more favorable than pure opening mode propagation in sandstone, while the opposite is true in granite. Furthermore, following initiation, propagation in granite occurs with negligible mode II component, irrespective of the initiation conditions, while fractures in sandstone maintain a non-negligible mode II component during propagation across the sample.

Citation
Braunagel, M. J., & Griffith, W. A. (2021, 08). Microstructural Controls on Mixed Mode Dynamic Fracture Propagation in Crystalline and Porous Sedimentary Rocks. Poster Presentation at 2021 SCEC Annual Meeting.

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