Failure of sandstone with preexisting damage under dynamic tensile loading

Zachary D. Smith, & William A. Grifith

Published September 11, 2022, SCEC Contribution #12277, 2022 SCEC Annual Meeting Poster #144

Loading by earthquakes and alteration through heating and chemical processes can alter the physical properties of rocks and reduce their strength. During subsequent earthquakes altered rock may influence earthquake rupture processes. Heat treatments alter mineralogy and induce microfractures making them ideal for studying the influence of mineral alteration and preexisting damage on the failure of rocks. In this study we investigate the influence of preexisting damage on the behavior of heat-treated Berea Sandstone under dynamic tensile loading. A modified sample configuration for a Split Hopkinson Pressure Bar (SHPB) apparatus is used to induce tensile fragmentation in Berea Sandstone samples heat-treated at 250 °C, 450 °C, 650 °C, and 850 °C. Optical and scanning electron microscopy are used to characterize postmortem damage at the sample and grain scale. In dynamic tension experiments strain is accommodated by the formation of mode-I fractures, dilation bands, and pore space expansion that can result in localized increases in porosity of up to 25%. Tensile strength increases with heat treatments up to 450 °C and then decreases for higher heat treatments above the alpha-beta-quartz transition. The elastic properties of Berea Sandstone change with heat treatments, and in a single orientation the Poisson’s Ratio becomes negative at heat treatments above 250 °C. Whereas fractures induced by SHPB experiments are primarily intergranular for sandstone heat treated at temperatures up to 450 °C, intragranular fractures become more prevalent in sandstone heat-treated at 650 °C and 850 °C. Hertzian-like fractures are observed in samples loaded under tension, but the mechanism of their formation is different than morphologically similar fractures formed under compression. Here we propose a model for contact emanated tensile fractures formed under a remote tensile stress field that resemble, to some degree, Hertzian fractures formed under compressive loading. Our results have implications for dynamic tensile brittle fragmentation of sandstone with preexisting damage during earthquake rupture and for the interpretation of loading conditions based on damage observed in the field.

Key Words
Fault damage, Pulverization

Citation
Smith, Z. D., & Grifith, W. A. (2022, 09). Failure of sandstone with preexisting damage under dynamic tensile loading. Poster Presentation at 2022 SCEC Annual Meeting.

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