Effect of heat-treatment on the pure- and mixed-mode fracture properties of a homogeneous sandstone

Fracture-mechanical properties of the sedimentary rocks at elevated temperature strongly control hydraulic fracturing in rocks, geothermal energy extraction, deep tunnelling, and spent fuel disposal in geological medium. This paper investigates the relationship between the strength properties (e.g. tensile strength, brittleness index and Young's modulus) and the pure-mode and mixed-mode fracture toughness of Dholpur sandstone at temperatures ranging from room temperature to 500 degrees C. A suitable brittleness index definition was identified to quantify the deformational changes. Additionally, evolution of the stress-strain behaviour, change in the degradation degree (DD), peak strain, and evolution in porosity values were closely studied to comment on the potential brittle-ductile transition of the rock. The fracture toughness remained unchanged between 25-150 degrees C, sharply increased 3%- 28% between 150-200 degrees C, and followed by a 75%-80% decrease from 200-500 degrees C. All the pure- and mixed-mode fracture toughness were found to maintain an exponential relationship with the Young's modulus, tensile strength, and brittleness index of the heat-treated rocks across all the temperatures. A potential brittle to quasi-brittle/semi-ductile transition temperature of 225 degrees C was identified for Dholpur sandstone. Additionally, mixed mode fracture behaviour of the heat-treated rock was investigated using generalized maximum tangential stress (GMTS) criterion. It was noticed that GMTS is far better at predicting the KII/KI ratio of mode-II fractures than the conventional maximum tangential stress (MTS) criterion. Statistical analysis of the test data indicate that a 3 - parameter Weibull function can be successfully employed to predict the mode-II fracture toughness of heat-treated sandstone in terms of the mode-I data.(c) 2022 Elsevier Ltd. All rights reserved.