Comprehensive investigation of rock fracture behaviour in clay-rich rocks under the effect of temperature: Experimental study under three loading modes (I, I/II, II)

Abstract

Rock fracture behaviour is a crucial aspect in many geological and engineering projects and can be affected by different environmental factors such as temperature, especially in projects where the rocks are exposed to a change in temperature such as geothermal energy, oil and gas exploitation, nuclear waste areas, etc. Therefore, this research aims to investigate the effect of temperature on rock fracture behaviour under four loading modes; pure mode I, pure mode II, and two mixed-mode loadings I/II in SCB specimens of clay-rich rocks (mudstone). Thirty-six SCB specimens were prepared and tested under three temperatures (Ambient temperature (AT), 250 and 500 °C), and some important parameters such as fracture toughness, fracture envelope, crack propagation speed, and crack acceleration were measured and discussed as a function of temperature. The main results concluded that due to the gradual loss of water from the structure of high clay content in the mudstone during thermal treatment, the behavior of mudstone gradually changed from ductile to brittle behaviour, and as a result, fracture toughness and crack propagation speed increased significantly. In addition, our results showed that the area under the fracture envelope which represents fracture resistance to growth under mixed-mode loading increased by 1.86 and 2.68 orders in magnitude after thermal treatment at temperatures of 250 and 500 °C, respectively. Under the four loading mode mixities, fracture grows through three main stages, high acceleration (0–5 mm), high deceleration (5–10 mm), and low deceleration (10–15 mm) stages. Crack propagation speed and crack acceleration depend strongly on the mode mixity ratio (KII/KI) and it increased by changing the loading mode from pure mode I to mixed mode loading I/II and pure mode II. Keywords: Temperature, Mixed-mode loading I/II, Mudstone, Fracture toughness, Fracture envelope, Crack propagation speed, Crack acceleration