Initiation and propagation behaviour of cohesive cracks at high temperatures: A dynamic experimental study

•The crack initiation and propagation images are captured through the high-speed camera.•A new method for identifying the cohesive crack tip are proposed.•The cohesive crack initiation and propagation dynamic process at high temperatures is elaborated.•The dynamic propagation process and characteristics change of the cohesive crack at different temperatures are analyzed. Temperature, as an important factor, affects the fracturing effect in deep reservoirs, so it is urgent to carry out high-temperature crack propagation experiments. Here, by combining a high-speed camera and DIC technology, Brazilian splitting tests of three types of rocks at different temperatures are carried out to study the crack initiation and propagation characteristics from a mesoscopic perspective. A new way to identify the cohesive crack tip is proposed based on the high-temperature cohesive zone model and opening displacement patterns. In addition, the dynamic propagation process and changes in the characteristics of cohesive cracks at different temperatures are analysed. The experimental results demonstrate that dynamic process of high-temperature cohesive crack initiation and propagation can be subdivided into three stages: stress concentration, cohesive microcrack initiation and propagation and cohesive macrocrack penetration. Interestingly, multiple interconnected rather than single cohesive microcracks develop in the centre region of sandstone and limestone specimen surfaces at 260 ℃. Notably, the duration of the stress concentration stage of sandstone and limestone shows an increasing trend with temperature. The average propagation speed and the opening displacement change rate of the cohesive crack in sandstone, limestone and shale specimens show a declining trend with increasing temperature. The results can provide some insights into the high-temperature crack propagation mechanism in deep reservoirs.