Experimental and numerical studies on the mechanical behaviour of Australian Strathbogie granite at high temperatures: An application to geothermal energy

The effect of temperature on the mechanical behaviour of Strathbogie granite (fine-grained) was studied under unconfined stress conditions. Fracturing behaviour of test specimens was studied using an acoustic emission (AE) detection system and some crack propagation was also performed using electron microscopy scanning (SEM). The stress-strain curves showed plastic and post-peak behaviour for temperatures above 800 °C and the brittle-plastic transition was observed to occur between 600 and 800 °C for the uniaxially tested Strathbogie granite at a strain rate of 0.1 mm/min and room humidity. Specimens were heated at a rate of 5 °C/min with a 1 h holding period before testing. The AE results showed that the increasing temperature reduces the stress thresholds for crack initiation and crack damage and extends the duration of stable crack propagation. Prevalence of ductile properties with increasing temperature was also observed from AE results. The stress-strain and AE results reveal that the failure modes of Strathbogie granite specimens changed from brittle fracturing to quasi-brittle shear fracturing and eventually to ductile failure with increasing temperature. Temperature was observed to influence the colour of granite, and the initial white/grey colour changed to an oxidated reddish colour with increasing temperature. The stress-strain data of tested specimens were incorporated into a finite element model (ABAQUS 6.7.1), so that both plastic and ductile behaviour of the Strathbogie granite could be predicted over a wide range of temperatures.