An experimental study on tensile characteristics of granite rocks exposed to different high-temperature treatments

Investigation of temperature dependent tensile strength characteristics of rocks provides essential inputs to continue the development of deep geo-engineering applications, including enhanced geothermal systems, deep mining and deep geological disposal of nuclear waste. The aim of this study is, therefore, to identify the influence of temperature (from room temperature to 1000 °C) and different heating and cooling treatments (constant high temperature, slow and rapid cooling) on Australian granite’s tensile strength and to investigate the corresponding microstructural alterations due thermal treatment. Brazilian tensile strength tests were performed on two types of granites collected from Australia, Strathbogie, and Harcourt. ARAMIS photogrammetry, Acoustic Emissions (AE) and Scanning Electron Microscopy (SEM) were employed to examine the crack initiation and development during the experiment. The results concluded that with the increase of temperature for all three thermal conditions, there was a slight increase in tensile strength for Strathbogie granite between room temperature and 200 °C for constant high-temperature condition and slowly cooled condition whilst beyond this there was a clear negative trend. Harcourt granite experienced a negative trend immediately from room temperature conditions. Further, the influence of rapid cooling was much higher than that of slow cooling, due to the intense thermal shock. Beyond 500 °C, there was a significant reduction in the range of strength values which evidence α to β quartz mineral transition. Therefore, the failure mechanism of granite at considered temperature transitioned from a brittle to a quasi-brittle state. This was confirmed with AE testing and ARAMIS photogrammetry such that, higher temperatures resulted in a considerable crack closure and an increase in the unstable crack propagation. This was mainly due to the closure of pre-existing thermally induced cracks followed by crack re-bonding due to the melting followed by re-crystallization of grains, particularly at 1000 °C which was confirmed with SEM.