Dynamic tensile behaviour of Barakar sandstone under high-temperature conditions

Rocks are very much susceptible to deformation in tension, especially under elevated temperatures. Therefore, the study of the dynamic tensile behaviour of rock exposed to high temperature is highly significant to understand the tensile deformation behaviour in dynamic loading conditions which will be proved useful in a variety of engineering problems such as quantifying the blast load impact in fire affected underground/opencast coal mine regions; assessment of ground subsidence due to coalmine fire coupled with blast loading etc. The Jharia coalfield region, known as the coal capital of India, is affected by pervasive underground coalmine fire for decades resulting in small to large-scale surface fracturing. So,the present study focuses on the effect of high temperature on dynamic tensile behaviour and its relation with micro-mineralogical properties of subsurface coal-bearing sandstone samples from a fire-affected mine.  To achieve the objective, the prepared samples were kept in the furnace for 24h with a heating rate of 5°C/min and then allowed to cool down naturally within the furnace. Samples were divided into nine groups based on the thermal treatment at 25 °C, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C. Using the Split-Hopkinson Pressure Bar (SHPB), the indirect dynamic tensile strength was measured for each group. Based on the obtained results, the indirect dynamic tensile strength of heat-treated specimens is characterized into three zones; viz.: Zone 1 (25-400°C), Zone 2 (400-600°C) and Zone 3 (600-800°C). In zone 1, an increase in average indirect dynamic tensile strength is observed with elevated temperature. However, in zone 2, a sharp decreasing trend in indirect dynamic tensile strength was observed with increasing temperature. This zone is characterised by a progressive increase in thermal cracks and porosity which is possibly the prime reason for a sharp transition in thermal properties. An overall reduction in indirect dynamic tensile strength is observed within zone 3, however, the rate of reduction is gentle. The plasticity that occurred due to high temperature was responsible for a slow rate of reduction in indirect dynamic tensile strength.