Investigation on the dynamic mechanical behavior and thermal damage mechanism of rocks under confining pressure after high-temperature treatment

Temperature and in-situ stress both affect the properties of formation rocks. Marble and sandstone were heat treated and the changes in their composition were analysed by X-ray diffraction (XRD). A thermal-mechanical-damage (TMD) model was established to characterise the thermal damage before impact. Dynamic compression tests under active confining pressure were carried out in a split Hopkinson pressure bar (SHPB), and the fracture surfaces were examined by scanning electron microscopy (SEM). Thermal damage in marble appears as external tensile and internal shear failure, whereas sandstone shows mainly internal tensile failure. Without confining pressure, the dynamic peak stress of marble decreases with temperature, while that of sandstone first increases and then decreases. Under confining pressure the dynamic peak stress of marble increases with temperature up to 600 °C and drops sharply at 800 °C, while for sandstone the temperature of maximum dynamic peak stress rises with the confining pressure, from 200 °C at 2 MPa to 600 °C at 6 MPa; both rocks are strongest at 600 °C and 6 MPa. Under confinement, marble treated at 400–600 °C mostly separates along fracture surfaces after impact, whereas sandstone cracks far less at its strongest temperature than at room temperature.