Effect of Direct Thermal Heating on Cracking in Uniaxially Compressed Specimens

Abstract—A number of published papers have suggested that the influence of electromagnetic action on the seismic regime in natural conditions or on crack formation in laboratory experiments can be explained by the role of Joule heating when an electric current passes through fluid-saturated rocks. Heating of the fluid in the pore space can lead to an increase in fluid pressure in pores and cracks, which in turn can trigger additional cracking of the solid skeleton and decrease in rock strength. In this paper, the results of direct heating of artificial sandstone samples under uniaxial compression conditions are presented in order to identify the role of the thermal factor in rock fracture. The samples were taken from the same batch as previously studied in experiments on the effect of passing an electric current through a sample on the acoustic emission (AE) characteristics and on its failure process. In the presented experiments, the heat flow was supplied to the opposite side faces of the sample using Peltier elements, which are reverse thermoelectric converters. The heat flow density on the sample surface reached almost 10⁴ W/m²; the local temperature of the outer surface of the sample near the heating element increased during the heating process by 10–15°C depending on the duration of the switched-on current. Experiments have shown that even with such significant heat flows, the effect of heating is only evident for loads close to fracturing (Kp ≥ 0.95–0.97). The effect of heating was that the process of crack formation in the sample intensified and the sample itself gradually passed into a overcritical state, eventually failing. At lower loads, direct heating of the sample does not have a noticeable effect on the crack formation process.