Spatiotemporal evolution and combined action mechanisms of thermal-mechanical fields in soundless cracking agents with varying charging time intervals

Rolled-soundless cracking agents (R-SCAs) are widely used for fracturing hard coal mine roofs, but their thermal-mechanical fracturing mechanism remains unclear, limiting efficiency improvements. This study investigated R-SCA fracturing through experiments with three charging time intervals (0, 15, and 40 min between two boreholes), employing infrared thermal imaging, temperature acquisition systems, and non-contact full-field strain measurement systems to reveal the spatiotemporal evolution of thermal-mechanical fields. Results showed that temperature fields exhibited dual-center gradient distribution across all time intervals, with the effective thermal influence radius approximately 2.9 times the borehole diameter. The SK-15 specimen exhibited the lowest peak thermal stress, reduced by 38.46%. Longitudinal strain fields displayed asymmetric, single-peak distributions around boreholes, with increased charging intervals weakening the stress enhancement effect between holes. Mechanism analysis reveals that thermal stress dominates the early fracturing, inducing micro-damage through differential thermal expansion of mineral components within the rock. In the later stage, expansive stress becomes dominant, promoting crack initiation and propagation in thermally weakened zones. The 15 min charging interval significantly optimized synergistic fracturing efficiency by homogenizing the inter-borehole temperature field, prolonging thermal stress duration, and increasing expansive stress proportion. This study advances understanding of R-SCA fracturing mechanism, providing theoretical guidance for engineering practice optimization.