Thermal evolution of mechanical properties and energy partitioning in mudstone with an entropy-based damage index

: High temperatures induced by tunnel fires can significantly deteriorate surrounding rock; however, understanding of its mechanical behavior, energy evolution, and damage development remains limited. In this study, tunnel mudstone was selected as the research object. A room-temperature control group and thermally treated groups at 100–500°C were considered. Mass and P-wave velocity were measured, and uniaxial compression tests were conducted to determine key mechanical parameters, including peak strength, peak strain, and elastic modulus. In addition, TG-DTA and SEM were employed to reveal the mechanisms underlying high-temperature evolution. Based on the stress-strain curves, an energy partitioning relationship was established, while the fracture fractal dimension and a multi-parameter information-entropy-based thermal damage variable were used to characterize the failure pattern and damage degree. The results show that the mechanical properties of mudstone evolve with temperature in a staged manner, transitioning from drying-related strengthening to thermal-damage-dominated behavior. The failure mode shifts from a single throughgoing main crack to sheet-like spalling failure with high energy consumption. Meanwhile, the thermal damage variable increases continuously with temperature and reaches a normalized maximum value of 1.000 at 500°C, indicating a pronounced nonlinear cumulative effect. This study establishes an integrated quantitative evaluation method for high-temperature mudstone by incorporating physical properties, mechanical behavior, energy evolution, and damage characteristics, thereby providing a basis for assessing the integrity and stability of tunnel surrounding rock after fire.