Progressive damage characteristics and constitutive model construction of perforated coal under sub-high temperature

• Mechanical properties of coal under sub-high temperatures were studied. • Characteristics of progressive coal destruction under sub-high temperatures were studied. • A coupled heat-stress damage model based on AE energy was constructed. • The mechanism of sub-high temperatures on coal strength was revealed. In order to explore the integrity of deep coal seam gas extraction boreholes in a sub-high temperature environment, uniaxial compression and scanning electron microscopy experiments were carried out on coal with boreholes at different pretreatment temperatures (ranging from 25℃ − 80℃). The combined acoustic emission (AE) and XTDIC scattering systems were employed to investigate the effects of sub-high temperature on the mechanical properties and progressive damage characteristics of coal. A damage constitutive model appropriate for heating-cooled coal was established, which elucidated the mechanisms of augmentation and degradation of coal’s mechanical characteristics under sub-high temperature conditions from a microscopic perspective. The research results demonstrate that within the temperature range of 25℃-80℃, the compressive strength and elastic modulus of coal initially increase and subsequently decline with rising temperature, with 65℃ serving as the inflection point for coal strength. The surface strain and AE signals of coal exhibit nonlinear variations with temperature, while the maximum main strain shows a “V” shaped pattern as temperature rises. The AE count and energy release progressively diminish after reaching a peak at 50℃. Considering the positive correlation between coal damage and energy accumulation, a temperature-corrected damage constitutive model was constructed based on the time series characteristics of AE energy, which can effectively characterize the pre-peak mechanical behavior of coal under sub-high temperature conditions. The combined effect of microcrack closure, crack growth, and the absorption and release of water caused by high temperatures is the main factor contributing for changes in coal’s mechanical properties, which significantly affects the dynamic evolution of its mechanical behavior.