Creep properties and energy evolution of anthracite coal with different loading rates

The mechanical properties of creep coal bodies under the influence of dynamic pressure have important engineering significance for the stability and control of remained coal pillars in the re-mining area. The strain characteristics, elastic modulus and creep rate evolution characteristics of creep process of anthracite coal were studied by carrying out the graded loading creep and loading-unloading tests under different loading rates. The energy dissipation law of creep process was characterized based on the linear energy storage law of anthracite coal, and the “hardening-damage” mechanism of creep of anthracite coal was further explained. The results show that when the loading rate is less than 0.04 mm/s, the axial strain of the anthracite specimen shows obvious transient characteristics, but the radial strain shows obvious hysteresis and creep. With the increase of the loading rate, the axial hardening and radial expansion of the specimen in the loading stage are significantly enhanced, and the creep rate decay characteristics appear more obvious. The actual yield stress of the specimen meet the linear decreasing relationship with the loading rate, and its energy storage coefficient and creep limit elastic energy show the trend of increasing and then stabilizing with the increase of loading rate. The evolution of energy dissipation rate of each stress level shows the trend of “decreasing-stabilizing-increasing”, which is corresponded to the compression-density, elasticity and yielding damage stages of the specimen, respectively. The hardening-damage mechanism of anthracite specimens was analyzed from the perspective of energy dissipation. The hardening and damage effects act on the whole creep process, and the hardening and damage effects are comparable in the elastic stage, resulting in a stable energy dissipation rate of the specimen, and the damage effect dominates after the specimen enters the yield damage stage, resulting in a rapid increase in the energy dissipation rate. The specimen hardening and damage effects gradually become significant with the increase of loading rate. When the loading rate is less than 0.04 mm/s, the specimen hardening effect is stronger than the damage effect, resulting in a rapid increase in the ultimate elastic energy of the specimen. The yield stress reduction caused by the increase of loading rate will accelerate the evolution of the damage of the specimen, when the loading rate is greater than 0.04 mm/s, the damage effect of the specimen is significantly enhanced, and the mutual constraints of hardening and damage lead the ultimate elastic energy of the specimen to be stabilized.