Experimental study on the effect of unloading rate on the damage of coal with boreholes

Abstract

In coal mining, the gas pre-drainage boreholes in front of working faces are often repeated loading and unloading. Studying the failure characteristics of boreholes under cyclic loading has a guiding significance for clarifying the failure mechanism of gas pre-drainage boreholes. To simulate the influence of the advancing speed of the working face on the deformation instability of the borehole, a cyclic loading path under the influence of different unloading rates was designed. In this paper, the uniaxial cyclic loading test was conducted by the DYD-10 testing machine, and the variation law of mechanical strength characteristics with the unloading rate was obtained. Combined with digital image technology (XTDIC), the sample deformation characteristics were analyzed. The samples' compressive strength rose with the unloading rate. The sample was damaged in the tensile-shear composite form at different unloading rates. Crack opening displacement (COD) with the increase in the number of cycles shows a “stepwise” increase. Near the moment of destruction, COD suddenly increased. The number of surface fractures that could be seen reduced as the unloading rate increased, and reduced fragmentation when the sample failed. The various types of energy of the sample increase nonlinearly with the increase of the number of cycles, and the plastic damage energy drops as the unloading rate increases. When the external stress state enters the next stress state, the internal stress adjustment of the sample takes a particular time, which is manifested as lagging behind the change of the external stress state. The influence mechanism of stress adjustment lag on strength, crack evolution and damage under different unloading rates was proposed. The results of the study contribute to the research of destabilization control technology for gas pre-drainage boreholes.