Study on deformation and failure characteristics of coal containing echelon crack-boreholes under the influence of loading rates
In order to investigate the deformation and failure characteristics of coal around gas drainage boreholes under the influence of loading rates,uniaxial compression tests of coal samples with echelon crack and borehole composite defects under different loading rates were carried out.Combined with digital image correlation technology,the mechanical properties,failure modes and deformation field evolution of coal samples with composite defects under different loading rates were analyzed.Through computer vision technology,the characteristics of borehole failure and echelon crack propagation under the influence of the loading rate were further studied,and the loading rate effect of deformation and failure characteristics of coal samples with composite defects was revealed.The results show that:(1)The echelon crack and borehole composite defect samples have the characteristics of high strength at low loading rate and low strength at high loading rate under different loading rates.The critical loading rate of the sample is 0.2 mm/min,and the change trends of peak stress,peak strain and elastic modulus of the sample change at the critical loading rate.(2)The samples present progressive failure characteristics at different loading rates,and the failure mode is dominated by tensile shear mixed failure.With the increase of the loading rate,the tensile effect gradually dominates.Under the induction of echelon cracks,the sample shows shear dislocation failure along the penetration surface formed by new cracks and flying cracks at low loading rate,and shows tensile failure along the horizontal opening of new cracks at high loading rate.(3)The borehole shrinkage C is introduced to quantitatively characterize the degree of borehole deformation.The borehole is opened at low loading rate,and C is greater than 1.With the increase of loading rate,C gradually decreases.At high loading rate,the borehole first opens and then collapses rapidly,and finally C is less than 1.(4)Under the action of loading,the echelon cracks are staggered and opened in the counterclockwise direction,and the deformation is mainly concentrated in the unstable crack propagation stage.With the increase of loading rate,the dislocation momentum and opening amount increase significantly.With the increase of loading rate,the dislocation momentum and opening amount of the echelon crack increase significantly,and the tensile effect of the sample is enhanced,and the opening deformation ratio is gradually higher than the dislocation deformation ratio.
rock mechanicsloading rateextraction drillingechelon cracksdrilling deformationcrack propagation
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