Experimental study on impact dynamics and failure characteristics of coal specimen under true triaxial conditions

Deep coal is mostly in a three-dimensional unequal stress state. In order to study the dynamic characteristics of coal under impact load, the dynamic failure mechanics experiment of coal was carried out by true triaxial Hopkinson experimental system. This system can be used to provide uniaxial (σ₁≥σ₂=σ₃=0), biaxial (σ₁≥σ₂ > σ₃=0) and true triaxial (σ₁≥σ₂≥σ₃ ≠ 0) pre-stressing for coal samples respectively(where, σ₁ is axial prestress; σ₂ is vertical prestress; σ₃ is horizontal prestress), so as to realize the mechanical property test of coal samples at high strain rates. By analyzing the dynamic mechanical characteristics and differences of coal under uniaxial and true-triaxial static pre-stress, the relationships of dynamic peak strength, peak strain and macroscopic fracture morphology with three-dimensional static pre-stress and strain rate were studied. The results show that the mechanical curves of coal under true triaxial impact load are basically the same, including three stages: linear elastic stage, plastic stage and failure stage. The peak strength and strain of coal samples under uniaxial and true triaxial impact loads have obvious strain rate effects .With the increase of strainrate, the peak strength and peak strain of coal under uniaxial and true triaxial impact loading increase gradually. Compared with uniaxial and true triaxial impact, the three-dimensional static load pre-stress has obvious constraint on the dynamic failure of coal specimen, and the mechanical strength of coal specimen is higher than that of uniaxial impact. The mechanical peak strength and strain of the coal sample decrease with the increase of the principal stress σ₁ while the stress σ₂ and σ₃ remain unchanged. With the increase of strain rate, the coal under uniaxial impact load is gradually broken into particles or powder, and the average particle size of the fragments decreases linearly with the increase of strain rate. However, the coal under three-dimensional static pre-stress presents soma macroscopic fractures, and the average particle size of fragments decreases linearly with the increase of σ₁ while the damage degree is lower than that of uniaxial impact.