Dynamic Mechanical Properties and Fragment Fractal Characteristics of Fractured Coal-Rock-Like Combined Bodies in Split Hopkinson Pressure Bar Tests

To investigate the effect of fracture distribution on the mechanical responses and failure forms of coal-rock combined masses under impact loading, dynamic impact tests on coal-rock-like combined bodies containing pre-existing cracks were carried out using split Hopkinson pressure bar. The dynamic mechanical properties and the failure and fractal characteristics of specimens under different impact velocities were investigated. The obtained results showed that the strength of fractured combined bodies was influenced by the positions and angles of cracks. It was found that fractures in coal sections had great influence on specimen strength and the maximum influence was obtained for the angle of 30 degrees. An obvious strain rate effect was also observed on the dynamic strength of RCB (combined body with fractures in rock section) specimens. Fractal dimensions of the coal sections of cracked combined bodies were close to those of intact ones when at angles 60 degrees and 90 degrees, while they were smaller at angle 30 degrees. The existence of cracks changed stress concentration in specimens under impact loading, changing their dynamic mechanics and failure characteristics, where fractures in coal sections with lower strength had a greater effect on the overall stability of combined bodies. Therefore, strengthening supporting work on the weak part of combined or layered rock masses has to be conducted in engineering practice and the failure of rock masses due to crack propagation in adjacent rock masses should be prevented.