Effect of bedding on the fractal characteristics of dynamic crack propagation in coal rocks

The research on the crack propagation behavior of coal and rock materials under different strain rates provides an important theoretical reference for mining engineering practice.In order to study the crack propagation behavior and development mechanism of anisotropic coal samples with bedding structure under impact load,Hopkinson loading device was used to test the fracture characteristics of semi-circular coal samples with straight cut slots under impact.The high-speed camera was used to photograph the crack propagation characteristics during the impact process.The influence of bedding angle and loading rate on the fractal characteristics of dynamic crack propagation in coal was further analyzed by using image processing software such as MATLAB and Image J.The results show that the bedding structure has an important influence on the dynamic crack propagation path and velocity in coal.The fractal crack propagation velocity of 45° bedding coal sample is the largest,while the fractal crack propagation velocity of 0° coal sample is the smallest.The fractal dimension of crack propagation path in coal samples with 45° bedding is the largest,followed by bedding angle of 22.5° and 67.5°,and the smallest is 0° and 90° respectively.The velocity peak and acceleration peak value of the same crack appear alternately,and the acceleration peak appears before the velocity peak value.The driving force of crack growth can be indirectly expressed by the acceleration index.Considering the instantaneous effect and fractal characteristics,the crack propagation velocity of several coal samples is close to Rayleigh wave velocity.The coal samples with the bedding angles of 22.5°,45° and 67.5° are more likely to appear the highest peak value.The experimental results are consistent with the previous theoretical assumptions.Therefore,fractal effect and instantaneous characteristics should be fully considered while obtaining the dynamic crack propagation velocity of coal and rock materials with bedding structures.The research results are useful to further understand the fractal characteristics of crack propagation and the evolution law of crack velocity of coal and rock mass with structural plane under dynamic load disturbance.