层理对煤岩动态裂纹扩展分形特征的影响

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

对于煤岩类材料在不同应变速率下的裂纹扩展行为研究为采矿工程实践提供了重要的理论参考,然而该类研究难度较大。为了研究含层理结构各向异性煤样在冲击载荷下的裂纹扩展行为及发育机制,采用霍普金森加载装置对直切槽半圆形煤样进行冲击条件下的断裂特性测试,并采用高速摄像机对冲击过程中裂纹扩展发育特征进行拍摄。利用Matlab和Image J等图像处理软件,进一步分析了层理和加载速率对煤中动态裂纹扩展分形特征的影响。研究表明:层理结构对煤中动态裂纹扩展路径及速度均具有重要影响,45°层理煤样分形裂纹扩展速度最大,而层理倾角为0°煤样的分形裂纹扩展速度最小;层理倾角对裂纹扩展路径的分形维数也具有显著影响,45°时煤样中裂纹扩展路径分维值最大,其次为层理角度22.5°和67.5°,0°和90°最小;同一裂纹的速度峰值和加速度峰值交替出现,且加速度峰值出现在速度峰值之前,裂纹扩展的驱动力可由加速度指标间接表示。考虑瞬时效应和分形特征的若干煤样裂纹扩展速度测试值与瑞利波速接近,且22.5°,45.0°和67.5°层理角度的煤样测试时易出现最高峰值。该试验结果与先前理论假设相吻合,因此获取含结构面煤岩类材料动态裂纹扩展速度时应当充分考虑其分形效应和瞬时特征。研究成果有助于进一步理解掌握含结构面煤岩体受动载扰动下的裂纹扩展分形特征及裂纹速度演变规律。

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.