冲击载荷对煤样表面裂纹扩展特征影响研究

Experimental research on evolution of surface cracks in coal mass under impact load

以型煤和原煤试样为研究对象,以自行研发的落锤式冲击加载试验装置、基于细观分析的微结构演化三维自动测控试验装置和煤岩表面微结构动态演化三维细观监测软件为主要研究设备,对冲击载荷作用下煤样表面裂纹扩展规律进行了试验研究.结果表明:冲击荷载使两类试样表面产生自上而下与冲击方向呈锐角分布的裂纹;表面裂纹形态呈锯齿状,且裂纹扩展遇坚硬颗粒时表现为穿晶与绕晶扩展模式相结合,并可能产生分岔,总体呈S型扩展;与型煤相比,原煤表面裂纹的变化频率、形态的粗糙度、分岔频率和穿晶扩展的比例均较低;随着冲击次数的增加,煤样表面裂纹宽度扩展表现为渐进式与突变式2种,表面裂纹扩展方向表现为随着冲击次数增加呈"高速变化-平缓发展-急速变化"3段式;坚硬颗粒阻碍煤岩表面裂纹扩展,原生裂隙对其具有双重作用,非均质性使其起裂位置和扩展路径具有很强的随机性;提出了1种表征煤岩表面裂纹扩展路径的函数;受相同作用后,原煤表面裂纹的分形维数大于型煤;随着冲击载荷增加,煤样表面裂纹分形维数呈减小趋势,各裂纹间分形维数的差异也逐渐减小.

In this experiment,briquette samples and raw coal sample were taken as research objects,a self-developed drop hammer impact loading experimental device,three-dimensional automatic measurement and control experimental device for micro structure evolution based on meso analysis and three-dimensional micro monitoring software of coal rock surface microstructure dynamic evolution were used as main devices to investigate the crack propagation and evo-lution of coal samples under impact load.The results show that the cracks with a sharp angle distribution from top to bottom to the impact direction appear on the surface of two kinds coal samples under under impact load.The shape of surface cracks show serrated border,and manifests transcrystalline and bypassing the crystal mode at the same time when the crack growth meets the hard particles.The evolution of surface cracks may be bifurcated and shown as S type on the whole.Compared with briquette samples,the frequency of change of surface cracks,roughness of shape,frequency of bifurcation and through the crystal mode of raw coal is lower.As the increase of impact times,the evolution law of surface cracks width of coal samples in the same position shows progressive mode and abrupt mode,and the direction of crack propagation representes as three paragraph style that " high speed change-gentle development-rapid change".Hard particles impede the evolution of surface cracks,and original cracks plays two roles in the evolution of surface cracks,in addition,strong randomness of initiation position and propagation path of surface crack is caused by heterogeneity of coal.A function containing five parameters determining the crack growth path of surface cracks is proposed.The fractal dimension of surface cracks for raw coal samples is larger than briquette samples under the same impact effect.The fractal dimension of surface cracks decreases as the increase of impact load,and the difference of fractal dimension between each crack is gradually reduced.