Quantitative analysis of crack expansion in specimens of coal having a single pre-existing hole

Experimental tests about the crack propagation of coal specimens having a central borehole under local load were conducted with high definition video camera,infrared thermal imaging instrument and relevant software. The char- acteristics in the whole process of crack initiation,propagation,coalescence under different local loads were analyzed. The results showed that the crack propagation in the specimens was caused by the coupling effect of load conditions and macro center borehole,and the route of crack was also dominated by it. However,the anisotropism rooted in micro- pores played a subordinate role. It would cause tensile or shear stress area around the borehole under uniform load, which was a key factor for crack propagation on that conditions. Macro center borehole was the main factor to cause stress concentration,but the dominate position of it would subvert with the change of load area,and the sequence of stress concentration occurred suddenly,the position of crack initiation was turned from peripheral borehole to the criti- cal point of the load. The influence of load area on the crack propagation in the specimens having a central borehole had a regional characteristic,which occurred obviously in the region of load area. With local load increase,the scale of the initial crack(crack width and length) increase,and the crack direction showed the trend of non-straight.