Micro- and macroscopic study of crack propagation in coal: theoretical and experimental results and engineering practice

摘要

Coal is a material that fractures naturally. To determine the physical laws that govern crack propagation in coal, a fracture mechanics model was established and uniaxial compression experiments were conducted on coal samples. During the experiments, a high-definition camera, scanning electronic microscope and an optical microscope were used to record the crack propagation process. These techniques illustrate an intuitive procession of events: damage - crack - crack propagation - macroscopic failure. Pre-existing and induced fractures were studied separately, and results show that induced fractures are formed by the evolution of damage and always occur near these pre-existing cracks where the damage is severe. In a complete stress-strain curve a bending point exists after which transverse induced cracks begin to develop. The pre-existing crack experiences shear slip, Mode II propagation and stable bending propagation before the plastic point after which it switches to shear propagation resulting in the macroscopic failure at the peak point. Pre-existing cracks tend to branch towards the damage in the coal matrix while induced fractures tend to connect with other cracks nearby. According to crack propagation, the abutment pressure area in an underground mine was divided into six zones and the field data are consistent with the theoretical and experimental results.