Experimental study on directional propagation of rock type-I crack

To obtain the rock type-I crack propagation process accurately, a simple crack directional propagation device was used to test the crack propagation of different rock types. The crack propagation process of rock is monitored with acoustic emission (AE) and digital image correlation method (DICM). A mechanical model of crack directional propagation was established. Then, AE and deformation field evolution laws during crack propagation process are analyzed, and a new crack propagation energy index (CE) is proposed to evaluate the difficulty of rock type-I crack propagation and the crack initiation and propagation mechanism is discussed. The results show that, the simple crack directional propagation device can effectively realize the stable propagation of type-I crack along the predetermined direction, the crack initiation angle is less than 10, and the deviations between the peak strength of crack propagation calculated by simplified mechanical model and the tensile strength of Brazilian splitting are 22.76% and 7.53% for white sandstone and grey sandstone, respectively. According to the deformation field evolution law, the crack propagation can be divided into three stages: microcrack development (speckle deformation field is still uniform), main crack propagation (zone phenomenon appears in speckle deformation field) and main crack propagation. Four stages including quiet stage, slow increase stage, rapid increase stage and decreasing stage can be identified in the AE evolution process. Compared with white sandstone, the gray sandstone is denser and the main cracks are fully developed, resulting in a long quiet stage of AE, while the last three stages are short. The area ratio of pre-peak area to post-peak areaof load-displacement curve was defined as C-E, the C-E of gray sandstone and white sandstone are 13-16, 1-2, respectively, which shows that the C-E can effectively evaluate the difficulty of type-I crack propagation. Rock type-I crack initiation and propagation mechanism can be summarized as follows: before the peak, the prefabricated crack tip is subjected to the maximum tensile stress, the elastic energy increases rapidly, and the dissipated energy increases slowly; nevertheless after the peak, elastic energy exceeds its energy storage limit and is released rapidly, most of the input energy is converted into dissipated energy, which results in a propagation rapid of crack. In the future, the crack directional propagation device will be optimized and improved, and it is expected to provide a new method for the study of crack propagation mechanism, rock fail precursor information and crack arrest principle, and to provide theoretical guidance for the optimization of coal strata directional blasting, fracturing, fracture arrest and other related technologies in the engineering site.