Relationship Between Mode I Fracture Toughness and Strength Parameters of Brittle and Plastic Coal and the Fracture Process

Underground roadway excavation will form a plastic circle within the surrounding rock. The brittleness and plasticity of coal at different positions relative to the plastic circle are significantly different. Through drilling and sampling of the roadway roof, the KIC and strength parameters of coal with different brittle and plastic at different positions of roadway plastic circle were tested. Coal samples were collected at borehole depths of 0.7 m, 2.5 m, and 4.4 m, which are respectively located in the plastic circle, on the edge of the plastic circle, and outside of the plastic circle. The KIC and strength parameters of coal at different positions relative to the plastic zone are quite different. The KIC of coal at 0.7 m, 2.5 m, and 4.4 m borehole depth is 0.2436 MPa, 0.1736 MPa, and 0.1504 MPa, respectively. With increasing borehole depth, the compressive strength of coal increases, and the tensile strength decreases. For specimens with high compressive strength, the elastic modulus is relatively large, but Poisson ratio is relatively small. The linear fitting of KIC and strength parameters of coal shows that the KIC of coal negatively correlates with compressive strength and elastic modulus; the KIC of coal positively correlates with tensile strength and Poisson's ratio. Compared with the compressive strength, the tensile strength of coal has a stronger influence on the KIC and cracks can more easily expand with increasing tensile stress. Compared with the elastic modulus, Poisson's ratio has a stringer influence on crack growth and has a stronger correlation with the KIC of coal. And the relationship between the brittle index and the KIC is studied. The deeper the borehole is, the greater the brittleness index of coal is. There is a linear relationship between brittleness and KIC. KIC can be characterized by coal brittleness. Digital image correlation (DIC) was used to analyze the mode I crack growth characteristics and nonlinear failure process of semicircular three-point bending specimens. The accurate crack initiation point (PS) and the critical crack tip opening displacement (W0) of different brittle plastic coals are obtained. Compared with experimental results and numerical simulation, the method for calculating the W0 in this paper is more accurate. By using this method defined in this paper, the W0 of coal at 0.7 m, 2.5 m, and 4.4 m borehole depth are 0.0149 mm, 0.0138 mm, and 0.0115 mm, respectively. Mode I cracks in coal with stronger brittleness outside the plastic zone are easier to propagate. The non-linear fracture process of coal with stronger plasticity is more significant at shallow depths, and the load at the PS point at 0.7 m borehole depth is only 64.05% of the peak load. With increasing borehole depth, the more brittle the coal is, the closer it is to elastic failure. The load at the PS point of coal at 4.4 m borehole depth is 98.07% of the peak load, proving the importance of nonlinear fracture behavior analysis of coal. This study is significant to analyzing the crack propagation behavior of coal at different positions relative to the plastic circle of roadway and to analyzing the mechanism and features of different forms of roadway failures like roof fall and rock burst.