Effect of liquid nitrogen on structural damage of coal samples with different joints

The joint structure of coal body is the main storage space and seepage channel of coal bed methane,and the low permeability of coal seam is the main reason for the low efficiency of coal bed methane extraction.Because of the different angles of joints and the degree of penetration,the physical and mechanical properties of coal body are quite different.In order to increase the permeability of coal body,different kinds of jointed coal samples were prepared by a cold loading of coal body with liquid nitrogen.The long flame coal of Fuxin basin was used to produce 50 mm×50 mm×50 mm square coal samples.The coal samples were immersed in liquid nitrogen for 4 h and placed at room temperature for 20 h,which is a cold loading cycle (T).With multiple cycles of cold loading,the coal samples were ruptured.Using laser confocal microscope,acoustic wave tester,uniaxial compression tester and other instruments to observe the surface of coal samples,the overall fracture damage and the change degree of uniaxial compressive strength,it revealed the damage law of different jointed coal sample structures under the cold loading of liquid nitrogen at low temperature.The results show that:① The joint structure of coal samples (Ipb15°,Iam45° and Ial60°) has accumulated damage to reach the strength limit of coal and rock after 8T,6T and 7T cold loading,respectively;② Under the same cold loading period,the penetration joint is faster than the semipenetrating joint and the nonpenetrating joint;③ After the cold loading of coal sample,the joint expansion with bedding 45° is obvious.Cold loading and action period are important factors of joint structure damage of coal samples.The experimental results accord with the theory of fracture mechanics and is useful for the further study of coal structure damage.