Mechanical properties and failure characteristics of composite coal and rock mass under dynamic and static loading

In order to understand roadway instability mechanism under gas explosions and coal and rock dynamic dis⁃ asters，the damage characteristics of composite coal and rock mass under static and dynamic loading，as well as the effect of thickness ratio， combination angle and impact direction on damage characteristics were investigated using the MTS servo press and Hopkinson pressure bar(SHPB)system. Cylindrical samples with the height and diame⁃ ter of 50 mm were adopted in the tests. Bituminous coal and sandstone were selected as experimental materials. The coal and rock thickness ratios in the tests are 0 ∶ 1，1 ∶ 2，1 ∶ 1，2 ∶ 1，and 1 ∶ 0，while the coal and rock combi⁃ nation angles are 0°， 15°， 30°， 45°， 60° and 90°. In the dynamic impact test， two loading directions were applied(strike coal sample and strike rock sample)respectively to observe the influence of impact direction on the mechanical properties of composite coal and rock mass. The results show that the higher the thickness ratio of coal to rock，the smaller the compressive strength and elastic modulus of composite coal and rock mass. The elastic modulus of the composite coal and rock mass increases gradually with the increase of the combination angle. The maximum and minimum elastic modulus is obtained under the combination angle of 90° and 0°，respectively. The compressive strength firstly decreases and then increases. When the combined angle is 45°，the compressive strength is lowest. The compressive strength is highest when the combined angle is 90°. Compared with striking coal sample，the compres⁃ sive strength and elastic modulus of the composite coal rock mass are larger when striking rock sample，while the ulti⁃ mate strain is slightly lower. Under uniaxial loading， when the combination angle of the composite specimen is less than 45°，the splitting failure occurs. The splitting failure and compression shear failure coexist when the combina⁃ tion angle is 45°. When the combination angle is greater than 45°， the shear failure occurs along the interface. Under the dynamic loading，the stress⁃strain curve at the combination angle of 45° does not show springback phenom⁃ enon due to slip on the interface of coal and rock，and the dynamic compressive strength is lowest under this condition.