Experimental investigation into interface effect and progressive instability of coal-rock combined specimen

The characteristics of the coal-rock mass combination determine the deformation and instability of the overall structure formed by the coal-rock combination. In order to analyze the influence of the coal-rock height ratio on the mechanical characteristics,energy conversion laws,and the instability failure characteristics of the combination under load,“Rock-Coal-Rock”(RCR) test specimens with three different proportions of height ratio were made. Subsequent- ly,a uniaxial compression test and a PFC2D numerical simulation of combined test pieces were carried out. Based on the analysis of acoustic emission and macro-destructive characteristics, the meso-damage characteristics, macro-me- chanical characteristics,and acoustic emission energy evolution laws of the assembly before and after transient instabil- ity were obtained. The research shows that with the increase of the height ratio of coal and rock,the uniaxial compres- sive strength,elastic modulus and dynamic intensity at the failure of the combination decrease. The damage of the specimens is an asymptotic discontinuity. The damage first occurs and develops in the coal body in the specimens. Then when the coal crack develops to the interface of coal and rock,it is blocked. Finally,due to the influence of the Poisson effect of the composite body and the increase of the elastic energy storage of the composite body,the cracks propagate from the coal body to the rock,and the composite body undergoes an overall failure. Due to the difference in the structure and elastic energy storage of the RCR specimens,the crack propagation ability,speed and angle are dif- ferent. Furthermore,the combination has different failure shapes,but the failure patterns are similar. Among them,the coal body is dominated by shear failure,and the top and bottom rocks are dominated by split failure. The acoustic e- mission signal of RCR specimens has obvious phase characteristics when it is loaded. When the overall instability oc- curs,the acoustic emission energy rises rapidly and reaches a maximum. According to the regular characteristics of a- coustic emission events,the instantaneous damage of RCR specimens can be monitored and warned,which provides a reference for the mechanism and prediction of rock burst.