Deformation and failure characteristics of rock-coal-rock combinations with different coal thickness ratios under constant amplitude cyclic loading and unloading

In response to the multiple disturbance effects of periodic weighting on the "roof-coal-floor" composite structure during the working face mining process, uniaxial conventional loading and constant amplitude cyclic loading-unloading tests were conducted on rock-coal-rock combinations. The deformation and failure characteristics of the combinations were analyzed using digital speckle correlation method (XTDIC), revealing the failure mechanisms of combinations with varying coal thickness ratios under constant amplitude cyclic loading-unloading paths. Research shows that: under the constant amplitude cyclic loading and unloading path, the peak compressive strength of the combination is negatively correlated with the proportion of coal thickness. Compared with the conventional uniaxial loading, the proportion of coal thickness is from low to high, and the compressive strength is reduced by 13.98%, 22.34%, 11.82% and 6.36% from 16.10 MPa, respectively, which are between the strength of single rock and single coal; With the increase of the proportion of coal thickness in the combination, the peak value of elastic modulus in the loading stage decreased from 2.13 GPa to 1.69 GPa, 1.31 GPa and 0.87 GPa, except for the first loading, the unloading elastic modulus is always lower than the loading elastic modulus; When the coal rock ratio is 1∶0.86∶1 and 1∶1.33∶1, the maximum principal strain field concentration area of the combination is dispersed and shows a multi regional development trend, and the overall failure is a tensile shear composite failure. When the coal rock ratio increases to 1∶2∶1 and 1∶3∶1, the strain concentration area develops to a directional strip, showing an overall shear failure; With the loading and unloading of axial stress, the overall displacement of each part of the combination shows the phenomenon that the elastic displacement in the initial compaction stage increases to the rebound fluctuation of the cyclic elastic-plastic displacement and finally increases sharply before the peak failure, while the lateral displacement shows the phenomenon that the lateral compression expansion in the initial compaction stage reaches the cyclic loading response, the residual expansion in the unloading stage reaches the sharp expansion before the failure and finally stabilizes; The fracture opening presents the characteristics of "ladder wave" transition growth and evolution, and shows strong nonlinear acceleration when it is close to the peak strength threshold.