Wave velocity evolution and fracture distribution of soft coal under uniaxial compression

In order to study the ultrasonic propagation law and damage characteristics of the soft coal body, coal briquette is selected to replace raw coal, three wave propagation paths were set, and MTS rock mechanics experiment machine and PCI-II acoustic emission instrument were used to carry out the multi-path wave synchronization monitoring test of the coal body under the condition of uniaxial compression, and the anisotropy index was introduced to analyze the wave velocity evolution law. Three transverse and longitudinal sections were selected, and CT scanning equipment was used to carry out damage characterization experiments, reconstruct the micro-structure of the coal body after failure, and compare and study the differences in the distribution of the transverse and longitudinal fissures of two-dimensional slices, and to explore the relationship between the wave velocity and the three-dimensional volume of the fissures. Results show that: ① The parallel and vertical loading directions and the average wave velocity of the coal body show the basic law of smoothness followed by decrease and then smoothness with the increase of axial strain.The wave velocity anisotropy index has a smooth, then increasing and then smooth trend with the increase of axial strain, and the wave velocity in the vertical loading direction is the first to decrease, and the decrement is the largest. ② Compared with the vertical plane, the fracture spectral peak ratio and fractal dimension of the transverse plane of the damaged coal body are larger, the fracture density is higher, and the fracture morphology is also more complicated. ③ With the increase of fissure volume, the average wave velocity of the damaged coal body decreases roughly linearly, and the expansion of internal fissures in the coal body under uniaxial loading is dominated by the parallel loading direction, and the vertical linear expansion of the fissures is the main cause of the variability of the wave velocity in all directions and the damage characteristics of the transverse and longitudinal sections. ④ Compared with the parallel loading direction, the damage variables obtained by using the wave velocity in the vertical loading direction are larger and the evaluation of the damage state of the coal body is more reliable. When sound waves are used in the field to evaluate the damage state of coal , it is beneficial to improve the reasonableness of the prediction of instability accidents by making the sound wave propagation path perpendicular to the direction of the coal body loaded (maximum principal stress).