The mechanism of meso-crack damage evolution in the failure process of porous soft coal specimens

The accumulation of meso-crack damage is one of the main reasons for the formation of large-area fractured air leakage channels around the drainage boreholes. The damage accumulation of mesoscopic cracks around the hole is manifested in three stages：incubation，stable development and rapid development. In order to explore the damage evolution and crack propagation characteristics of the porous soft coal at different stages，a study on the meso-crack damage law of the porous soft coal sample during the uniaxial compression failure process was carried out. Gypsum and water were mixed at a mass ratio of 7∶3 to make a square sample of soft coal with holes. The uniaxial compressive test was performed and the digital speckle correlation measurement method (DSCM)was used to obtain the complete deformation of the sample surface. Based on this，a method of using the DSCM system to determine the instant Poisson’s ratio was proposed，and it was used to calculate the micro-crack density and the establishment of the constitutive relationship of the micro-crack damage when the soft coal was loaded. The results show that：①based on DSCM technology，the Poisson’s ratio of porous soft coal samples can be obtained accurately and reliably. ②during the incubation and stable development stage of meso-crack damage，the density of meso-cracks grows slowly，and no obvious macro-cracks appear 〖JP2〗on the surface of the specimen. During the rapid development stage of damage，the density of mesoscopic cracks increased exponentially. At this time，the area far from the edge of the hole has visible macroscopic cracks. ③The growth of microcracks in the elastic stage is very slow，and the growth rate of microcracks in the plastic stage is accelerated，and the stress peak point is the catastrophic point of the acceleration of microcrack growth. ④The damage evolution model of the soft media under load is established，and the instant Poisson’s ratio can reflect the inelastic and other mechanical effects of the soft coal body’s meso-damage.