Investigation on fracture mechanism of coal rock on three-point bending tests under different gas pressures

To investigate the opening-mode ( mode-I) fracture toughness and the crack propagation characteristic of coal containing gas,semi-circular bend coal specimens with central straight cracks were tested with the self-developed visible gas-solid coupling test equipment under different molding pressures. The results show that mode-I fracture toughness and rupture energy decrease of coal area obviously influenced by gas. The fracture toughness and the rupture energy decrease with the increase in gas pressure,but the deflection and plastic deformation of coal increases with the increase in gas pressure. Crack propagation velocity becomes faster as gas pressure increases. When gas pressure in- creases from 0 to 1. 5 MPa,the crack velocity increases form 78 m / s to 239 m / s,and when cracks are gradually propa- gating,the crack velocity sharply decreases. On the basis of the digital speckle correlation method,the full deformation field of gas-containing coal at the mesoscopic scale has been successfully measured,and the whole failure process and damage evolution of the coal containing gas is revealed by the lateral displacement and strain of coal specimens.