Investigation on damage laws of loading coal samples under cyclic cooling treatment

Coal is a solid medium in which contains pore and fissure structure. The extent of interconnection of pores and the width of the cracks play important roles in the permeability of coal samples. To fracture the coal and enhance its permeability,the cooling loading experiment of the coal samples under triaxial compression through liquid nitrogen injection was performed. The test equipment of liquid nitrogen injection into drilled coal samples was developed. The confining pressure of 4 MPa and several periods cooling loading were executed,respectively. The high magnification camera,metallurgic microscopy,sound wave velometer tester and CT imaging system were employed to investigate the evolution of the surface crack width,wave velocity and inner damage structure. According to the uniaxial compression stress-strain curve,the inner micro-structural change law of coal samples with cooling loading periods was studied. The results show that:① With the increasing periods of the cooling loading,the damage becomes much more serious. The damage velocity increases firstly,then decreases. So it is nonlinear. ② Particles drop off from the surface of coal sam- ples after 6 periods cooling loading,and the macroscopic fragmentation appears after a 7 periods cooling loading. ③ The damage and fracturing effects of liquid nitrogen cooling loading are much more remarkable for the initial fissures,and the expansion of the cracks mainly extends along the bedding direction. Liquid nitrogen cooling loading can evi- dently damage and fracture coal bed,and is a new method to prevent from rock and gas outburst.