Experimental Study on Mechanical Properties of Coal Under Repeated Loading

The mechanical properties of coal samples from Yunhe coal mine under repeated loading are studied through experiments of MTS 815.03 servo-controlled rock mechanical test system.The results indicate that compared with the ultimate strength under uniaxial compression test,the ultimate strength of coal samples under repeated loading decreases by 23.65%;compared with the first loading,the elastic modulus under the second loading increases by 21.08%.Because the coal samples have many initial fissures and lower intensity,the close and splay of the initial and newborn damage fissures and the non-recover macroscopic cracks offer a reasonable explanation for the increase of elastic modulus and the decrease of the ultimate strength.Under repeated loading,more than a third of the energy is consumed by the way of the close of interior fissures,the yielding,failure,breakage of the material and the sliding friction of fractures.The failure processes of coal go through pressure consolidation,strain hardening and strain softening.