Mechanical properties of coal under repeated loading

Compared with ultimate strength under uniaxial compression, ultimate strength of coal samples under repeated loading decreases by 23.65%; compared with first loading, elastic modulus under second loading increases by 21.08%. Because coal has many initial fissures and a lower intensity, close and splay of initial and newborn damage fissures and non-recover macroscopic cracks offer a reasonable explanation for elastic modulus increase and strength limit decrease. More than a third of energy is consumed by the way of interior fissures close, material yielding, failure, breakage and sliding friction of fractures. Failure processes go through pressure consolidation, strain hardening and strain softening.