Research on fatigue damage difference of soft and hard coal samples under cyclic loading

In order to compare and analyze the difference of fatigue damage process between soft and hard coal samples, the deformation and acoustic emission characteristics of coal samples are analyzed by cyclic loading test of constant amplitude triangular wave.The results show that the stress-strain curves of soft coal samples show continuous hysteresis loops under cyclic loading, while the hysteresis loops of hard coal samples coincide almost.The acoustic emission parameters show obvious cyclic characteristics.During the whole test process, the peak ringing number of hard coal samples is equivalent, and the soft coal samples show the law of stabilizing first and then decreasing in turn; the cumulative acoustic emission energy and impact number of hard coal samples show a ladder like straight-line upward trend, while soft coal samples show an obvious turning point.The difference of deformation characteristics of coal samples indicate that the fatigue damage process of hard coal samples had brittle failure characteristics, while the fatigue damage process of soft coal samples had plastic failure characteristics.The differences of acoustic emission characteristics indicate that cracks develop slowly under cyclic loading, and only small local cracks form.However, the small cracks in soft coal samples continue to develop, and gradually converge and coalesce with the increase of the number of cycles.The results provide guidance for the study of fatigue damage mechanism of different coal properties and the prevention and control of coal and rock dynamic disasters.