Using an analysis of the uniaxial compression process of Datong coal samples, the change of transient charge signals on coal surfaces is observed, and the influence of sampling directions (perpendicular to bedding planes and parallel to bedding planes) on the transient charge signals is studied. The intensity in perpendicular to bedding planes is 4.6~10.2 MPa, parallel to bedding planes is 2.1~5.3 MPa. The results show that the change of the charge signals on sample surfaces is instantaneous and pulsing, and such a change is always in accord with stress change and the alternation of positive and negative charge occurring over a short time period. Under uniaxial compression, the surface charge signal characteristics of coal sample in perpendicular to and parallel to the bedding are different. With a higher value of limiting stress, the transient charge signals on coal sample surfaces perpendicular to the bedding exhibit higher strength than those of coal samples oriented parallel to the bedding. However, the number of signal pulses during the failure process, for the samples perpendicular to the bedding, is less than that for the samples oriented parallel to the bedding. According to the variation law for transient charge signals on coal surfaces, we conclude that changes in the transient charge can serve as a tool to characterize crack propagation within coal specimens and provide an important reference for the prediction of coal and rock dynamic disasters.
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