An experimental study on the precursory characteristics of EP before sandstone failure based on critical slowing down

Abstract

The electric potential (EP) generated during coal rock deformation and fracture reflects coal rock mass stress and damage states. However, identifying the EP signal precursory signs that characterize a coal rock mass tending toward instability and failure is difficult. In this paper, we established an acoustic-electric multiparameter test experimental system, synchronously tested and collected acoustic emission (AE), surface potential (SP), and axial strain (AS) and observed surface crack propagation during the sandstone sample failure process. Meanwhile, by introducing the critical slowing down theory, the EP signal precursory characteristics before the sandstone sample unstable failure were explored. The results showed that EP signal time series autocorrelation coefficients and variances increased abruptly and continuously before the main fracture of the sandstone samples, which could be used as sandstone failure precursory information. Compared with the autocorrelation coefficient, the variance mutation point was easy to identify and was not affected by lag length selection, which was an effective precursory point to predict sandstone sample failure. From the precursory point to the main fracture of the sandstone samples, the AE events gradually gathered at the fracture damage location, high-energy AE events gradually increased, and the crack propagation rate gradually accelerated. Compared with the AE count time series, the variance mutation point (precursory point) of the EP signal time series appeared earlier. The research results were of theoretical significance for the monitoring and early warning of coal rock mass instability and failure and for identifying precursory characteristics using the EP response.