Resistivity and damage of coal under cyclic loading and unloading

The stability of the coal in underground coal mining can be reduced greatly by mining activities and seepage water. Studying the failure and damage evolution of water-saturated coal under cyclic loading is therefore of great importance. Conducting single-axis equal-amplitude cyclic loading and unloading tests of water-saturated coal, this paper studied the resistivity response and analyzed the relationship between resistivity and strain field with Digital Image Correlation (DIC) in coal cyclic loading. The damage in coal cyclic loading was quantitatively characterized by the damage variables and damage constitutive equations based on electrical resistivity. The results show that the response of resistivity and DIC is regular and complementary. In the loading stage, the strain field has an opposite trend to the resistivity, which decreases as the load force increases; In the unloading stage, the resistivity increases correspondingly, but the shearing strain decreases. The above responses correlate with the sample failure: as the number of cycles increases, the sample's internal damage accumulates, and the resistivity and strain increase accordingly. The two damage variables and the trend of plastic strain are in good agreement. The damage constitutive model's calculation agrees with the experimental results. The resistivity method is able to objectively describe the whole damage process of saturated water coal under constant amplitude cyclic loading, thus contributing to the understanding of the damage mechanism of coal and rock subjected to complex stresses, which is of great practical significance and value to the safe and efficient production of coal mines.