Laboratory Investigations of a New Method Using Pressure Stimulated Currents to Monitor Concentrated Stress Variations in Coal

Abstract

Coal bumps occur frequently during coal mining, and monitoring of stress variations of coal mass is a prerequisite for coal bump prediction. To investigate a new method to reflect coal stress variation based on the pressure stimulated current (PSC), localized cyclic loading–maintaining–unloading experiments on coal samples were carried out and weak currents were measured. The response characteristics of PSCs with stress variations were analyzed, physical models showing the mechanisms of PSCs were established, and mathematical models explaining the relationships between PSCs and mechanical behaviors were derived. The results show that PSCs (tens of nA) flowing from the stressed volume to the un-stressed one can be observed upon load application. The currents are repeated with cyclic loading, load maintenance and unloading, which indicate that PSCs correspond well to stress variations. During loading, the PSC increases to a peak value at approximately 0.6 MPa and then it fluctuates until the stress stops increasing. When load is kept constant, the current decays exponentially to a stable value. During unloading, the PSC decreases at a lower rate when stress is greater than 0.6 MPa, after which it decreases at a higher rate to a minimum when stress is removed. Fitting results show that PSC increases exponentially with loading rate and that the stable current increases linearly with the maintained stress. Positive holes activated by the fracture of chemical bonds in the side chain of coal macro-molecules are believed to be the carriers, and the PSCs are generated due to the movement of the holes along stress gradients. Deduced formulas relating PSCs with coal mechanical behaviors show that PSC is proportional to strain rate; that is, PSC is inversely proportional to elastic modulus when stress rate is constant, as verified by the experimental results. These research results are expected to provide a new idea for monitoring of stress variations of coals during mining, which is significant for the prediction and early-warning of coal bumps.