Barrier formation of micro-crack interface and piezoelectric effect in coal and rock masses

• The micro-cracks of coal or rock expand constantly under external loads.
• The slip, dislocations and inhomogeneous deformation of crack cause charge breakthrough.
• Crack interface value of potential barrier increases in the compression process of coal sample.
• The transfer speed of charge is based on potential crack interface value.
• Crack interface barrier and potential amplitude get change dramatically before coal sample fracture.

The interface barrier formation mechanism of micro-cracks of coal and rock mass is investigated by using theoretical analysis and piezoelectric experiments. It is found that the micro-cracks of coal or rock expand constantly under external loads, while the slip, dislocations and inhomogeneous deformation of crack interface and lattice cause charge breakthrough, and then, barrier form on the crystal interface. The generation process of charge is accompanied by the friction of micro-crack slip, and the dislocation and deformation are analyzed theoretically. A theoretical model of micro-crack slip friction charge is established. Furthermore, piezoelectric law of coal or rock deformation and fracture is studied experimentally. The results show that cracks expand vertically more seriously than horizontally, and vertical cracks expand first, followed by horizontal ones, which expand continuously till crashed at the end of the coal sample. Crack interface value of potential barrier increases constantly in the compression process of coal sample. And before the coal sample gets crashed, sliding friction between crack interfaces reaches the peak, together with the potential barrier. Moreover, extreme acceleration of transfer speed of charge is based on extreme multiplying potential value of crack interface. Both crack interface barrier and potential amplitude get change dramatically before the coal sample fracture, which will help to get the precursor information of coal sample rupture. More importantly, the monitoring results can predict the extent and state of coal and rock rupture.