Evolution law of plasma-induced fractures in porous media materials: A case study on coal samples

Plasma based on electrical discharge has attracted extensive attention in various fields for cracking hard coal, yet the behavior characteristics of plasma channels and the evolution law of fractures in coal have not been investigated in depth. In this study, the evolution law of plasma-induced fractures in coal was revealed by means of physical experiments and numerical simulations. Moreover, the propagation behavior of plasma channels in coal was analyzed. The main conclusions were drawn as follows. The propagation of fractures in coal manifests four behavior characteristics due to numerous minerals and pores in coal bodies: 1) Newly formed fractures are interconnected and develop towards primary fractures; 2) Mineral enrichment areas have an attraction effect on fractures; 3) Fractures propagate along interfaces between minerals and coal matrix; 4) Fractures penetrate through minerals. Besides, the development law of plasma channels in coal under different voltage conditions was explored based on the WZ (Wiesmann H J and Zeller H R. A.) breakdown model of solid dielectrics. It was found that plasma channels in coal develop from the positive electrode to the negative electrode in a “dendritic” shape. The influences of pores and pyrite in coal differ significantly on the patterns and development trends of electrical branches. Specifically, pores in coal have an attraction effect on electrical branches. This indicates a sudden increase in the electric field intensity around fractures, which leads to a surge of local stress and thereby induces the generation of fractures. The research findings are of crucial significance for theoretical study on the propagation mechanism of plasma-induced fractures in coal.