Energy criterion of coal breaking by abrasive jets based on stress wave effect

Abrasive jet is a green processing technology with easy operation and high cutting efficiency. It is widely used in mining,oil and gas development and other fields to break coal rock. Abrasive jet coal breaking is a process of jet energy transfer,transformation and release. The jet energy is directly loaded on the jetcoal rock contact area in the form of cyclic and impact load. At the same time,it is transferred to the interior of the coal rock in the form of spherical stress waves. When the stress wave energy is higher than the coal damage threshold,coal damage occurs. However,the current stress wave propagation law and effective coal breaking range in the process of coal breaking by abrasive jet are not clear. Therefore,in this paper,the energy density of coal rock at crack initiation during local microcracking was analyzed based on the stress wave effect,and the energy criterion describing coal breaking by abrasive jet was established by combining with the characteristics of abrasive jet. The energy equivalence method was used to study the energy consumption law and damage characteristics of coal samples under the same incident energy based on SHPB (Split Hopkinson Pressure Bar) experiment,and the coal rock parameters in the energy criterion were calculated. On this basis,the stress wave propagation law under the impact of abrasive jet was obtained by DIC (Digital Image Correlation) experiment,and the theoretical range of coal breaking by abrasive jet was calculated by using the energy criterion. The energy criterion of coal breaking by abrasive jet was verified by comparing the actual and theoretical coal breaking depth. The results show that the energy threshold for coal destruction during abrasive jet impact is determined by the stress wave velocity,the energy density of coal rock breaking,the average particle size of broken pieces and the abrasive jet impact frequency. With the increase of incident energy,the strain rate of the coal sample increases and the energy consumption density increases,but the effective utilization of incident energy decreases. The energy absorbed by the coal body is mainly used for microcrack expansion,and the average diameter of broken pieces after crushing is a primary function of the strain rate of coal sample. The spherical stress wave propagation process has obvious spatial and temporal characteristics,and the energy attenuation occurs along the propagation direction. However,due to the development of primary pores and fissures in the coal,the stress wave is aggregated and superimposed,resulting in an increase of local energy. If the stress wave energy is higher than the energy threshold of the coal body when the cracks start to crack,the coal body appears to be damaged.