Study On The Strength Criterion Of The Coal

Coal resources have become one of our country’s primary energy for its hugestorage. However, there are many coal accidents happened during coal mining, such asroof accidents, gas explosions and so on, which severely limit the development of thecoal industry, and some even seriously threaten our national economy as well as thepeople’s safety. Indeed, all these coal accidents are caused by the destruction of coal, soit needs to do further investigation of the failure criterion for the coal. Because ofplentyof micro-crack system and micro-inhomogeneous feature of the coal,it is difficult toaccurately describe its failure state only using the classic strength theory of the rockmechanics.So it is necessary to do in-depth research of the strength criteria of coal fromthe perspective of the micro-fracture mechanism.This thesis is supported by the state key laboratory of coal mine disaster dynamicsand control, with the research “Constitutive relations and yield criterion of the coal”(No.0222002109107). A new strength criterion of the coal isproposed based on fracturemechanics and micromechanics, which is derived from the growth of three-dimensionalpenny-shaped micro-cracks. The main conclusions are summarized as follows,①Different failure modes of coal are analyzed from the destructive mechanism,yet the factors affecting the intensity coal are investigated.Then the micro-fracturemechanism of coal are further studied from its microstructure and micromechanics,finally the strength criterion of the coal is obtainedthat it will be damaged when thedensity of micro-fracture reaches a critical value. When considering the rheologicalproperties of coal and adding the time factors, the strength criterion will become thecreep strength criterion.②A three-dimensional micromechanical model of coal is established, and yet themathematical formulasof the creep stress intensity factor are obtained. The micro-failureorientation angle isapplied to characterize the inside density of micro-fracture of thecoal under tri-axial compressive loads, yet the relationship between the angle domain and its external load is derived based on the brittle fracture theory of the mixed-modecracks. Based on the micromechanics, cos1is selectedas the failurecharacteristic parameter, then the creep strength criterion and the general criteria of thecoal are derived. ③The uniaxial compression experiments and tri-axial compression experimentsare carried out using the MTS tri-axial testing system, and creepexperiments are carriedout usingrheological test system. The experimental result is analyzed, and then therelevant strength parameters of the coalare determined. Compared with experimentalcurves, the theoreticalcurves are in good agreement with experimental ones.