DEM Simulation Of Dynamic Characteristics Of Bulk Iron Ore Concentrates During Maritime

Iron ore concentrates(IOCs)is one of the indispensable industrial materials in economic development,and its main transportation method is bulk ocean transportation.However,due to its high moisture content(MC),it is very easy to liquefy during shipping,which causes major marine accidents.In order to reveal the formation mechanism of bulk IOCs during maritime,this paper uses discrete element numerical simulation to study the dynamic characteristics of bulk IOCs under cyclic loading from a meso-scale.Firstly,a particle-fluid coupling analysis module is constructed with the initial MC of IOCs as a variable,and the model is verified by the results of the preliminary hollow cylinder torsional shear test.Secondly,the effects of factors such as load,frequency,and confining pressure on the dynamic characteristics of unsaturated IOCs are explored.At last,the microcosmic evolution laws of porosity,coordination number and contact force chain in the shear failure process of element samples are analyzed to study the dynamic characteristics of IOCs in bulk under circular coupled cyclic load.The research in this paper lays a foundation for further revealing the liquefaction mechanism of IOCs,and provides a theoretical basis for effectively preventing and controlling the liquefaction of bulk IOCs in marine transportation.The main research results are as follows:(1)Considering the unsaturated characteristics of IOCs,a particle-fluid coupling model with initial MC as a variable is proposed,and a numerical coupling computing framework is built on the particle flow platform.In this model,the measurement sphere is used to divide the measurement domain.By solving the motion equation of particles and fluid separately and changing the stiffness of the water-air mixed pore fluid,the strain induced pore fluid pressure in the measurement domain and the fluid pressure gradient between the measurement domains can be calculated.The model can realize the undrained mechanical behavior of IOCs under different MC conditions,and can monitor each measuring sphere to obtain the dynamic change of local pore pressure of the sample.(2)In the particle flow program,the hollow cylindrical unit sample of IOCs with laminated wall as the model boundary is established,and then the vertical-torsional circular coupling cyclic load is realized by controlling the axial loading speed and the particle torque of the bottom torque layer.At the same time,the particle-fluid coupling model is added,and the micro parameters of IOCs with different MC are calibrated according to the previous hollow cylinder test results.Finally,the hollow cylinder discrete element samples with different MC are established.(3)The generalized shear strain curves of IOCs with MC of 8% and 9% under different loads,vibration frequencies and confining pressures are compared,and the influences of various factors on the dynamic characteristics of IOCs under cyclic load are discussed.The results show that under the action of low dynamic stress and low frequency,if the MC exceeds 8%,as long as the vibration times are enough,the IOCs will still be damaged.Additionally,the confining pressure has a significant effect on the anti-liquefied ability of IOCs.The IOCs with MC of 8% can be damaged under the action of low confining pressure,while the IOCs with MC of 9% in the simulated confining pressure environment at the burial depth of 8m,its dynamic strain growth rate is weak in the middle and later period,the strength of liquefaction resistance is high,and the possibility of liquefaction formation is low.(4)Based on the measurement sphere distribution model,it is found that the local porosity,average coordination number and contact force chain distribution of IOCs with MC of 8% and 12% are significantly different during dynamic loading.With the development of dynamic strain,the local porosity of IOCs with MC of 8% gradually decreases,the coordination number fluctuates up and down slightly,and the strong and weak contact force chains uniformly distributed during the loading process make the particle structure stable.However,for the sample with MC of 12%,the porosity first drops and then rises,and the coordination number fluctuates violently.Combining with the position of the measuring ball,it is inferred that there is a shear band in the direction of about 45°-50° spiral.Meanwhile,the strength of the contact force chains decrease gradually,and the strength chains disappear,which imply that the particle skeleton basically loses its strength.The results show that the micro evolution of IOCs under cyclic loading is closely related to the macro mechanical behavior.