Discrete Element Analysis On Press And Fracture Of Propellant Bed

The press and fracture degree of propellant bed is the key to evaluate the launch safety of propellant charge.Launch safety accidents caused by propellant charge are closely related to the press and fracture degree of propellant bed.The press and fracture process of propellant bed under the mechanical environment of bore is extremely complicated.Considering the cost and safety of physical test,it is of great theoretical significance and engineering application value to study the press and fracture process and fracture degree of propellant bed by numerical method.Based on Discrete Element Method,the numerical model of propellant grain and propellant bed were established,and the press and fracture processes of propellant grain and propellant bed were simulated.After the press and fracture process of propellant bed,the fracture degree of propellant bed was counted and compared with the physical test result.Firstly,the discrete element analysis on press and fracture process of propellant grain was carried out,and the fracture form,crack propagation and energy evolution of propellant grain were studied.Secondly,the influence factors analysis on the press and fracture process of propellant grain was carried out,and the effects of loading position,confining pressure and initial defect on the press and fracture of propellant grain were studied.Finally,the discrete element analysis on press and fracture process of propellant bed was carried out.The force chain transmission,crack evolution and fracture degree of propellant bed were studied,and the effects of initial packing structures on the press and fracture of propellant bed were studied.The following conclusions were obtained.1)The fracture mode of propellant grain under unconfined uniaxial compression test is shearfailure along the direction of maximum shear stress.The press and fracture process ofpropellant grain can be divided into four phases: elastic deformation phase,crackinitiation phase,crack stable propagation phase and crack unstable propagation phase.Thepress and fracture process of propellant grain can be regarded as a evolution process fromenergy dissipation to instability,and the boundary work is mainly stored in the contactsand bonds between particles in the form of strain energy and bond energy.2)Loading position,confining pressure and initial defect have significant effects on the peakstrength and fracture form of propellant grain.The compressive capacity of propellantgrain under lateral compression is lower than axial compression,and the peak strength ofpropellant grain under lateral compression is less than the peak strength of propellantgrain under axial compression.The fracture form of propellant grain under axialcompression is shear failure with single shear plane,and the fracture form of propellantgrain under lateral compression is shear failure with X-conjugate shear plane.Theexistence of confining pressure can enhance the compressive capacity of propellant grain.As confining pressure increases,the peak strength of propellant grain increases linearly,and the fracture form of propellant grain gradually changes from brittle failure to ductilefailure.The existence of initial defects weakens the compressive capacity of propellantgrain,and the peak strength of the propellant grain with surface and internal initial defectsboth decrease with the increase of initial defects.The fracture form of the propellant grainwithout initial defects is global shear failure,and the fracture form of the propellant grainwith surface initial defects is local shear failure,while the fracture form of the propellantgrain with internal initial defects is global tensile failure.3)The numerical test results of the fracture degree of regular and random packing propellantbed are both in good agreement with the physical test results.The press and fractureprocess of regular packing propellant bed can be divided into compaction stage,failureinitiation stage,local failure stage and failure development stage.The force chaintransmission,crack evolution and fracture degree of regular packing propellant bed haveobvious spatial stratification.The force chain transmission of random packing propellantbed is random,and its crack evolution and fracture degree have better spatial uniformity.The initial packing structures of propellant bed have significant influences on the initialpacking density,bottom compression stress and fracture degree of propellant bed.Theinitial packing density of regular packing propellant bed is larger,and it is more uniformand dense than random packing propellant bed.The bottom compression stress curve ofregular packing propellant bed drops after reaching the peak value,and the bottomcompression stress curve of random packing propellant bed shows a fluctuating upwardtrend.The fracture degree of random packing propellant bed is greater than regularpacking propellant bed,and the fracture degree of random packing propellant bed has agood consistency.The random packing structure of propellant bed is more in line with theactual packing and fracture condition of the propellant grains in propellant bed.