Research On The Dispersion Characteristics Of Fragments From A Cylindrical Casing Under Eccentric Initiation

Cylindrical casing filled with charge is one of the most common structures used in defending aerial targets,called fragmentation warhead.Fragmentation warhead with eccentric initiations(also called velocity enhanced warhead,abbreviated as VEW)is a kind of typical aimable fragmentation warhead.In the traditional fragmentation warhead design,the detonators are usually deployed at the geometrical center of the cylindrical charge.In order to achieve aimable damage effects,the off-axis detonators are employed in the VEW,by which it changes the characteristics of the energy output resulting to higher fragmentation velocity in an aimable direction.In this paper,both theoretical methods and simulation techniques are employed to study the radial and axial velocity distribution,projection angles and the circumferential abruption characteristics of the fragments from a cylindrical casing under eccentric initiations.The main research contents are as follows:Firstly,both the two-dimensional and three-dimensional control equation and the corresponding discretization forms are deduced based on the generalized interpolation material point(GIMP)method.The material models and corresponding functions used to model the materials in the simulation,the parallel strategy for the GIMP based on OpenMP are also presented.According to the theory above,the in-house code using GIMP is programmed and debugged.A series of simulations based on two-dimensional planar,axis-symmetric and three-dimensional MPM codes are conducted,the simulation results agree well with the experimental and theoretical results,which indicates the high efficiency and accuracy of the present MPM in simulating the dynamic fly characteristics of the fragments from cylindrical warheads.Secondly,according to the one-dimensional projection and oblique impact on the condensed medium theory,a new method is established to calculate the fragments velocity from the VEW under single eccentric initiation according to the basic equations of motion and the detonation wave characteristics.Based on this method,the maximum velocity gain is calculated and compared with the results from the literature.The fly characteristics of the fragments along the circumference are also analysed according to the established method.Thirdly,the velocity enhancement mechanism for the VEW with double symmetric initiations is analysed theoretically,after that a new method to evaluate the fragment velocity in the Mach loading area is presented.Then,in order to predict the velocity distribution in the whole circumferential direction,a new formula is proposed based on the theoretical analysis and simulations.Additionally,the corresponding simulations are conducted to close this formula.Fourthly,a new formula is proposed to calculate the characteristic acceleration time of the fragments from a cylindrical casing.By the combination of this formula and Chou's analytical projection angle formula,a revised formula is established to predict the projection angles of fragments,and a revised formula for velocity axial distribution for fragmentation warhead under center point initiation is also proposed.In this part,the differences of the VEW with single point(double points)and line(double lines)are studied by a series of simulations.Then the formula proposed for two-dimensional rings under double symmetric initiations is tested for the three-dimensional condition.Finally,the exponential acceleration model of fragment is used to establish a method for evaluating the fracture radius and corresponding strain rate for the fragmentation warhead.Then a revised formula for predicting the circumferential abruption number is proposed and used to study the dynamic abruption of VEW theoretically.Simultaneously,the Stochastic model is introduced to the in-house code for the simulation of the dynamic abruption.The abruption characteristics of the casing from the VEW along the circumference are analysed.