Study On Dynamic Fragmentation Of Variable Wall Thickness Shell Under Internal Explosive Loading

With the increasing damage capabilities of weapons and equipments, the protectivecapability of important military targets have became stronger, the positions continued todevelop in the direction of deep underground. For the anti hard target penetration warhead,in order to effectively destroy underground bunkers, it needed to ensure that penetrationblast warhead would not appear instability phenomena in the process of penetration on thetarget. In order to improve the penetration warheads’ bending strength and bucklingresistance, variable wall thickness cone structure was often used, but the fragmentationmechanism of variable wall thickness shell were still unclear. The basis and premise ofeffectively evaluating the warheads’ damage was properly describe the characteristics ofnatural fragments generated after dynamic fragmentation of variable wall thickness shell, italso has the great significance for the damage effectiveness evaluation and design of highpenetration explosive warhead. In this dissertation, explosion experiment of the shell withcharge inside, theoretical analysis and numerical simulation were used to study the dynamicfragmentation of variable wall thickness shell under internal explosive loading. The lawbetween shell structure, location of the detonation point and morphology, fracture pattern,mass distribution of fragments were analysised. The main works are as follows.Under the internal explosive loading, a series of variable wall thickness shells’fragmentation experiments were carried out, which initiated from the both ends of the shellsand used the sand box to recover fragments of the experiments. The morphology andfracture mode of the fragments were studied, and the statistical regularity of fragment massdistribution were analyzed. Morphology and fracture mode results indicated that variablewall thickness shell of the same structure would produce more small and medium-sizedfragments when it was initiated from the big end; the shell would be more prone to producesuper-long fragments when it was initiated from the small end; large cone angle shells’fracture mode was significantly affected by the initiation point, and the area oftension-shear mixed fracture were expanded when the TNT was initiated from the big end.Weibull and Mott distribution were adopted to fit and analysis the mass statistical data of fragments, the results were as follows: the large taper shell corresponds with morenonuniform mass distribution of fragments; when it was initiated from the big end of theshell, mathematical expectation of fragments mass distribution were smaller; thecharacteristic mass of fragments were proportional to the shell mass.The finite length cylindrical shell final speed Gurney formula’s applicability ofvariable wall thickness shell were analysised, computed shell rupture time at each axialposition. Combined with dynamic fracture mechanics and the degree of mutual influencebetween adjacent axial cracks of the variable wall thickness shell surface were analysised.The mechanism of the large cone angle shell fracture mode affected by the initiation pointin the experiment were explained.Based on the Gurney theory of finite length cylindrical shells, the average strain ratewas estimated by the average radius of shell, and the differences between shear fracturelength and radial wall thickness were considered, then the Grady-Kipp method wasmodified to give the complete expression of cylindrical shell’s circumferential segmentsnumber nθ. The average circumferential width of the fragment and the circumferentialsegments number on each axial location of shell were obtained, calculation methods of theaverage characteristics mass and the fragments characteristic aspect ratio of the shell wereproposed too.Based on stochastic fracture model of dynamic finite element software AUTODYN,the dynamic fragmentation of variable wall thickness shell under the internal explosiveloading were simulated. The cumulative mass distribution of fragments were in goodagreement with the experimental data. The distribution law of fragments’ characteristicmass which obtained by fitting the numerical simulation data was consistent with theexperimental results.The shells’ axial crack propagation characteristics were analysised,when variable wall thickness shell was initiated from the small end, axial crack wouldkeeped a stable shape and propagation path more easily. The errors between numericalsimulation results of surface crack propagation velocity along the axial direction andtheoretical value were less than10%.