Study On Structural Evolution And Stress Response Of TATB Modeling In Molding Process

TATB based PBX (polymer bonded explosive) is made of granules in various shapes and sizes in which TATB crystals are mixed with thermoplastic polymer. It is pressed in a mould, and be processed into a specific shape. When PBX is compressed, the TATB granules undergo friction, extrusion, and propagation of pressure, thus affecting the microstructure and internal stress distribution of the compression molding. These microstructures are important factors to confirm the physical, mechanics, and other characteristics of the explosive and will directly affect the density distribution and stress concentration of PBX, thus affecting the safety performance and usability of the explosive. To investigate the relationship between the changes in the microstructure caused by compression and macroperformance, the various characteristics of microstructure should be accurately evaluated. Nowadays, with the unique characteristics of X-ray tomography, the deficiency of traditional inspecting method has been compensated. Meanwhile, combined with the discrete element model for inter-granules forces and stress distribution analysis can explore the evolution process of deformation, displacement, movement and mechanical characteristics of explosive granules during compression in the mesoscopic level.In this study, The x-ray tomography technology and deformation characteristics of deformable material were combined to study the TATB granule deformation and structural evolution during compaction process. The interaction between granules and the effect of structural transformation on load fluctuation were analyzed. The CT images show that the deformation orientations of granules in the central part of mould are mostly downwards. In the meanwhile, there are arching between the granules near the edge of mould. The porosity changing mainly happens in the initial stage of compaction which under OMPa to 10MPa forming pressures. The porosity of each part decrease 73%,62% and 58% respectively. The porosity increase from the top down during each compacting phase. The displacements mainly take place along the axis. Friction between the mould and granule is greater than that between the granules, which makes the displacements of the granules in the central part are larger than other parts. The results show that X-ray Micro-tomography can nondestructively characterize inner structural evolution of TATB granules during compaction. Morphology changes and contact relations of granules reveal the interaction mode between the granules. The displacements of granules indirectly reflect the internal stress transfer in the explosive.Moreover, the TATB granules were compressed in a warm mould to study the stress and strain state in PBX during the molding process. By implanting strain markers and conducting CT inspection, the deformation characteristics and changing trend had been observed. With the strain analysis of strain markers at different locations, local strain states at different locations were obtained. So the local strain field could be established, and the local stress state could be deduced. The stress response and stress distribution were studied during the compression process. The results showed that each strain marker deformed axisymmetrically, The strain ellipsoid was flat and was uniaxial flattened. In the central area the stress was mainly in an axial direction, and the shear force was small. There were not only axial stresses but also large shear stresses in the surrounding area of the explosive. The stress gradient in the central area was greater than that in the surrounding area. The surrounding area was squeezed by the mould, so the inner stress was greater.The granule discrete element method was used to conduct numerical simulations on the compression of the granules of TATB explosive, and the contact force between the internal granules of the specimen after the loading and the displacement vector were used to study the transformation of the specimen and the motions of the granules. Moreover, the structure of the TATB granules under different pressures and the stress response were studied. The results showed that the porosity in the upper part was greater than what in the bottom part at the early stage. There was little difference of porosity between each part at the later stage of compaction. At the initial stage of compaction, there were greater contact forces in the upper and bottom part. The inner stress decreased progressively from top to bottom. And the axial stresses were stronger than radial stress. At the later stage of compaction, the stresses in the top and bottom were greater than that in the middle. The inner stresses in the central part were weaker than the stress nearby mold wall by reason of lateral pressure influence.This paper had made an innovation in characterizing the microstructure and inner strain measurement. A combination of strain markers and CT inspection were presented to study granule contact, contact force transmission, force chain origin and evolution and inner stress distribution. The results and understandings could be used for molding and densifying mechanic analysis in explosive compression process. This method could break through the limitation in explosive inner strain measuring. It should solve the problem of inner structural evolution and stress(strain) quantification to a certain extent.