| Hot spot mechanism in plastic bonded explosives (PBX) under shock loading is the key of shock initiation in explosives. The formation of hot spot is the result of the interaction of mesoscale structure which can link the macroscale and microscale in the multiscale responses of explosives, so mesoscale numerical simulation is an effective approach to solve initiation problems in explosives. In this work, mesoscale responses and hot spots formation processes of heterogeneous explosives under shock loading have been investigated using the discrete element method. The mesoscale numerical simulation reveals mechanism of hot spots formation without considering chemical reaction.The main points of this paper and results of the numerical simulations are as follows:(1) By investigating the theory and calculation process, we upgrade the two-dimensional discrete element code DM2 to three-dimensional code DEM3D. And the central force was replaced by a modified Hugoniot relation. In addition, the calculation efficiency was improved after replacing the intrinsic neighbor calculation method by the link-cell method.(2) The three-dimensional geometrical model which can approximatively mimics the mesoscopic structure of the plastic bonded explosives was created based on the Voronoi tessellation.(3) Based on the above progress, hot spots formation processes of explosives under shock loading have been investigated using the discrete element method. In the case of shocked PBX explosives high temperature regions mostly locate near the interface between HMX crystals and binder, the temperature rise of HMX crystals is lower than the binder, and the surrounding parts of HMX crystals have higher temperature rise than the inner parts. The binder is softer than HMX explosives, so it can cushion the explosive, causing the temperature of HMX in PBX much lower than that of HMX crystal. Temperature of hot spot under shock loading in explosives containing a void is much higher than that in explosives without void. We also find that temperature from three-dimensional simulation is higher than that from two-dimensional simulation. For pure HMX explosives, higher hot spot temperature is obtained due to void collapsing from big void than from small void, and temperature obtained from spherical void is higher than from cubic void with the same size.(4) We also developed a three-dimensional combined finite-discrete element code. A part of models that mentioned above are also calculated using this code, and the calculated results almost agree with the discrete element method. |
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