Thermal cycling and ratchet growth of as-pressed TATB pellets

The explosive 1,3,5-triamino-2,4,6-trinitirobenzene (TATB) contains a unique crystal morphology that imparts a degree of texture to the compacted material. This is also true for TATB-containing plastic bonded explosives (PBXs), thus inducing anisotropy to the mechanical and thermal properties of these materials. The plate-like TATB crystals possess very anisotropic coefficient of thermal expansion (CTE) values, the through-plate direction being similar to 10 times greater than the other two directions. Although the mechanism is not well-understood, in solid compactions of TATB and TATB-based PBXs, the highly anisotropic CTE gives rise to an irreversible volume expansion that accompanies thermal cycling. This phenomenon, called "ratchet growth", is believed to arise from internal stresses induced by thermal expansion. For this study, TATB was die-pressed into cylindrical pellets. These pellets were thermally cycled to hot and cold temperatures within a thermal mechanical analyzer (TMA) to measure the ratchet growth response. The results were compared to the response of PBX 9502, measured in a previous study, to highlight the role of the Kel-F binder in the ratchet growth phenomenon. The comparison may be somewhat complicated by the effects of texture due to the difference in sample preparation (i.e. isostatically machined versus "as-pressed" parts). The detailed evaluation of porosity changes, before and after ratchet growth, is simplified in the absence of binder. LA-UR-17-27817.