PBX 9502 is a plastic‐bonded explosive that contains 95 wt.‐% TATB, a graphitic‐structured high explosive known to undergo “ratchet growth,” i.e., irreversible volume change that accompanies temperature excursions. Earlier studies have reported changes in TATB‐based composites as a function of thermal cycling and density change, however, a clear distinction between density and ratchet‐growth effects has not been made. In the work reported here, an “as‐pressed density” baseline for the mechanical response of recycled PBX 9502 is established over a density range of interest, then high‐density specimens are thermally cycled between −55 and 80 °C to achieve “ratchet‐grown” parts in the same low‐density region. As‐pressed and ratchet‐grown specimens with identical densities are then analyzed using microX‐ray computed tomography and USANS techniques to obtain information about pore‐size distributions. Data show that after ratchet‐growth, PBX 9502 specimens contain, in general, more numerous and smaller voids than specimens that were pressed with lower compaction pressures to match the same density. The mechanical response of the ratchet‐grown material is consistent with damage, showing lower tensile stress and modulus, lower compressive modulus, and higher tensile and compressive strain, than as‐pressed specimens of the same density.

[a] 1IntroductionMany plastic-bonded explosive(PBX)applications use precision-made componentsw ith requirements of thermal and mechanical integrity throughout the lifetime of the part. This is the basis for av ariety of PBX characterization studies that correlate engineering response with formulationa nd environmental variables, includingt hermal changes associated with storage and aging. TATB, triaminotrinitrobenzene, is an insensitive explosive [1].I nr ecent years, there has been increasing interest in the use of TATB-basedP BX composites due to the safety advantages derived from their insensitive nature.D etails can vary (i.e. with binderp roperties or particle-size distribution), but generally speaking, TATB is ar emarkably safe explosive that detonates only in the presenceo ft riggering shocks in appropriately-engineered configurations.I nadvertent detonation is extremely unlikely.TATB crystals are sheet-like and graphitic in nature.K olb and Rizzo measured highly anisotropic coefficient of thermal expansion (CTE) values for TATB single crystals [2]. They also showedt he effects of different binders on the thermal properties of TATB-basedc omposites [3].F or decades since, various observations have beenm ade [4][5][6][7] showing that solid pressings of TATB, with and without binder,c an possess highly anisotropic CTE and irreversible expansion( ratchet growth).C unningham [ 4] and Cady [5] measured anisotropic CTE in specimens removed from larger PBX pressings, providing strong evidence that the compactionp rocess induces preferred TATB orientation. The cause of these observationsw as confirmed by Schwarz et al.[6] using transmission X-raya nalysis, where the preferred orientation of TATB crystals in pressed PBX 9502 (95 wt-% TATB) charges was quantified and correlated directly with the observed anisotropy in the PBX 9502 CTE. Schwarz also proposedamodelf or how compactionm ethods and geometry are responsible for the direction and magnitude of TATB texture. The quantified TATB texture has similarlyb een correlated with anisotropy in ratchet growth magnitudes [7].Observations and models of ratchet growth [8-10] have typicallyi nvolved the iterative applicationo ft hermal excursions while monitoring the irreversibleg rowth, usually as Abstract:T ATBi sa ni nsensitive high explosive, attractive for use because of its safety aspects. TATB compactions, with or without binder,u ndergoi rreversible volumee xpansion (or ratchet growth)u pon thermal cycling. In the past, experimental elucidation of this phenomenon has focused on irreversiblee xpansion as af unction of the number of thermal excursions over ag iven temperature range, where growth is asymptotic with increasing cycle number.I nt his paper,w ed emonstrate that ratchet growth also occurs as af unction of time at constant temperature, and that growth is substantial at elevated temperatures. We have measured strain response in PBX 9502, aT ATB-based composite, by performing thermal-cycling tests with different durations at high temperature. Irreversible...

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The irreversible volume expansion, or ratchet growth, of TATB and PBX 9502 (95 weight% TATB) compactions has been quantified over a wide range of thermal cycles. While the precise TATB texture distributions of these specimen sets are likely different from each other, we believe they are consistent within each set, as the expansion data show reproducibility and consistency. These data provide a baseline characterization of the ratchet growth phenomenon in these materials. The increased expansion that comes with changes in the temperature range of the cycles is quantified, repeated hot cycles growing far more than cold. For thermal cycles above ambient where the temperature range is increased in subsequent cycles, the growth of a given cycle is shown to be dictated by previously‐established growth trajectories, and the specimen will grow according to the growth potential associated with the temperature range of the present cycle. Alternating hot‐cold cycles greatly enhances the cold‐cycle contributions as compared to the growth of cold cycles alone. These “rules” of ratchet growth are first established, then observed to hold true for more complex sequences of hot and cold cycles. A simple equation is used to parameterize the response of individual data sets.

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