Thermal Cycling and Ratchet Growth of TATB and PBX 9502

Abstract

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.

Coefficient of Thermal Expansion during Ratchet Growth of TATB Compactions

“…In Figure 3, the magnitude of change in the CTE values correlates with the high-temper- ature limit (X°C). Elsewhere [2] we have demonstrated that as the upper bound of temperature is increased, the ratchet growth also significantly increases. Figures 3 and 4 distinguish data from ascending temperature ramps (solid symbols) and data from descending ramps (open symbols).…”

“…Some tests were performed with many repeated cycles to the same X(°C) endpoint temperature and other tests were conducted with alternating hot/cold X(°C) endpoint temperatures. Note that the tests listed in Table 1 are a subset of those analyzed earlier [2] for their irreversible thermal expansion, specifically strain values measured at ambient and caused by thermal cycles. Here we analyze the strain evolution during the ascending and descending temperature ramps.…”

“…These values are plotted versus the Accumulated Irreversible Strain per Cycle, ΣΔɛ hot or ΣΔɛ cold . Analysis of these values over a wide range of test parameters provides a great deal of information about the irreversible expansion behavior [2,16,17]. On the bottom of Figure 2, we plot the strain versus temperature response of the material during the ascending temperature ramps (23 to 153°C) from repeat cycles, after we have subtracted off all of the strain values at 23°C.…”

“…The insensitive properties of TATB (triamino trinitrobenzene) make it a unique and attractive high explosive for use in weapons applications [1]. The crystal structure of TATB is graphitic, like a stacked deck of cards, which endows TATB-based compactions (with or without binder) with unusual properties such as thermal and mechanical anisotropy and irreversible volume expansion or ratchet growth [2][3][4][5][6]. The anisotropy is caused by preferential alignment of the TATB crystals during the compaction process [7] and the significant underlying thermal expansion and mechanical anisotropy of those crystals [8,9].…”

“…PBX 9502 is a plastic bonded explosive with 5 weight% (wt %) FK-800 binder and 95 wt% TATB crystals. We have previously characterized the detailed response of ratchet growth by performing high-fidelity thermal expansion measurements under a wide variety of conditions on drypressed TATB specimens and on machined PBX 9502 specimens from isostatic pressings [2,16,17]. These data sets are comprised of carefully controlled thermal ramps and holds, with an analysis of the corresponding specimen strains caused by those thermal excursions.…”