The Effects of TATB Ratchet Growth on PBX 9502

Abstract

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.

Thermal cycling and ratchet growth of as-pressed TATB pellets

“…Immersion density methods are not reliable for such small specimens; therefore the dimensions and mass were used to estimate the density of each specimen. The average density of tested specimens was 1.8927 g/cm 3 . Thermal cycling was performed using a thermal mechanical analyzer (TMA), model Q400 by TA instruments and measured strain as the specimen displaced the probe head.…”

“…TATB crystals have more growth in one direction than in the other two directions. The irreversible thermal expansion, or ratchet growth, of the TATB-based composite, PBX 9502, was studied previously [3][4][5]. A recent study was performed on PBX 9502 cylinders cored out of isostatically pressed charges [6].…”

The effects of PBX 9502 ratchet growth on detonation failure as determined via the lanl failure cone test

“…Both affect microstructure, which controls hot spot formation and thus influences detonation performance. At present, data currently exists for the recycled lot 006 pore size distributions [3], but is needed for the virgin lot 008 distributions as well.…”

“…Through continued thermal cycling phases, its volume progressively ratchets towards an eventual limit in diminishing fashion. This ratchet effect has been observed and studied for over 30 years [1,2,3,4].…”

“…The microstructures for the recycled lot 006 materials have been characterized by Thompson et al [3]. Pore size distributions were determined for each material via micro X-ray computed tomography image analysis and ultra-small-angle neutron scattering data analysis.…”