Coefficient of Thermal Expansion during Ratchet Growth of TATB Compactions

Abstract

Triamino trinitrobenzene (TATB) is an insensitive high explosive, meaning that it is quite immune from accidental detonation and only performs in appropriately engineered configurations. While this is advantageous for applications, the TATB crystal is graphitic and possesses significant thermal and mechanical anisotropy. Compactions of TATB crystals, with or without binder, have been shown to have oriented TATB crystals that relate to the pressing geometry and process, and this TATB texture necessarily imparts microscale and macroscale anisotropy to the compaction. By a mechanism not fully understood, thermal cycling results in irreversible volume expansion (also called ratchet growth) of the compaction likely by creating stress points on the microscale. Changes in density caused by thermal cycling can certainly effect the explosive performance. Our past characterization of the ratchet growth phenomenon has focused on the magnitude of the irreversible strain as measured before and after a thermal cycle. In the work presented here, we analyze the evolving thermal expansion behavior during the ascending and descending temperature ramps, providing further insight into the material changes taking place during the ratchet growth phenomenon.

Coefficient of Thermal Expansion Evolution during Ratchet Growth of PBX 9502 and Neat TATB

“…For all ascending/descending CTE pairs (ascending and descending ramp of the same cycle), the ascending CTE values always have a larger magnitude than the descending CTE values. Consistent with our earlier TATB paper [17], we propose “Rule One”, that for “hot only” cycles, damage occurs during the increasing temperature ramps, and that increased damage and increased porosity results in a decreased value of CTE. After many thermal cycles, the ascending/descending CTE pairs coalesce toward a common value.…”

“…Previously we investigated the CTE evolution of Neat TATB compactions [17]. The work presented here is a continuation of this investigation, now specifically comparing and contrasting the differences observed due to the presence of 5 wt% FK-800 binder.…”

“…Specimen strain is measured using a flat probe head (2.8 mm diameter) applied vertically and, except where noted, the applied probe force was 0.1 N to the top of the cylindrical PBX 9502 specimens, providing an axial strain value. (The probe force on Neat TATB specimens were all 0.1 N, except where noted [17]). We have demonstrated with a linear material (aluminum) that accurate CTE values can be measured on the TMA by turning the specimen 90°a nd measuring the thermal expansion across the diameter (data not shown here).…”

“…Table 1 lists the PBX 9502 specimens and tests analyzed in this paper. The TATB specimens and tests analyzed here are listed elsewhere [17]. 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.…”

“…All previous analysis of these data evaluated the expansion observed after each thermal excursion away from ambient, specifically following the irreversible strain changes after each thermal cycle. Here we evaluate the thermal expansion data during the repeated ascending and descending thermal ramps, similar to the process performed on the Neat TATB compactions [17]. The CTE is the slope of the strain versus temperature curve, and for a linear material (e. g. copper or aluminium) it is quite linear over a wide range of temperatures.…”