Effect of mechanical impact on PBX disk investigated by DMA

One safety aspect during use of polymer (plastic) bonded high explosives (PBX) is their resilience against mechanical impacts by set-back of the HE charge during firing. To investigate this feature PBX disks with 140 mm in diameter and with 14 to 18 mm thickness were manufactured and, held in a fixture, exposed to projectile impacts to simulate the set-back effect. The PBX disk was covered with a thin steel plate to simulate to some extend the base casing. The frame composition of the PBX is HTPB (hydroxyl-terminated polybutadiene) based binder cured with IPDI (isophorone diisocyanate), HMX, plasticizer DOA (dioctyl adipate) and a phenolic antioxidant. The projectile impact was performed with such a velocity that the steel plate was not perforated but clear destruction effects on the PBX diske were created. The PBX disk was then sectioned into DMA sample stripes with different distances to the impact center (DTIC). The DMA (dynamic mechanical analysis) measurements were done in torsion and the rectangular samples were about 10 mm wide and 18 to 35 mm long. The temperature dependence was evaluated with a stepwise increase by 1°C from -120°C to +90°C. The deformation frequencies were in the range of 0.1 Hz to 30 Hz with nine frequencies in total and applied on each temperature step after an equilibration period. The measurements revealed a clear indication of changes with the DTIC. Observed was a shift of the glass-to-rubber transition temperature (GRT-T) to lower temperatures with decreasing DTIC. With each measurement frequency, a clear dependence of the GRT-T with DICT was found. The dependencies of GRT-T on deformation frequency are evaluated with two equations of type standard Arrhenius (providing f0, Eaf) and of modified Arrhenius (providing f0M, Ea0M and T0M). The change of the parameters Eaf and Ea0M, T0M are discussed in terms of DTIC.