The mechanical properties of composite explosives are being studied as a function of mechanical confinement and three techniques for confinement were used. These are: (a) a constant confining pressure obtained by oil immersion, (b) radial confinement of a cylindrical sample by a thick walled steel cylinder that surrounds the sample (negligible radial strain), and (c) radial confinement of thin wafers by the platen-sample friction. while many energetic materials fail by crack growth when unconfined (significant surface area free of stress), with all of these forms of confinement, they appear to fail by yield and plastic flow. For crystalline explosives (e.g., TNT and Composition B), the yield strength and the modulus are independent of confining pressure so that useful results can be easily obtained by use of the steel cylinder technique. However, for materials containing polymer binders, such as plastic bonded explosives, a constant confining pressure is used because these properties are found to significantly increase with this pressure. These results indicate the very significant role of the polymer binders in determining the mechanical properties of these energetic materials.

Pressed plastic-bonded explosives, PBXs, are commonly used by defense and private industry. PBX 9501 is composed of HMX crystals held together with a plastic binder 'softened' with plasticizers. The detonation behavior of any explosive is very dependent upon its density, with the desire to have a uniform, high density throughout the explosive component. A parametric study has been performed pressing PBX 9501 hydrostatically and uniaxially. The effects of several pressing parameters on the bulk density and density profile, as well as mechanical properties, have been measured. The parameters investigated include pressure, temperature, number of cycles, dwell time, rest time, sack thickness, and particle distribution and size. Density distributions within the pressed explosives were also compared.

Two applications of thermal expansion measurements on plastic-bonded explosive (PBX) composites are described. In the first dilatometer application, thermal expansion properties of HMX-based PBX 9501 are measured over a broad thermal range that includes glass and domain-restructuring transitions in the polymeric binder. Results are consistent with other thermal measurements and analyses performed on the composite, as well as on the binder itself. The second application used the dilatometer to distinguish the reversible and irreversible components of thermal expansion in PBX 9502, a TATB-based explosive. Irreversible expansion of the composite is believed to derive from the highly-anisotropic coefficient of thermal expansion (CTE) values measured on single T A TB crystals, although the mechanism is not well understood. Effects of specimen density, thermal ramp rate, and thermal range variation (warm first or cold first) were explored, and the results are presented and discussed. Dilatometer measurements are ongoing towards gaining insight into the mechanism(s) responsible for PBX 9502 irreversible thermal expansion.

The response of small confined disks of PBX 9501 to cookoff has been investigated with high-speed photography through a transparent toughened-glass window, observing both the ignition and propagation of reaction. External strain gauges and microwave interferometry have been used to measure the expansion of the confining ring. The results show that when reaction starts, cracks propagate from the ignition site and that these cracks may be effective in leading to the fast transfer of ignition to other sites within the charge.

A series of plastic-bonded explosives (PBX) has been formulated with more binder than is normally contained in high-energy formulations. Adding a relatively small amount of binder to a material such as PBX 9501 (95/2.5/1.25/1.25 wt % HMX/Estane/BDNPA/BDNPF (the BDNPA and BDNPF form a eutectic that is frequently called simply the eutectic)) was found to decrease the shock sensitivity while not decreasing the energy of the explosive. The best compromise for a PBX 9501-type material contains about 92 wt % HMX. Adding additional binder does not continue to decrease the gap sensitivity of the formulation; however, the energy of the PBX decreases as expected. The higher-binder formulations are of potential use because of the possibility of formulating a PBX with energy similar to TATB formulations, such as PBX 9502 (95/5 wt % TATB/Kel-F 800), and with a higher strain to failure. 2 refs., 4 figs., 1 tab.