| The dislocation mechanics based properties of solid energetic materials, particularly, of high explosives, are of particular interest in connection with issues of intrinsic chemical stability and with their fast chemical decomposition when employed as propellants or in explosive formulations. The ballistic impact and shock-associated plasticity responses of such materials present great experimental and model challenges for establishment of predictable performances. As demonstrated in the present report, much has been learned through direct investigation with a full range of scientific tools of the individual crystal and composite material properties and, also, through their comparison with relevant inert ionic and metallic material behaviors. Thus, in relation to other solid material structures, energetic crystals are elastically compliant, plastically strong, and fracture prone. Somewhat surprisingly perhaps for such materials, individual dislocation self-energies are indicated to be relatively large while the intrinsic crystal-determined dislocation mobility is restricted because of the complicated and rather dense molecular packing of awkwardly-shaped molecules that are self-organized within the exhibited lower-symmetry crystal structures. Because crack surface energies are low, cleavage is able to be initiated by relatively small dislocation pile-ups and, with the restricted dislocation mobility, there is little additional plastic work requirement associated with cleavage crack propagation. Nevertheless, when compared with indentation fracture mechanics prediction, crack propagation appears to be controlled by the behavior of very limited dislocation activity at the crack tip. Adiabatic heating associated with dislocation pile-up avalanches provides an important mechanism for the thermal hot spot model of explaining the initiation of rapid chemical decompositions and relates directly to predicted influence of crystal (or particle) sizes. On such basis, desired characteristics of greater mechanical insensitivity to initiation but afterwards greater power dissipation are predicted to occur for energetic composite formulations comprising smaller particle-sized ingredients. |
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