Plasticity in Crystalline Molecular Explosives – A Key to Unraveling “Unpredictable” Responses

This is the authors' abstract. We don't add key points for this paper.

Last year, in the 40th anniversary issue of PEP, we published a review of elasticity in molecular explosives 1. Following this, we had a conversation about the approach we might take to develop a follow-on review of plasticity. Marc Cawkwell's first response was: “what we know, really know, about plasticity in molecular explosives, could be written on a beer mat with space left over.” We've been working together on elastic-plastic response of explosives for more than 10 years, and many others have studied the topic since well before we were born. We mean no discredit to those that have explored plasticity in these materials and, indeed, a review of the progress to date is a good place to start. What we mean, though, is that we don't know nearly enough about plasticity in these materials, and we assert that it is important that we dig in to really know. The application of stress to a material, whether by mechanical loading or thermal gradients, gives rise to reversible elastic deformation, irreversible plastic flow once the yield stress is exceeded, and possibly fracture. While plastic deformation and fracture are of clear importance to load-bearing materials, their impact on the performance and safety of crystalline energetic materials may also be pivotal. Why? It's been known for nearly of a century that mechanical localization – “hot spots” – is what gives rise to the first stages of chemical reactivity in both shock and non-shock stimuli. While we expect some will argue with us about whether the plastic response matters in strongly shocked explosives, it is inarguable that, in sub-shock insults, the likelihood of unintended explosive response depends on how the material dissipates stress at the microstructure scale. These are the scenarios we are used to calling “unpredictable” or statistical responses. Unraveling this unpredictability starts with knowing about plasticity and failure in these materials. We would also argue very strongly for the importance of plasticity in all localization events in explosives, whether or not the discussion is over a beer. Elasticity, plasticity, and fracture manifest themselves in many areas of explosive science. All forms of deformation are prominent processes during pressing to consolidate explosives into polycrystalline compacts. Plastic bonded explosives with high loading fractions (typically greater than 90 wt-%) must withstand, without fracture or compromised integrity, machining to tolerances on the order of 25 µm, structural loads in assembled munitions, and inertial forces during deployment. Compared to compacts of molecular crystals in other industries, for example tablets in the pharmaceutical industry, typically with less than 50 wt-% crystalline loading fractions, the material requirements for plastic bonded explosives are extreme. Intriguing connections exist between crystal plasticity and the impact sensitivity of explosives. For instance, Jerry Dick proposed that anisotropic plasticity accounts for the pronounced orientation dependence of the impact sensitivity of PETN single crystals. Dick argued that dislocation-mediated plasticity can rapidly relax shear stresses during impacts on single crystals oriented such that shear stresses are resolved on the slip systems 2. Most of the mechanisms for the formation of hot spots in energetic materials involve localized plastic flow to some extent. Plasticity during void collapse is traditionally considered to be hydrodynamic in nature, but for weaker impacts or small voids the anisotropy and native deformation mechanisms of the parent phase must affect rates of heating and dissipation. Moreover, Ron Armstrong and colleagues proposed that the sudden release of a dislocation pile-up could liberate sufficient heat to initiate reactions 3. This mechanism is supported by corollaries such as melting in shear bands in metals. Crystal plasticity is inherently complex since dislocations interact with each other and other crystal defects, boundaries, and interfaces. Additional challenges specific to molecular crystals include their exceptional brittleness, and the difficulty in applying the transmission electron microscope, the workhorse of the metallurgists, to such chemically fragile samples. Furthermore, in the low symmetry lattices common to energetic molecular crystals, multiple slip systems may exist that are not related by symmetry, each with different dependences on the resolved shear stress, strain rate, and temperature. Understanding the mechanical response of molecular crystals requires embracing their anisotropy and characterizing in detail their deformation mechanisms. John Sherwood and colleagues, and several others, showed that non-destructive techniques such as indentation and X-ray topography are capable of piecing together this puzzle even in the absence of transmission electron microscopy 4. Though out of fashion, these methods are effective 5, and should be revisited and applied more broadly to energetic materials while groundbreaking new diagnostics should be pursued in earnest. Recently, real-time in situ X-ray techniques have been developed that can be applied to this problem, opening up many exciting possibilities for discovery 6. Additionally, theory and simulation play an increasingly important role in the identification of slip systems in molecular crystals 7. Advances in theory and the availability of ever-more-powerful computational hardware complement the new diagnostic techniques. Atomistic simulation methods are promising but are often limited by the quality or availability of interatomic potentials suited to the study of extended crystal defects containing thousands of atoms. Detailed experimental determinations of elastic-plastic properties enable many improvements to these simulation tools as well. Given the undoubted importance of mechanics to the properties of explosives and the rapidly evolving experimental and computational capabilities, the materials science of energetic materials is brimming with exciting possibilities. Advances in experimental techniques and computational capabilities put knowing about plasticity in molecular explosives within our reach. It's time to fill up more beer mats with this knowledge! Daniel E. Hooks, Marc J. Cawkwell, and Kyle J.