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The results of recent researches are overviewed focusing on two emerging topics: 1—genesis of reaction spot when the PBX structure is undergoing a shock-induced deformation, and 2—mechanisms of energy dissipation from the reaction spots and their related small-scale perturbations in the detonation reaction zone and in the PBX-driven inertial confinement (IC). The dominant role of shear driven plastic deformation in the reaction spot origination is disclosed in wide range of PBX sample sizes: from 1 mm3 (single HMX crystal within the binder) up to few cm3. Experimental evidences are presented showing that in the case of copper IC, a boundary layer, wherein the PBX-induced local perturbations dissipate to heat, is of order of few sizes of coarse HMX grains, ∼1–1.5 mm. [ABSTRACT FROM AUTHOR]

Our latest contribution to the meso-scale study of PBX detonations is presented. We apply a 96-channel optical analyzer for simultaneous measurements of light irradiation from the detonation front surface, and the stress field induced by the chemical reaction zone in optical monitor. This paper addresses three major emerging topics: the dominant role of shear at the shock initiation of PBX, the origination of DW oscillations and related cellular structure of DF, and the relation between failure diameter and the size of detonation cells. New experimental evidence of the existence of a thermal precursor at SDT is presented and discussed. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

This describes an experiment to test the hypothesis that the energy dissipated due to shear driven plastic deformation provides the hot spots needed for shock initiation of explosives. To develop controlled shear, sinusoidal shock wave fronts were generated and used to initiate PBXN-110 explosive charges. A threshold level was found for nearly flat shock wave fronts that did not initiate the explosive charges yet sinusoidal shock waves of lower amplitude caused initiation in the regions of maximum shear and not in the regions of minimum shear in identical explosive charges. As predicted, initiation occurred as long rows centered over the regions of maximum slope — maximum shear of the corrugated sinusoidal surface. These experiments will be discussed in detail. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Initiation of hot spots-reaction sites in PBX explosives by a linear shock generator has been studied under wide variation of the input shock pressure, from 3 GPa up to 20 GPa. A multi-channel high-resolution optical method based on application of a 96 optical fiber strips has been used for a comprehensive registration of the reaction spot initiation and following the processes of reaction growth or extinction. We have obtained strong evidence that reaction originates in the shear regions established by the induced shock. This suggests that shear has a major role in shock initiation. The pressure field in the front of reacted flow and the minimum size of reaction sites, which results in reaction growth, are defined as a function of the input shock pressure. Effect of shear stress is discussed concerning the grain sizes and mass concentration of HMX in the PBX’s. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Recent theoretical calculations have demonstrated a relationship between the rote of energy dissipation and the rate of plastic deformation in crystalline solids subjected to plastic flow due to shock or impact. In the case of explosive crystals the energy dissipated locally within the crystals during plastic deformation forms the hot spots from which chemical reaction can be initiated. Prompted by this prediction relating the plastic deformation rate with initiation, a series of experiments were undertaken to measure the plastic deformation rate at the initiation site at the moment of initiation for a number of polycrystalline explosives when subjected to impact or mild shock. The experiment and the results will be reviewed here. [ABSTRACT FROM AUTHOR]

Atomic Force Microscopy (AFM) of molecular crystals subjected to impact or shock has shown increasing lattice and molecular damage with increasing levels of plastic flow. While AFM measures only surface distortions, evidence exists showing that the molecular and lattice distortions persist deep within the crystal interior. This evidence of lattice damage has been used to provide a physical model of lattice softening and eventual failure during large plastic deformation due to impact or mild shock. For a simple geometry, the onset of crystal failure and crack trajectory during impact is determined and compared with experiment. [ABSTRACT FROM AUTHOR]

Nano-crystallites of RDX produced by aquarium shock, were examined using atomic force microscopy and found to contain sharply defined, apparent shear-type (Mode II) fracture steps having heights less than the size of an RDX molecule. The sub-molecular steps run for substantial distances along crystallite surfaces, thus indicating concerted disregistry in depth between juxtaposed molecules across the crack surfaces. The observation of locally jumbled regions of displaced/misoriented molecules suggests an obstacle barrier, not previously considered, to either shear crack propagation or dislocation pile-up release with sufficient stress intensity to cause hot spot initiation. Recent follow-on nanostructural results are reported for separate cleaved RDX crystals containing nano-indentations showing only plastic deformation with evidence of cracking. [ABSTRACT FROM AUTHOR]

Internal porosity within laboratory-grown crystals of RDX has been investigated with an atomic force microscope (AFM) in extension of previous work [1]]. The crystals were cleaved along {001} planes so as to provide cross-sections of the defects. The pores showed up as nano-caverns, ranging in size from 10 nm to a few microns, with very complicated shapes and tentacle-like arms. They were only 5-200 nm deep, and were seen in various orientations. The complex shapes seem to arise from anisotropic local crystal growth and from thermal influences with associated redistribution of solvent and solute. RDX crystals, grown in an ultracentrifuge were also investigated. They did not show any bubbles or solvent inclusions. Both kinds of crystals studied, frequently showed one molecule high terraces running over distances of microns. These seem to have been left by edge dislocations which have egressed out of the crystals altogether. [ABSTRACT FROM AUTHOR]