W. L. Elban

Crystal size dependence for impact initiation of cyclotrimethylenetrinitramine explosive

The dislocation pile‐up avalanche model is used to explain the crystal size dependence for hot spot‐controlled initiation of chemical decomposition in cyclotrimethylenetrinitramine crystals subjected to drop‐weight impact testing. Deformation‐induced temperature rises, hot spot sizes, and lifetimes are related to previously reported values for direct thermal decomposition. A reasonable chemical reaction yield is estimated from available kinetic data.

Adiabatic heating at a dislocation pile-up avalanche

Abstract Most model calculations of the adiabatic heating which can be produced by plastic deformation give only small increases in the bulk temperature of a material. This occurs because a very significant averaging and presumed continuity of the plastic flow processes are built into the description of what otherwise is undoubtedly an extremely localized and sudden action of energy conversion on the microstructural scale. For the opposite case description, an appreciable localized heating is estimated to occur within a material if a dislocation pile-up is catastrophically released in an avalanche configuration from a collapsed obstacle. The estimated temperature rise at such a hot spot is found at the limiting dislocation velocity to be directly proportional to the Hall-Petch stress intensity factor for the strength of the internal obstacle originally impeding the growth of a slip band (or twin or crack). Thus, this description of adiabatic heating has been shown to be a particularly important effect for materials which exhibit sudden load drops in their deformation responses and for materials which are susceptible to the onset of brittle fracturing. Although this work gives emphasis to metallic materials, the same considerations are relevant for other types of crystalline solids as well, such as explosive molecular crystals and inorganic crystalline oxidizer ingredients incorporated in propellant formulations.

Investigation of hot spot characteristics in energetic crystals

Abstract The model hot spot characteristics of energetic crystals, particularly, of cyclotrimethylenetrinitramine (RDX) and related materials, are described within a molecule-to-crystal–lattice-to-dislocation defect framework, as developed under the guidance of R.S. Miller. The purpose was to trace the dimensional scale at which controlling influences occurred for energetic material responses to imposed mechanical forces and/or deformations. Important consequences of the work include: (1) explanation of a substantial energetic crystal lattice resistance to shear-type deformations; (2) molecular dynamics modeling demonstration of significant heating associated with defect relaxations; and (3) dislocation pile-up prediction of a greater drop-weight impact height requirement for initiation of smaller crystals.

Materials science and technology aspects of energetic (explosive) materials

Abstract Crystallographic, phase diagram, X-ray diffraction and mechanical property aspects of energetic materials are described in relation, first, to the properties of individual crystals as employed in composite explosive material formulations and then, second, to initiation of chemical decomposition by imposed mechanical forces and deformations. The mechanically induced decomposition properties are tied to the special character of dislocations in the molecularly bonded energetic crystal structures formed by individual covalently bonded molecules. Microindentation fracture mechanics, hardness stress–strain and drop weight impact measurements on energetic crystals are assessed. Of particular interest are the material granular compaction properties associated with a mechanically induced deflagration to detonation transition (DDT) behaviour. Shock induced initiations of detonation involve experimental and theoretical model considerations at nanometric dimensions. The total results point to improved mechanical insensitivities and higher energy release rates at smaller particle dimensions.

Quasi-static compaction study of coarse HMX explosive

Abstract The quasi-static compaction behavior of granular beds of coarse HMX (cyclotetramethylenetetranitramine) explosive has been investigated, in some detail, by simultaneously measuring applied force, transmitted force and porous bed displacement. A narrow sieve-cut of both HMX and Class D HMX was selected for study. These two materials have a similar average particle size (∼900 μm) but differ appreciably in their particle size distribution. The principal difference in the compaction behavior of these two materials was observed in the early stages, below 85% TMD (theoretical maximum density), and is associated with the difference in their initial particle packing efficiency. Above 85% TMD, the difference in behavior was small, suggesting that the physical state of the compacted bed had become very similar for the two materials. Optical examination of recovered sieve-cut HMX samples revealed that the onset of widespread fracture of particles occurred a little above 58% TMD (average initial density was 56.6% TMD) and widespread fracture occurred, in apparently increasing amounts up to 75% TMD. Above 80% TMD, the recovered samples were intact, a condition which precluded direct determination of the extent of fracture.

Temperature rise at a dislocation pile-up breakthrough

Abstract Relatively large temperature rises are theoretically estimated for collapsing dislocation pile-ups in MgO, the explosive RDX (cyclotrimethylenetrinitramine) and LiF, as compared with α-Ti and α-Fe, where localized heating effects are known to be important and are estimated to be much larger than those which can occur for aluminum.

Nanofractography of shocked RDX explosive crystals with atomic force microscopy

Examination with atomic force microscopy has revealed apparent shear-type cleavage steps with heights as small as 0.05 nm, smaller than the size of cyclotrimethylenetrintramine (RDX) molecules, on the fracture surfaces of crystals that were subjected to aquarium shocks of 61.6 or 129 kbar, both greater than the pressure (38 kbar) required for the alpha-to-gamma phase transformation. The shocked centimeter size, originally transparent crystals became opaque and white from prolific fractures and internal cracks that are associated with their breakup into nanocrystallites of sizes extending from 500 down to 20 nm. The submolecular steps are related geometrically to the macroscale (K∥) fracture mechanics mode of shear fracturing that has obvious consequences at the nanoscale level for nonregistry between molecules across the crack surfaces. The results are of interest in relation to lattice trapping of crack fronts and as a contribution to the possibility of deformation-induced chemical decomposition/detonations.

X-ray orientation and hardness experiments on RDX explosive crystals

RDX (cyclotrimethylenetrinitramine) explosive crystals, typically approaching 5 mm in size, were grown by evaporation from acetone solution using production-grade crystals as starting material. Two distinctly different morphologies resulted, including one that apparently has not been previously reported in other investigations. These morphologies were characterized using Laue X-ray diffraction methods and an optical trace analysis, both involving a stereographic projection description. Microindentation experiments were performed on different prominent growth surfaces of several selected laboratorygrown crystals having the conventional morphology type. The hardness results are compared with measurements made directly on several production-grade crystals having a different morphology, and are compared with preceding measurements on a crystal having the previously unreported morphology. The latter crystal exhibited highly localized plastic deformation at the indentations as revealed by dislocation etch-pitting. Observations are made regarding the dislocation structure and cleavage properties of RDX based on its orthorhombic unit cell.

ELASTIC, PLASTIC, CRACKING ASPECTS OF THE HARDNESS OF MATERIALS

The hardness properties of materials are tracked from early history until the present time. Emphasis is placed on the hardness test being a useful probe for determining the local elastic, plastic and cracking properties of single crystal, polycrystalline, polyphase or amorphous materials. Beginning from connection made between individual hardness pressure measurements and the conventional stress–strain properties of polycrystalline materials, the newer consideration is described of directly specifying a hardness-type stress–strain relationship based on a continuous loading curve, particularly, as obtained with a spherical indenter. Such effort has received impetus from order-of-magnitude improvements in load and displacement measuring capabilities that are demonstrated for nanoindentation testing. Details of metrology assessments involved in various types of hardness tests are reviewed. A compilation of measurements is presented for the separate aspects of Hertzian elastic, dislocation-mechanics-based plasticity and indentation-fracture-mechanics-based cracking behaviors of materials, including elastic and plastic deformation rate effects. A number of test applications are reviewed, most notably involving the hardness of thin film materials and coatings.

Adsorption surface energy and crystal growth in lron-3 pct silicon

The effect of surface energy and sulfur adsorption on secondary recrystallization and texture development in thin sheets of Fe-3 pct Si has been delineated. In the absence of adsorbed sulfur, a {110} secondary recrystallization texture develops. The influence of sulfur on the surface energy hierarchy of the {100} and {110} planes was determined by observing grain boundary motion in thin bicrystals exposed to controlled gaseous environments. High temperature sulfur adsorption isotherms were obtained from a radioactive tracer study and it was found that sulfur adsorbs preferentially on the {100} plane. The Gibbs adsorption equation was used to analyze this preferential adsorption, demonstrating a reversal in the surface energy hierarchy with the {100} becoming the minimum energy plane. Under these conditions a {100} secondary texture develops.