Frank J. Zerilli

Description of tantalum deformation behavior by dislocation mechanics based constitutive relations

Dislocation mechanics based constitutive equation constants are determined for temperature, strain rate, work hardening, and polycrystal grain size influences on the deformation behavior of various tantalum materials. An analysis of the maximum load point strain provides a useful method of determining the work hardening constants. Different athermal stress constants and thermal activation related constants are obtained for certain groupings of the different tantalum materials. The variations are correlated with the annealing history of the materials and related to dislocation model parameters involved in the thermal activation strain rate analysis. Computed tantalum deformation results based on these constants are shown to agree with Gourdin’s reported expanding ring test measurements and with the deformed shape of a Taylor cylinder impact test specimen.

Influences of strain rate and grain size on yield and serrated flow in commercial Al-Mg alloy 5086

Much attention has been paid to the rather special stress-strain characteristics of Al-Mg alloys. Beginning with the pioneering work of Portevin and LeChatelier in 1923, the focus has been on the appearance of tensile band-type deformation markings that are observed coincident with the appearance of serrated flow in stress-strain curves. The serrations are the result of dynamic strain aging (DSA). A major influence of the DSA phenomenon is to produce both a higher flow stress and, very importantly, greater strain hardening at lower strain rates than for higher ones at which serrations do not appear. Physically-based constitutive equations, for example, of Zerilli-Armstrong type, that are derived for thermally-activated dislocation-lattice or dislocation-dislocation interactions, are susceptible to interference from DSA. For this reason, clarification of the importance of strain aging in accounting for laboratory tests of the stress-strain behavior of the alloy has been investigated. Of special interest was the solute influence on the combined yield and strain hardening behavior.

Dislocation mechanics of copper and iron in high rate deformation tests

Different dislocation processes are shown to be operative under high rate loading by impact-induced shock tests as compared with shockless isentropic compression experiments (ICEs). Under shock loading, the plastic deformation rate dependence of the flow stress of copper is attributed to dislocation generation at the propagating shock front, while in shockless ICEs, the rate dependence is attributed to drag-controlled mobile dislocation movement from within the originally resident dislocation density. In contrast with shock loading, shockless isentropic compression can lead to flow stress levels approaching the theoretical yield stress and dislocation velocities approaching the speed of sound. In iron, extensive shock measurements reported for plate impact tests are explained in terms of plasticity-control via the nucleation of deformation twins at the propagating shock front.

Equation of state and structural changes in diaminodinitroethylene under compression

Structural changes in 1,1-diamino-2,2-dinitroethylene (DADNE, FOX-7) compressed to high pressure in diamond anvil cells were investigated using angle-dispersive x-ray diffraction analysis, Raman spectroscopy, and optical polarizing microscopy. The x-ray results show several changes above 1 GPa. When the x-ray data are indexed according to the ambient-pressure structure, DADNE shows anisotropic compression, with higher compression along the b axis than along the a or c axis. An ambient-temperature isothermal equation of state of DADNE was generated from these data. In addition, the experimentally obtained Raman spectra were matched with vibrational normal modes calculated using quantum chemistry calculations. The shifts in vibrational modes indicate changes in H-wagging vibrations with pressure.

Shear-strain induced decomposition of 1,1-diamino-2,2-dinitroethylene

The structural and electronic features of shear strains in the molecular crystal 1,1-diamino-2,2-dinitroethylene and their effect on decomposition of the material are investigated. The authors demonstrate that shear strains lower the decomposition barrier and narrow the band gap of the solid and thus facilitate thermal chemistry in molecular materials. The appearance of defect-related electronic states in the band gap is consistent with previous results for dislocation modeling in molecular solids and with experiments on energetic materials. The dynamic behavior of the band gap contains a rich variety of information about the details of the decomposition processes at the initiation stage.

Predicting CH/π Interactions with Nonlocal Density Functional Theory

We examine the performance of a recently developed nonlocal density functional in predicting a model noncovalent interaction, namely the weak bond between an aromatic pi system and an aliphatic C--H group. The new functional is a significant improvement over traditional density functionals, providing results which compare favorably to high-level quantum-chemistry techniques, but at considerably lower computational cost. Interaction energies in several model C--H/pi systems are in good general agreement with coupled-cluster calculations, though equilibrium distances are consistently overpredicted when using the revPBE functional for exchange. The new functional predicts changes in energy upon addition of halogen substituents correctly.

Electronic excitations and decomposition of 1,1-diamino-2,2-dinitroethylene

We present first-principles density-functional calculations of the atomic and electronic structure of the molecular crystal 1,1-diamino-2,2-dinitroethylene (FOX-7). Under either an isotropic or uniaxial applied stress, the ideal crystal lattice of this material accumulates elastic energy without any chemical or significant electronic structure changes. The presence of “reversed-orientationmolecule” defects narrows the band gap and lowers the decomposition barrier of the material in the solid phase.

Ab initio studies of crystalline nitromethane under high pressure

We have studied the mechanical compressibility and band structure of solid nitromethane both in equilibrium and compressed states using Hartree-Fock and density functional theory (DFT) with atom-centered all-electron linear combination of atomic orbitals basis sets. Hartree-Fock calculations with a 6-21G basis set, uncorrected for basis set superposition error, gave the best agreement with experimental compression studies. These results may be due to the cancellation of basis set superposition error with dispersion force errors. The equilibrium DFT band gap is comparable to the lowest-energy feature in electron-impact spectroscopy of nitromethane but underpredicts the optical absorption gap; we interpret these features in terms of the presence of tightly bound excitons. Only minor changes in the gap are observed under hydrostatic compression.

Experimental and computational study of the impact deformation of titanium Taylor cylinder specimens

Abstract Gas-gun reverse-ballistic Taylor cylinder impact deformation experiments have been performed with specimens of 99.7% titanium. The flat-ended, 6.35 mm diameter, 25.4 mm long solid cylindrical specimens were impacted by sabot-mounted 34.0 mm diameter, 19.0 mm thick disks of maraging 350 steel. Experiments were performed at 225 and 294 m/s; a soft recovery system prevented post-impact damage to the specimens. Plastic deformation extended over approximately 57 and 64% of the deformed specimen length, respectively, with little lateral spreading. The specimen shapes were digitized via a toolmakers microscope connected to a computer. Metallographic examination of longitudinal sections through the specimens revealed extensive deformation twinning, with twinned grains appearing throughout the deformed regions. Distributions of twins as a function of distance from the impact end were determined for each specimen. The importance of twinning in the computational modelling of deformation in body centered cubic (BCC) iron was shown by Zerilli and Armstrong. It is also very important for hexagonal close packed (HCP) titanium which exhibits a BCC type of behavior. Deformation twinning can cause a reduction in the effective size of the twinned grains; an associated increase in the yield stress could explain the relatively small amount of lateral spreading observed for the titanium Taylor specimens. Computational simulations of the impact experiments performed without provision for twinning show significant differences between the simulations and the experimental shapes. When twinning effects are included, good fits to the experimental shapes are obtained.

Theoretical equation of state for aluminized nitromethane

The simple fluid constituents of the reaction products of an aluminized nitromethane explosive are described by a perturbation technique based on their intermolecular interactions. Liquid metal constituents are treated with a Grover scaling model. Standard solid‐state approaches are applied to the solid components of the reaction products. Calculated detonation velocities compared favorably with experimental data.