Search

We investigate with molecular dynamics the dynamic response of Cu bicrystals with a special asymmetric grain boundary (GB), (111)//(112)<110>, and its dependence on the loading directions. Shock loading is applied along the GB normal either from the left or right to the GB. Due to the structure asymmetry, the bicrystals demonstrate overall strong left-right loading dependence of its shock response, including compression wave features, compression and tensile plasticity, damage characteristics (e.g., spall strength), effective wave speeds and structure changes, except that spallation remains dominated by the GB damage regardless of the loading directions. The presence or absence of transient microtwinning also depends on the loading directions. [ABSTRACT FROM AUTHOR]

We investigate with nonreactive molecular dynamics simulations the dynamic response of phenolic resin and its carbon-nanotube (CNT) composites to shock wave compression. For phenolic resin, our simulations yield shock states in agreement with experiments on similar polymers except the 'phase change' observed in experiments, indicating that such phase change is chemical in nature. The elastic-plastic transition is characterized by shear stress relaxation and atomic-level slip, and phenolic resin shows strong strain hardening. Shock loading of the CNT-resin composites is applied parallel or perpendicular to the CNT axis, and the composites demonstrate anisotropy in wave propagation, yield and CNT deformation. The CNTs induce stress concentrations in the composites and may increase the yield strength. Our simulations suggest that the bulk shock response of the composites depends on the volume fraction, length ratio, impact cross-section, and geometry of the CNT components; the short CNTs in current simulations have insignificant effect on the bulk response of resin polymer. [ABSTRACT FROM AUTHOR]

A detailed derivation and discussion is given of our diagrammatic theory of temperature-dependent resonance Raman (RR) scattering and the optical absorption for multimode systems having general quadratic plus linear electron-vibrational coupling. By combining the time-correlator reformulation of RR scattering with suitably developed nonzero temperature many-body diagrammatic techniques, we obtain the RR excitation profiles and the absorption as one-dimensional Fourier transforms of analytic expressions involving just the model parameters and the temperature. The expressions are very convenient for explicit multimode model calculations. In addition, the theory brings out in a natural way the relation between RR profiles and the absorption, such that within well-defined special cases useful ‘‘transform’’ techniques canbe developed for computing profiles directly from the observed temperature-dependent absorption. The many practical advantages of the theory for the analysis of experimental data have been demonstrated in earlier papers dealing with specific systems. In this paper we provide a comprehensive discussion of the theoretical details, which have not been given previously. The theory applies for any number of normal modes, and for arbitrary normal coordinate mixing, mode frequency shifts, and atomic equilibrium position shifts under electronic excitaiton. It involves products of phonon operators having both positive and negative time ordering, necessitating specialized combinatorial arguments. The use of an appropriate linked cluster expansion is shown to lead very naturally to a separation of the RR scattering into ‘‘orders,’’ which is the essential component producing the important general features listed above.Detailed derivations are given of the exact expressions for the first-order RR profiles in the most general model, and for first- , second- , and third-order profiles in the frequency-shift... [ABSTRACT FROM AUTHOR]

Examines plasticity path effects in metals under shock compression. Methods used to extract the shock-wave plasticity from shock-wave data; Underlying concepts of plastic flow described by one internal state variable.

Presents a model for the relation between shock velocity and particle velocity in weak shock waves in metals. Background on Wallace's weak-shock theory; Calculation of shock velocity numerically and algebraically based on the theory; Sources for elastic constant and shock-wave velocity data.

Presents a study that showed how the percolation threshold in a standard continuum percolation model corresponds to the plastic yield transition in a dislocation model with infinitely strong point obstacles and with no dislocation cancellation allowed. Simulation of the cluster properties of areas defined to be open to dislocation loop expansion; Use of a lattice model and Monte Carlo methods; Experimental results and discussion.

In a recent paper (Az I), well-structured T=300 K resonance Raman (RR) profiles for the 1400, 1260, 900, and 2×825 cm-1 lines of azulene in CS2 and for the 825 cm-1 line of azulene in methanol were reported. Previously developed transform techniques were used to (1) compute RR profile line shapes directly from measured optical absorption spectra, and (2) extract ratios of Stokes loss parameters from the line shape scale factors. The transform analysis indicated that (1) our model assumptions (adiabatic and Condon approximations, harmonic phonons, atomic equilibrium position shifts, and small vibrational frequency shifts upon excitation to a single electronic state) are basically correct allowing forminor modifications, and (2) any deviations from these assumptions are likely to be larger for the 900 cm-1 mode and smaller for the 1400 and 1260 cm-1 modes. In this paper (Az II), we report model calculations of the optical absorption spectra, RR profile line shapes, and relative RR intensities. In these calculations, we use a recently proposed nonzero temperature multimode time-correlator modeling procedure. Compared with the conventional sum-over-states method, our time-correlator modeling procedure is superior in that (1) our optical absorption spectra and RR profiles computed via fast Fourier transform techniques have a practically unlimited spectral range and (2) the computing times are short for nonzero temperature multimode calculations. In our basic model, we adopt the assumptions of Az I and use seven azulene modes to obtain simultaneous good fits of the well-structured RR profile line shapes and optical absorption spectra. However, we find that the basic model does not account for the intensity of the 900 cm-1 Raman line relative to that of the 1400 cm-1 line, even though the individual profile line shape fits for these modes are very good. The basic model is therefore modified to allow mixing of the normal coordinates of these two modes. By... [ABSTRACT FROM AUTHOR]