Numerical thermo-elasto-plastic analysis of residual stresses on different scales during cooling of hot forming parts

In current research, more and more attention is paid to the understanding of residual stress states as well as the application of targeted residual stresses to extend e.g. life time or stiffness of a part. In course of that, the numerical simulation and analysis of the forming process of components, which goes along with the evolution of residual stresses, play an important role. In this contribution, we focus on the residual stresses arising from the austenite-to-martensite transformation at microscopic and mesoscopic level of a Cr-alloyed steel. A combination of a Multi-Phase-Field model and a two-scale Finite Element simulation is utilized for numerical analysis. A first microscopic simulation considers the lattice change, such that the results can be homogenized and applied on the mesoscale. Based on this result, a polycrystal consisting of a certain number of austenitic grains is built and the phase transformation from austenite to martensite is described with respect to the mesoscale. Afterwards, in a two-scale Finite Element simulation the plastic effects are considered and resulting residual stress states are computed

One-body energy dissipation in fusion reaction from mean-field theory

Information on dissipation in the entrance channel of heavy-ion collisions is extracted by macroscopic reduction procedure of Time-Dependent Hartree-Fock theory. The method gives access to a fully microscopic description of the friction coefficient associated with transfer of energy from the relative motion towards intrinsic degrees of freedom. The reduced friction coefficient exhibits a universal behavior, i.e. almost independent of systems investigated, whose order of magnitude is comparable with the calculations based on linear response theory. Similarly to nucleus-nucleus potential, especially close to the Coulomb barrier, there are sizable dynamical effects on the magnitude and form factor of friction coefficient.Comment: 7 pages, 10 figure

Van der Waals Density Functional for General Geometries

A scheme within density functional theory is proposed that provides a practical way to generalize to unrestricted geometries the method applied with some success to layered geometries [H. Rydberg, et al., Phys. Rev. Lett. 91, 126402 (2003)]. It includes van der Waals forces in a seamless fashion. By expansion to second order in a carefully chosen quantity contained in the long range part of the correlation functional, the nonlocal correlations are expressed in terms of a density-density interaction formula. It contains a relatively simple parametrized kernel, with parameters determined by the local density and its gradient. The proposed functional is applied to rare gas and benzene dimers, where it is shown to give a realistic description.Comment: 4 pages, 4 figure

The warm circumstellar envelope and wind of the G9 IIb star HR 6902

IUE observations of the eclipsing binary system HR 6902 obtained at various epochs spread over four years indicate the presence of warm circumstellar material enveloping the G9 IIb primary. The spectra show Si IV and C IV absorption up to a distance of 3.3 giant radii (R_g}. Line ratio diagnostics yields an electron temperature of ~ 78000 K which appears to be constant over the observed height range. Applying a least square fit absorption line analysis we derive column densities as a function of height. We find that the inner envelope (< 3 R_g) of the bright giant is consistent with a hydrostatic density distribution. The derived line broadening velocity of ~ 70 kms^{-1} is sufficient to provide turbulent pressure support for the required scale height. However, an improved agreement with observations over the whole height regime including the emission line region is obtained with an outflow model. We demonstrate that the common beta power-law as well as a P \propto rho wind yield appropriate fit models. Adopting a continuous mass outflow we obtain a mass-loss rate of M_loss= (0.8 - 3.4)*10^{-11} M_{sun}yr^{-1} depending on the particular wind model.Comment: 11 pages, 8 figures, submitted to Astronomy Astrophysics main Journa

Group Theory of Chiral Photonic Crystals with 4-fold Symmetry: Band Structure and S-Parameters of Eight-Fold Intergrown Gyroid Nets

The Single Gyroid, or srs, nanostructure has attracted interest as a circular-polarisation sensitive photonic material. We develop a group theoretical and scattering matrix method, applicable to any photonic crystal with symmetry I432, to demonstrate the remarkable chiral-optical properties of a generalised structure called 8-srs, obtained by intergrowth of eight equal-handed srs nets. Exploiting the presence of four-fold rotations, Bloch modes corresponding to the irreducible representations E- and E+ are identified as the sole and non-interacting transmission channels for right- and left-circularly polarised light, respectively. For plane waves incident on a finite slab of the 8-srs, the reflection rates for both circular polarisations are identical for all frequencies and transmission rates are identical up to a critical frequency below which scattering in the far field is restricted to zero grating order. Simulations show the optical activity of the lossless dielectric 8-srs to be large, comparable to metallic metamaterials, demonstrating its potential as a nanofabricated photonic material

Group Theory of Circular-Polarization Effects in Chiral Photonic Crystals with Four-Fold Rotation Axes, Applied to the Eight-Fold Intergrowth of Gyroid Nets

We use group or representation theory and scattering matrix calculations to derive analytical results for the band structure topology and the scattering parameters, applicable to any chiral photonic crystal with body-centered cubic symmetry I432 for circularly-polarised incident light. We demonstrate in particular that all bands along the cubic [100] direction can be identified with the irreducible representations E+/-,A and B of the C4 point group. E+ and E- modes represent the only transmission channels for plane waves with wave vector along the ? line, and can be identified as non-interacting transmission channels for right- (E-) and left-circularly polarised light (E+), respectively. Scattering matrix calculations provide explicit relationships for the transmission and reflectance amplitudes through a finite slab which guarantee equal transmission rates for both polarisations and vanishing ellipticity below a critical frequency, yet allowing for finite rotation of the polarisation plane. All results are verified numerically for the so-called 8-srs geometry, consisting of eight interwoven equal-handed dielectric Gyroid networks embedded in air. The combination of vanishing losses, vanishing ellipticity, near-perfect transmission and optical activity comparable to that of metallic meta-materials makes this geometry an attractive design for nanofabricated photonic materials

Scaling approach to itinerant quantum critical points

Based on phase space arguments, we develop a simple approach to metallic quantum critical points, designed to study the problem without integrating the fermions out of the partition function. The method is applied to the spin-fermion model of a T=0 ferromagnetic transition. Stability criteria for the conduction and the spin fluids are derived by scaling at the tree level. We conclude that anomalous exponents may be generated for the fermion self-energy and the spin-spin correlation functions below $d=3$, in spite of the spin fluid being above its upper critical dimension.Comment: 3 pages, 2 figures; discussion of the phase space restriction modified and, for illustrative purposes, restricted to the tree-level analysis of the ferromagnetic transitio