Intergranular stress distributions in polycrystalline aggregates of irradiated stainless steel

In order to predict InterGranular Stress Corrosion Cracking (IGSCC) of post-irradiated austenitic stainless steel in Light Water Reactor (LWR) environment, reliable predictions of intergranular stresses are required. Finite elements simulations have been performed on realistic polycrystalline aggregate with a recently proposed physically-based crystal plasticity constitutive equations validated for neutron-irradiated austenitic stainless steel. Intergranular normal stress probability density functions are found with respect to plastic strain and irradiation level, for uniaxial loading conditions. In addition, plastic slip activity jumps at grain boundaries are also presented. Intergranular normal stress distributions describe, from a statistical point of view, the potential increase of intergranular stress with respect to the macroscopic stress due to grain-grain interactions. The distributions are shown to be well described by a master curve once rescaled by the macroscopic stress, in the range of irradiation level and strain considered in this study. The upper tail of this master curve is shown to be insensitive to free surface effect, which is relevant for IGSC

Wrapping an adhesive sphere with a sheet

We study the adhesion of an elastic sheet on a rigid spherical substrate. Gauss'Theorema Egregium shows that this operation necessarily generates metric distortions (i.e. stretching) as well as bending. As a result, a large variety of contact patterns ranging from simple disks to complex branched shapes are observed as a function of both geometrical and material properties. We describe these different morphologies as a function of two non-dimensional parameters comparing respectively bending and stretching energies to adhesion. A complete configuration diagram is finally proposed

Strain gradient crystal plasticity with evolving length scale: Application to voided irradiated materials

International audienceA micromorphic crystal plasticity model is used to simulate slip band localization in single crystalsunder simple shear at finite deformations. Closed form analytical solutions are derived for singleslip in the case of positive, zero and negative strain hardening. Linear negative strain hardening,i.e. linear softening, leads to a constant localization slip band width, while non linear softening andsaturating behaviour results in an increasing band width. An enhanced model is therefore proposed inorder to maintain a bounded localization slip band width when considering an exponential softeningbehaviour. Analytical solutions are used to validate finite element computation of the same boundaryvalue problems. The enhanced micromorphic crystal plasticity model is then applied to predict theinteraction between localized slip bands and voids encountered in voided irradiated materials. For thatpurpose, periodic porous unit cells are loaded in simple shear with a strain gradient crystal plasticitymatrix material. The finite element simulation results show that, for a given void volume fraction, thelarger the voids, the wider the localization band. However, for a given void size, the larger the voidvolume fraction, the narrower the localization band. In addition a satisfactory qualitative agreementof the rotation and elongation of the voids with the experimental observations made in irradiatedmaterials is observed, where small voids are shown to remain ellipsoidal for larger shear strains thanlarge voids which deform into peanut-like shapes

On the turbulent α\alpha-disks and the intermittent activity in AGN

We consider effects of the MHD turbulence on the viscosity during the evolution of the thermal-viscous ionization instability in the standard $\alpha$-accretion disks. We consider the possibility that the accretion onto a supermassive black hole proceeds through an outer standard accretion disk and inner, radiatively inefficient and advection dominated flow. In this scenario we follow the time evolution of the accretion disk in which the viscosity parameter $\alpha$ is constant throughout the whole instability cycle, as implied by the strength of MHD turbulence. We conclude that the hydrogen ionization instability is a promising mechanism to explain the intermittent activity in AGN.Comment: 13 pages, 9 figures; ApJ accepte

Mira's wind explored in scattering infrared CO lines

We have observed the intermediate regions of the circumstellar envelope of Mira (o Ceti) in photospheric light scattered by three vibration-rotation transitions of the fundamental band of CO, from low-excited rotational levels of the ground vibrational state, at an angular distance of beta = 2"-7" away from the star. The data were obtained with the Phoenix spectrometer mounted on the 4 m Mayall telescope at Kitt Peak. The spatial resolution is approximately 0.5" and seeing limited. Our observations provide absolute fluxes, leading to an independent new estimate of the mass-loss rate of approximately 3e-7 Msun/yr, as derived from a simple analytic wind model. We find that the scattered intensity from the wind of Mira for 2" < beta < 7" decreases as beta^-3, which suggests a time constant mass-loss rate, when averaged over 100 years, over the past 1200 years.Comment: accepted for publication in the Astrophysical Journa

Interpreting the Variability of Double-Peaked Emission Lines in Active Galactic Nuclei with Stochastically Perturbed Accretion Disk Models

In an effort to explain the short-timescale variability of the broad, double-peaked profiles of some active galactic nuclei, we constructed stochastically perturbed accretion disk models and calculated H alpha line profile series as the bright spots rotate, shear and decay. We determined the dependence of the properties of the line profile variability on the spot properties. We compared the variability of the line profile from the models to the observed variability of the H alpha line of Arp 102B and 3C 390.3. We find that spots need to be concentrated in the outer parts of the line emitting region to reproduce the observed variability properties for Arp 102B. This rules out spot production by star/disk collisions and favors a scenario where the radius of marginal self-gravity is within the line emitting region, creating a sharp increase in the radial spot distribution in the outer parts. In the case of 3C 390.3, all the families of models that we tested can reproduce the observed variability for a suitable choice of model parameters.Comment: 27 pages, 8 figures, accepted for publication in Ap

New Analytical Formula for Supercritical Accretion Flows

We examine a new family of global analytic solutions for optically thick accretion disks, which includes the supercritical accretion regime. We found that the ratio of the advection cooling rate, $Q_{\rm adv}$, to the viscous heating rate, $Q_{\rm vis}$, i.e., $f=Q_{\rm adv}/Q_{\rm vis}$, can be represented by an analytical form dependent on the radius and the mass accretion rate. The new analytic solutions can be characterized by the photon-trapping radius, \rtrap, inside which the accretion time is less than the photon diffusion time in the vertical direction; the nature of the solutions changes significantly as this radius is crossed. Inside the trapping radius, $f$ approaches $f \propto r^0$, which corresponds to the advection-dominated limit ($f \sim 1$), whereas outside the trapping radius, the radial dependence of $f$ changes to $f \propto r^{-2}$, which corresponds to the radiative-cooling-dominated limit. The analytical formula for $f$ derived here smoothly connects these two regimes. The set of new analytic solutions reproduces well the global disk structure obtained by numerical integration over a wide range of mass accretion rates, including the supercritical accretion regime. In particular, the effective temperature profiles for our new solutions are in good agreement with those obtained from numerical solutions. Therefore, the new solutions will provide a useful tool not only for evaluating the observational properties of accretion flows, but also for investigating the mass evolution of black holes in the presence of supercritical accretion flows.Comment: 14 pages, 7 figures, accepted for publication in the Astrophysical Journa

Evolution of the X-ray spectrum in the flare model of Active Galactic Nuclei

Nayakshin & Kazanas (2002) have considered the time-dependent illumination of an accretion disc in Active Galactic Nuclei, in the lamppost model. We extend their study to the flare model, which postulates the release of a large X-ray flux above a small region of the accretion disc. A fundamental difference with the lamppost model is that the region of the disc below the flare is not illuminated before the onset of the flare. A few test models show that the spectrum which follows immediately the increase in continuum flux should display the characteristics of a highly illuminated but dense gas, i.e. very intense X-ray emission lines and ionization edges in the soft X-ray range. The behaviour of the iron line is different in the case of a "moderate" and a ``strong'' flare: for a moderate flare, the spectrum displays a neutral component of the Fe K$\alpha$ line at 6.4 keV, gradually leading to more highly ionized lines. For a strong flare, the lines are already emitted by FeXXV (around 6.7 keV) after the onset, and have an equivalent width of several hundreds of eV. We find that the observed correlations between $R$, $\Gamma$, and the X-ray flux, are well accounted by a combination of flares having not achieved pressure equilibrium, strongly suggesting that the observed spectrum is dominated by regions in non-pressure equilibrium, typical of the onset of the flares. Finally a flare being confined to a small region of the disc, the spectral lines should be narrow (except for a weak Compton broadening), Doppler shifted, and moving.Comment: 14 pages, 13 figures, accepted in A & A, english corrected versio

Observations of H3+ in the Diffuse Interstellar Medium

Surprisingly large column densities of H3+ have been detected using infrared absorption spectroscopy in seven diffuse cloud sightlines (Cygnus OB2 12, Cygnus OB2 5, HD 183143, HD 20041, WR 104, WR 118, and WR 121), demonstrating that H3+ is ubiquitous in the diffuse interstellar medium. Using the standard model of diffuse cloud chemistry, our H3+ column densities imply unreasonably long path lengths (~1 kpc) and low densities (~3 cm^-3). Complimentary millimeter-wave, infrared, and visible observations of related species suggest that the chemical model is incorrect and that the number density of H3+ must be increased by one to two orders of magnitude. Possible solutions include a reduced electron fraction, an enhanced rate of H2 ionization, and/or a smaller value of the H3+ dissociative recombination rate constant than implied by laboratory experiments.Comment: To be published in Astrophysical Journal, March 200