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

Estimation des niveaux d'inondation pour une crue éclair en milieu urbain : comparaison de deux modèles hydrodynamiques sur la crue de Nîmes d'octobre 1988

Lors des crues extrêmes en ville, une forte part des écoulements reste en surface. Pour simuler ces inondations, deux modèles sont présentés : le logiciel REM2 U unidimensionnel a pour objectif de simuler la propagation des débits de crue dans l'ensemble d'un réseau de rues alors que le logiciel Rubar 20 bidimensionnel vise à fournir plus d'information sur ces écoulements. Des calculs avec ces deux logiciels ont été menés sur la crue d'octobre 1988 dans un quartier de Nîmes. Lors de cet événement, les hauteurs d'eau maximales ont dépassé deux mètres en certains points et les vitesses 2 m/s ce qui entraînait des passages en régime torrentiel. A partir des données rassemblées sur les sections en travers des rues, des maillages de calcul limités au réseau de rues ont été construits pour les deux logiciels afin de permettre un calcul détaillé. La comparaison des résultats avec les laisses de crue montre des situations très contrastées d'un point à un autre pour une hauteur d'eau maximale moyenne sur l'ensemble de la zone inondée correctement simulée. L'écart sur cette hauteur est, en moyenne, de 1 m ce qui provient des incertitudes sur les observations, sur la topographie et sur les conditions aux limites, des approximations lors de la modélisation et de particularités locales non décrites. Entre les deux logiciels, l'évolution des hauteurs et des vitesses est généralement très proche bien que, comme pour la comparaison avec les laisses de crue, des différences locales importantes sont observées.The hydraulic models that are used to simulate floods in rural areas are not adapted to model floods through urban areas, because of details that may deviate flows and create strong discontinuities in the water levels, and because of the possible water flow running in the sewage network. However, such modelling is strongly required because damage is often concentrated in urban areas. Thus, it is necessary to develop models specifically dedicated to such floods. In the southern part of France, rains may have a high intensity but floods generally last a few hours. During extreme events such as the October 1988 flood in the city of Nîmes, most of the flow remained on the ground with high water depths and high velocities, and the role of sewage network can be neglected. A 1-D model and a 2-D model were used to calculate such flows, which may become supercritical. On the catchments of the streams which cross the city of Nîmes, the rainfall was estimated as 80 mm in one hour and 250 mm in six hours in October 1988, although some uncertainties remain. The return period can be estimated between 150 and 250 years. The zone selected to test the models was an area 1.2 km long and less than 1 km wide in the north-eastern part of the city. It includes a southern part with a high density of houses. The slope from the North (upstream) to the South (downstream) was more than 1 % on average and was decreasing from North to South. Various topographical and hydrological data were obtained from the local Authorities. The basic data were composed of 258 cross sections of 69 streets with 11 to 19 points for each cross section. Observations of the limits of the flooded areas and of the peak water levels at more than 80 points can be used to validate the calculation results. The inputs consisted of two discharge hydrographs, estimated from a rainfall-discharge model from rains with a return period of 100 years, which may result in an underestimate of these inputs. These two hydrographs correspond to the two main structures that cross the railway embankment, which constitutes an impervious upstream boundary of the modelled area. Whereas the western and eastern boundaries are well delimitated by hills above maximum water levels, the downstream southern boundary is somewhat more questionable because of possibilities of backwater and inflows from neighbouring areas.The 1-D software REM2U solved the Saint Venant equations on a meshed network. At crossroads, continuities of discharge and of water heads were set. The hydraulic jump was modelled by a numerical diffusion applied wherever high water levels were found. The Lax Wendroff numerical scheme was implemented. It included a prediction step and a correction step, which implied precise solving of these very unsteady and hyperbolic problems. The software was validated on numerous test cases (Al Mikdad, 2000) which proved the adaptation to problems of calculations in a network of streets.The 2-D software Rubar 20 solves 2-D shallow water equations by an explicit second-order Van Leer type finite volume scheme on a computational grid made from triangles and quadrilaterals (Paquier, 1998). The discontinuities (hydraulic jumps for instance) are treated as ordinary points through the solving of Riemann problems. For the Nîmes case, the grid was built from the cross sections of the streets. Four grids were built with respectively 4, 5, 7 or 11 points for every cross section and these points correspond to the main characteristics of the cross section: the walls of the buildings, the sidewalks, the gutters and the middle point. The simplest crossroads were described from the crossings of the lines corresponding to these points, which provide respectively 16, 25, 49 or 121 computational cells. The space step was about 25 metres along the streets but went as low as 0.1 m in the crossroads; due to the explicit scheme, which implies that the Courant number was limited to 1, the time step was very small and a long computational time was required.The computations were performed with a uniform Strickler coefficient of 40 m1/3/s. Both 1-D and 2-D models provided results that agreed well with observed water levels. The limits of the flooded area were also quite well simulated. However, locally, the differences between calculated and observed maximum water depths were high, resulting in an average deviation of about 1 metre. The reasons for such deviations could come from three main causes. First, the uncertainty of topographical data is relatively high, because of the interpolation between measured cross sections without a detailed complementary DEM (digital elevation model). Second, the observed levels were also uncertain and reveal local situations that are not reconstructed by the hydraulic models which provided maximum water levels averaged on one cell which may not coincide with the exact location of the observations. Finally, modelling means a simplification of the processes, which implies cancelling the level variations due to some obstacles, such as cars, which are not simple to identify.In conclusion, both software packages can model a flood, even a flash flood, in an urbanised area. Research is still necessary to develop methods to fully use urban databases in order to define details more precisely. The improvements to the 1-D software should include a better modelling of storage and of crossroads with an integration of adapted relations for the head losses. 2-D software has a greater potential but the difficulty to build an optimal computational grid means a long computational time, which limits the use of such software to small areas. For both software packages, methods still need to be developed in order to represent exchanges with the sewage network, storage inside buildings and inputs directly coming from rainfall

Realization of a semiconductor-based cavity soliton laser

The realization of a cavity soliton laser using a vertical-cavity surface-emitting semiconductor gain structure coupled to an external cavity with a frequency-selective element is reported. All-optical control of bistable solitonic emission states representing small microlasers is demonstrated by injection of an external beam. The control scheme is phase-insensitive and hence expected to be robust for all-optical processing applications. The motility of these structures is also demonstrated

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

Analysis of the genetic variation for adaptation to a short thermal stress on young « Bos taurus » cattle

In the present work, an attempt was made to determine the adaptation to heat of some cattle breeds, especially French ones. A total of 582 young male and female cattle, about 14 months old and coming from 41 elementary genetic combinations analyzed in 5 different non bioclimatological experiments, were subjected to a heat stress for 8 hours during which the room temperature was increased from 18°C to 38 °C. The usual reaction were observed i.e. increase in the rectal temperature during the stress (+ 0,54 °C), increase in the respiratory rate ( X 3), increase in the sweating rate, heart rate and skin temperature. The statistical analyses were made at three stages of stress (beginning, middle, end). In all cases, a very strong environmental effect was noticed, ie effect of the year and of experimental errors in the management of the theoretical schedule involving temperature and relative hygrometry. The data adjusted for environmental effects rather clearly show the genetic variability. However, a more accurate analysis shows that it is mainly proceeded from a between group variability when defining the groups according to « animal husbandry » parameters (dairy purpose, beef purpose, and local breed group). As a matter of fact, in most cases, there was no significant variability between the reactions of the genotypes belonging to the same group, whereas this variability existed between the groups..The satisfactory performance of the local breed group clearly appeared in comparison with the group including the improved dairy or beef breeds exhibiting only minor differences in the parameters analyzed. In the discussion, emphasis in laid ov the possible influence of thermogenesis on the results. Although our findings could have been afrected by disturbing events such as emotional stress, they agree rather well with data obtained is practice on the behaviour of the local French breeds in some hot countries.Le but de ce travail est d’essayer de caractériser l’adaptation à la chaleur d’un certain nombre de races bovines, notamment françaises. Pour cela, 582 jeunes bovins, mâles et femelles, d’environ 14 mois et issus de 41 combinaisons génétiques élémentaires analysées dans 5 expérimentations différentes, à objectif non bioclimatologique, sont soumis en chambre chaude, à un stress thermique de 8 heures pendant lequel la température externe a été portée de 11) De 18°C à 38 °C. Les réactions classiques sont observées : augmentation de la température rectale au cours du stress (+ 0,54°C), augmentation du rythme respiratoire ( X 3), augmentation du taux de sudation, du rythme cardiaque, de la température de la peau. Les analyses statistiques sont effectuées à trois stades du stress (début, milieu, fin). Dans tous les cas, on note de très forts effets dûs au milieu : effet de l’année et des erreurs expérimentales dans la conduite du protocole théorique, concernant la température et l’hygrométrie relative. Sur les données ajustées pour les effets de milieu, la variabilité génétique apparaît assez clairement. Cependant une analyse plus fine montre qu’elle provient essentiellement d’une variabilité inter groupe en définissant le groupe d’après des critères « zootechniques » ou «fonctionnels » (groupe à vocation laitière, groupe à vocation bouchère, groupe des races locales). En effet, dans la grande majorité des cas, il n’apparaît pas de variabilité significative entre les réactions des génotypes rangés dans le même groupe, alors qu’elle existe entre les groupes. La bonne performance du groupe des races locales apparaît en comparaison des groupes mettant en oeuvre les races améliorées, laitières ou bouchères, qui diffèrent peu entre eux pour les critères analysés. Dans la discussion, l’accent est mis sur l’incidence probable sur les résultats des phénomènes de thermogénèse. Si nos résultats ont pu être affectés par des phénomènes parasites, notamment ceux de stress émotionnel, il n’en reste pas moins qu’ils concordent assez bien avec l’expérience pratique acquise sur le comportement des races locales françaises dans certains pays chauds

What is the probability of connecting two points ?

The two-terminal reliability, known as the pair connectedness or connectivity function in percolation theory, may actually be expressed as a product of transfer matrices in which the probability of operation of each link and site is exactly taken into account. When link and site probabilities are $p$ and $\rho$, it obeys an asymptotic power-law behavior, for which the scaling factor is the transfer matrix's eigenvalue of largest modulus. The location of the complex zeros of the two-terminal reliability polynomial exhibits structural transitions as $0 \leq \rho \leq 1$.Comment: a few critical polynomials are at the end of the .tex source fil

Continuum limit of amorphous elastic bodies: A finite-size study of low frequency harmonic vibrations

The approach of the elastic continuum limit in small amorphous bodies formed by weakly polydisperse Lennard-Jones beads is investigated in a systematic finite-size study. We show that classical continuum elasticity breaks down when the wavelength of the sollicitation is smaller than a characteristic length of approximately 30 molecular sizes. Due to this surprisingly large effect ensembles containing up to N=40,000 particles have been required in two dimensions to yield a convincing match with the classical continuum predictions for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk systems. The existence of an effective length scale \xi is confirmed by the analysis of the (non-gaussian) noisy part of the low frequency vibrational eigenmodes. Moreover, we relate it to the {\em non-affine} part of the displacement fields under imposed elongation and shear. Similar correlations (vortices) are indeed observed on distances up to \xi~30 particle sizes.Comment: 28 pages, 13 figures, 3 table