Phase-Field Reaction-Pathway Kinetics of Martensitic Transformations in a Model Fe3Ni Alloy

A three-dimensional phase-field approach to martensitic transformations that uses reaction pathways in place of a Landau potential is introduced and applied to a model of Fe3Ni. Pathway branching involves an unbounded set of variants through duplication and rotations by the rotation point groups of the austenite and martensite phases. Path properties, including potential energy and elastic tensors, are calibrated by molecular statics. Acoustic waves are dealt with via a splitting technique between elastic and dissipative behaviors in a large-deformation framework. The sole free parameter of the model is the damping coefficient associated to transformations, tuned by comparisons with molecular dynamics simulations. Good quantitative agreement is then obtained between both methods.Comment: 4 pages, 3 figure

Investigating ramp wave propagation inside silica glass with laser experiments and molecular simulations

Under elastic shock compression silica glass exhibits a very specific behaviour. A shock propagating inside a material is usually seen as the propagation of a discontinuity. However in silica glass, shocks are unstable and lead to the propagation of a ramp wave where the shock front becomes gradually larger over time. Ramp waves were already reported in the literature, however their origin remain uncertain. This work presents an original study combining laser shock-induced experiments and molecular dynamics simulation aiming to improve the understanding of the mechanisms involved. Experimental ramp waves were directly observed using shadowgraphy technique allowing for an estimation of the head and tail velocities. Molecular dynamics simulations were carried out in order to reproduce ramp waves and to gain insight into the material properties. Ramp waves were observed for both elastic and plastic shockwaves. In the latter case, the plastic waves were preceded by an elastic ramp precursor. The sound speed, related to the material compressibility, was found to decrease with increasing pressure, as observed experimentally for quasi-static hydrostatic loading, thus providing an explanation for the instabilities that lead to the propagation of ramp waves

Constant entropy sampling and release waves of shock compressions

We present several equilibrium methods that allow to compute isentropic processes, either during the compression or the release of the material. These methods are applied to compute the isentropic release of a shocked monoatomic liquid at high pressure and temperature. Moreover, equilibrium results of isentropic release are compared to the direct nonequilibrium simulation of the same process. We show that due to the viscosity of the liquid but also to nonequilibrium effects, the release of the system is not strictly isentropic

Analyse des profils spectraux et calculs de fonctions potentielles vibrationnelles pour quelques molecules a basse temperature

SIGLECNRS T 57040 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Molecular dynamics of shock-wave induced structural changes in silica glasses

info:eu-repo/semantics/publishe

Etude du comportement dynamique de matériaux sous choc laser sub-picoseconde

Le travail présenté a pour objectif d'étudier le comportement de métaux soumis à un choc laser femtoseconde, amenant à des sollicitations dynamiques extrêmes (>107s-1). Dans ces conditions, on peut découpler les phénomènes en 3 parties : l'interaction laser-matière ultra-brève donnant naissance à l'impulsion en pression, la propagation de l'onde et sons atténuation dans la cible, très prononcée en raison de la brièveté du chargement et l'endommagement en face arrière par écaillage. Les modèles utilisés pour restituer ce comportement ont été validés dans ce régime par comparaison avec des données expérimentales de tirs effectués sur le laser fs 100TW du LULI (VISAR, post-mortem). En raison des ordres de grandeurs expérimentaux impliqués, proches de ceux employés en Dynamique Moléculaire ( m et ps), une étude microscopique de l'écaillage ductile a été effectuée au CEA-DAM. Les résultats obtenus, cohérents avec la physique des chocs et de l'endommagement, permet d'explorer des régimes ultra-dynamiques (>109s-1) où le seuil d'endommagement approche la force théorique de décohésion interatomique. Toutes les données déduites permettent de décrire le comportement d'un modèle en fonction de la vitesse de déformation, dans notre cas Kanel. L'ensemble de l'étude a ensuite été généralisé à des configurations 2D, permettant notamment de caractériser l'évolution du diamètre d'écaille, source complémentaire d'informations sur l'endommagement. Ce type d'approche peut être transposé à des cibles multicouches pour déterminer l'adhérence de revêtements (Procédé LASAT). Les sources ultra-brèves ont permis de provoquer la décohésion des couches sub-micrométriques et mesurer leur adhérence.This work consists in studying the behavior of metals submitted to femtosecond laser driven shocks, leading to highly dynamic solicitations (>107s-1). In these conditions, phenomena can be separated into 3 parts : ultra-short laser matter interaction which generates the pressure pulse, wave propagation and decay, very strong because of brief loading duration, and finally damage mechanics induced by spallation close to the target free surface. The numerical models used to reproduce this behavior have been validated in this regime by comparison with experimental data obtained on the LULI 100TW fs laser facility (VISAR, sample microscopy...). By considering the involved space-time characteristic scales, close to those actually available in molecular Dynamic approach ( m and ps), a microscopic study of ductile spallation has been performed with CEA-DAM. The related results are consistent with both shock and damage physics. Moreover, they allow to explore ultra-dynamic regimes (>109s-1) where the damage threshold is close to the theoretical interatomic decohesion force limit. All the data obtained make possible to describe a damage model behavior in function of strain rate, Kanel in this work. The study has been then generalized for 2D configurations, giving access in particular to the spall diameter evolution characterization which constitutes a complementary information source about damage mechanisms. This kind of approach can be transposed to multilayer targets in order to determine coatings adhesion (LASAT process). The ultra-short lasers allowed to debond sub-micrometric layers and then deduce their adhesion strength.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Construction of two genetic linkage maps of taro using single nucleotide polymorphism and microsatellite markers

UMR AGAP - équipe AFEF - Architecture et fonctionnement des espèces fruitièresLinkage maps are needed for genetic studies and molecular breeding of taro (Colocasia esculenta). In this study, we used genotyping-by-sequencing (GBS) to identify single nucleotide polymorphism (SNP) loci on two mapping populations: F31 (HLB11 × VU006) composed of 266 progenies and F32 [HLB01 × (VU370×ID316)] composed of 292 progenies. SNP calling generated an initial set of 22,734 SNPs for F31 and 16,744 for F32. A large proportion of individuals and loci were later removed by filtering on the proportion of missing data and segregation distortions. Linkage maps were constructed with filtered SNPs in association with 14 simple sequence repeat (SSR) markers, using the maximum likelihood method. In both populations, loci were successfully grouped into 14 linkage groups (LGs) with an independence logarithm of odds (LOD) threshold of 11.0 and 8.0 for F31 and F32, respectively. LGs ranged in size from 90 to 15 markers for F31 and from 92 to 12 markers for F32. Bridge markers (459 SNPs and 9 SSRs) were identified and revealed homologous groups between families. Although our maps presented unprecedented chromosome coverage, the colinearity between homologous groups was low (except for LG07), and map lengths were globally inflated. Putative effects of missing data, segregation distortion, and genotyping errors on map accuracy are discussed. This research work led to the identification of a reliable set of SNPs potentially useful as a tool for a wide range of genetic studies in taro

Assessment of an anisotropic coarse-grained model for cis -1,4-polybutadiene: a bottom-up approach

International audienceThe spherical representation usually utilized for the coarse-grained particles of soft matter systems is an assumption and pertinent studies have shown that both structural and dynamical properties can depend on anisotropic effects. On these grounds, we develop coarse-grained equations of motion which take into account explicitly the anisotropy of the beads. As a first step, this model incorporates only conservative terms. Inclusion of the dissipative and random terms is in principle possible but is beyond the scope of this study. The translational dynamics of the beads is tracked using the position and momentum of their center of mass, while their rotational dynamics is modeled by representing their orientation through the use of quaternions, similarly to the case of rigid bodies. The associated force and torque controlling the motion are derived from atomistic molecular dynamics (MD) simulations via a bottom-up approach and define a coarse-grained potential. The assumptions of the model are clearly stated and checked for a reference system of a cis-1,4-polybutadiene melt. In particular, the choice of the angular velocity as a slow variable is justified by comparing its dynamics to atomic vibrations. The accuracy of this approach to reproduce static structural features of the polymer melt is assessed

AMR-based molecular dynamics for non-uniform, highly dynamic particle simulations

International audienceAccurate simulations of metal under heavy shocks, leading to fragmentation and ejection of particles, cannot be achieved by simply hydrodynamic models and require to be performed at atomic scale using molecular dynamics methods. In order to cope with billions of particles exposed to short range interactions, such molecular dynamics methods need to be highly optimized over massively parallel supercomputers. In this paper, we propose to leverage Adaptive Mesh Refinement techniques to improve efficiency of molecular dynamics code on highly heterogeneous particle configurations. We introduce a series of techniques that optimize the force computation loop using multi-threading and vectorization-friendly data structures. Our design is guided by the need for load balancing and adaptivity raised by highly dynamic particle sets. We analyze performance results on several simulation scenarios, such as the production of an ejecta cloud from shock-loaded metallic surfaces, using a large number of nodes equipped by Intel Xeon Phi Knights Landing processors. Performance obtained with our new Molecular Dynamics code achieves speedups greater than 1.38 against the state-of-the-art LAMMPS implementation

Skin surface nanoscale topography by light interferometry

International audienceThe skin is the first thing one perceives of a person. Unconsciously, its color, its radiance and its complexion give us a first impression of a person. This is why cosmetic care and makeup products are of great importance.But how to evaluate their activity?Many methods of in vivo measurements exist to evaluate its color or shine, but the evaluation of a product activity on skin surface is much more complicated.. The relief of the skin and even more its microrelief, are largely responsible for its optical properties and therefore the image it refers.Many products have a filling or a tensor effect, modifying the depth and width of the microfolds that compose the microrelief. It is consequently necessary to measure and characterize the skin surface before and after the application of a cosmetic product with great precision.More precise than 3D scanners, or the fringe projections in vivo, light interferometry allows making a topographical survey of the skin surface with an accuracy of a few tens of nanometers.The topography by light interferometry is developed by the Polytec company whose experience in measuring equipment is no longer to be proved.This technology used in particular in the control of mechanical parts of aeronautics, offers a lateral resolution of approximately 10 μm and a vertical resolution of less than 20 nm.Depending on the light interference reflected from the surface of the skin, this technology is sensitive to vibrations which avoids its in vivo use.To benefit from this technology usually reserved for the world of precision micromechanics, we have developed a support to put under tension a human living skin explant and to maintain it in survival ex vivo.This support specially designed and printed in 3D allows stretching the skin on a reservoir of culture medium while respecting a tension similar to the in vivo conditions.The tension of the skin was measured and verified by cutometry. This measurement can also be performed during the study which can last up to 10 days.The skin thus stretched can be topographed and treated by different products. So it will be very simple to measure the roughness of the surface of the skin, the depth of its folds or their width.We applied on the surface of the skin three products, two with smoothing and filling activity. We showed a decrease in the average depth of the cutaneous microfolds as well as a decrease of the roughness of the skin.If an immediate effect can be observed in a few minutes, the ex vivo character of this model also allows evaluating a biological activity which reinforces in depth this surface-visible effect.This new and original approach, coupling light interferometry technology and human living skin explant, offers all the precision and finesse necessary for an immediate evaluation of the activity of a product on skin surface proprieties and to investigate in parallel the cutaneous-linked biological effects