Archive 2019

Research engineer in materials modelling

Job : Research engineer

Academic level : PhD degree

Location: LEM, Châtillon

ONERA, a major player in aerospace research, employs around 2,000 people. Placed under the tutelage of the Ministry of Defense, it has a budget of 230 million euros, more than half of which comes from commercial contracts. State expert, ONERA prepares the defense of tomorrow, meets the aeronautics and space challenges of the future, and contributes to the competitiveness of the aerospace industry. ONERA masters all disciplines and technologies of the field. All the major civil and military aerospace programs in France and in Europe carry a part of ONERA’s DNA.

The Materials and Structures Department is involved in the development of new materials. By integrating all the stages (development, characterization, dimensioning), the characteristic time of development of a new class of materials is measured in tens of years. The developments observed today in the field of modeling and the increase of the computational capabilities should allow decreasing this delay by a factor 2 or 3. It is in this context that ONERA develops a strategic axis “Digital material” to deploy a digital modeling software suite. This suite aims to establish the link between chemistry, microstructures and properties to eventually replace many experimental campaigns by arbitrary virtual sample simulations.

The models concerned are obviously multiphysics (thermodynamics, mechanics, damage) and call for a sequential multi-scale approach because of the huge gap between the space and time scales that separate the basic laws of physics and dimensions and lifetime of an aeronautical part. The challenge is to master the transfer of information between the different scales in order to consolidate the quantitative and predictive nature of the approach.

Within the Laboratoire d’Etude des Microsctructures, ONERA-CNRS Joint Research Unit integrated into ONERA’s Materials and Structures Department, you are in charge of research activities in the field of mesoscopic modeling of material microstructures. This includes the development of continuous multiphysics methods to model the microstructural evolution of materials and the impact of these microstructures on macroscopic properties (mechanical behavior, lifetime). It will involve implementing different theories (phase field methods, continuous mechanics, discrete and continuous plasticity, damage) in a coarse-graining spirit (discrete-continuous transition and meso-macro upscaling). You will also develop interactions with academic partners in the relevant fields.

The Ph.D. candidate must have an expertise in material physics and numerical modeling at different scales. A solid knowledge of statistical physics, out of equilibrium thermodynamics and solids mechanics, at discrete and continuous scales, is important. The practice of English is essential.

Please send your application (CV and cover letter) to: Alphonse.Finel@onera.fr

Phd Defence:” Investigation of grain size and shape effects on crystal plasticity by dislocation dynamics simulations”

Phd candidate: Maoyuan JIANG

Directeur de thèse : Benoit DEVINCRE

Co-directeur de thèse : Ghiath MONNET

Dislocation Dynamics (DD) simulations are used to investigate the Hall-Petch (HP) effect and back stresses induced by grain boundaries (GB) in polycrystalline materials.

The HP effect is successfully reproduced with DD simulations in simple periodic polycrystalline aggregates composed of 1 or 4 grains. In addition, the influence of grain shape was explored by simulating grains with different aspect ratios. A generalized HP law is proposed to quantify the influence of the grain morphology by defining an effective grain size.  The average value of the HP constant K calculated with different crystal orientations at low strain is close to the experimental values. 

The dislocations stored during deformation are mainly located at GB and can be dealt with as a surface distribution of Geometrically Necessary Dislocations (GNDs). We used DD simulations to compute the back stresses induced by finite dislocation walls of different height, width, density and character. In all cases, back stresses are found proportional to the surface density and their spatial variations can be captured using a set of simple empirical equations. The back stress calculation inside grains is achieved by adding the contributions of GNDs accumulated at each GB facet.

These back stresses are found to increase linearly with plastic strain and are independent of the grain size. The observed size effect in DD simulations is attributed to the threshold of plastic deformation, controlled by two competing mechanisms: the activation of dislocation sources and forest strengthening. Due to strain localization in coarse-grained materials, the pile-up model is used to predict the critical stress. By superposing such property to the analysis we made from DD simulations in the case of homogeneous deformation, the HP effect is justified for a wide range of grain sizes.

Tuesday 04/05/2019, 13h30
Amphi 3, Bâtiment Eiffel, CentraleSupélec, 8-10 rue Joliot-Curie, 91190 Gif-sur-Yvette

“HDR” Defence: Mathieu FEVRE

Mathieu Fèvre will defend his HDR thesis entitled  “Modelling and characterization of alloys and alloy nanoparticles” in order to obtain the diploma of  “Habilitation à Diriger des Recherches”, Tuesday 29th January2019 at 2 p.m., in the BLANDIN amphitheatre of the Laboratoire de Physique des Solides on the site of the Faculté des Sciences d’Orsay (Building 510).
The jury will be composed by  Pascal ANDREAZZA (ICMN), Alexis DESCHAMPS (SIMAP), Alphonse FINEL (ONERA), Philippe GOUDEAU (INSTITUT P’), Christine MOTTET (CINAM) and Sylvain RAVY (LPS).