Hydrogen - Plasticity interactions : modelling and experiments in hydrogenated nickel alloys

International audienceThe effect of hydrogen on tensile tests of nickel and binary nickel - 16 wt. % chromium is analysed in terms of solute drag phenomenon. Static Strain Ageing experiments are used to measure the saturated dislocation pinning force as a function of the H concentration. First order hydrogen - dislocation interactions causes a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the selfenergy and line tension of curved dislocations, the critical force for the expansion of a dislocation loop and the dissociation mechanism. These results are used to interpret experimental results on the plastic flow of hydrogen-charged nickel single crystals oriented for easy glide. This study illustrates the mechanisms of H - dislocation interactions and their consequences on the different contributions of hydrogen to the flow stress of nickel

Experimental and numerical analysis of hydrogen interaction with plastic strain in a high strength steel

8 pagesInternational audienceCyclic loading tests were performed on micro-notched samples of high-strength steel S690QT in air and under cathodic polarisation in a saline solution. These specimens were modelled and their behaviour simulated by finite elements calculations with a combined nonlinear isotropic-kinematic hardening constitutive law. This model can simulate cyclic softening and ratcheting effect of the high-strength steel. Stress and strain fields in the vicinity of the notch-tip were calculated. Results show that a strong dependence of the crack initiation with plastic strain accumulation. Hydrogen assisted cracking mechanism is discussed based on arrangements of dislocations structures

Effects of Cyclic Plastic Strain on Hydrogen Environment Assisted Cracking in High-Strength Steel

8 pagesInternational audienceCyclic loading tests were performed on micro-notched samples of high-strength steel S690QL (standard EN 10137-2) in air and under cathodic polarization in a saline solution. These specimens were modeled and their behavior simulated by finite elements calculations with a combined nonlinear isotropic-kinematic hardening constitutive law. This model can simulate cyclic softening and ratcheting effect of the high-strength steel. Stress and strain fields in the vicinity of the notch-tip were calculated. Results show a strong dependence of the crack initiation with plastic strain accumulation. Hydrogen assisted cracking mechanism is discussed based on arrangements of dislocations structures

Experimental investigations of internal and effective stresses during fatigue loading of high-strength steel

International audienceLow cycle fatigue tests are performed on a high strength tempered martensitic steel at different plastic strain amplitudes at room temperature. Internal and effective components of the flow stress are analyzed using Handfield and Dickson's method. The internal stress is affected by the plastic strain amplitude. Conversely, the evolution of the athermal component of the effective stress with the number of cycles is independent of the plastic strain amplitude. The thermal part of the effective stress increases with the plastic strain amplitude, but remains constant with plastic strain accumulation. Microstructural changes in the cyclically deformed material are investigated by means of transmission electronic mycroscopy and X-Ray characterizations. Internal and effective stress evolutions are discussed based on these observation

Effects of strain and trapping on hydrogen-induced cracking in high strength low alloy steels

International audienceIn pearlitic steels, hydrogen trapping at interphase boundaries may induce hydrogen- cracking in the absence of external applied stress such as in blistering and Hydrogen Induced Cracking. However, in low alloy steels containing a much lower density of such trapping sites and/or when hydrogen activity is reduced, most instances of hydrogen-induced cracking involve strong interactions between local plasticity and hydrogen effects. We investigated the effects of these variables on the crack growth rate in two quenched and tempered high strength low alloy (HSLA) steels. Slow strain rate tensile and low-amplitude cyclic tests were conducted on micro-notched specimen in conditions of variable hydrogen activity. The results indicate a strong dependence of crack growth rate with the tempering on the one hand and both the notch-tip plastic zone and hydrogen activity on the other. At low activity, dynamic strain seems to be the major factor affecting hydrogen embrittlement, while trapping effects appear dominant at higher hydrogen activity

Mobility and trapping of hydrogen in high-strength steel

6 pagesInternational audienceElectrochemical permeation and thermo-desorption tests are performed to evaluate hydrogen mobility in high strength steel. Experimental parameters are used in a Krom like phenomenological diffusion model. This model is developed to simulate hydrogen diffusion and trapping in processing zones of specimens subjected to fatigue loadings

Palladium coating on quenched-tempered martensitic steel for hydrogen electrochemical permeation tests

A coating process is proposed to obtain a thin palladium layer on high-strength steel. The quality of the layer is investigated by SEM observation. Potentiokinetic curves are performed to ensure no hydrogen formation during the coating process. Coated samples are tested by hydrogen permeation test and compared to non-coated samples. Permeation data analysis and boundary conditions are discussed based on permeation results and 1D simulations

Rendu visuel de surfaces nano-structurées (effet de l'ordre à courte distance)

Les surfaces nanostructurées permettent d obtenir des effets colorés gonio-apparents lorsque les nanostructures présentent des dimensions de l ordre des longueurs d onde du spectre visible. ces couleurs sont habituellement modélisées par le biais d interactions de types interférences ou cristaux photoniques entre le rayonnement lumineux et une structure modèle. dans cette thèse, l anodisation d aluminium est utilisée comme méthode de structuration à l échelle submicronique. cette technique présente l avantage d être mature industriellement et de permettre de structurer de grandes surfaces. des effets colorés sont observés même si les structures obtenues ne sont pas parfaitement ordonnées. le but de cette thèse est de comprendre les phénomènes optiques mis en jeu dans l obtention de ces effets. ce manuscrit se divise donc en deux parties principales, toutes deux basées sur une étude de la littérature existante. afin d établir un parallèle entre caractérisation expérimentale et simulation numérique, la première partie présente l outil de caractérisation optique développé. la seconde est dédiée à l étude des effets colorés de certaines surfaces d aluminium anodisé. cette partie propose une compréhension des phénomènes d interaction de la lumière avec la structure d aluminium anodisé se basant sur les caractérisations optiques et microstructurales des échantillons, associées à une modélisation de l interaction entre rayonnement et matière structurée. cette étude montre que les structures réelles présentent un ordre à courte distance. les effets colorés sont simulés par la méthode modale de fourier par le biais de structures modèles avec un certain niveau de désordre.Nano-structured surfaces allow obtaining of colored gonio-apparent effects when the nano-structures dimensions are of the order of the visible spectrum wavelengths. these colors are usually modeled by means of the interactions type so-called interferences type ou photonic crystals type between the luminous radiation and a model structure. in this thesis, the anodization of aluminum substrates is used to produce surface structures at the submicron scale. this technique is industrially mature and allows to structure large surfaces. color effects were observed even if obtained structures are not perfectly ordered. the aim of this thesis is the understanding of the optical phenomena involved in the production of such effects. this manuscript is divided into two main parts, both based on the existing literature analysis. in order to draw a parallel between experimental characterization and numerical simulations, one part presents the instrumental development of the optical characterization instrument. the second one is dedicated to the study of color effect of certain anodized aluminum surfaces. this part proposes an understanding of the interaction phenomena between the light and the anodized aluminum structure, based on the optical and microstructural characterization of the samples, associated to a modeling of the interaction between light and structured matter. this study shows that such structures present a short-distance order. the color effects are simulated numerically by fourier modal method by the means of model-structures with certain disorder degree.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Oxidation of austenitic stainless steels in PWR primary water

International audience304L and 316L SS samples are SCC-tested in PWR water for various conditions: strain level, duration, pH, surface finish. The passive Chromium-rich oxide layer, the consistent oxidation feature, is characterized using Transmission Electron Microscope analysis. EDX analysis and Energy-filtered images are used to map the oxide penetration at the surface of the sample and at contiguous grain boundaries. Statistical data obtained by these characterizations are exploited to considerate the effects of deformation, pH and duration on oxidation. Complementary analyses of intergranular crack initiations allow to invetigate the oxidation phenomenon at grain boundaries and its influence on intergranular cracking

Functional materials for Design

The main problematic of the research is to connect the stimuli-responsive behaviour of functional material to the end-user experience. To make this connection, the research was divided in layers, from the most technical at the bottom, to the most designerly at the top. The objective is to propose a set of chained tools that will eventually allow a seamless journey through all the layers and provide support for designers to use functional material in their projects