Modeling Bake Hardening Effects in Steel Sheets-Application to Dent Resistance

International audienceThis study is dedicated to the experimental characterisation and phenomenological modeling of the bake hardening effect of a thin steel sheet, to predict the static dent resistance and perform an experimental validation on a bulged part. In a first step, rectangular samples are submitted to a thermo-mechanical loading to characterise the bake hardening magnitude in tension. A three-step procedure is considered, involving first a pre-strain in tension up to several values followed by unloading. Secondly, a heat treatment during a fixed time and a given temperature is performed, and finally, a reloading in tension in the same direction as the pre-strain is applied. Then, a specific device is developed to perform dent tests on a bulged specimen, to evaluate the influence of bake hardening on the dent resistance. A three-step procedure is also considered, with a pre-strain applied with a hydraulic bulge test followed by a heat treatment and then static dent test at the maximum dome height. An original phenomenological model is proposed to represent the yield stress increase after the heat treatment and the second reloading. Material parameters are identified from the tensile tests and are input data to a finite element model. The numerical prediction of the load evolution during the dent test is then compared with experimental data and shows an overall good correlation

Déformation décrochante d’un bassin à litage rhéologique : modélisation analogique à l’échelle crustale

International audienceIn sedimentary basins, sediment type and deposit and burial conditions generate successivelayers with various rheologies reacting differently to boundary stresses. We used analoguemodelling to explore how a rheological-layered basin reacts to strike-slip displacements appliedto boundaries. Several experimental set-ups were tested by varying horizontal rheological layering(alternating viscous and cohesive layers) and displacement rates. All experiments generated a complexthree-dimensional deformation pattern localized into a narrow vertical band with thickened, thinnedand folded domains alternating along both strike and depth. This results in a succession of positiveand negative flower structures connected to the basal velocity discontinuity at depth and in a decouplingof deformation from one layer to another. Thinning and thickening variations display an alongstrikewavelength proportional to the layer thickness, an amplitude controlled by rheological layering,and in particular the strength contrast between each layer. The simultaneous genesis of shorteningand stretching structures during only one deformation stage is symptomatic of strike-slip boundaryconditions. These results indicate that caution should be exercised when postulating polyphaseddeformation fromoverprinting of different deformation styles.Dans les bassins sédimentaires, la nature des sédiments, leurs conditions de dépôt et d’enfouissementproduisent un litage mécanique dont les niveaux rhéologiques réagissent différemmentaux contraintes appliquées aux limites. Une modélisation analogique a été déployée pour explorer ladéformation d’un système sédimentaires multicouche subissant des conditions décrochantes à ses limites.Plusieurs conditions expérimentales ont été testées : variation de la stratification rhéologique horizontale (alternance de couches visqueuses et cohésives) et quantité de déplacement. Toutes lesexpériences montrent une déformation complexe localisée dans un couloir vertical étroit, formée parune succession de structures en fleur positives et négatives connectées à la base du modèle. Les niveauxprésentent des variations dépaisseur (zones épaissies, amincies et ployées). Les variations destyle de déformation se produisent à chaque interface qui découple la déformation d’une coucheà l’autre. Les variations d’épaisseur d’un niveau présentent une longueur d’onde proportionnelle àl’épaisseur de la couche dont l’amplitude est contrôlée par la stratification rhéologique, et en particulierle contraste de résistance entre chaque niveau. La co-existence des structures en raccourcissementet en étirement pendant la même phase de déformation sont caractéristiques des conditionsaux limites décrochantes. Ce résultat remet en cause l’interprétation systématique d’une déformationpolyphasée, la présence de structures accommodant de déformation différentes

Evaluating conditions for the formation of chitosan/gelatin microparticles

Chitosan/gelatin microparticles were prepared by complex coacervation. Three chitosan (CH) samples, with different acetylation degrees and intrinsic viscosities, were used together with commercial gelatin (G) samples. Microparticles formation was investigated at various CH/G ratios, within the pH range of 3.5 to 6.0. Reactions were carried out at 40 and 60 ºC, for 2, 4, and 6 hours. Turbidity measurements performed at 633 nm were used to monitor the process. The resulting curves revealed maximum values, which were correlated to the glucosamine content of CH samples. After isolation, yields were determined, and the microparticles were characterized by infrared spectroscopy (FTIR) and thermogravimetry (TGA). Both techniques evidenced the formation of coacervate microparticles. The highest yields in microparticles were determined for the system comprising the CH sample with the lowest degree of acetylation and intrinsic viscosity, and the gelatin sample with the lowest bloom strength