Apport de la simulation numérique à l'essai de Small Punch Test

Lors de l’essai de Small Punch Test (SPT) (essai de micro emboutissage d’un disque d’épaisseur 0,5 mm et de diamètre 8,9 mm), le matériau est sollicité en contraintes bi-axiales, déformé plastiquement jusqu’à fissuration et, rompu. Des essais de Small Punch Test, réalisés sur l’acier inoxydable 316L, montrent le comportement ductile de l’acier à partir des courbes d’évolution de la force exercée sur l’échantillon en fonction du déplacement. Des essais interrompus couplés à une analyse par microscopie électronique à balayage mettent en évidence l’amorçage de la fissuration et une rupture ductile. Parallèlement, nous avons développé une simulation numérique de l’essai SPT par éléments finis à l’aide du solveur 2D du logiciel implicite FORGE2005. La plasticité des matériaux est modélisée par une relation de type Hollomon ; l’implémentation d’un critère de rupture permet de mettre en évidence l’apparition de la fissuration. La simulation permet de montrer l’évolution des contraintes et des déformations au cours de l’essai, et la propagation de fissures dans l’échantillon

Apport de la simulation numérique à l'essai de Small Punch Test

Lors de l’essai de Small Punch Test (SPT) (essai de micro emboutissage d’un disque d’épaisseur 0,5 mm et de diamètre 8,9 mm), le matériau est sollicité en contraintes bi-axiales, déformé plastiquement jusqu’à fissuration et, rompu. Des essais de Small Punch Test, réalisés sur l’acier inoxydable 316L, montrent le comportement ductile de l’acier à partir des courbes d’évolution de la force exercée sur l’échantillon en fonction du déplacement. Des essais interrompus couplés à une analyse par microscopie électronique à balayage mettent en évidence l’amorçage de la fissuration et une rupture ductile. Parallèlement, nous avons développé une simulation numérique de l’essai SPT par éléments finis à l’aide du solveur 2D du logiciel implicite FORGE2005. La plasticité des matériaux est modélisée par une relation de type Hollomon ; l’implémentation d’un critère de rupture permet de mettre en évidence l’apparition de la fissuration. La simulation permet de montrer l’évolution des contraintes et des déformations au cours de l’essai, et la propagation de fissures dans l’échantillon

Influence de la teneur de nickel sur le comportement tribologique et électrochimique de l’alliage TiNi.

Parmi les matériaux métalliques, les alliages de TiNi sont employés principalement pour des applications biomédicales et/ou dentaires dues à leur meilleure compatibilité mécanique avec les tissus, leur module de Young proche de celui de l’os et une résistance élevée à la corrosion dans les fluides du corps et une bonne biocompatibilité. Pour cette raison, le comportement à la corrosion et à l’usure de l’alliage TiNi avec des teneurs en Ni varie de 40 à 60% en poids, ont été étudiés afin de vérifier l’effet du Ni sur la biocompatibilité de cet alliage pour des applications dentaires

Nanoindentation hardness and macroscopic mechanical behaviors in filled elastomeric nanocomposites

Carbon black (CB) filled semi-crystalline ethylene butyl acrylate (EBA) copolymer networks are investigated to probe for the CB particles dependence of the deformation behavior from nano-to micrometers length scales of samples which are submitted to nanoindentation characterization. With respect to this purpose, the phenomenology for hardness (H) response in these materials indicates a typical increase of the hardness by decreasing the indentation depth (h) similar to the observed behavior in elastomeric materials. This behavior can be related to the change of the mesostructure, formed by the heterogeneous three-dimensional interconnected network of polymer and of aggregates of CB particles. Furthermore, The CB amount is found to increase the resistance of composite under the action of a mechanical stress. The H-h curves were then compared to some analytical models and correlated to a tensile macroscopic behavior in order to highlight the involved deformation mechanisms with length scale. A complementary set of characterizations such as profilometry and atomic force microscopy probes were also employed to best understand of those mechanisms

Nanoindentation hardness and macroscopic mechanical behaviors in filled elastomeric nanocomposites

Carbon black (CB) filled semi-crystalline ethylene butyl acrylate (EBA) copolymer networks are investigated to probe for the CB particles dependence of the deformation behavior from nano-to micrometers length scales of samples which are submitted to nanoindentation characterization. With respect to this purpose, the phenomenology for hardness (H) response in these materials indicates a typical increase of the hardness by decreasing the indentation depth (h) similar to the observed behavior in elastomeric materials. This behavior can be related to the change of the mesostructure, formed by the heterogeneous three-dimensional interconnected network of polymer and of aggregates of CB particles. Furthermore, The CB amount is found to increase the resistance of composite under the action of a mechanical stress. The H-h curves were then compared to some analytical models and correlated to a tensile macroscopic behavior in order to highlight the involved deformation mechanisms with length scale. A complementary set of characterizations such as profilometry and atomic force microscopy probes were also employed to best understand of those mechanisms

Indentation Hardness Measurements at Macro-, Micro-, and Nanoscale: A Critical Overview

The Brinell, Vickers, Meyer, Rockwell, Shore, IHRD, Knoop, Buchholz, and nanoindentation methods used to measure the indentation hardness of materials at different scales are compared, and main issues and misconceptions in the understanding of these methods are comprehensively reviewed and discussed. Basic equations and parameters employed to calculate hardness are clearly explained, and the different international standards for each method are summarized. The limits for each scale are explored, and the different forms to calculate hardness in each method are compared and established. The influence of elasticity and plasticity of the material in each measurement method is reviewed, and the impact of the surface deformation around the indenter on hardness values is examined. The difficulties for practical conversions of hardness values measured by different methods are explained. Finally, main issues in the hardness interpretation at different scales are carefully discussed, like the influence of grain size in polycrystalline materials, indentation size effects at micro-and nanoscale, and the effect of the substrate when calculating thin films hardness. The paper improves the understanding of what hardness means and what hardness measurements imply at different scales.Funding Agencies|Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University ((Faculty Grant SFO Mat LiU) [2009 00971]</p

Influence of processing conditions on nanoindentation properties of spark plasma sintered ptfe

The Spark Plasma Sintering (SPS) is considered as fast sintering route using self-heating action from inside the powder similar to microwave sintering and self-propagating high temperature. High sintering rate, low temperature processing, ease of operation and accurate control of sintering energy are the major advantages of this non-melting technique with regard to classical methods. The aim of this investigation is to take benefit from the advantages offered by the SPS process in order to sinter the polytetrafluoroethylene (PTFE) polymer materials from PTFE powder particles. The effect of the process’s parameters, namely the sintering temperature and heating rate, on the nanoindentation deformation is explored at room temperature. Empirical data on depth-dependent hardness are confronted to some theoretical models with a special focus on the apparent surface stress changes with the indentation depth. Furthermore, the obtained results are compared to the macro-behavior deduced from tensile and bending properties in order to assess the change in both surface and bulk deformations of the SPSed samples and according to the processing parameters

Requirements for translation re-initiation in Escherichia coli: roles of initiator tRNA and initiation factors IF2 and IF3

Despite its importance in post-transcriptional regulation of polycistronic operons in Escherichia coli, little is known about the mechanism of translation re-initiation, which occurs when the same ribosome used to translate an upstream open reading frame (ORF) also translates a downstream ORF. To investigate translation re-initiation in Escherichia coli, we constructed a di-cistronic reporter in which a firefly luciferase gene was linked to a chloramphenicol acetyltransferase gene using a segment of the translationally coupled geneV–geneVII intercistronic region from M13 phage. With this reporter and mutant initiator tRNAs, we show that two of the unique properties of E. coli initiator tRNA – formylation of the amino acid attached to the tRNA and binding of the tRNA to the ribosomal P-site – are as important for re-initiation as for de novo initiation. Overexpression of IF2 or increasing the affinity of mutant initiator tRNA for IF2 enhanced re-initiation efficiency, suggesting that IF2 is required for efficient re-initiation. In contrast, overexpression of IF3 led to a marked decrease in re-initiation efficiency, suggesting that a 30S ribosome and not a 70S ribosome is used for translation re-initiation. Strikingly, overexpression of IF3 also blocked E. coli from acting as a host for propagation of M13 phage

Transient Phenomena in Gene Expression after Induction of Transcription

When transcription of a gene is induced by a stimulus, the number of its mRNA molecules changes with time. Here we discuss how this time evolution depends on the shape of the mRNA lifetime distribution. Analysis of the statistical properties of this change reveals transient effects on polysomes, ribosomal profiles, and rate of protein synthesis. Our studies reveal that transient phenomena in gene expression strongly depend on the specific form of the mRNA lifetime distribution