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No description available.

Ce travail est une contribution à l'analyse du comportement à l'endommagement des matériaux ductiles. L'endommagement ductile désigne le processus de dégradation progressive des propriétés physiques et mécaniques d'un matériau lorsqu'il subit, sous l'effet d'un chargement et avant rupture, une déformation plastique importante à température ambiante ou peu élevée. L'objectif de ce travail est de déterminer un critère de propagation de fissures pour simuler la déchirure des matériaux ductiles en se basant sur les modèles d'endommagements locaux. Nous avons mené une campagne expérimentale sur 3 matériaux structurellement très différents: un acier 12NC6, un aluminium 2024T351 et un polymère PA66. Le modèle de croissance des cavités Rice-Tracey s'est révélé très limité à décrire la rupture des matériaux testés, nous avons alors proposé une modification de ce modèle en l'enrichissant de paramètres micromécaniques permettant de prendre en compte l'effet de la croissance et des interactions entre les cavités. Le modèle proposé a été implanté dans un code de calcul par éléments finis pour simuler l'amorçage d'une fissure et sa propagation au cours du chargement. Nous avons validé ce modèle analytique à l'aide des résultats expérimentaux des trois matériaux. Les résultats numériques obtenus en termes de courbes R et «charge-déplacement» comparés aux résultats expérimentaux ont permis de valider le modèle sur les trois matériaux étudiés. / The purpose of this thesis is to contribute to the study of ductile fracture of materials on micromechanical models based on local approach. Ductile fracture of metals occurs mainly by nucleation, growth and coalescence of voids. Concerning the local approach of ductile fracture, the emphasis is laid on the micromechanical model of Rice-Tracey. The latter based on growth cavities is verified in order to describe the ductile tearing in stainless steel NiCr(12NC6), 2024T351 aluminium alloy and polymer PA66. The experimental study is carried out on compact tension ct , central crack panel ccp, single edge notched tension sent and tensile axisymetric notched specimens to characterize ductile failure in different materials. The comparison of the experimental results with the classical Rice-Tracey model shows that it cannot make a good prediction using "j -da" curve. On the basis of fitting procedure and numerical simulation a correction of coefficients is introduced to improve the Rice-Tracey model. A methodology of identification of parameters is presented. Ductile crack growth, defined by the" j -da" curve, has been correctly simulated using the numerical calculations by adjusting the three parameters of " modified Rice-Tracey model in the calibration procedure. The proposed model is implemented into a finite element code. Close agreement between the computed results aj'ld experimentally measured values justifies the usefulness of the present modified model.

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Nb3Al base alloys were produced in ingot or ribbon form using cold hearth non-consumable tungsten arc melting and chill block casting by melt overflow on a Mo wheel. The following alloys were studied: Nb-25Al, Nb-18Al, Nb-18Al-xMo (x=20,30,40) and Nb-18Al-9Cr-5Ti (at%). Low levels of interstitials (< 200 wppm O2, < 10 wppm H2) were achieved in all alloys. The effects of alloy chemistry and method of solidification processing on phase selection, solidification microstructures and solid state phase transformations were studied. Room temperature microhardness and hardness of the alloys were evaluated. The A2 structure (disordered beta Nbss) transformed to the B2 structure (ordered beta Nbss) by solid state transformation in the Nb-18Al and Nb-18Al-xMo (x=20,30,40) alloys. The experimental results showed that Al promoted A2&rarr;B2 ordering in Nb rich binary Nb-Al alloys and confirmed the predictions of thermodynamic modelling of A2&rarr;B2 ordering in the Nb-Al system. Mo addition inhibited the A2&rarr;B2 ordering whereas Cr+Ti additions favoured ordering. Formation of an o-phase was observed for the first time in the binary Nb-Al system after prolonged annealing of the Nb-18Al ribbons (973 K / 500 h). It is suggested that the o-phase forms as a transition phase in the transformation B2&rarr;o&rarr;Al5. While o-phase formation has been reported in alloys of the Nb-Al-Ti system, it is shown that Mo addition does not favour the o-phase in Nb-18Al-xMo alloys (x=20,30,40). Formation of o diffuse in the beta phase supports a previous hypothesis that addition of Al to transition metal elements such as Nb,Mo,V can enhance formation of the o phase in the beta phase. Mo addition in the Nb-Al solid solution increases the electron density of A2 or B2 phases and consequently the deviation | Delta | of the o diffuse from crystalline o. A new ternary o type phase, o*, was formed as a transition phase in the B2?Al5 transformation in heat treated Nb-18Al-xMo (x=20,30,40) alloys (973 K / 500 h). The orientation relationship of o* is [111]B2//[0001]o* and (110)B2//(0110)o* and the lattice parameters are ao* = 61/2aB2 and = c o* = 31/2aB2. Rapid solidification refined the alloy microstructures by two orders of magnitude. The room temperature solid solubility of Al in Nb was extended from 6 at% to 25 at% Al. In Nb-18Al-9Cr-5Ti up to 5at%Ti+8.4at%Cr were retained in solid solution. Suppression of Nb3Al and Nb2Al formation was achieved in the ribbons of Nb-25Al. Nb3Al formation was suppressed in the ribbons of Nb-18Al, Nb-18Al- xMo (x=20,30,40) and Nb-18Al-9Cr-5Ti. The A2-*B2 ordering was suppressed in the ribbons of all Nb-18Al-xMo alloys and in the as cast ingots of Nb-18Al-40Mo. Aluminium additions in Nbss increased its microhardness by 16.6 kgmm-2/at%Al and Mo addition hardened the Nbss (16.1at%Al) by 4.2 kgmm-2/at%Mo. Regarding the oxidation behaviour, the Nb-18Al-20Mo and Nb-18Al-9Cr-5Ti alloys exhibited lower weight gain rates as compared to Nb3Al. No protective scale was formed on any of the alloys studied in this work.

No description available.

This work deals with problem of analyzing cooling curves of ductile iron and discovering possibility to predict shrinkage tendency of melt from thermal analysis results. This work was executed in foundry production conditions where obtained data from thermal analysis data linked to experimental castings where amount of shrinkage was estimated. Data were analyzed and looked for correlations between acquired thermal data and shrinkage defects.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19394号 / 理博第4125号 / 新制||理||1593(附属図書館) / 32419 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 平島 崇男, 教授 土`山 明, 准教授 河上 哲生 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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A mulitscale approach is used to model the coalescence of voids. At the microscale, cylindrical and spherical voids in nickel and the magnesium alloy AM60 are simulated through finite element analyses. The nickel cylindrical void simulations are compared to a set of experiments to validate this micromechanical finite element approach used to study void coalescence. At the macroscale, the coalescence portion of a microstructure-property material model is modified to reflect the behavior of three-dimensional spherical voids using results from the micromechanical simulations. An analysis of an automotive component illustrates the influence of void coalescence at the structural scale.