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A physics-based, uncoupled damage model is calibrated using cylindrical notched round tensile specimens made of Ti5553 and Ti-6Al-4V alloys. The fracture strain of Ti5553 is lower than for Ti-6Al-4V in the full range of stress triaxiality. This lower ductility originates from a higher volume fraction of damage sites. By proper heat treatment, the fracture strain of Ti5553 increases by almost a factor of two, as a result of a larger damage nucleation stress. This result proves the potential for further optimization of the damage resistance of the Ti5553 alloy. The damage model is combined with an elastoviscoplastic law in order to predict failure in a wide range of loading conditions. In particular, a specific application involving bolted sectors is addressed in order to determine the potential of replacing the Ti-6Al-4V by the Ti5553 alloy.

The elastoviscoplastic behavior of the Ti5553 alloy is characterized and compared to the classical Ti-6Al-4V alloy. The true stress-strain curves are determined based on tensile tests performed under different strain rates at room temperature and at 150°C, from which the elastic constants and the parameters of a Norton-Hoff viscoplastic model are identified. The strength of the Ti5553 alloy is 20-40% higher than the strength of the Ti-6Al-4V alloy. The Ti5553 alloy constitutes thus a promising candidate for advanced structural applications. In view of modeling structural applications of forming operations, the elastic and plastic initial anisotropy of the two alloys is investigated by combining compression on cylinders with elliptical sections, uniaxial tensile tests in different material directions, plane strain and shear tests. The initial anisotropy of the different alloys is very weak which justifies the modeling of the mechanical behavior with an isotropic yield surface. The identified elastoviscoplastic model is validated by comparing experimental results with FE predictions both on cylindrical notched specimens subjected to tensile tests and on flat specimens subjected to plane strain conditions.

Nowadays, virtual testing is widely used in several sectors (automotive, aeronautics and space, nuclear, ...) as a mean to reduce product development time and cost. A large part of the virtual testing and simulation effort is dedicated to structural integrity assessment of parts. Continuum damage mechanics [1] and fracture mechanics [2] are two sciences that study the failure of materials in different ways. The former describes failure through the gradual reduction of the material properties (elastic or plastic) due to the development of spatially continuous damage, the latter would start from an existing macro-crack or discontinuity and use indicators of stress concentration or distribution to propagate the crack via an appropriate geometric update of the discontinuity. Recent attempts have tried to bridge the gap between damage initiation and crack propagation, using continuum damage mechanics until insertion of a discontinuity. The approach is appealing but it suffers from several difficulties (on top of the traditional difficulties associated with material softening): energetic equivalence, initial crack insertion criterion, initial crack shape and size definition. In this contribution, the level set method and eXtended Finite Element Method are used in combination with basic continuum damage mechanics to easily incorporate a discontinuity and allow for crack propagation. Starting from an uncracked / undamaged part, the method is able to model the whole failure history, from damage initiation to the propagation of a macro-crack. The method is demonstrated on several examples (metals and composites).

In a previous study, a generic micro-macro mechanical model based on the coupling of an incremental mean-field model [1, 4, 5] with the Gurson-Tvergaard (GT) porous plastic law [2, 6] has been developed for the prediction of the mechanical properties of multiphase materials with a large volume fraction of voids. This model has been used in the framework of the FP6 PROHIPP European project to investigate the link between the microstructure and the mechanical properties of spheroidal cast iron. In this contribution, experimental results are presented and an inverse analysis procedure is proposed to adjust the parameters of the model.

In the framework of the European project PROHIPP (New design and manufacturing processes for high pressure fluid power product - NMP 2-CT-2004-50546), CENAERO develops a library of constitutive models used to predict the mechanical response of a family of cast iron. The present contribution focuses on one particular microstructure, corresponding to a ferrite matrix containing spheroidal graphite and isolated inclusions of pearlite. An incremental mean field homogenisation scheme such as the one developed by Doghri and Ouaar is used. In the present application, the ferrite matrix is described by a Gurson type constitutive law (porous plasticity) while the pearlite inclusions are assumed to obey the classical isotropic J2 plasticity. The predictions of the micromechanical model are compared to the results of Finite Element simulations performed on three-dimensional representative volume elements (RVEs).

info:eu-repo/semantics/published

info:eu-repo/semantics/published

The stability of bimetallic silver-cobalt clusters with less than 50 atoms is studied experimentally and their associated geometries are predicted by classical modeling. The clusters are created by laser vaporization and inert gas condensation. Their mass distribution is analyzed with time-of-flight mass spectrometry. For clusters containing mainly silver, we find strong quantum size effects related to itinerant behavior of the silver and cobalt valence electrons. In the case of clusters containing mainly cobalt, no pronounced size effects appear in the mass spectra. Photofragmentation experiments reveal that neutral silver atom evaporation is the favorable channel, suggesting that the Ag{single bond}Co bonds are weaker than the Co{single bond}Co bonds. Consistently, and for both sets of clusters, Metropolis Monte-Carlo simulations predict these clusters to have icosahedral based structures that may depend on temperature. In clusters containing mainly silver, cobalt sits at the cluster center and fragmentation proceeds by the evaporation of silver surface atoms. In clusters containing mainly cobalt, silver atoms also locate at the periphery and are more weakly bound to the cluster than cobalt surface atoms. © 2006 Elsevier B.V. All rights reserved., info:eu-repo/semantics/published

Doctorat en Sciences, info:eu-repo/semantics/nonPublished

info:eu-repo/semantics/published

info:eu-repo/semantics/published

FLWIN, info:eu-repo/semantics/published

info:eu-repo/semantics/published

info:eu-repo/semantics/published

Semi-empirical potentials used for atomic scale modeling in metals are not designed for modeling low coordinance interactions. A popular second moment tights binding potential is revisited and modified for low coordinance systems. Cu3Au nanoclusters containing no more than a few hundred atoms are used as a case study. The parameterization of this potential is improved and a coordinance correction is presented, based on available experimental and ab initio data for reduced coordinance systems. It is found that a fine tuning of the parameters for bulk materials allows a significantly better description of the order-disorder transition, as predicted by Metropolis Monte Carlo simulations. Short range order does not vanish at high temperature. Accounting for a coordinance correction does not conclusively affect the structural and thermodynamic properties of the cluster cores. At equilibrium, cluster cores display the same stoichiometry as bulk Cu3Au and the surface gold segregation is a continuously decreasing function of temperature, irrespective of the first order-disorder phase transition in the cluster. © 2003 Elsevier B.V. All rights reserved., SCOPUS: cp.j, info:eu-repo/semantics/published