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Advanced hardening models accurately describe the transient plastic behavior (reyielding, stagnation, resumption...) after various strain-path changes (reverse, orthogonal...). However, a common drawback of these models is that they usually predict monotonic loading with lower accuracy than the regular isotropic hardening models. Consequently, the finite element predictions using these models may sometimes lose in accuracy, in spite of their tremendous theoretical superiority. This drawback has been eliminated in the literature for the Chaboche isotropic-kinematic hardening model. In this work, a generic approach is proposed for advanced hardening models. Arbitrary models could be successfully compensated to preserve rigorously identical predictions under monotonic loading. A physically-based model involving a 4th order tensor and two 2nd order tensors was used for the demonstration. The parameter identification procedure was greatly simplified by rigorously decoupling the identification of isotropic hardening parameters from the other parameters.

The effect of kinematic hardening on springback predictions in sheet metals was rigorously clarified using a modular bending-under-tension test. DP600 sheet samples were tested over a wide range of tool radii and tension forces. Specifically modified isotropic hardening and combined isotropickinematic hardening models were compared, which provide strictly identical predictions under monotonic loading. Loading configurations (combinations of tool radius and sheet tension) were identified which require the use of one or the other type of hardening model, in order to ensure the predictive accuracy. The two models predicted similar springback for high backforces or for high R/t ratios. In turn, both models failed to predict the springback accurately for small R/t ratios, in particular when combined with small backforces. This combination of loading conditions appeared as the most discriminant one, in which the combined hardening model clearly appears qualitatively superior although not entirely sufficient, designating the potential application areas for more advanced material models. published

The two-stage abrupt strain path change method is used to investigate the subsequent yield surface evolution, as predicted by associated J2 plasticity models. Experimental results from the literature have revealed several specificities of the yield surface evolution following such a strain-path change, including formation of a vertex and apparent non-normality. The current study showed that some features of this behavior can be simulated using models based on associated plasticity. Kinematic hardening had the most important effect, together with rate sensitivity and elasticity. The results suggest that some apparent violation of the normality condition can result from the combined effects of kinematic hardening, rate sensitivity and elasticity, even when the normality rule is built into the model.

Flow drill screw (FDS) driving is a thermomechanical assembly process that allows for single-sided access multi-material joining, with a high application potential for modern lightweight vehicle design. The process combines three consecutive operations which partially overlap: flow drilling, thread forming, and tightening. The paper extensively investigates the process parameter effects on the flow drilling operation of thin sheet AA5182-O 2.5-mm thick and DP600 1.4-mm thick. Drilling defects have been associated with specific ranges of control parameters. Mechanical tests have shown that the drilling defects do not have a significant impact on the mechanical strength of the assembly under static loading. These results were used to determine both the optimal joining parameters and the robust process window. The variation of all process-relevant mechanical quantities was recorded over the whole available process window. This data may further serve for process simulation validation, or alternatively for the construction of data-driven meta-models.

A new analytical method to determine the effective tool width in contact with the sheet workpiece in the plane strain compression test, which changes during the test if a tool with a radius is used, is proposed. A detailed description of this method and the corresponding procedure of the flow curve determination for high-strength sheet steels are presented. The underpinning assumptions of the method are validated with the help of the FEA and the validation results are presented. Furthermore, with the help of the FEA, the main disadvantages of the plane strain compression test – strain inhomogeneity and possible tool misalignment – are investigated. It is shown that these disadvantages become negligible if a tool with a sufficiently large radius is used. The experimental validation of the proposed method was performed with the help of the uniaxial tensile test, the plane strain compression test and hydraulic bulge test on ten common high-strength and advanced high-strength sheet steels in the ultimate tensile strength range between 460 and 1260 MPa and the thickness range between 0.8 and 3.1 mm. The paper demonstrates that with the proposed analytical method for determination of the effective tool width, the plane strain compression test equipped with a tool with a sufficiently large radius becomes more appealing as a cost-efficient alternative to the hydraulic bulge test for the flow curve determination of high-strength sheet steels than it has been considered until now.

In this paper, a new elastic viscoplastic micromechanical modelling is proposed to represent the semi-solid behaviour and predict the ductile-brittle transition of the C38LTT near the solidus. It is based on a viscoplastic modelling previously presented in [1]. The originality of the new model comes from three main enhancements: the transition between the solid state and the semi-solid state was included meaning that the material properties were taken temperature-dependent, the elastic properties was taken into account similarly as [2] and the evolution of the internal variable describing the degree of agglomeration of the solid phase was enhanced. The model was implemented in the commercial software FORGE©. Tensile tests representing the experimental thermal conditions and obtained using a GLEEBLE© machine were simulated. The comparison of the predicted and experimental results shows that, for the first time to our knowledge, the three steps of the load-displacement response and ductile-brittle transition were successfully described.

Large strain characterization of sheet metals has become increasingly important with the generalization of advanced high strength steels, for which the tensile test provides data over a very reduced strain range. Among the numerous alternative characterization tests, the plane strain compression test (PSCT) requires a small amount of material and classical testing machine and acquisition. PSCT was mainly used for hot forming characterization, but recently it has been proved sufficiently accurate for application in cold metal forming. This work provides an in-depth validation of the PSCT by means of the finite element method. When converting the PSCT force-displacement curve into a stress-strain curve (flow curve), several analytical corrections are applied. Several sets of such corrections terms were proposed in the literature, some of which are consistently used by all authors, while others being only used in some papers. The FE simulation of the test was used in order to validate these correction terms and their hypotheses. The originality of the approach is the design of a sequence of test configurations which allow for the individual validation of each and every one of the correction terms concerning the effect of several test parameters. The FE simulations showed that the analytical exploitation of the PSCT provides a very good accuracy. They helped identifying the most suitable correction terms to consider.

Constitutive laws for hot forming typically consider the effects of strain, strain rate, and temperature. Their parameter identification requires experiments performed on wide ranges of these three factors. Usually, specialized equipment is utilized to ensure uniform strain, strain rate, and temperature. Alternatively, parameter databases provide parameter sets for various materials and constitutive laws at a low cost. This work explores the potential of using compression experiments on industrial presses for parameter identification of such constitutive models, in order to balance the cost and accuracy of these two existing alternatives. The results are compared with the reference results produced on specialized large temperature/strain rate compression equipment. Two materials are used to prove the usefulness of the proposed methodology.

International audience; The bending process of an industrial connector is considered and investigated via numerical simulation using a crystal plasticity finite element model (CPFEM). The process consists of sequentially bending a 0.1 mm thick copper-based alloy (CuBe2) with progressive tools into a miniature cylindrical connector of around 1 mm in diameter. The paper focuses on the prediction of springback through the influence of several key-parameters of the numerical simulations. The finite element characteristics, single crystal plasticity model features and the number of grains in the sheet thickness are investigated in order to highlight relevant and influential parameters in CPFEM based microforming process simulations. The influence of the elastic properties is analyzed and a modification of the Peirce-Asaro-Needleman single crystal hardening law is taken into account in order to improve the description of reverse strain path changes. Finally, the numerical results are discussed and compared to the springback measured during the industrial process of the cylindrical connector. It is demonstrated, through the very good agreement with the experimental results, that such approach can be useful to simulate industrial processes.

The accuracy of numerical simulation models for forming processes depends highly on the method for the flow curve determination. Numerous experimental tests exist to identify the flow curves of sheet metals, each with certain characteristics regarding stress states and achievable strains. In this paper stress state analysis are presented for the mechanical joining techniques clinching as well as self-pierce riveting with semi-tubular rivet (SPR). Based on these findings flow curves for DC04 material are determined by selected experimental materials tests in order to compare the influence of the flow curve determination method on the accuracy of the process simulation models.

The automobile manufacturing industry, until recent years, has been using steel for car body components and the main method for joining these components has always been Resistance Spot Welding. However, since the global trends toward CO2 reduction and resource efficiency have significantly increased, the importance and usage of lightweight materials has enhanced as well. New lightweight materials such as aluminum and magnesium alloys, carbon-fiber-reinforced plastics, etc., have become a reality, thanks to the new fastening technologies. Flow drill screw driving (FDS) is a one–sided thermomechanical assembly process based on heat generation by frictional force and plastic deformation. A special screw, known as hole forming and self-tapping screw, is used in this process as both fastener and tool. Moreover, rotational and translation movements are applied to the screw to create special friction conditions with the workpiece. Furthermore, unlike traditional drilling and thread milling processes, there is no chip or waste of material in FDS and the machining operations are realized through plastic deformation. This paper explores flow drilling steps and the parameters which influence heating and local softening of the aluminum sheet 5182-0. An experimental study has been carried out by varying process parameters (rotational speed, drilling force), coating and geometry of the screw. As a result, an increase of rotational speed and drilling force allows significant reduction in drilling time and introduce an important variation of the torque installation. In addition, a strong dependence is observed between drilling time and torque on the one hand, and related to the screw parameters geometry and coating on the other hand. Finally, an evaluation of the heating effect on the thread forming operation is also undertaken.

In the EU, sheep bred for dairy and meat purposes are of low quality, their economic value is not even enough to cover shearing costs, and their wool is generally seen as a useless by-product of sheep farming, resulting in large illegal disposal or landfilling. In order to minimize environmental and health-related problems considering elemental compositions of discarded materials such as waste wool, there is a need to recycle and reuse waste materials to develop sustainable innovative technologies and transformation processes to achieve sustainable manufacturing. This study aims to examine the application of waste wool in biocomposite production with the help of a sustainable hydrolysis process without any chemicals and binding material. The impact of superheated water hydrolysis and mixing hydrolyzed wool fibers with kraft pulp on the performance of biocomposite was investigated and characterized using SEM, FTIR, tensile strength, DSC, TGA, and soil burial testing in comparison with 100% kraft pulp biocomposite. The superheated water hydrolysis process increases the hydrophilicity and homogeneity and contributes to increasing the speed of biodegradation. The biocomposite is entirely self-supporting, provides primary nutrients for soil nourishment, and is observed to be completely biodegradable when buried in the soil within 90 days. Among temperatures tested for superheated water hydrolysis of raw wool, 150 °C seems to be the most appropriate for the biocomposite preparation regarding physicochemical properties of wool and suitability for wool mixing with cellulose. The combination of a sustainable hydrolysis process and the use of waste wool in manufacturing an eco-friendly, biodegradable paper/biocomposite will open new potential opportunities for the utilization of waste wool in agricultural and packaging applications and minimize environmental impact.

International audience; Accurate sheet metal simulation often requires advanced strain-path dependent material models, in order to predict the material response under complex loading conditions, including monotonic, reverse and orthogonal paths. More and more flexible models imply higher and higher costs in terms of parameter identification, computer implementation and simulation time, and robust comparison is often compromised by the inconsistent predictions of advanced models under monotonic loading. In this paper, a simple and general approach is proposed for the alteration of advanced hardening models in order to make them rigorously identical to each other under monotonic loading. This objective was reached without any drawback other than the addition of the corresponding equations. On the contrary, the flexibility and accuracy of the selected models was improved, and the parameter identification procedure became simpler, more accurate and more robust. Three material models of increasing complexity were selected to demonstrate the interest of this approach with respect to a complete set of characterisation experiments for a DP600 sheet steel.

Three water-soluble chitosan derivatives (ChDs)- carboxymethyl- chitosan (CCh), alkyl-chitosan (ACh), and quaternary-chitosan (QCh)- were evaluated as new materials for paper conservation. Several series of samples were prepared by coating different paper types with ChDs or methylcellulose (MC). The ChDs’ effectiveness were analyzed by their effects on the strength (tensile energy absorption (TEA), double folds) and water barriers (Cobb60, contact angle (CA)). The coatings on laboratory paper showed strength improvements for the CCh/QCh coatings that were consistent with an increase in the coating weight (CW). The ACh had little effect on the strength, but developed an effective barrier to water. The coatings on printing paper were performed at a constant CW by applying two layers of the same ChD or MC, and by combining CCh or QCh in the first layer with ACh in the second layer. Homogenous coatings based on the CCh or QCh resulted in high strength improvements, comparable to MC, but only ACh coatings developed an effective barrier to water. Combinations of the CCh or QCh with ACh provided the best relationship between the strength and barrier properties and proved their effectiveness as strengthening/protective materials in the treatment of natural aged paper.

Morphine has been a target for synthetic chemists since Robinson proposed its correct structure in 1925, resulting in a large number of total syntheses of morphine alkaloids. Here we report a total synthesis of (±)-morphine that employs two key strategic cyclizations: (i) a diastereoselective light-mediated cyclization of an O-arylated butyrolactone to form a tricyclic cis-fused benzofuran and (ii) a cascade ene-yne-ene ring closing metathesis to forge the tetracyclic morphine core. This approach enables a short and stereoselective synthesis of morphine in an overall yield of 6.6%.

Anisotropy is a key feature of the plastic behavior of aluminum alloy sheets. Significant research efforts have been dedicated during the past few decades to characterize and model this behavior. Advanced anisotropic yield criteria are available today in the scientific literature. This article reviews the most important yield criteria, in historical order, with an emphasis on the models particularly adapted to aluminum alloys. Along with the mathematical description, illustrations of their performance are provided together with the parameter identification procedure and the required input data. The mechanical parameters that describe plastic anisotropy are also summarized. The relevance of the available models for practical applications is analyzed, and guidance is provided for the selection and aware use of these yield criteria in engineering.

International audience; Large strain hardening is the main material description ingredient for cold bulk forming process simulations. Hardening identification of large strains is a trade-off between cost, standardization and the ability to represent the experiment, but there are no standard procedures to date. In this proposed approach, large strains (larger than 1) are reached using an industrial wire drawing process, and measured data for identification are obtained from standard tensile tests. Fast semi-analytical post-processing was possible despite the significant process-inherited strain heterogeneity. The parameters of state-of-the-art hardening models were identified, and the robustness was demonstrated to reach far beyond the strain level attained in the experiments. As a consequence, accurate and robust large strain hardening modelling was achieved from standard (tensile test) acquisition by using industrial wire drawing pre-strains.

This paper is a contribution to the phenomenological modeling of damage evolution in DP steels in the framework of Gurson's approach. It is based on recent results of X-ray tomography in-situ tensile tests and subsequent one-dimensional metallurgical void nucleation models proposed in [C. Landron et al., Scripta Materialia 63 (2010) 973–976]. A macroscopic void nucleation law for DP steels is proposed, covering a wide range of stress triaxialities. The respective effects of nucleation, growth and coalescence are clearly separated. Validations with respect to experimental porosity measurements were performed for several monotonic loading cases and for two loading sequences involving large strains and strain-path changes. Keywords: GTN model, Phenomenological void nucleation law, DP steels, Finite element simulation, Damage evolution

The patellazoles are a family of complex marine macrolides that exhibit potent cytotoxicity against cancer cell lines. However, despite extensive characterisation efforts, their full stereochemical assignment has remained elusive. We report our approach towards the synthesis-enabled structural elucidation of patellazole B (4), a 24-membered macrolide with 16 stereocentres and a signature thiazole-containing side chain. Our plan hinges upon isolating the unknown stereocentres into a single C20–C25 fragment to facilitate the flexible assembly of various possible diastereomers of an advanced C1–C25 fragment. Towards this end, a highly convergent and modular synthesis of one candidate diastereomer 37, corresponding to the patellazole B macrocyclic skeleton, has been achieved based on the strategic application of stereocontrolled aldol methodology, combined with Suzuki and Heck cross-coupling reactions.

Abrupt strain path changes without elastic unloading have been used to investigate the yield surface of sheet metals, both experimentally and theoretically. These investigations emphasized an apparent non-normality phenomenon, following such a strain-path change. Subsequently, these results inspired the development of plasticity models including non-associated flow rules and non-normality theories. In this study, this type of abrupt strain-path changes is investigated using kinematic hardening models in the context of associated plasticity. The aim is to emphasize the contribution of kinematic hardening ingredient to the apparent violation of the normality condition. The results show that kinematic hardening slope and curvature of the material have a strong influence to contribute to this apparent non-normality.

International audience; Abrupt strain path changes without elastic unloading have been used in the literature to investigate the yield surface of sheet metals, both experimentally and theoretically. Such pioneering studies emphasized an apparent non-normality of the plastic strain rate tensor with respect to the trace of the yield surface in stress space, following such a strain-path change. They inspired numerous subsequent developments of plasticity models including non-associated flow rules. In this paper, this type of abrupt strain-path changes is investigated using state-of-the-art plasticity models. The aim is to emphasize the respective contributions of elasticity, isotropic-kinematic hardening, and rate sensitivity, to the apparent violation of the normality condition. The results show that these classical ingredients of plasticity models significantly contribute to the apparent vertex and loss of normality. These effects are quantified for typical sheet metals subject to biaxial-to-shear orthogonal strain path change.

In the expanding context of device miniaturization, forming processes of ultra thin sheet metals are gaining importance. Numerical simulation of these processes requires accurate material modeling. In this study, both the phenomenological modeling approach and the crystal plasticity finite element method (CPFEM) are considered. Theoretical definitions of both models, numerical implementation as well as their parameter identification procedures are outlined. Subsequently they are compared on a one to one basis, mainly with regards to their ability to predict mechanical responses for a variety of strain loading paths. Agence Nationale de la Recherche, ANR-12-RMNP-0009-02

Forging in semi-solid state significantly extends the possibilities of classical hot forging. In order to fully exploit its potential, the process requires a specific and demanding environment, penalizing its industrial deployment. In this context, an alternative route is proposed. In the proposed process, semi-solid zones at the heart of the material coexist with surrounding solid zones within the part. The outcome is an optimized process where the benefits of thixoforging are reached at a significant extent within the classical process framework of hot forging. The paper investigates this proposal up to a full-scale proof-of concept in an industrial setting. Thixofranc project (Région Lorraine and Région Champagne Ardennes, TACA project (IRT M2P) and ISEETECH (forging equipment)

International audience; In this contribution, the formability of sheet metal cut edges is investigated using a damage model. Classical forming limit diagrams are known not to apply properly to the cut edges. However, with mild steels the sheet edges usually behave better than the Forming Limit Diagram predictions, so this phenomenon has not been given sufficient attention. In contrast, for Advanced High Strength Steels the cut edges exhibit reduced formability as compared to the plain sheet; this effect is very sensitive to the quality of the cutting process. The current investigation is aimed to evaluate the ability of available damage models to predict this effect on sample applications. The Gurson-Tvergaard-Needleman model is used and the material parameters for a Dual Phase steel are considered [1]. The effect of the cutting process is described by means of initial fields of equivalent plastic strain and porosity. The geometrical distribution and typical values for these two initial fields are devised based on literature. Numerical simulations of flat notched tensile tests are used within the FE code Abaqus/Explicit to illustrate the impact of the initial fields on the moment and the location of the failure initiation. The influence of the mesh size in the cutting-affected area is also investigated. The hole expansion test and a flat bending test are further simulated to investigate the influence of the cut edge.

This numerical investigation of an advanced Gurson-Tvergaard-Needleman (GTN) model is an extension of the original work of Ben Bettaiebet al. (2011 [18]). The model has been implemented as a user-defined material model subroutine (VUMAT) in the Abaqus/explicit FE code. The current damage model extends the previous version by integrating the three damage mechanisms: nucleation, growth and coalescence of voids. Physically based void nucleation and growth laws are considered, including an effect of the kinematic hardening. These new contributions are based and validated on experimental results provided by high-resolution X-ray absorption tomography measurements. The current damage model is applied to predict the damage evolution and the stress state in a tensile notched specimen experiment.

The search for new scaffolds to complement current HTS and fragment libraries is an active area of research. The development of novel strategies to synthesise compounds with 3D character in order to expand the diversity of a fragment library was explored. A range of substituted bicyclo[2,2,1]spirooxindoles were synthesised using a Diels-Alder [4+2] cycloaddition reaction. Both diastereoisomers were isolated from the reactions and these 3D fragment scaffolds were screened against the cytochrome P450 enzyme CYP121 from Mycobacterium tuberculosis. A number of hits were identified to bind to CYP121 and were shown to exhibit Type I binding interactions with the heme group.

Lien vers la version éditeur : http://www.sciencedirect.com/science/article/pii/S0020740312000677 The springback behavior of four advanced high-strength sheet steels (Dual-Phase, TRIP, ferrite-bainite) with thicknesses ranging from 1.2 to 4 mm was investigated by means of the bending-under-tension (BUT) test. The applicability of several guidelines from the literature was investigated experimentally and numerically. The monotonic decrease of springback as back force increased was confirmed for this category of sheet steels, and a general trend for the non-linear influence of the tool radius was observed. The influence of numerical factors on the predicted values of springback was investigated, and conclusions and simple guidelines were drawn from the analysis with industrial sheet forming applications in mind.

This present contribution consists of implementing an advanced GTN damage model as a "User Material subroutine" in the Abaqus FE code. This damage model is based on specific nucleation and growth laws in order to predict the void coalescence properties of the material. When applied, this implementation predicts the damage evolution and the stress state of notched specimens made from dual phase steel. By comparing numerical predictions with experimental results, the numerical approach was improved and then validated.

Modelling of plastic anisotropy requires the definition of stress potentials (coinciding with the yield criteria in case of the associated flow rules) or, alternatively, plastic strain-rate potentials. The latter approach has several advantages whenever material parameters are determined by means of texture measurements and crystal plasticity simulations. This paper deals with a phenomenological description of anisotropy in elastoplastic rate-insensitive models, by using strain-rate potentials. A fully implicit time integration algorithm is developed in this framework and implemented in a static-implicit finite element code. Algorithmic details are discussed, including the derivation of the consistent (algorithmic) tangent modulus and the numerical treatment of the yield condition. Typical sheet-forming applications are simulated with the proposed implementation, using the recent non-quadratic strain-rate potential Srp2004-18p. Numerical simulations are carried out for materials that exhibit strong plastic anisotropy. The numerical results confirm that the presented algorithm exhibits the same generality, robustness, accuracy, and time-efficiency as state-of-the-art yield-criterion-based algorithms.; International audience; Modelling of plastic anisotropy requires the definition of stress potentials (coinciding with the yield criteria in case of the associated flow rules) or, alternatively, plastic strain-rate potentials. The latter approach has several advantages whenever material parameters are determined by means of texture measurements and crystal plasticity simulations. This paper deals with a phenomenological description of anisotropy in elastoplastic rate-insensitive models, by using strain-rate potentials. A fully implicit time integration algorithm is developed in this framework and implemented in a static-implicit finite element code. Algorithmic details are discussed, including the derivation of the consistent (algorithmic) tangent modulus and the numerical treatment of the yield condition. Typical sheet-forming applications are simulated with the proposed implementation, using the recent non-quadratic strain-rate potential Srp2004-18p. Numerical simulations are carried out for materials that exhibit strong plastic anisotropy. The numerical results confirm that the presented algorithm exhibits the same generality, robustness, accuracy, and time-efficiency as state-of-the-art yield-criterion-based algorithms.

Sheet metal forming processes generally involve large deformations together with complex loading sequences. In order to improve numerical simulation predictions of sheet parts forming, physically-based constitutive models are often required. The main objective of this paper is to analyze the strain localization phenomenon during the plastic deformation of sheet metals in the context of such advanced constitutive models. Most often, an accurate prediction of localization requires damage to be considered in the finite element simulation. For this purpose,an advanced, anisotropic elastic-plastic model, formulated within the large strain framework and taking strain-path changes into account, has been coupled with an isotropic damage model. This coupling is carried out within the framework of continuum damage mechanics.In order to detect the strain localization during sheet metal forming, Rice’s localizationcriterion has been considered, thus predicting the limit strains at the occurrence of shear bands as well as their orientation. The coupled elastic-plastic-damage model has been implemented in Abaqus/Implicit. The application of the model to the prediction of Forming Limit Diagrams (FLDs) provided results that are consistent with the literature and emphasized the impact of the hardening model on the strain-path dependency of the FLD. The fully threedimensional formulation adopted in the numerical development allowed for some new results – e.g. the out-of-plane orientation of the normal to the localization band, as well as more realistic values for its in-plane orientation.; International audience; Sheet metal forming processes generally involve large deformations together with complex loading sequences. In order to improve numerical simulation predictions of sheet parts forming, physically-based constitutive models are often required. The main objective of this paper is to analyze the strain localization phenomenon during the plastic deformation of sheet metals in the context of such advanced constitutive models. Most often, an accurate prediction of localization requires damage to be considered in the finite element simulation. For this purpose,an advanced, anisotropic elastic-plastic model, formulated within the large strain framework and taking strain-path changes into account, has been coupled with an isotropic damage model. This coupling is carried out within the framework of continuum damage mechanics.In order to detect the strain localization during sheet metal forming, Rice’s localizationcriterion has been considered, thus predicting the limit strains at the occurrence of shear bands as well as their orientation. The coupled elastic-plastic-damage model has been implemented in Abaqus/Implicit. The application of the model to the prediction of Forming Limit Diagrams (FLDs) provided results that are consistent with the literature and emphasized the impact of the hardening model on the strain-path dependency of the FLD. The fully threedimensional formulation adopted in the numerical development allowed for some new results – e.g. the out-of-plane orientation of the normal to the localization band, as well as more realistic values for its in-plane orientation.

The plastic anisotropy of sheet metals can be described using phenomenological yield criteria or strain- rate potentials. Linear transformations provide one of the most effective ways to describe anisotropy in this framework. In this work, an anisotropic strain-rate plastic potential with an arbitrary number of transformations is formulated as an extension of former potentials involving one or two transformations, respectively. The parameter identification becomes an important issue since the number of parameters increases along with the number of transformations. A specific parameter identification method is used, based on a micromechanical model. Several sheet aluminium alloys and sheet steels are used to investigate the gain in accuracy when the number of linear transformations in the model increases. It is shown that when five or more transformations are used, the new model is able to describe the initial plastic anisotropy as accurately as a micromechanical model - provided that its parameters are identified accordingly.

This paper reviews a class of anisotropic plastic strain-rate potentials, based on linear transformations of the plastic strain-rate tensor. A new formulation is proposed, which includes former models as particular cases and allows for an arbitrary number of linear transformations, involving an increasing number of anisotropy parameters. The formulation is convex and fully three-dimensional, thus being suitable for computer implementation in finite element codes. The parameter identification procedure uses a micromechanical model to generate evenly distributed reference points in the full space of possible loading modes. Material parameters are determined for several anisotropic, fcc and bcc sheet metals, and the gain in accuracy of the new models is demonstrated. For the considered materials, increasing the number of linear transformations leads to a systematic improvement of the accuracy, up to a number of five linear transformations. The proposed model fits very closely the predictions of the micromechanical model in the whole space of plastic strain-rate directions. The r-values, which are not directly used in the identification procedure, served for the validation of the models and to demonstrate their improved accuracy.

This work aims to study the strain localization in different sheet metal forming processesby the finite element method using advanced behaviour models. In order to improve the prediction of the formability limit of sheets under forming it is important to consider constitutive models that take into account the degradation of material properties due to large strain localization. For this purpose, an elastic-plastic model has been coupled with the classical damage model of Lemaitre and Chaboche [2,3]. Two hardening models are considered in an attempt to more accurately reproduce the work-hardening phenomenon at large strains. The first one represents the classical cyclic hardening model of Chaboche-Marquis [3,4] while the second corresponds to the so-called dislocation-based microstructural model of Teodosiu and Hu [5,6]. These two models are able to reproduce some transient hardening phenomena at strain path changes - with a better description obtained with the microstructural model. The coupling with damage is carried out within the framework of continuum damage mechanics thanks to the introduction of a scalar variable describing the degradation of the elastic proprieties and consequently softening behaviour when damage occurs. The coupled elastic-plastic damage model is implemented into the Abaqus software using an implicit integration scheme. The resulting code is used for the prediction of forming limit diagrams.; International audience; This work aims to study the strain localization in different sheet metal forming processesby the finite element method using advanced behaviour models. In order to improve the prediction of the formability limit of sheets under forming it is important to consider constitutive models that take into account the degradation of material properties due to large strain localization. For this purpose, an elastic-plastic model has been coupled with the classical damage model of Lemaitre and Chaboche [2,3]. Two hardening models are considered in an attempt to more accurately reproduce the work-hardening phenomenon at large strains. The first one represents the classical cyclic hardening model of Chaboche-Marquis [3,4] while the second corresponds to the so-called dislocation-based microstructural model of Teodosiu and Hu [5,6]. These two models are able to reproduce some transient hardening phenomena at strain path changes - with a better description obtained with the microstructural model. The coupling with damage is carried out within the framework of continuum damage mechanics thanks to the introduction of a scalar variable describing the degradation of the elastic proprieties and consequently softening behaviour when damage occurs. The coupled elastic-plastic damage model is implemented into the Abaqus software using an implicit integration scheme. The resulting code is used for the prediction of forming limit diagrams.

This paper shows that non-linearity of mechanical behaviour of metal in the elastic regime has an influence on the forming process. Discrete Dislocation Dynamics simulations show that pure elastic behaviour is altered when reversible dislocation displacements occur even in the very beginning of the elastic stage. The influence of such a non-linearity has an impact on the results of numerical simulations of industrial forming of very thin metal sheets of copper.

International audience; The aim of this work is to develop a dislocation density based model for IF and DP steels that incorporates details of the microstructure evolution at the grain-size scale. The model takes into account (i) the contribution of the chemical composition for the prediction of the initial yield stress, (ii) the description of initial texture anisotropy by incorporating grain-size dependent anisotropy coefficients in Hill’48 yield criterion, (iii) the contribution of three dislocation density “families” that are associated with forward, reverse and latent structures. It reproduces the macroscopic transient behaviors observed when strain-path changes occur. The model is implemented in FE code in order to assess its predictive capabilities in case of industrial applications.

The paper presents a new yield criterion for orthotropic sheet metals under plane-stress conditions. The criterion is derived from the one proposed by Barlat and Lian in 1989. Two additional coefficients have been introduced in order to allow a better representation of the plastic behaviour of the orthotropic sheet metals. The predictions of the new yield criterion are compared with the experimental data for two materials.

The paper presents a new yield criterion for orthotropic sheet metals under plane-stress conditions. The criterion is derived from the one proposed by Barlat and Lian (Int. J. Plasticity 5 (1989) 51). Three additional coefficients have been introduced in order to allow a better representation of the plastic behaviour of the sheet metals. The predictions of the new yield criterion are compared with the experimental data for an aluminium alloy sheet and a steel sheet.