Identification methodology and comparison of phenomenological ductile damage models for complex loading paths

National audienceUnderstanding and modeling of ductile damage mechanisms remains a major issue for many industrial cold forming processes. The ability of numerical modeling to predict the ductile fracture is indeed crucial. However, this modeling is limited because of the complexity to take into account the influence of the complex loading paths (multi-axial and non-monotonic, nonproportional, shear effects, etc.) on damage mechanisms

Comparative study of phenomenological coupled damage models - Application to industrial forming processes

International audienceXue's damage model [Xue, 2007] accounts for the third invariant of deviatoric stress tensor, allowing a better balance between respective effects of shear and elongation on damage. It has been implemented in Forge® Finite Element (FE) software to investigate ductile damage occurring during industrial forming processes. Material and damage parameters identification has been carried out for both Xue and Lemaître models. Application to wire drawing followed by flat rolling shows that in such shearinducing processes, both models predict damage at different locations, due to their different emphasis on shear with respect to elongational strain damage

Damage Prediction Using Several Types of Macro-scale Damage Models in Different Cold Wire Production Lines

International audienceThe purpose of the present paper is to show how and to what extent the introduction of refined, shear sensitive models improves on previous ones, based on triaxiality only, for the phenomenological description of ductile damage in bulk cold metal forming processes. Wire-drawing and wire rolling are taken as examples. A set of mechanical tests has been conducted: round bar tension, notched bar tension, plane strain tension, and torsion for pure shear deformation. Both constitutive and damage model parameters have been carefully identified, with back-computation of the laboratory tests for validation. Application of the models to the cold forming processes, described here, shows the superiority of the shear-enhanced models for locating maximum damage in flat wire rolling, where a significant amount of shear is present (“blacksmith's cross” deformation pattern). On the contrary, it proves unnecessary for low-shear processes such as wire-drawing. The cavity-growth Gurson-Tvergaard-Needleman model seems to be the best basis for damage prediction in patented high carbon steel, a very ductile material

Ductile damage prediction in different cold forming processes

The purpose of the present paper is to show how and to what extent the introduction of refined, shear sensitive models improves on previous ones, based on triaxiality only, for the phenomenological description of ductile damage in bulk cold metal forming processes. Wire-drawing and wire rolling are taken as examples. A set of mechanical tests has been conducted: round bar, notched bar and plane strain tensions as well as torsion for pure shear deformation. Both constitutive and damage models parameters have been carefully identified, with back-computation of the laboratory tests for validation. Application of the models to the cold forming processes, described here, shows the superiority of the shear-enhanced models for locating maximum damage in flat wire rolling, where a significant amount of shear is present (``blacksmith's cross'' deformation pattern). On the contrary, it proves unnecessary for low-shear processes such as wire-drawing. The cavity-growth Gurson-Tvergaard-Needleman model, with a modified formulation for nucleation, seems to be the best basis for damage prediction in patented high carbon steel

Analysing factors influencing land use planning for sustainable land resource management in Vietnam: a case study of Dan Phuong district in Hanoi city

Land use planning is an important task for every country in the world to ensure the sustainable use of land resources for economic and social development activities. However, many land use plans have not achieved their desired effectiveness due to a lack of consideration and evaluation of factors affecting the land use planning implementation process. This study was conducted to answer the following question: what are the main factors influencing the land use planning process in Vietnam? In this study, we use multivariate regression to identify the main factors influencing land use planning in Vietnam, including the economic factor group; institutional and policy factor group; urbanisation factor group; planning factor group; social factor group; environmental factor group; and organisational and implementation factor group. The results identified five groups of factors influencing land use planning in the Dan Phuong district through the influencing factor analysis method, including policy (Po), economy (Ec), society (So), environment (En), and others (Ot). In addition, the linear regression model obtained for land use planning in Dan Phuong district is as follows: land use planning = 0.408Po + 0.454Ec + 0.398So + 0.368En + 0.259Ot. In which the group of factors with the strongest influence was the Eco factors (β = 0.454), the second was the Po factors (β = 0.408), the third was the So factors (β = 0.398), the fourth was the En factors (β = 0.368), and lastly, there were the Ot factors (β = 0.259). The results of the analysis were used to plan land use effectively and according to local conditions. This can help managers to find solutions for the sustainable use of land resources in the future

Spatiotemporal evolution of SARS-CoV-2 Alpha and Delta variants during large nationwide outbreak of COVID-19, Vietnam, 2021

We analyzed 1,303 SARS-CoV-2 whole-genome sequences from Vietnam, and found the Alpha and Delta variants were responsible for a large nationwide outbreak of COVID-19 in 2021. The Delta variant was confined to the AY.57 lineage and caused >1.7 million infections and >32,000 deaths. Viral transmission was strongly affected by nonpharmaceutical interventions

Modeling ductile damage for complex loading paths

Ce travail de thèse vise à une meilleure compréhension et prise en compte des mécanismes d'endommagement ductile au cours des procédés de mise en forme à froid. Le tréfilage, le laminage de fils et le laminage à pas de pèlerin sont pris comme exemples d'application. Une grande attention est portée également à la méthodologie d'identification des paramètres des modèles d'endommagement implémentés. Trois approches de l'endommagement ductile ont été étudiées : des critères de rupture phénoménologique, des modèles phénoménologiques couplés et des modèles micromécaniques. Ces modèles ont été implémentés dans Forge®, et les algorithmes ont dû être adaptés à sa formulation mixte vitesse-pression et à son élément fini (P1+/P1). Parallèlement aux développements numériques, différents essais mécaniques ont été effectués sur trois matériaux différents (acier haut carbone, acier inoxydable et alliage de zirconium) pour identifier les paramètres des modèles d'écrouissage et d'endommagement. Des essais de traction in situ sous micro-tomographie aux rayons X ont été exploités pour l'identification des mécanismes d'endommagement ductile (germination, croissance et coalescence), ainsi que l'identification du modèle micromécanique. Enfin, la dernière partie de ce travail a consisté à réaliser des études comparatives de ces différents modèles d'endommagement pour les différents procédés de mise en forme et différents matériaux. Concernant le tréfilage et le laminage de l'acier inoxydable, un bon accord entre les simulations numériques et les résultats expérimentaux a été obtenu. Pour l'acier à haute teneur en carbone (perlitique), le modèle micro-mécanique GTN a donné le meilleur résultat à la fois qualitativement et quantitativement pour le tréfilage ultime. Par ailleurs, la comparaison entre les différents modèles sur différents procédés (laminage du fil avec l'acier haut carbone, laminage à pas de pèlerin avec l'alliage de zirconium) a aidé d'une part à mettre en évidence le rôle important du troisième invariant de la contrainte déviatorique sur la localisation de l'endommagement dans les procédés de mise en forme où le cisaillement est dominant. D'autre part, elle montre que le processus d'identification lui-même devrait se fonder sur la mesure de la microstructure afin de garantir un résultat précis lors de l'application aux procédés.The present PhD thesis aims at a better understanding and modeling of ductile damage mechanisms during cold forming processes, with wire drawing, rolling and cold pilgering as examples. In addition, special attention is paid to implemented damage models parameters identification methodology. All three approaches of ductile damage were investigated: uncoupled phenomenological fracture criteria; coupled phenomenological models; micromechanical model. These models have been implemented in Forge®, which required adaptation of algorithms to its mixed velocity-pressure formulation and to its finite element (P1+/P1). Parallel to the numerical work, various mechanical tests on three different materials (high carbon steel, stainless steel and zirconium alloy) were carried out for work hardening, and damage models parameters identification. In situ X-ray micro-tomography tensile tests have also been exploited for the identification of ductile damage mechanisms (nucleation, growth and coalescence) as well as the identification of micromechanical model. Finally, we carried out comparative studies of these models on our three abovementioned forming processes and materials. Regarding wire drawing and rolling of stainless steel, good agreement between numerical simulations and experimental results was found. For high carbon perlitic steel ultimate wire drawing, the GTN micro-mechanical model has given the best result, both qualitatively and quantitatively. Moreover, the comparison of the different models on different processes (wire rolling on high carbon steel, cold pilgering on zirconium alloy) highlights on the one hand the important role of the third deviatoric stress invariant in damage localization for shear-dominated forming processes. It shows on the other hand that the identification process itself should be based on microstructure measurements to provide accurate results in forming application

Models for ductile damage and fracture prediction in cold bulk metal forming processes: a review

International audienceDuctile damage and fracture prediction in real size structures subjected to complex loading conditions has been of utmost interest in the scientific and engineering community in the past century. Numerical simulations with nonlinear finite element (FE) codes allow investigating various complicated problems for damage and fracture prediction in real scale models, which is an important topic in many industries, including metal forming industry. For all industrial cold forming processes, the ability of numerical modeling to predict ductile fracture is crucial. However, this ability is still limited because of the complex loading paths (multi-axial and non-proportional loadings) and important shear effects in several forming processes. The development robust damage and fracture prediction models is essential to obtain realistic results for both geometry precision and mechanical properties. The present article reviews the models in three approaches of ductile damage, namely: uncoupled phenomenological model (or fracture criteria), coupled phenomenological models, and micromechanics-based models, which have been developed to predict ductile fracture in metal forming processes. The objective is to supply to engineers and scientists an overview on a “top-down” procedure to be able to construct predictive tools for metal forming processes

Numerical simulation of 3D ductile cracks formation using recent improved Lode-dependent plasticity and damage models combined with remeshing

International audienceDamage to fracture transition has become a popular topic in the ductile fracture scientific community. Indeed, the transition from a damage continuous approach to a discontinuous fracture is not straightforward both from mechanical and numerical points of view. In the present study, a new improved Lode dependent phenomenological coupled damage model is used to investigate the ductile fracture in different mechanical tests. The remeshing and elements erosion techniques are employed to propagate the ductile cracks in 3D models using Forge® finite element code. This code is based on a mixed velocity-pressure formulation using the MINI element P1+/P1P1+/P1. In addition, the plasticity behavior is modeled by a Lode-dependent plasticity criterion. Applications to different mechanical tests at different loading configurations, using identified damage model parameters, show good agreement in terms of fracture prediction between experimental and numerical results

Modélisation de l’endommagement ductile sous trajets de chargement complexes

The present PhD thesis aims at a better understanding and modeling of ductile damage mechanisms during cold forming processes, with wire drawing, rolling and cold pilgering as examples. In addition, special attention is paid to implemented damage models parameters identification methodology. All three approaches of ductile damage were investigated: uncoupled phenomenological fracture criteria; coupled phenomenological models; micromechanical model. These models have been implemented in Forge®, which required adaptation of algorithms to its mixed velocity-pressure formulation and to its finite element (P1+/P1). Parallel to the numerical work, various mechanical tests on three different materials (high carbon steel, stainless steel and zirconium alloy) were carried out for work hardening, and damage models parameters identification. In situ X-ray micro-tomography tensile tests have also been exploited for the identification of ductile damage mechanisms (nucleation, growth and coalescence) as well as the identification of micromechanical model. Finally, we carried out comparative studies of these models on our three abovementioned forming processes and materials. Regarding wire drawing and rolling of stainless steel, good agreement between numerical simulations and experimental results was found. For high carbon perlitic steel ultimate wire drawing, the GTN micro-mechanical model has given the best result, both qualitatively and quantitatively. Moreover, the comparison of the different models on different processes (wire rolling on high carbon steel, cold pilgering on zirconium alloy) highlights on the one hand the important role of the third deviatoric stress invariant in damage localization for shear-dominated forming processes. It shows on the other hand that the identification process itself should be based on microstructure measurements to provide accurate results in forming application.Ce travail de thèse vise à une meilleure compréhension et prise en compte des mécanismes d'endommagement ductile au cours des procédés de mise en forme à froid. Le tréfilage, le laminage de fils et le laminage à pas de pèlerin sont pris comme exemples d'application. Une grande attention est portée également à la méthodologie d'identification des paramètres des modèles d'endommagement implémentés. Trois approches de l'endommagement ductile ont été étudiées : des critères de rupture phénoménologique, des modèles phénoménologiques couplés et des modèles micromécaniques. Ces modèles ont été implémentés dans Forge®, et les algorithmes ont dû être adaptés à sa formulation mixte vitesse-pression et à son élément fini (P1+/P1). Parallèlement aux développements numériques, différents essais mécaniques ont été effectués sur trois matériaux différents (acier haut carbone, acier inoxydable et alliage de zirconium) pour identifier les paramètres des modèles d'écrouissage et d'endommagement. Des essais de traction in situ sous micro-tomographie aux rayons X ont été exploités pour l'identification des mécanismes d'endommagement ductile (germination, croissance et coalescence), ainsi que l'identification du modèle micromécanique. Enfin, la dernière partie de ce travail a consisté à réaliser des études comparatives de ces différents modèles d'endommagement pour les différents procédés de mise en forme et différents matériaux. Concernant le tréfilage et le laminage de l'acier inoxydable, un bon accord entre les simulations numériques et les résultats expérimentaux a été obtenu. Pour l'acier à haute teneur en carbone (perlitique), le modèle micro-mécanique GTN a donné le meilleur résultat à la fois qualitativement et quantitativement pour le tréfilage ultime. Par ailleurs, la comparaison entre les différents modèles sur différents procédés (laminage du fil avec l'acier haut carbone, laminage à pas de pèlerin avec l'alliage de zirconium) a aidé d'une part à mettre en évidence le rôle important du troisième invariant de la contrainte déviatorique sur la localisation de l'endommagement dans les procédés de mise en forme où le cisaillement est dominant. D'autre part, elle montre que le processus d'identification lui-même devrait se fonder sur la mesure de la microstructure afin de garantir un résultat précis lors de l'application aux procédés