Finite element simulation of ductile fracture in polycrystalline materials using a regularized porous crystal plasticity model

In the present study, a hypoelastic–plastic formulation of porous crystal plasticity with a regularized version of Schmid’s law is proposed. The equation describing the effect of the voids on plasticity is modified to allow for an explicit analytical solution for the effective resolved shear stress. The regularized porous crystal plasticity model is implemented as a material model in a finite element code using the cutting plane algorithm. Fracture is described by element erosion at a critical porosity. The proposed model is used for two test cases of two- and three-dimensional polycrystals deformed in tension until full fracture is achieved. The simulations demonstrate the capability of the proposed model to account for the interaction between different modes of strain localization, such as shear bands and necking, and the initiation and propagation of ductile fracture in large scale polycrystal models with detailed grain description and realistic boundary conditions

The relation between grain boundary precipitate formation and adjacent grain orientations in Al-Mg-Si(-Cu) alloys

The occurrence of grain boundary precipitates was investigated with respect to the crystallographic orientation of the adjacent grains in extruded AA6110, AA6063 and AA6061 alloys brought to T6 temper. It was found that the requirement for grain boundary precipitate formation is for the adjacent grains to have Al directions or {100}Al planes parallel to the grain boundary plane. The highest density of grain boundary precipitates was present when this requirement was fulfilled by both adjacent grains.publishedVersio

Numerical simulations of submerged and pressurised X65 steel pipes – COMPLAS XII

While in service, pipelines may from time to time be exposed to impact loads from anchors or trawl gear. A lot of parameters influence the behaviour of the pipeline during impact, e.g. the diameter and thickness of the pipeline, the impactor’s mass and velocity, and of course the material used. Also potentially influencing the deformation pattern is the presence of surrounding water, which can be a difficult parameter to include experimentally. The pressurised contents of the pipeline can also be an influencing factor. To gain some insight into how the water and possible pressure inside the pipeline affect the global impact behaviour, numerical investigations have been carried out using FSI- techniques available in the explicit finite element code Europlexus. One case was set up for validation against available experimental data, and additional cases examined numerically the effect of including pressure and/or surrounding water. The simulations generally captured the deformation and load levels from the experiments well, and may be assumed to represent the events with reasonable accuracy. Adding internal pressure generally led to a higher peak load, and created a more localised deformation. Submerging the pipe in water seemed to be of minor importance with respect to global and local deformation, but this depended heavily on the FSI conditions. Other parameters than the surrounding water, which for design purposes may be omitted, appear to be more significant to the global and local response

Étude du comportement mécanique des matériaux dans des conditions étendues de vitesses et de températures (application à l'acier C68 dans le cas d'une opération de formage incrémental)

L'objectif de ce travail de thèse est la caractérisation et la modélisation du comportement thermo-mécanique d'un acier à haut taux de carbone dans des conditions de chargement identiques à celles d'un procédé de formage incrémental à 720C. Le regain d'intérêt des industriels pour ce type de procédé provient du fait qu'ils sont moins énergivores et permettent un ratio matière valorisée sur matière brute intéressant pour des propriétés mécaniques améliorées de la pièce formée. Le recours à l'outil numérique devient aujourd'hui une solution intéressante pour optimiser au mieux la mise au point du procédé. Son application demande, entre autres, une description fine du comportement du matériau dans les conditions de sollicitations de celui-ci, c'est-à-dire sur une large plage de vitesses de déformation et de températures. Pour cela, une caractérisation du matériau est nécessaire dans ces conditions. Le comportement mécanique d'un acier à haut taux de carbone a été étudié au travers d'essais de traction quasi-statiques et dynamiques afin de déterminer la sensibilité du matériau à la température et à la vitesse de déformation. Cette caractérisation repose sur la combinaison novatrice d'un système de chauffage par induction contrôlé par pyrométrie et d'un système d'acquisition d'images numériques. Ce dernier est utilisé pour la détermination des déformations à partir des champs de déplacement obtenus par corrélation d'images. En particulier, les effets de la température sur le module d'élasticité, l'anisotropie et l'écrouissage isotrope ont été étudiés. Les résultats de ces essais ont ensuite constitué une base de données expérimentales pour l'identification des paramètres de divers modèles de comportement thermo-élasto-visco-plastique. Ces derniers sont plus ou moins complets selon le nombre de paramètres utilisés pour décrire le comportement du matériau sur l'ensemble du domaine étudié. Cela permet d'identifier le modèle avec le meilleur ratio qualité/coût-délai pour une application donnée. Chacun des modèles étudiés dispose donc d'un domaine de définition propre.The aim of this thesis work is the characterization and the modelisation of the thermo-mechanical behaviour of a high-carbon steel with loading conditions corresponding to those imposed by a forming process at 720C. The renewal interest for this kind of industrial process comes from the fact that they use less energy and provide a ratio of material valued/raw material interesting to improved mechanical properties of the formed part. The use of numerical tool becomes today an interesting solution to optimize the development process. Its application requires a detailed description of the material behaviour in the loading conditions of the process, that is over a wide range of strain rates and temperatures. So, a material characterization is required in those conditions. The mechanical behaviour of a high-carbon steel was studied through quasi-static and dynamic tensile tests in order to determine the temperature and strain rate sensitivity of the material. This characterization is based on the innovative combination of an induction system controlled by pyrometry and a digital image acquisition system. This one is used for the determination of strains from displacement elds obtained by digital image correlation. In particular, e ects of temperature on the modulus of elasticity, anisotropy and isotropic hardening were studied. Results of these tests allow us to establish an experimental database for the identi cation of parameters of various thermo-elasto-visco-plastic models. These are more or less complete depending on the number of parameters used to describe the material behaviour throughout the studied area. The model with the best ratio quality/cost-delay for a given application can be identi ed. Each of studied models has a speci c domain of validity.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Fuzzy variable linear programming with fuzzy technical coefficients

Fuzzy linear programming is an application of fuzzy set theory in linear decision making problems and most of these problems are related to linear programming with fuzzy variables. In this paper an approximate but convenient method for solving these problems with fuzzy non-negative technical coefficient and without using the ranking functions, is proposed. With the help of numerical examples, the method is illustrated

Influence of constituent particles on fracture of aluminum alloys under high-triaxiality loading

Single-edge notch bending tests are conducted to study the influence of constituent particles on the fracture resistance of aluminum alloys 6061, 6063, and 6110 under high-constraint loading conditions. The alloys are tested in the as-cast state after homogenization and artificial aging to temper T6. Each alloy type was delivered with two different volume fractions of constituent particles to enable a quantitative assessment of its impact on the toughness of these aluminum alloys. One variant corresponds to the commercial alloy, whereas the other variant is tailor made with an increased amount of constituent particles by adding Fe and Si to the commercial alloy. All alloys exhibit a dendritic structure with particles clustered at grain boundaries and dendrite arm boundaries. The increased content of constituent particles in the tailor-made alloys is shown to be purely detrimental for the toughness and reduces relevant fracture energy parameters by more than 50% in the alloys tested herein. In the plane-strain-dominated regions of the specimens where the stress triaxiality is highest, crack propagation was found to take place on grain boundaries and dendrite arm boundaries due to void nucleation, growth, and coalescence from the constituent particles. Differences in toughness between the alloys are primarily related to variations in the content, size, and spacing of the constituent particles. A comparison between the three different alloy types, i.e. 6061, 6063, and 6110, shows that strength affects the toughness, but it does not follow the commonly reported trade-off between strength and ductility.publishedVersio

Texture gradients and strain localisation in extruded aluminium profile

Through-thickness crystallographic texture gradients may develop in extruded profiles and rolled sheets of aluminium alloys. These texture gradients are often modelled using the crystal plasticity theory in order to predict strain localisation more accurately. In this work, an experimental and numerical study was carried out for a flat extruded profile with texture gradients made of the aluminium alloy AA6063. Uniaxial tension and plane-strain tension specimens were produced from this profile in two orthogonal material directions and heat treated to different tempers to study the role of crystallographic texture and work-hardening on strain localisation. The microstructure, including the orientation, morphology and position of the grains, was obtained from EBSD scans. The plane-strain tension tests were simulated using two crystal plasticity finite element models: the first represents accurately the measured microstructure, whereas the second one only represents correctly the global texture but not the texture gradients and grain morphology, i.e., the grain orientations are assigned randomly. In addition, a coarser model was used to identify the material parameters based on data from the plane-strain tension tests. The simulation results showed that accurate modelling of the microstructure did not influence the simulation results significantly. The model with the same global texture but with random assignment of grain orientations gave similar predictions for both the global stress-strain behaviour and the local deformation patterns.acceptedVersio

ATLAS of yield surfaces for strongly textured FCC polycrystals

Discrete yield surfaces for several generic texture components, including randomness, found in aluminium alloys have been generated and used to calibrate the yield function Yld2004-18p. It is generally observed that the roundness of the corners of the yield surface increases, and the stress and strain ratios flatten towards isotropic values, as the ratio of random component increases. A short investigation on the effect of number of points and homogenization approach on calibration of the yield function seems to indicate that the number of points used in the calibration has a stronger effect than the homogenization approach. Furthermore, it is also shown that setting the exponent as a free parameter in calibrating the yield function could lead to better fits.acceptedVersio

Volumetric strain measurement of polymeric materials subjected to uniaxial tension

A novel method for measuring and calculating volumetric strain in circular cylindrical uniaxial tension samples made from polymeric materials is proposed. It is shown that special considerations must be taken when calculating volumetric strain when a sample is in a postnecking state. Solely based on surface data, the key feature of the proposed correction is that it allows for an inhomogeneous distribution of longitudinal strain through the diameter of the sample, where a more traditional approach would be to assume a homogeneous distribution. These two approaches are evaluated by applying them to data from a close‐to‐incompressible steel sample. Whereas the proposed method indicates only a small positive increase in volume, the assumption of a homogeneous distribution results in substantial negative volumetric strains. Applying the two methods to tension samples made from HDPE and PVC, where plastic dilatation is nonlinear, again shows an initial negative volumetric strain for HDPE with the assumption of a homogeneous longitudinal strain. The proposed method predicts close‐to‐zero early‐stage volumetric strain for the same test. The differences are more subtle for samples of PVC. Micrographs obtained with scanning electron microscope show that the dilatation of PVC is related to voiding of the material around filler particles, while the underlying mechanism for HDPE is less clear. The results indicate that earlier reports of negative volumetric strain in polymers subjected to uniaxial tension might be artefacts of the implicit assumption made when calculating the volumetric strain.acceptedVersio