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See detailComparison of a Fluid and a Solid Approach for the Numerical Simulation of Friction Stir Welding with a Non‐Cylindrical Pin
Bussetta, Philippe ULg; Dialami, Narges; Boman, Romain ULg et al

in Steel Research International (2014), 85(6), 968-979

Friction stir welding (FSW) process is a solid‐state joining process during which materials to be joined are not melted. As a consequence, the heat‐affected zone is smaller and the quality of the weld is ... [more ▼]

Friction stir welding (FSW) process is a solid‐state joining process during which materials to be joined are not melted. As a consequence, the heat‐affected zone is smaller and the quality of the weld is better with respect to more classical welding processes. Because of extremely high strains in the neighborhood of the tool, classical numerical simulation techniques have to be extended in order to track the correct material deformations. The Arbitrary Lagrangian–Eulerian (ALE) formulation is used to preserve a good mesh quality throughout the computation. With this formulation, the mesh displacement is independent from the material displacement. Moreover, some advanced numerical techniques such as remeshing or a special computation of transition interface is needed to take into account non‐cylindrical tools. During the FSW process, the behavior of the material in the neighborhood of the tool is at the interface between solid mechanics and fluid mechanics. Consequently, a numerical model of the FSW process based on a solid formulation is compared to another one based on a fluid formulation. It is shown that these two formulations essentially deliver the same results in terms of pressures and temperatures. [less ▲]

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See detailNumerical simulation of T-bend of multilayer coated metal sheet using solid-shell element
Ben Bettaieb, Amine ULg; Tuninetti Vásquez, Victor ULg; Duchene, Laurent ULg

in Steel Research International (2012)

The main aim of this paper is to model the T-bend test performed on multilayer coated metal sheets in order to measure the coating flexibility. Because of important uses of polymer coatings in many ... [more ▼]

The main aim of this paper is to model the T-bend test performed on multilayer coated metal sheets in order to measure the coating flexibility. Because of important uses of polymer coatings in many industrial applications and higher requirement on the quality of products, an accurate modeling of the T-bend process is therefore essential. During the modeling with the finite element method, the large thickness ratio between the different layers is likely to produce elements with an unfavorable aspect ratio. Therefore, to avoid obtaining inaccurate results linked to the shape of the elements, solid-shell elements are used in this study. These elements are based on the Enhanced Assumed Strain (EAS) technique and the Assumed Natural Strain (ANS) technique. These techniques permit to avoid locking problems even in very bad conditions (nearly incompressible materials, very thin elements conducting to large aspect ratios, distorted element geometry…). The EAS technique artificially introduces additional degrees of freedom (DOFs) to the element. They permit to increase the flexibility of the element which is very efficient for several locking issues. On the other hand, the ANS technique modifies the interpolation scheme for particular strain components. The ANS technique proved to eliminate the transverse shear locking from the element in bending dominated situations. Besides, a numerical integration scheme dedicated to Solid-Shell elements was implemented. It uses a user-defined number of integration points along the thickness direction, which permits to increase the element accuracy with a mesh containing a reduced number of elements along the thickness direction. The results obtained from numerical simulations are compared with some analytical results in order to check the strain predicted in the coated layer by FEM. This information helps to investigate the coating layer ductility in the real process. [less ▲]

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See detailCompression test for plastic anisotropy characterization using optical full-field dis-placement measurement technique
Tuninetti Vásquez, Victor ULg; Gilles, Gaëtan ULg; Milis, Olivier ULg et al

in Steel Research International (2012)

In this work, a constitutive law, its identification and its validation for TA6V titanium alloy are presented. The material characteri-zation is achieved by quasi-static monotonic tensile and compression ... [more ▼]

In this work, a constitutive law, its identification and its validation for TA6V titanium alloy are presented. The material characteri-zation is achieved by quasi-static monotonic tensile and compression tests at several strain rates and in three orthogonal material orientations. Constitutive model as Johnson-Cook which describes the strain rate sensitivity of the plastic material response is used in order to identify the initial yield locus at a common strain rate (10-3 s-1). The macroscopic orthotropic yield criterion chosen is the well-known CPB06 proposed by Cazacu et al., which describes both the tension/compression asymmetry and the anisotropic behavior of hcp metals and alloys. Voce type isotropic hardening model is used for simplicity reasons. A compression test on ellipsoidal cross-section specimen is selected for validation purpose, where 3 Digital Image Correlation 3D systems are used in order to acquire the evolution of the full field displacement/strain during the test. Coulomb friction coefficient between the dies and the specimen is computed by means of inverse modelling. A good agreement on the prediction of the load as a function of axial displacement of the specimen is found. [less ▲]

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See detailEffect of a decrease in sample dimensions on plasticity: application to the micro-forming of nickel
Keller, Clément ULg; Afteni, Mitica; Banu, M. et al

in Steel Research International (2010), 81(9), 1173-1177

The mechanical behavior of polycrystalline nickel is investigated in tension as a function of the dimensions. Experimental tests and finite element simulations with a crystalline plasticity code are ... [more ▼]

The mechanical behavior of polycrystalline nickel is investigated in tension as a function of the dimensions. Experimental tests and finite element simulations with a crystalline plasticity code are carried out for samples with thicknesses ranging between 12.5 μm and 3.2 mm and grain size around 100 μm. Results show three different kinds of behavior depending on the number of grains across the thickness due to the apparition of stress and dislocation density gradients. Bulge tests are then used to study the formability of nickel polycrystals with thickness lower than 400 μm. [less ▲]

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See detailComparison of mechanical efficiency of two microstructures of Ti5553 alloy in the case of bolted flanges submitted to tensile loading
Van Hoof, T.; Ben Bettaieb, Mohamed ULg; Lani, F. et al

in Steel Research International (2010), 81(9), 1148-1152

A combination of numerical and experimental methods is used to compare the efficiency of two microstructures of Ti5553 titanium alloy in an industrial test case. The test case consists in two parts of ... [more ▼]

A combination of numerical and experimental methods is used to compare the efficiency of two microstructures of Ti5553 titanium alloy in an industrial test case. The test case consists in two parts of sectors of aircraft engine stators linked together by an Inconel bolt and submitted to tensile loading. The efficiency of these microstructures is evaluated by using finite element (FE) models. The chosen FE models take into account elasto-visco-plastic material behaviors, contact with friction (bolt heads/flanges and flange/flange), and the initial bolt tightening. The parameters of the elasto-visco-plastic (EVP) material law are adjusted from experimental results extracted from tensile tests performed on Inconel, and Ti5553 alloys. Viscous properties are observed at different strain rates. The studied alloy is assumed to be plastically isotropic. In order to compare the fracture resistance of both microstructures, a micro-mechanical (damage based) rupture model ([4]) is used as a FE post-processing step. [less ▲]

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See detailRelative Influence of Aluminium and Silicon on the Kinetics of Bainite Formation from Intercritical Austenite
Mertens, Anne ULg; Jacques, Pascal J.; Sietsma, Jilt et al

in Steel Research International (2008), 79(12), 954-959

Bainite formation from intercritical austenite is of great practical importance for the production of TRIP-assisted steels. Silicon and aluminium play important roles during this transformation by ... [more ▼]

Bainite formation from intercritical austenite is of great practical importance for the production of TRIP-assisted steels. Silicon and aluminium play important roles during this transformation by delaying carbide precipitation, thus favouring the carbon enrichment of untransformed austenite, which makes its stabilisation down to room temperature possible. Previous studies have shown a strong dependence of bainite formation kinetics on both chemical composition and transformation temperature. In the present work, the effect of silicon and aluminium contents on bainite formation kinetics is investigated experimentally using dilatometry combined with microscopical observations. The experimental results are analysed by comparison with thermodynamic parameters, such as the activation energy G* for nucleation of bainite and the carbon content CT0 corresponding to the T0-curve. It is shown that the faster transformation kinetics induced by the substitution of silicon by aluminium can be ascribed (i) to a higher driving force for nucleation, (ii) to a higher carbon content CT0 at the T0-curve and (iii) to the precipitation of carbide in austenite in steels with a low Al content. [less ▲]

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See detailAnalytical formulation for actual mechanical properties of stainless steel structural members
Rossi, Barbara ULg; Degée, Hervé ULg; Habraken, Anne ULg

in Steel Research International (2008), 79(2), 265-272

One of the fastest growing market segments for stainless steel is its use for cold-formed profiled structural members. It is wellknown that the forming process is likely to significantly enhance the ... [more ▼]

One of the fastest growing market segments for stainless steel is its use for cold-formed profiled structural members. It is wellknown that the forming process is likely to significantly enhance the mechanical properties of the profile by strainhardening. This leads to increased resistance compared to a resistance assessment based on nominal properties, even if this effect is partly counterbalanced by the introduction of residual stresses. It is thus necessary to be able to accurately determine its actual mechanical properties. For this purpose, experimental data and theoretical investigations aiming at determining the material behaviour of coldformed stainless steel profiles including the effects of the forming process are reported in this paper. [less ▲]

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See detailStudy of bendability of steel sheets
Ben Bettaieb, Mohamed ULg; Lemoine, Xavier; Duchene, Laurent ULg et al

in Steel Research International (2008), (79/1), 225-232

In the present contribution bending tests are modeled and the bendability of steel sheets is evaluated. Bendability refers to the ratio of the minimum bend radius to the initial sheet thickness at which ... [more ▼]

In the present contribution bending tests are modeled and the bendability of steel sheets is evaluated. Bendability refers to the ratio of the minimum bend radius to the initial sheet thickness at which the bending proc-ess is successfully accomplished [1]. The metallurgic microstructure of the studied sheet consists in two principal phases: a fully dense matrix (which may be itself composed by several metallurgic phases) and spherical voids. For that purpose, the Gurson Tvergaard Needleman law ([2], [3], [4]) is used and significantly modified. The behavior of the fully dense matrix is de-fined by the anisotropic Hill 48 function and the Swift hardening law. The width of the sheet is assumed to be large enough to neglect the transversal strains and the stress component in the thickness direction is also neglected. The bending operation can thus be modelled by a plane strain-plane stress loading. The influence of mechanical pa-rameters such as the initial porosity, the Lankford coefficient and the strain harden-ing exponent on the bendability is studied herein. The failure here is defined by the onset coalescence of neighbour voids and is checked by using Thomason [5], Pardoen [6] and Brunet [7] coalescence models. So the influence of other phenomena (like the shear band development and the localized necking) on the bendability is neglected here. [less ▲]

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