Papers by Author: Frédéric Barlat

Abstract: A succinct description of advanced constitutive models for applications to forming process simulations is provided. These models are continuum-based because they are more efficient in terms of computation time than microstructure–based models. However, they are so–called advanced because they are considered in many scientific studies but rather scarcely used in industrial applications. In addition, the relationship between these continuum constitutive models and multi-scale approaches based on crystal plasticity, dislocation dynamics and mechanics of multi-phase materials, such as advanced high strength steels, is substantiated.

Abstract: The effects of the stress state and temperature on the martensitic phase transformation behavior in TRIP 780 steel were investigated using multi-axial experimental techniques. Various mechanical experiments are performed to differentiate the stress state and temperature effects. For this purpose, five different stress states were considered; i.e., uniaxial tension, uniaxial compression, equibiaxial tension, plane strain tension and simple shear. A temperatures both 25 and 60 °C for each stress state condition except the simple shear test were investigated. In-situ magnetic measurements were performed to mesure the evolution of the martensite content throughout each experiment. Finally, a new martensitic transformation kinetics model for the TRIP 780 steel is proposed to take the effect of stress state and temperature into account.

Abstract: In order to predict the shape change in hot press forming of a TWB made with HPF1470 boron steel and HSLA340 micro-alloyed steel sheets, a coupled thermo-mechanical-metallurgical finite element model was developed to simulate the process. The simulation consisted of air cooling, forming, die-quenching and, finally, by a treatment designed to relax residual stresses. It is shown in this paper that the experimentally observed distortion in the TWB part was reasonably captured by the simulations.

Abstract: Experimental and numerical investigations of the ridging in ferritic stainless steels were presented in this paper. Two kinds of ferritic stainless steels exhibiting different levels of ridging were selected as model materials. The measured roughness of the uniaxially elongated specimens up to 15% in rolling direction (RD) was compared to the prediction using a rate-dependent crystal plasticity FEM (CPFEM). Initial textures of the two materials on 5 equi-spaced sequential RD planes were obtained by EBSD measurement. The initial textures were utilized as input data for the constitutive parameters of the crystal plasticity. Measured respective single planar textures were collected all together so that the 5-layer textures complete 3-dimensional structure and they were mapped onto the FE mesh. Ridging profiles predicted by the CPFEM using both every single layer texture and multilayer texture were compared to the experimental results. Predicted ridging profile of a material exhibiting weak ridging by using 5-layer EBSD mapping was in good agreement with the experimental result. On the other hand, prediction by using only single layer texture was efficient to estimate the ridging in a material exhibiting severe ridging due to the elongated cluster of analogous orientations along RD.

Abstract: In the present work the disc compression test used to determine the balanced biaxial strain-ratio $r_b$ is analyzed in terms of the influence of contact friction using non-linear finite element analysis (FEA). The FEA results reveal an unexpectedly strong sensitivity of the $r_b$-value on contact friction, which is discussed in detail. The most important outcome of the present work is that the FEA can reproduce the $r_b$-value imposed by the utilized yield function very well, but only when the prescribed Coulomb friction coefficient has a very small value; for increasing friction coefficients a gradual deviation from the imposed $r_b$-value can be observed. This finding implies that in experiments contact friction must be eliminated to a larger extent than commonly expected, otherwise the determined $r_b$-value using disc compression testing will considerably deviate from the actual one, particularly when $r_b$ is far from one.

Abstract: Experimental and numerical investigations using Forming Limit Curve (FLC) and Forming Limit Stress Curve (FLSC) were carried out for two Advanced High Strength Steel (AHSS) grades DP780 and TRIP780. The forming limit curves were experimentally determined by means of Nakazima stretching test. Then, both FLC and FLSC were analytically calculated on the basis of the Marciniack-Kuczinsky (M-K) model. The yield criteria Barlat2000 (Yld2000-2d) were employed in combination with the Swift and modified Voce strain hardening laws to describe plastic flow behavior of the AHS steels. Hereby, influence of the constitutive models on the numerically determined FLCs and FLSCs were examined. Obviously, the forming limit curves predicted by the M-K model applying the Yld2000-2d yield criterion and Swift hardening law could fairly represent the experimental limit curves. The FLSCs resulted from the experimental data and theoretical model were also compared.

Abstract: Improved formability has been reported due to stress relaxation when the continuous forming cycle is interrupted with steps by adjusting the punch motion. The contribution of stress relaxation and its parameters on the ductility of materials has not been established so far. In the present work, the stress relaxation behavior of three materials, low carbon steel, DP and TRIP steels are studied. The influence of strain rate and strain on the ductility enhancement due to stress relaxation is analyzed. It is observed that stress relaxation improved the ductility of materials in all the cases and therefore can be used as a potential method to improve formability in sheet metal forming.

Abstract: The present paper aims at a theoretical study of the forming limits of a sheet metal subjected to double strain path changes by using as reference material the AA6016-T4 aluminum alloy sheet. The simulation of plastic instability is carried out through the Marciniak-Kuczynski analysis. The initial shape of the yield locus is given by the Yld2000-2d plane stress yield function. The strain hardening of the material is described by the Voce type saturation law. Linear and several complex strain paths involving single and double strain path changes are taken into account. The validity of the model is assessed by comparing the predicted and experimental forming limits under linear and selected one strain path change. A good accuracy of the developed software on predicting the forming limits is found. A sensitive analysis of the influence of the type and value of the double prestain in the occurrence of the plastic flow localization is performed. A remarkable effect of the double strain path change on the sheet metal forming limits is observed.

Abstract: In this work we propose a new model (SOFTMAR) to describe the transient softening observed during severe plastic deformation of Al-30wt%Zn alloys. The model is divided in two main parts. The first one describes the softening process based on the evolution law of the mean free path of dislocations (L) with plastic strain . In the second part of the model emphazis is given to the relationship between the grain size and strain rate that in turn depends on diffusion-driven grain-boundary sliding.

Abstract: Two non-quadratic orthotropic yield functions called Yld2011-18p (containing 18 param-eters) and Yld2011-27p (containing 27 parameters) are proposed. The formulations are based on theestablished concept of linear transformations operating on the stress deviator. Application examplesreveal the capabilities of both yield functions to accurately describe complex plastic anisotropy ofsheet metals.