E Hoferlin

Finite element modeling of plastic anisotropy induced by texture and strain-path change

Abstract Consideration of plastic anisotropy is essential in accurate simulations of metal forming processes. In this study, finite element (FE) simulations have been performed to predict the plastic anisotropy of sheet metals using a texture- and microstructure-based constitutive model. The effect of crystallographic texture is incorporated through the use of an anisotropic plastic potential in strain-rate space, which gives the shape of the yield locus. The effect of dislocation is captured by use of a hardening model with four internal variables, which characterize the position and the size of the yield locus. Two applications are presented to evaluate the accuracy and the efficiency of the model: a cup drawing test and a two-stage pseudo-orthogonal sequential test (biaxial stretching in hydraulic bulging followed by uniaxial tension), using an interstitial-free steel sheet. The experimental results of earing behavior in the cup drawing test, maximum pressure and strain distribution in bulging, and transient hardening in the sequential test are compared against the FE predictions. It is shown that the current model is capable of predicting the plastic anisotropy induced by both the texture and the strain-path change. The relative significance of texture and strain-path change in the predictions is discussed.

Assessment of crystal plasticity based calculation of the lattice spin of polycrystalline metals for FE implementation

Abstract When texture is incorporated in the finite element simulation of a metal forming process, much computer time can be saved by replacing continuous texture and corresponding yield locus updates by intermittent updates after strain intervals of e.g. 20%. The hypothesis that the evolution of the anisotropic properties of a polycrystalline material during such finite interval of plastic deformation can be modelled by just rotating the initial texture instead of continuously updating it by means of a polycrystal deformation model is tested in this work. Two spins for rotating the frame have been assessed: the classical rigid body spin and a crystal plasticity based “Mandel spin” (calculated from the rotated initial texture) which is the average of the spins of all the crystal lattices of the polycrystal. Each of these methods was used to study the evolution of the yield locus and the r-value distribution during the 20% strain interval. The results were compared to those obtained by simulating the texture evolution continuously using a polycrystal deformation model. When the texture was not updated during deformation, it was found that for most initial textures the Mandel spin does not perform better than the rigid body spin, except for some special initial textures for which the Mandel spin is much better. The latter ones are textures which are almost stable for the corresponding strain mode. When the texture was updated after each strain interval of e.g. 20% the Mandel spin performed much better than the rigid body spin.

The design of a biaxial tensile test and its use for the validation of crystallographic yield loci

A biaxial tensile test has been designed for the experimental determination of yield locus points of thin steel sheets. Using texture-based anisotropic finite-element simulations, the geometry of the test sample has been optimized. A detailed accuracy analysis is presented and the range of accuracy of the new specimen is derived. Experimental tests have been carried out on a ultra-low carbon and a bake-hardening steel. Both the yield stresses and the ratios of plastic strains have been compared to the theoretical predictions obtained with the Taylor-Bishop-Hill model using the experimentally-determined crystallographic textures.

Application of a Texture‐Based Plastic Potential in Earing Prediction of an IF Steel

In the present study, an anisotropic yield locus represented by using a texture-based plastic potential in strain-rate space, together with an isotropic hardening law, has been applied in the FE simulations of cup drawing of an IF steel. Only a texture measurement and a tensile test are needed to obtain the material properties required fy a FE simulation. The experimentally observed effects of blankholder force abd frictions of the cup heights are well reproduced in the simulations. The predictions of the earing behavior as well as the average cup heights are also in good agreemebt with the experimental results

Biaxial tests on cruciform specimens for the validation of crystallographic yield loci

Abstract A biaxial tensile test has been designed for the experimental determination of yield locus points of thin steel sheets. Using texture-based anisotropic finite-element (FE) simulations, the geometry of the test sample has been optimised and the accuracy of the conversion procedure has been validated. It was found that the present technique has a high precision for principal stress ratios σ y / σ x between tan(20°) and tan(70°). Experimental tests have been carried out on five steel qualities (low and ultra-low carbon) with thickness between 0.8 and 1.5 mm. Both yield stresses and ratios of plastic strains have been compared to the theoretical predictions obtained with the Taylor–Bishop–Hill (TBH) model using the experimentally determined crystallographic textures. The {110}〈111〉 and {112}〈111〉 slip systems were considered using both full-constraints (FC) and relaxed-constraints (RC) assumptions for the TBH model. These comparisons are clearly in favour of the TBH-FC model.

Side-Bulging during Tensile Tests of IF-Steels with Cross-Thickness Texture Gradients

Sheet tensile specimens sometimes exhibit peculiar shape changes during testing, such as a striking out-of-plane bending or the less eye-catching bulging of the cross-thickness edges. By means of anisotropic finite-element simulations it is demonstrated that both deformation phenomena may be caused by the presence of cross-thickness texture gradients.