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18 results on '"Jöchen, Katja"'

3. Homogenization of the Linear and Non-linear Mechanical Behavior of Polycrystals

Author

Jöchen, Katja and Böhlke, T.

Subjects
polycrystalline materials, homogenization, inelastic properties, ddc:500, mean-field approach, NATURAL sciences & mathematics
Abstract

This work is dedicated to the numerically efficient simulation of the material response of polycrystalline aggregates. Therefore, crystal plasticity is combined with a new non-linear homogenization scheme, which is based on piecewise constant stress polarizations with respect to a homogeneous reference medium and corresponds to a generalization of the Hashin-Shtrikman scheme. This mean field approach accounts for the one- and two-point statistics of the microstructure.

Published
2013

16. Micromechanical Modeling of Metal Forming Operations.

Author

Van, Tung Phan, Jöchen, Katja, and Böhlke, Thomas

Subjects
*METAL formability, *FERRITIC steel, *MICROMECHANICS, *MATERIALS compression testing, *PLASTIC crystals, *ELECTRON backscattering, *FINITE element method, *DEEP drawing (Metalwork)
Abstract

In this work, a ferritic stainless steel (DC04) is investigated in the following three steps. First, we use micropillar compression test data for the identification of a large strain single crystal plasticity model. In the second step the model is verified based on Electron Backscatter Diffraction (EBSD) measurements in a small specimen subjected to a large strain uniaxial tensile test. The two-dimensional EBSD data have been discretized by finite elements and subjected to homogeneous displacement boundary conditions for the second step. Finally, we apply a two-scale Taylor type model at the integration points of the finite elements to simulate the deep drawing process based on initial texture data. The texture data required for the specification of the two-scale model is determined based on the aforementioned EBSD data and by using a texture component method simultaneously to improve the computation time. The finite element simulations were performed with differently textured sheet metals and compared with experiment. [ABSTRACT FROM AUTHOR]

Published
2011
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17. Qualitative study on texture evolution in rolled sheet metals using homogenization methods.

Author

Jöchen, Katja and Böhlke, Thomas

Subjects
*SHEET metal, *METALS, *CRYSTAL texture, *ROLLING (Metalwork), *QUALITATIVE research, *ASYMPTOTIC homogenization, *MAXIMA & minima, *CRYSTALLIZATION
Abstract

The estimation of the texture in a sheet metal induced by rolling is an important issue for the accurate description of forming operations as, e.g., deep drawing. This work deals with comparing the prediction of the development of rolling textures in aluminum sheets based on different homogenization schemes. The crystallite orientation distribution function (codf) is evaluated by a class of homogenization methods based on a so-called comparison material and is compared to the widely used Taylor-type prediction. It is demonstrated that using the model based on the comparison material, the particular choice of the latter strongly influences the intensity distribution of the codf and also the location of the obtained β-fiber. The proposed homogenization method gives much better results for the reproduction of the codf than the Taylor-type model. When qualitatively comparing the computational results to experimental data, the location of the maxima in the codf generated by the rolling process are satisfactorily reproduced. [ABSTRACT FROM AUTHOR]

Published
2011
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18. Representative reduction of crystallographic orientation data.

Author

Jöchen, Katja and Böhlke, Thomas

Subjects
*CRYSTALLOGRAPHY, *ELECTRON backscattering, *CRYSTAL grain boundaries, *TESSELLATIONS (Mathematics), *CLUSTER theory (Nuclear physics), *MICROMECHANICS
Abstract

Experimental techniques [ e.g. electron backscatter diffraction (EBSD)] yield detailed crystallographic information on the grain scale. In both two- and three-dimensional applications of EBSD, large data sets in the range of 105-109 single-crystal orientations are obtained. With regard to the precise but efficient micromechanical computation of the polycrystalline material response, small representative sets of crystallographic orientation data are required. This paper describes two methods to systematically reduce experimentally measured orientation data. Inspired by the work of Gao, Przybyla & Adams [ Metall. Mater. Trans. A (2006), 37, 2379-2387], who used a tessellation of the orientation space in order to compute correlation functions, one method in this work uses a similar procedure to partition the orientation space into boxes, but with the aim of extracting the mean orientation of the data points of each box. The second method to reduce crystallographic texture data is based on a clustering technique. It is shown that, in terms of representativity of the reduced data, both methods deliver equally good results. While the clustering technique is computationally more costly, it works particularly well when the measured data set shows pronounced clusters in the orientation space. The quality of the results and the performance of the tessellation method are independent of the examined data set. [ABSTRACT FROM AUTHOR]

Published
2013
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