Mr. Jens Nellesen Profile

Jens Nellesen

at RIF eV

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Proceedings Article | 18 September 2008 Paper

Three-dimensional analysis of MMC microstructure and deformation by µCT and FE simulations

Horst-Artur Crostack, Jens Nellesen, Gottfried Fischer, Ulrich Weber, Siegfried Schmauder, Felix Beckmann

Proceedings Volume 7078, 70781I (2008) https://doi.org/10.1117/12.795206

KEYWORDS: Composites, Diamond, Particles, Metals, 3D modeling, 3D microstructuring, Failure analysis, Tomography, 3D image processing, Spatial resolution

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A better understanding of micro deformation and damage processes that the microstructure of particle reinforced metal matrix composites (MMC_p) undergoes at microscale before macroscopical failure gives the right direction for the microstructural design of these materials. To this end, a μCT-based analysis was performed that combines μCT-experiments and FE simulations: The gauge length of tiny tensile specimens (cross-section A = 2 x 1 mm²) consisting of the MMC_p systems Cobalt/Diamond and Al/B₄C was imaged by tomography at different stages of deformation. 3D strain tensor fields and displacement vector fields were determined by digital image correlation of the reconstructed tomograms. Based on tomograms of the analyzed volume at the undeformed state, FE meshes were generated that model the microstructure close to reality. Using these meshes and the displacement vector fields measured at the volume boundaries, FE simulations of the deformation and damage behavior were carried out. In both composites volume strains below 1% have been found experimentally. The spatial resolution of deformation fields is limited by the characteristic microstructural length which depends on the particle diameter and the particle spacing. The results of the experiments and the simulations are compared on the basis of 3D-strain fields sampled within the analyzed microstructural region. Additionally, the impact of microstructural features on the localization of strain, the initiation of localized damage and the successive failure of the composite materials is discussed.

Proceedings Article | 7 September 2006 Paper

Tomographic analysis and FE-simulations of MMC-microstructures under load

Horst-Artur Crostack, Jens Nellesen, Gottfried Fischer, Siegfried Schmauder, Ulrich Weber, Felix Beckmann

Proceedings Volume 6318, 63181A (2006) https://doi.org/10.1117/12.680493

KEYWORDS: Particles, Diamond, Tomography, Composites, Cobalt, Metals, X-rays, 3D modeling, 3D acquisition, Interfaces

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Microstructural changes like micro deformation and damaging due to tensile load precede the macroscopical failure of a component. In order to contribute to the understanding of such processes, the microstructure of tensile test specimens was imaged by microtomography in the course of deformation. The specimens consist of particle reinforced metal matrix composites (the MMCs Cobalt/Diamond and Al/TiN) manufactured on a powder metallurgical route. Tomograms of a volume in the gauge length of the specimens were reconstructed from the projection data acquired at different deformation stages. Both polychromatic radiation of a microfocus X-ray tube and monochromatic synchrotron radiation were used for projection data acquisition. With the help of 3D data processing 3D surface nets were extracted from the tomograms which indicate the particle/matrix interface. These nets which are composed of triangles were afterwards optimized with respect to the shape of the triangles. Using the triangles as seeds a 3D FE-mesh without gaps consisting of tetrahedra was generated. 3D FE-simulations were carried out utilizing both arbitrary and realistic boundary constraints. Realistic conditions were derived from an iterative matching procedure of tomograms. The effect of finite element type (tetrahedron or hexahedron) on the simulated distribution of stresses was analyzed. The appearance and development of plastic zones in the metal matrix depending on externally applied displacements were studied in the simulations. The calculated peak stresses are compared with the loci of cracks found in the tomograms.

Proceedings Article | 26 October 2004 Paper

Analysis of the material behavior of metal-matrix composites under tension by synchrotron radiation-based microtomography and FE calculations

Horst-Artur Crostack, Jens Nellesen, Heribert Blum, Thomas Rauscher, Felix Beckmann, Gottfried Fischer

Proceedings Volume 5535, (2004) https://doi.org/10.1117/12.559598

KEYWORDS: Particles, Composites, Tin, Metals, 3D microstructuring, Surface finishing, Tomography, 3D modeling, 3D image processing, Synchrotron radiation

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Micro-deformation and -damage processes running before the macroscopical failure of a component determine its macroscopically observable behaviour. Such processes in metal matrix composites (MMCs) can be imaged during tensile tests with high resolution microtomography utililising synchrotron radiation. To improve the understanding of the material's behaviour the microstructural changes in tensile experiments were studied by tomography and compared with FE computer simulations. Miniaturised tensile test specimens consisting of the particle reinforced MMC Al/10% TiN were manufactured on a powder metallurgic route. From a sub-volume in the gauge length of a specimen high resolution tomograms were created at different deformation stages deploying monochromatic synchrotron radiation supplied by the wiggler beamline BW2 in HASYLAB at DESY in Hamburg. After segmentation and binarization, wherein to each voxel of the 3D tomogram a phase property like e.g. surrounding air, particle or matrix is assigned, the FE-model of the area of interest was set up: Two and three dimensional micro-tomographical sections of interest were discretized using different element shapes to apply a non-linear finite-element method on the real microstructure. The ductile metal matrix was modelled using von Mises flow theory with isotropic hardening. Displacements computed by iterative matching of the tomograms of different deformation states were applied to the FE-model as boundary conditions. The FE-simulations show the appearance and development of plastic zones in the metal matrix as well as high stress concentrations on particles' surfaces, which are areas of crack initiation as the experiments reveal. In future work, criteria for micro-damaging like particle or matrix cracking and delamination can be derived from the comparison of the real with the computer experiments.

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