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Yasmin Ahmed Salem
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Scientific Events

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Composite voxels for nonlinear mechanical problems

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Composite voxels fornonlinear mechanical problems

Two-scale simulations of components classically  rely upon finite element simulations  on boundary- and interface-fitted  meshes on both the macro and the micro scale. For complex microstructures fast and memory-efficient  solvers posed on regular voxels grids, in particular the FFT-based homogenization method [1], provide a powerful  alternative to FE simulations on unstructured  meshes and can be used to replace the micro-solver [2, 3]. Since representative volume elements of the microstructure  consist of up to 80003  voxels, even this micro-solver reaches its limits for nonlinear elastic computations.This talk focuses on the composite voxel technique [4], where sub-voxels  are merged into bigger voxels to which an effective material law based on laminates is assigned. Due to the down-sampled grid, both the memory requirements and the computational effort are severely reduced. We discuss the extensions of linear elastic ideas [4, 5] to the physically non-linear setting  and assess the accuracy  of reconstructed solution fields by comparing them to direct full-resolution computations.References[1] H. Moulinec and P. Suquet. A numerical method for computing the overall response of nonlinear composites with complex microstructure.Computer Methods in Applied Mechanics and Engineering, 157(1-2):69–94, 1998. [2] J. Spahn, H. Andra, M. Kabel, and R. Mueller.A multiscale approach for modeling pro- gressive damage of composite materials using fast Fourier transforms. Computer Methods in Applied Mechanics and Engineering, 268(0):871 – 883, 2014. [3] J. Kochmann,  S. Wulfinghoff, S. Reese, J. R. Mianroodi,  and B. Svendsen.  Two-scale FEFFT- and phase-field-based computational modeling of bulk microstructural evolution and macroscopic material behavior. Computer Methods in Applied Mechanics and Engi- neering, 305:89 – 110, 2016. [4] M. Kabel, D. Merkert, and M. Schneider. Use of composite voxels in FFT-based homog- enization. Computer Methods in Applied Mechanics and Engineering, 294(0):168–188,2015. [5] L. Gelebart and F. Ouaki. Filtering Material Properties to Improve FFT-based Methodsfor Numerical Homogenization.  J. Comput. Phys., 294(C):90–95, 2015. [more]

In situ HR-EBSD characterization during micro-mechanical testing

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In situ HR-EBSD characterization during micro-mechanical testing

Quantification of the mechanical properties of crystalline materials at micro and nano length-scales is as important as it is challenging. In situ mechanical testing inside the SEM using a micro-indenter offers the great advantage of a direct observation of the progressive deformation in materials and has been applied for many years to assess the deformation mechanisms at small scale. This is now reinforced by the capability of doing in situ EBSD and HR-EBSD in order to map the evolution of the microstructure, the stress field and the GNDs distribution in the materials at several steps during progressive deformation. We apply this technique to estimate the size of the plastic zone underneath the crack tip during micro-cantilever bending in tungsten and NiAl intermetallic for fracture toughness determination. HR-EBSD is used to map the evolution of the stress field around the notch tip and to estimate the GNDs in the plastically deformed zone. In situ EBSD has been also applied to micropillar compression in alpha Titanium in order to study the formation and evolution of compressive twins during the deformation. The results show dislocation driven twin formation and twin propagation and thickening according to the local resolve shear stress. [more]

Deformation mechanisms of TWIP steel: from micro-pillars to bulk samples

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Deformationmechanisms of TWIP steel: from micro-pillars to bulk samples

  • Date: Jul 6, 2016
  • Time: 13:30 - 15:00
  • Speaker: Prof. Mingxin HUANG
  • Department of Mechanical Engineering, The University of Hong Kong, Hong KongShort biography of the speaker Dr. Mingxin HUANG is currently an Associate Professor at The University of Hong Kong, Hong Kong. His research interests focus on two areas: (1) fundamentals of microstructure-property relationship and phase transformation of advanced steels, and (2) development of lightweight materials for automotive applications. Both experimental and modelling works are involved in his research. His research projects have been well funded by government funding agents as well as industries from Europe and China (e.g. ArcelorMittal France, General Motors, Ansteel, Baosteel). Dr. Huang received his Bachelor as well as Master degrees from Shanghai Jiao Tong University (SJTU) in 2002 and 2004, respectively, and his PhD in 2008 from Delft University of Technology (TU Delft), The Netherlands. From 2008 to 2010, he worked as a research engineer at ArcelorMittal R&D centre in Maizieres-les-Metz, France. His research work in ArcelorMittal focused on the development of new advanced steels for automotive applications. Dr. Huang joined University of Hong Kong in 2010 as an Assistant Professor and was promoted to Associate Professor with tenure in 2016. Dr. Huang has published 50+ journal papers on major international journals in his field such as Acta Materialia and Scripta Materialia. Dr. Huang is an editorial board member of Materials Science and Technology, the Key Reader for Metallurgical and Materials Transactions A and has received twice “Outstanding Reviewer of Scripta Materialia” awards.
  • Location: Max-Planck-Institut für Eisenforschung GmbH
  • Room: Seminarraum 1
  • Host: Prof. Dierk Raabe

Twinning-induced plasticity (TWIP) steels have excellent combinationof strength and ductility and are potential lightweight materials forautomotive applications. Understanding the deformation mechanisms in TWIPsteels is essential for the successful application of TWIP steels. The firstpart of this work is to employ micron-sized single crystalline pillars toinvestigate the nucleation and growth mechanism of deformation twins. It isfound that the nucleation and growth of deformation twins are due to emissionand glide of successive partial dislocations. A physical model is proposed tosimulate the nucleation and growth of deformation twins. The second part of thepresentation discusses the deformationmechanism of bulk samples. Deformation mechanism of bulk samples at high strainrates will be discussed firstly. By synchrotron X-raydiffraction experiments, the present work demonstrates that a higher strainrate leads to a lower dislocation density and a lower twinning probability,which is opposite to other fcc metals. Furthermore, it has been demonstratedthat the contribution of twins to the flow stress is very limited. Instead,dislocations strengthening via forest hardening accounts for up to 90% of theflow stress. In other words, the contribution of twins to flow stress of TWIPsteels may have been overestimated in the existing literature. Finally, thepresent talk will discuss a nanotwinned steelwhich is manufactured by a simple thermomechanical treatment consisting of coldrolling and recovery annealing and possesses a high yield strength (1450 MPa)and considerable uniform tensile elongation (20%).   [more]

 
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