Department Seminar by Dr. Tuncay Yalçınkaya

Multiscale Modeling of Plastic Microstructure Evolution in Metals

Dr. Tuncay Yalcinkaya, Institute for Energy and Transport – European Commission

Date: Wednesday, October 31, 2012
Time: 13:40
Place: EA 202

Plastic deformation and its possible combination with other loadings (thermal, irradiation etc.) generate various types of dislocation microstructures, which result in a spatially heterogeneous deformation field. Different type of dislocation microstructures exist at different length scales in metallic materials. Typical examples at macro scale are Lüders bands, Portevin-Le Chatelier (PLC) bands, while dislocation cell structures, labyrinth, mosaic, fence or carpet structures develop at meso scale, which are mainly due to self-organization of dislocations. These microstructures macroscopically manifest themselves through softening and plastic anisotropy under strain path changes, which could lead to fracture and the ultimate failure of the material and eventually the whole structural component.

The plastic localization induces macroscopic softening-hardening and stress-plateau type of responses, arising numerical issues in the solution procedure. The application of standard finite element methods yields mesh-dependent post-critical results due to loss of ellipticity of the incremental boundary value problem. In order to remedy these problems during the formation of microstructures several models have been proposed including, methods using calculus of variations in an incremental setting, non-local methods, viscous regularization techniques and Cosserat theories. A complete understanding of models which can simulate the patterning of dislocation slip or formation of dislocation substructures is not at hand. In order to contribute to this, inspired by the success of phase field models, an approach is proposed to illustrate the ability of non-convex field models to predict the emergence and evolution of dislocation slip microstructures in a rate dependent strain gradient crystal plasticity framework

The framework studies the plastic slip patterning (localization) in a system with energetic hardening. Both the displacement and the plastic slip are considered as primary variables. These fields are determined on a global level by solving simultaneously the linear momentum balance and slip evolution equation which is derived from thermodynamical considerations. The slip law used in this context differs from the classical ones in the sense that it includes the non-convex free energy term leading to the patterning of this field. The non-convexity is treated as an intrinsicproperty of the free energy of the material. The numerical examples illustrate the microstructure evolution due to different types of non-convex contributions in a multi-slip 2D plane strain analysis.

Short bio:

Tuncay Yalcinkaya was born on January 31, 1980 in Ankara, Turkey. He studied Aerospace Engineering at Middle East Technical University in Ankara and got his B.Sc. degree with high honor in 2003. Directly thereafter, he continued his studies with a full DAAD scholarship at the University of Stuttgart and got his M.Sc. degree on computational mechanics of materials and structures (COMMAS) in 2005. After his graduation he was employed by the Materials Innovation Institute (M2i) in Delft and has worked on his PhD thesis at the Mechanical Engineering Department of the Eindhoven University of Technology under the supervision of M.G.D. Geers. He defended his PhD thesis with the title: Microstructure evolution in crystal plasticity: strain path effects and dislocation slip patterning in October 2011. He is currently a post-doctoral researcher at the Institute for Energy and Transport – Joint Research Centre (JRC) of European Commission.

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