Civil Engineering / İnşaat Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/13
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Article Citation - WoS: 1Citation - Scopus: 1A Magnetically Driven Elastic Rod Type Bi-Directional Swimmer at Stokes Flow(Springer, 2022) Özdemir, İzzetIn this paper, a flexible rod type micro-swimmer is proposed which achieves swimming direction reversal on the fly by forming a chiral helix-like geometry through external magnetic excitation. Furthermore an accompanying low Reynolds number flow-structure interaction analysis framework is developed which effectively combines a geometrically non-linear shear deformable beam model with regularized Stokeslet method in a monolithic implicit solution algorithm. This framework is used to investigate the basic characteristics of the proposed micro-swimmer in terms of dimensionless groups reflecting the interplay between different forces involved.Article Citation - WoS: 22Citation - Scopus: 27Micromechanical Modeling of Intrinsic and Specimen Size Effects in Microforming(Springer Verlag, 2018) Yalçınkaya, Tuncay; Özdemir, İzzet; Simonovski, IgorSize effect is a crucial phenomenon in the microforming processes of metallic alloys involving only limited amount of grains. At this scale intrinsic size effect arises due to the size of the grains and the specimen/statistical size effect occurs due to the number of grains where the properties of individual grains become decisive on the mechanical behavior of the material. This paper deals with the micromechanical modeling of the size dependent plastic response of polycrystalline metallic materials at micron scale through a strain gradient crystal plasticity framework. The model is implemented into a Finite Element software as a coupled implicit user element subroutine where the plastic slip and displacement fields are taken as global variables. Uniaxial tensile tests are conducted for microstructures having different number of grains with random orientations in plane strain setting. The influence of the grain size and number on both local and macroscopic behavior of the material is investigated. The attention is focussed on the effect of the grain boundary conditions, deformation rate and the grain size on the mechanical behavior of micron sized specimens. The model is intrinsically capable of capturing both experimentally observed phenomena thanks to the incorporated internal length scale and the crystallographic orientation definition of each grain.Article Citation - WoS: 1Citation - Scopus: 1Resistive Force Theory-Based Analysis of Magnetically Driven Slender Flexible Micro-Swimmers(Springer Verlag, 2017) Özdemir, İzzetResistive force theory is concise and reliable approach to resolve flow-induced viscous forces on submerged bodies at low Reynolds number flows. In this paper, the theory is adapted for very thin shell-type structures, and a solution procedure within a nonlinear finite element framework is presented. Flow velocity proportional drag forces are treated as configuration-dependent external forces and embedded in a commercial finite element solver (ABAQUS) through user element subroutine. Furthermore, incorporation of magnetic forces induced by external fields on magnetic subdomains of such thin-walled structures is addressed using a similar perspective without resolving the magnetic field explicitly. The treatment of viscous drag forces and the magnetic body couples is done within the same user element formalism. The formulation and the implementation are verified and demonstrated by representative examples including the bidirectional swimming of thin strips with magnetic ends.Article Citation - WoS: 4Citation - Scopus: 4A Numerical Solution Framework for Simultaneous Peeling of Thin Elastic Strips From a Rigid Substrate(Springer Verlag, 2017) Özdemir, İzzetSimultaneous peeling of multiple strips is commonly observed particularly at small-scale detachment processes. Although theoretical treatment of this problem is addressed, numerical solution procedures for geometrically arbitrary multiple-peeling problems are still missing. In this paper, a finite element-based numerical solution procedure for 3-D large displacement multiple-peeling problems is presented. Loading/unloading of peeling strips are expressed in the form of optimality conditions, and the current positions of the peeling fronts are determined locally adapting the multiplicative decomposition and the return mapping algorithm of finite strain plasticity theories. Within an incremental-iterative solution framework, peeling fronts and the current position of other nodes are determined in a staggered way instead of using an active set-based solution algorithm. The effectiveness of the approach is demonstrated by a series of example problems including multiple peeling of an assembly of randomly oriented strips.Article Citation - WoS: 4Citation - Scopus: 4Predicting and Measuring Surface Enlargement in Forward Rod Extrusion(The American Society of Mechanical Engineers(ASME), 2016) Duran, Deniz; Özdemir, İzzetSurface enlargement during bulk metal forming processes is one of the key parameters controlling the tribology at the tool-workpiece interface. Not only the surface roughness evolution but also the integrity of the lubricant layer critically reposes on surface enlargement. As an attempt to address this issue, in the first part of this work, a general, deformation gradient based surface enlargement description is implemented in a commercial finite element program. In the second part, forward rod extrusion tests with different area reductions are conducted using customized steel workpieces in which cylindrical copper rods are embedded through the depth. By sectioning the extruded parts and by identifying the position of the copper rods on the lateral surface, average surface enlargement values could be measured locally at different positions along the extrudate. Comparison of experiments and numerical predictions reveal that the deformation gradient based description performs reasonably well in capturing surface enlargement profiles both qualitatively and quantitatively.