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Item type:Publication, Access status: without files , Modeling twinning-induced lattice reorientation and slip-in-twin deformation(2019-01); The deformation behavior of hexagonal close packed (hcp) materials involves dislocation slip as well as deformation twinning, including a competition between those two. This is due to the limited number of easy to activate slip modes. We present a model that captures dislocation slip, dislocation interaction, size-dependent hardening and deformation twinning. It also accounts for the sudden change of the crystal lattice orientation at the very final stage of the twinning shear process which allows to describe dislocation glide deformation within a twinned region. Along with this, the model further considers the change in the elastic properties associated with the new lattice orientation. Finite element results are presented for the specific example of magnesium (Mg) and for different characteristic loading conditions in order to mimic local stress distributions as they occur in different regions within a single crystal or grain, e.g., close to a boundary and far off. In addition, the impact of twinning on the mechanical response of Mg single crystal during microcompression is investigated. We show that the suggested model is well capable of predicting the complex microstructural deformation behavior in Mg and compare our numerical results to experimental data.Publicationtype: Journal Article254 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Modeling of surface effects in crystalline materials within the framework of gradient crystal plasticityA finite-deformation gradient crystal plasticity theory is developed, which takes into ac- count the interaction between dislocations and surfaces. The model captures both ener- getic and dissipative effects for surfaces penetrable by dislocations. By taking advantage of the principle of virtual power, the surface microscopic boundary equations are obtained naturally. Surface equations govern surface yielding and hardening. A thin film under shear deformation serves as a benchmark problem for validation of the proposed model. It is found that both energetic and dissipative surface effects significantly affect the plastic be- havior.Publicationtype: Journal ArticleScopus© Citations 10 338 524 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Effect of surface elasticity on the elastic response of nanoporous gold(American Society of Civil Engineers (ASCE), 2017-06-29); ; In this work, the effect of surface elasticity on the effective elastic properties of nanoporous gold is studied. To this end, a theoretical framework for surface elasticity effects in submicron-sized solids is implemented as a user-defined finite element subroutine. This allows the use of the theory in large-scale engineering problems. The theory suggests a zero-thickness surface accommodating unique energetic properties and surface tension. For the example of ball-and-stick diamond cubic unit cell structures for nanoporous gold, it is shown that incorporation of surface excess elasticity and surface tension allows prediction of the size effect associated with the change of the surface area-to-volume ratio by capturing, e.g., the increase in the effective Young’s modulus and decrease in the effective Poisson’s ratio with decreasing ligament diameter, a phenomenon that is not accessible to classical continuum elasticity approaches.Publicationtype: Journal ArticleScopus© Citations 10 295 276 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The role of geometrically necessary dislocations in cantilever beam bending experiments of single crystals(2017-03-16); The mechanical behavior of single crystalline, micro-sized copper is investigated in the context of cantilever beam bending experiments. Particular focus is on the role of geometrically necessary dislocations (GNDs) during bending-dominated load conditions and their impact on the characteristic bending size effect. Three different sample sizes are considered in this work with main variation in thickness. A gradient extended crystal plasticity model is presented and applied in a three-dimensional finite-element (FE) framework considering slip system-based edge and screw components of the dislocation density vector. The underlying mathematical model contains non-standard evolution equations for GNDs, crystal-specific interaction relations, and higher-order boundary conditions. Moreover, two element formulations are examined and compared with respect to size-independent as well as size-dependent bending behavior. The first formulation is based on a linear interpolation of the displacement and the GND density field together with a full integration scheme whereas the second is based on a mixed interpolation scheme. While the GND density fields are treated equivalently, the displacement field is interpolated quadratically in combination with a reduced integration scheme. Computational results indicate that GND storage in small cantilever beams strongly influences the evolution of statistically stored dislocations (SSDs) and, hence, the distribution of the total dislocation density. As a particular example, the mechanical bending behavior in the case of a physically motivated limitation of GND storage is studied. The resulting impact on the mechanical bending response as well as on the predicted size effect is analyzed. Obtained results are discussed and related to experimental findings from the literature.Publicationtype: Journal ArticleScopus© Citations 19 505 685 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Size affected dislocation activity in crystals : advanced surface and grain boundary conditionsExtended crystal plasticity theories are well established to study size-dependent hardening of metals. Surface and inner grain boundary conditions play a significant role for crystals at small scales as they affect the dislocation activity and, hence, alter strength and strain hardening behavior. Conventional micro boundary conditions, i.e., microhard and microfree, are unable to capture the underlying physics as they describe ideal and over-simplified surface/interface conditions. In this work, advanced boundary conditions for gradient extended crystal plasticity are introduced to map realistic conditions at external surfaces, interphases, or grain boundaries. They relate the magnitude of plastic slip to surface defect density and slip directions with respect to the surface normal. Characteristic features are highlighted, including the effect of surface yielding and size dependent surface strengthening.Publicationtype: Journal ArticleScopus© Citations 19 395 434 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Materials based design of structures: computational modeling of the mechanical behavior of gold-polymer nanocompositesThis is an open access article under the CCBY-NC-ND license. The impact of small scale geometric confinement on deformation mechanisms is subject of intense research in materials sciences nowadays. Nanoporous metals have a microstructure with an extremely high volume-specific surface content. Due to a very high local strength and a relatively regular interconnection of the nanoconstituents as well as a low mass density, nanoporous metals are very good candidates for strong and light-weight structural materials. The modeling of a modern nanocomposite material of gold-polymer is in the focus of the contribution. A gradient extended crystal plasticity theory is applied to the computation of the mechanical response of the metal part of the composite and an elastic-viscoplastic continuum model is used for the simulation of the polymer material. The gradient hardening contribution is included into the crystal plasticity model in order to study the influence of the ligament size. Numerical results of the deformation of the gold-polymer nanocomposite under compression are presented. Simulation results are compared to the corresponding experimental data.Publicationtype: Journal ArticleScopus© Citations 15 233 332 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Determination of average dislocation densities in metals by analysis of digitally processed transmission-electron microscopy images [Bestimmung von durchschnittlichen Versetzungsdichten in Metallen durch Analyse digital bearbeiteter Transmissionselektronenmikroskopie‐Aufnahmen]This paper describes an effective and simple procedure to derive information about the dislocation density distribution in metals by applying standard techniques of digital image processing on gray scaled microstructure images obtained from transmission-electron microscopy. In a representative transmission-electron microscopy image, two local dislocation density values were investigated by classical methods and were used as input parameter for further processing. A correlation between dislocations and image intensities is assumed such that dark areas in microstructural images are seen as a dense concentration of dislocations. Then, the contrast is increased for each transmission-electron microscopy photography. In the next step, posterization, a gradation of tone, is applied to these images. From this, a pixel weighted average distribution of gray level correlated dislocation densities is obtained as well as an average value for a given set of images. The implementation of the several processing steps is done in Matlab employing graphical user interfaces.Publicationtype: Journal Article51