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Item type:Publication, Access status: with files , Generation of 3D representative volume elements for heterogeneous materials: a review(Elsevier, 2018); ; ; ; This work reviews state of the art representative volume element (RVE) generation techniques for heterogeneous materials. To this end, we present a systematic classification considering a wide range of heterogeneous materials of engineering interest. Here, we divide heterogeneous solids into porous and non-porous media, with 0 < void volume fraction < 1 and void volume fraction = 0, respectively. Further subdivisions are realized based on various morphological features. The corresponding generation methods are classified into three categories: (i) experimental methods targeting reconstruction through experimental characterization of the microstructure, (ii) physics based methods targeting simulation of the physical process(es) responsible for the microstructure formation and evolution, and (iii) geometrical methods concentrating solely on mimicking the morphology (ignoring the physical basis of the microstructure formation process). These comprise of various mathematical tools such as digital image correlation, tessellation, random field generation, and differential equation solvers. For completeness, relevant up-to-date software tools, used at various stages of RVE generation – either commercial or open-source – are summarized. Considered methods are reviewed based on their efficiency and predictive performance with respect to geometrical and topological properties of the microstructures.Publicationtype: Journal ArticleScopus© Citations 337 861 3101 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Fully periodic RVEs for technological relevant composites : not worth the effort!The setup of a finite element model for homogenization featuring a fully periodic geometry and a fully periodic mesh topology in combination with a high quality discretization is a cumbersome task and might significantly reduce the overall efficiency in multiscale finite element simulations. In this work, we examine multiple methodologies of setting up finite element models for homogenization purposes that extenuate these difficulties. Approaches resulting in periodic and nonperiodic representative volume element topologies in the microstructural generation process are introduced. Furthermore, we review and analyze various types of boundary conditions that either enforce periodicity or do not require periodicity of the underlying discretization. Approximate periodic boundary conditions are discussed in detail. The benchmark study proves that a fully periodic topology and mesh discretization with periodic boundary conditions is not necessary in order to identify effective macroscopic material parameters for technologically relevant composites.Publicationtype: Journal Article109 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Fourth-order strain-gradient phase mixture model for nanocrystalline fcc materialsThe proposed modeling approach for nanocrystalline materials is an extension of the local phase mixture model introduced by Kim et al (2000 Acta Mater. 48 493–504). Local models cannot account for any non-uniformities or strain patterns, i.e. such models describe the behavior correctly only as long as it is homogeneous. In order to capture heterogeneities, the phase mixture model is augmented with gradient terms of higher order, namely second and fourth order. Different deformation mechanisms are assumed to operate in grain interior and grain boundaries concurrently. The deformation mechanism in grain boundaries is associated with diffusional mass transport along the boundaries, while in the grain interior dislocation glide as well as diffusion controlled mechanisms are considered. In particular, the mechanical response of nanostructured polycrystals is investigated. The model is capable of correctly predicting the transition of flow stress from Hall–Petch behavior in conventional grain size range to an inverse Hall–Petch relation in the nanocrystalline grain size range. The consideration of second- and fourth-order strain gradients allows non-uniformities within the strain field to represent strain patterns in combination with a regularization effect. Details of the numerical implementation are provided.Publicationtype: Journal ArticleScopus© Citations 5 345 509 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The effect of yield surface curvature change by cross hardening on forming limit diagrams of sheetsThe paper aims at clarification of the role of reduction in yield locus curvature on forming limit diagrams. To this end,a cross-hardening model showing a reduction of yield surface curvature is used which accounts for dynamic and latent hardening effects associated with dislocation motion during loading.The model's three-dimensional tensorial as well as reduced plane-stress vector formulations are given.The first quadrants of forming limit diagrams are numerically producedusing finite elementmodels of the Marciniak-Kuczyński test with spatially correlated random defect distribution as localization triggering mechanism. The effect of cross hardening is investigated in detail. It is demonstrated that for plane strain loading path there occurs no difference in localization predictions of the models with and without cross hardening whereas for biaxial strain paths a delayed localization is observed in the cross hardening model as compared to the one without cross hardening effects.This is in accordance with the relative bluntness of the yield surface at the points of load path change towards localization.These results are complemented by Nakazima test simulations where similar observations are made.Publicationtype: Journal ArticleScopus© Citations 8 238 374 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Automatic three-dimensional geometry and mesh generation of periodic representative volume elements for matrix-inclusion compositesThis paper introduces an efficient method to automatically generate and mesh a periodic three- dimensional microstructure for matrix-inclusion composites. Such models are of major importance in the field of computational micromechanics for homogenization purposes utilizing unit cell models. The main focus of this contribution is on the creation of cubic representative volume elements (RVEs) featuring a periodic geometry and a periodic mesh topology suitable for the application of periodic boundary condi- tions in the framework of finite element simulations. Our method systematically combines various mesh- ing tools in an extremely efficient and robust algorithm. The RVE generation itself follows a straightfor- ward random sequential absorption approach resulting in a randomized periodic microstructure. Special emphasis is placed on the discretization procedure to maintain a high quality mesh with as few elements as possible, thus, manageable for computer simulations applicable to high volume concentrations, high number of inclusions and complex inclusion geometries. Examples elucidate the ability of the proposed approach to efficiently generate large RVEs with a high number of anisotropic inclusions incorporating extreme aspect ratios but still maintaining a high quality mesh and a low number of elements.Publicationtype: Journal ArticleScopus© Citations 48 380 608 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A directional modification of the Levkovitch-Svendsen cross-hardening model based on the stress deviatorAbstract In the original Levkovitch-Svendsen cross-hardening model parallel and orthogonal projections required for the yield surface evolution with respective dynamic and latent hardening effects are associated with the unit plastic flow direction np=Ėp/|Ėp|. This work gives a detailed investigation regarding the consequences and proposes the use of the so-called radial direction ns=[S-X]/|S-X| instead where S=dev(σ). It is shown that for an initially plastically anisotropic material under load paths with proportional stresses the original model brings a continuous directional change in the plastic strains. Eventually, even if the dynamic hardening component is bypassed, the material model predicts additional strengthening in loading direction due to latent hardening. In this undesired response, the broken coaxiality of the stress deviator and plastic strain rate tensor with initial anisotropy is the cause. This entanglement of isotropic/kinematic hardening and latent hardening creates difficulties - especially in the parameter identification even for the simplest uniaxial loading. The introduced modification to the model remedies this undesired feature and, hence, makes it possible to isolate the hardening sources during parameter identification stage. The discussions are supported by analytically and numerically derived yield loci for various scenarios. Our analytical studies allow definition of critical material parameter limits for the latent hardening parameter sl in terms of the initial anisotropy and the constant stress deviator ratio.Publicationtype: Journal Article29 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , The plastic yield and flow behavior in metallic glasses(American Institute of Physics (AIP), 2015-02-04); ; ; Metallic glasses have vast potential applications as components in microelectronics- and nanoelectronics-type devices. The design of such components through computer simulations requires the input of a faithful set of continuum-based constitutive equations. However, one long-standing controversial issue in modeling the plastic behavior of metallic glasses at the continuum level is the use of the most appropriate plastic yield criterion and flow rule. Guided by a series of molecular dynamics simulations conducted at low-homologous temperatures under homogeneous deformations, we quantitatively prove that the continuum plastic behavior in metallic glasses is most accurately described by a von Mises-type plastic yield criterion and flow rule.Publicationtype: Journal ArticleScopus© Citations 6 258 273 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Characterization of the microstructure evolution in IF-Steel and AA6016 during plane-strain tension and Simple Shear(Multidisciplinary Digital Publishing Institute (MDPI), 2015-01-15); ; ; In the current work, the evolutions of grain and dislocation microstructures are investigated on the basis of plane strain tension and simple shear tests for an interstitial free steel (DC06) and a 6000 series aluminum alloy (AA6016-T4). Both materials are commonly-used materials in the automobile industry. The focus of this contribution is on the characterization and comparison of the microstructure formation in DC06 and AA6016-T4. Our observations shed light on the active mechanisms at the micro scale governing the macroscopic response. This knowledge is of great importance to understand the physical deformation mechanisms, allowing the control and design of new, tailor-made materials with the desired material behavior.Publicationtype: Journal ArticleScopus© Citations 2 485 434 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Materials by design : an experimental and computational investigation on the microanatomy arrangement of porous metallic glassesThe correlation of a material's structure with its properties is one of the important unresolved issues in materials science research. This paper discusses a novel experimental and computational approach by which the influence of the pores on the mechanical properties of bulk metallic glasses (BMGs) can be systematically and quantitatively analyzed. The experimental stage involves the fabrication of a template whose pore configurations are pre-determined by computer-aided design tools, and replication of the designed patterns with BMGs. Quasi-static mechanical characterization of these complex microstructures is conducted under uniaxial tension and in-plane compression. For the numerical simulations, a non-local gradient-enhanced continuum mechanical model is established, using thermodynamic principles and periodic boundary conditions. The combination of the experimental and numerical results has identified the importance of the pore configuration, overall porosity and diameter to the spacing ratio of the pores to attain optimized material properties.Publicationtype: Journal Article33 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A computational study of a model of single-crystal strain-gradient viscoplasticity with an interactive hardening relationThe behavior of a model of single-crystal strain-gradient viscoplasticity is investigated. The model is an extension of a rate-independent version, and includes a new hardening relation that has recently been proposed in the small-deformation context (Gurtin and Reddy, 2014), and which accounts for slip-system interactions due to self and latent hardening. Energetic and dissipative effects, each with its corresponding length scale, are included. Numerical results are presented for a single crystal with single and multiple slip systems, as well as an ensemble of grains. These results provide a clear illustration of the effects of accounting for slip-system interactions.Publicationtype: Journal Article41