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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 Article252 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Automatic generation and discretization of fully periodic representative volume elements of plain woven composites(2018-12-01); ; This paper introduces an efficient and simple method to automatically generate and mesh geometrically parametrized representative volume elements for plain woven fiber-reinforced composites. The presented approach is capable of generating representative volume elements with 10%–55% fiber volume fraction. The practical feasibility of the model is demonstrated on numerical examples for isotropic and anisotropic tow materials which are compared to analytical solutions.Publicationtype: Journal Article274 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Finite element simulation of pole vaultingPole vaulting is one of the most spectacular disciplines in athletics. The evolution of world record heights is strongly influenced by the development of advanced poles and subsequent materials. Employing advanced, load-adjusted composites has resulted in a steady increase of the world record height. This study provides a framework for finite element simulations of pole vaulting with focus on the initial and boundary conditions as well as finite element choices. The influence of the pole bending stiffness on the achievable height is systematically simulated. Higher effective bending stiffness leads to higher pole vaulting heights. However, if a certain stiffness is exceeded, the vaulter will not be able to bend the pole enough which leads to failed attempts.Publicationtype: Journal ArticleScopus© Citations 7 316 610 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Functionalisation of metal-polymer-nanocomposites : chemoelectromechanical coupling and charge carrier transportElectrochemical actuation in nanoporous metals is achieved by impregnation of the material’s pore space with a ionic conductor, typically an aqueous electrolyte. These hybrid actuators exhibit fully reversible deformation and mechanical properties that can be controlled by electric signals. Recently, set-ups have been proposed in which the nanoporous metal’s surface is additionally coated with a conjugated polymer, resulting in a nanocomposite that exhibits strongly increased actuation strains compared to the pure metal while still retaining the mechanical strength of the metal backbone. In order to exploit the full potential of these nanocomposite actuators, a detailed understanding of the underlying ion transport mechanisms and means to predict the actuator’s response are necessary. We present an interface-extended continuum mechanical model to study actuation in pure nanoporous gold and nanoporous gold–polypyrrole nanocomposites. Simulations predict significantly enhanced actuation strains due to the presence of the polymer phase and show that both, the nanocomposite’s structure and the ions’ mobilities, greatly affect the actuator’s response.Publicationtype: Journal Article422 457 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , 3D stochastic bicontinuous microstructures: generation, topology and elasticity(2018-02-07); ; ; Motivated by recent experimental investigations of the mechanical behavior of nanoporous metal we explore an efficient and robust method for generating 3D representative volume elements (RVEs) with strikingly similar behavior. Our approach adopts Cahn's method of generating a Gaussian random field by taking a superposition of standing sinusoidal waves of fixed wavelength but random in direction and phase. In its theory part, our study describes closed-form xpressions for how the solid volume fraction affects the binarization evel, mean structure size, specific surface area, averages of mean and Gaussian curvature, and the scaled topological genus. Based on numerical studies we report on criteria for achieving representative realizations of the structure by proper choice of the number of waves and element size. We also show that periodic structures are readily created. We analyze the mechanical properties considering linear and infinitesimal elasticity and evaluate the residual anisotropy (which can be made small) and the effective values of the Young's modulus and Poisson's ratio. The numerical results are in excellent agreement with experimental findings for the variation of stiffness with solid fraction of nanoporous gold made by dealloying. We propose scaling relations that achieve naturally a perfect agreement with the numerical and experimental data. The scaling relation for the stiffness accounts for a percolation-to-cluster transition in the random field microstructure at a finite solid fraction. We propose that this transition is the origin of the previously reported anomalous compliance of nanoporous gold.Publicationtype: Journal ArticleScopus© Citations 148 391 876 - 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 10 338 521 - Some of the metrics are blocked by yourconsent settings
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 , Multiscale experimental and computational investigation of nature’s design principle of hierarchies in dental enamel(Springer, 2018); ; ; ; Dental enamel possesses extraordinary mechanical properties due to a complex hierarchical and graded microstructure. In this study, multiscale experimental and computational approaches are employed and combined to study nature’s design principle of the hierarchical structure of bovine enamel for developing bioinspired advanced ceramics with hierarchical microstructure. Micro-cantilever beam tests are carried out to characterize the mechanical properties from nano-to mesoscale experimentally. In order to understand the relationship between the hierarchical structure and the flaw-tolerance behavior of enamel, a 3D representative volume element (RVE) is used in a numerical analysis to study the deformation and damage process at two hierarchical levels. A continuum damage mechanics model coupled to hyperelasticity is developed for modeling the initiation and evolution of damage in the mineral fibers as well as protein matrix. Moreover, debonding of the interface between mineral fiber and protein is captured by a cohesive zone model. The effect of an initial flaw on the overall mechanical properties is analyzed at different hierarchical levels to understand the superior damage tolerance of dental enamel. Based on the experimental and computational investigation, the role of hierarchical levels on the multiscale design of structure in dental enamel is revealed for optimizing bio-inspired composites.Publicationtype: Book Part186 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Elastic behaviour at the nanoscale of innovative composites of nanoporous gold and polymerNanoporous gold is a material with unique properties that make it attractive for use in multiple areas. It has an extremely high compressive strain capability but is very brittle in tension which limits its usage. One means of addressing this limitation is to fill the voids of the nanoporous material with a polymer, resulting in a more ductile and stronger material. In this work, the mechanical properties of a nanocomposite representative sample are studied numerically. The study is based on homogenization with the use of three types of boundary conditions. The stress–strain response within the composite is also analysed. Under a pure compression test, the sample shows both compressive and tensile strains and stresses. This is due to the complex interaction within the microstructure. Within the composite, the polymer is still considerably more compliant than the gold material showing much higher strains. The resultant forces, moments and torques acting on representative ligament cross-sections are also investigated under compressive loading. Bending moments and normal forces dominate the modes of loading. Ligaments along the compression axis show markedly higher normal forces compared to ligaments perpendicular to this axis. The normal stress distributions of the ligaments show clear trends towards combined bending and normal forces while there is no discernible pattern to the shear stress distributions. These nevertheless generate high shear forces and torques.Publicationtype: Journal Article472 587 - 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 10 295 274