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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: 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 - 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 19 505 676 - 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 19 395 426 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Interface elasticity effects in polymer-filled nanoporous metalsA continuum formulation for electroactive composites made from nanoporous gold and ionconducting polymer is proposed. A novel extension of surface elasticity theory is developed to account for the high surface-to-volume ratio of nanoporous gold, and to capture the chemoelectromechanical coupling that occurs on the interface between the metal and the polymer. This continuum formulation accounts for the fully non-linear behaviour exhibited by the composite. The balance of linear momentum, Gauß's flux theorem and a relation for the transport of charge carriers are introduced in the bulk material as well as on the interface to describe the nonlinear multiphysics and highly coupled response of the actuator. The resulting system of nonlinear equations is solved using the finite element method. A series of numerical examples is presented to elucidate the theory.Publicationtype: Journal ArticleScopus© Citations 13 254 380 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Surface excess elasticity of gold: ab initio coefficients and impact on the effective elastic response of nanowiresPredicting the influence of the surface on the effective elastic properties of nanoscale structures and nanomaterials remains a challenge, which we here address on both levels, continuum and atomic. Density Functional Theory (DFT) computation at the atomic level yields the first reliable surface excess elastic parameters for the (111) and (001) surfaces of gold. At the continuum level, we derive closed-form expressions for the effective elastic behavior that can be combined with the DFT-derived excess elastic parameters to obtain the effective axial, torsion, and bending stiffness of circular nanowires with surface excess elasticity. The two approaches use different reference frames, and we emphasize the need for consistent stress definitions and for conversion between the separate stress measures when transferring results between the approaches. We present excess elastic parameters separately for Cauchy and 2nd Piola-Kirchhoff stresses, demonstrating that the conversion substantially modifies their numerical value and may even invert their sign. The results afford an assessment of the contribution of the surface excess elastic parameters to the effective elastic response of nanoscale beams or wires. This assessment sheds doubt on earlier suggestions relating experimental observations of an effective stiffening or softening at small size to the excess elasticity of clean surfaces.Publicationtype: Journal ArticleScopus© Citations 33 438 850