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Item type:Publication, Access status: with files , Modeling anisotropic atomic vibrations in diffusive molecular dynamics of magnesium hydride(2026-10-23); The coupled diffusion and deformation characterizing diffusive processes on the nanoscale provide significant challenges to many simulation methods due to the combination of fast atomic vibrations and slow diffusion. Diffusive molecular dynamics (DMD) models such processes by a scale separation modeling the thermal vibrations as a spatial position probability distribution and diffusion as atom sites with variable concentrations, which evolve based on kinetic equations. This study introduces a novel type of fully anisotropic spatial probability distribution to improve the approximations for atom sites with anisotropic potential energy wells, e.g. close to surfaces or lattice defects and atoms in lattices with a relatively high degree of intrinsic anisotropy. A comparative study between the classic isotropic and newly introduced anisotropic probability distributions is conducted for the case of magnesium hydride with molecular dynamics simulations using the same interatomic potential serving as reference solution. Tests range from lattice and elastic constants of perfect crystals to more complex cases like the bending of nanobeams and reveal previously unreported shortcomings of isotropic DMD, leading to a loss of lattice symmetry and nonphysical behavior. The results obtained using anisotropic shape functions avoid this behavior and follow the reference solution significantly better while maintaining comparable computational effort. This confirms the requirement for DMD to take anisotropic vibrations into account when simulating magnesium hydride and other systems with anisotropic atom environments.Publicationtype: Journal Article11 5 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Comparative analysis of diffusive molecular dynamics and traditional molecular dynamics in palladium hydrideDiffusive processes on the nanoscale, characterized by coupled diffusion and deformation, pose significant challenges for the assessment using conventional simulation methods due to their long time scales. Diffusive molecular dynamics (DMD) models such diffusive processes by optimizing positions and vibration frequencies of atom sites with variable concentrations evolving based on kinetic equations. Despite its various applications, validation studies for this method still remain limited due to a lack of suitable reference solutions and an absence of comparisons among different kinetic equations. This paper aims to fill these gaps by directly comparing DMD, utilizing different kinetic equations, with traditional molecular dynamics (MD) for the case of palladium hydride on thermal and diffusive time scales. Employing the same interatomic potential in both methods helps to isolate errors attributed solely to the assumptions inherent in DMD. The simulations conducted, which include measurements of lattice and elastic constants as well as surface diffusion and hydration processes of a nanocube, reveal that although DMD generally captures the trends observed in the reference solutions quite well, it exhibits non-negligible quantitative discrepancies in both static properties and diffusive behaviors. Among the tested kinetic equations, a linear approach and a master equation depending on vacancy formation energy showed the best agreement with the reference solutions. This study underscores the necessity for further refinement of DMD like enhanced kinetic equations that account for local environmental factors and improved mean-field approximations better suited to complex interatomic potentials. Given the significant advantages DMD offers as a method, efforts in this direction are doubtlessly an interesting avenue for future research.Publicationtype: Journal Article24 11 - 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 5 347 509 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Continuum damage modeling and simulation of hierarchical dental enamel(2016-04-25); ; Dental enamel exhibits high fracture toughness and stiffness due to a complex hierarchical and graded microstructure, optimally organized from nano- to macro-scale. In this study, a 3D representative volume element (RVE) model is adopted to study the deformation and damage behavior of the fibrous microstructure. 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 employing a cohesive zone model. The dependence of the failure mechanism on the aspect ratio of the mineral fibers is investigated. In addition, the effect of the interface strength on the damage behavior is studied with respect to geometric features of enamel. Further, the effect of an initial flaw on the overall mechanical properties is analyzed to understand the superior damage tolerance of dental enamel. The simulation results are validated by comparison to experimental data from micro-cantilever beam testing at two hierarchical levels. The transition of the failure mechanism at different hierarchical levels is also well reproduced in the simulations.Publicationtype: Journal Article80