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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 19 395 427 - 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 , Damage modeling of small-scale experiments on dental enamel with hierarchical microstructure(Elsevier, 2015-12); ; ; ; Dental enamel is a highly anisotropic and heterogeneous material, which exhibits an optimal reliability with respect to the various loads occurring over years. In this work, enamel's microstructure of parallel aligned rods of mineral fibers is modeled and mechanical properties are evaluated in terms of strength and toughness with the help of a multiscale modeling method. The established model is validated by comparing it with the stress-strain curves identified by microcantilever beam experiments extracted from these rods. Moreover, in order to gain further insight in the damage-tolerant behavior of enamel, the size of crystallites below which the structure becomes insensitive to flaws is studied by a microstructural finite element model. The assumption regarding the fiber strength is verified by a numerical study leading to accordance of fiber size and flaw tolerance size, and the debonding strength is estimated by optimizing the failure behavior of the microstructure on the hierarchical level above the individual fibers. Based on these well-grounded properties, the material behavior is predicted well by homogenization of a representative unit cell including damage, taking imperfections (like microcracks in the present case) into account.Publicationtype: Journal ArticleScopus© Citations 27 362 336 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Crashworthiness of magnesium sheet structures(Trans Tech Publications, 2013-07); ; ; ; A hollow rectangular profile, as an example of a typical structural component made of magnesium alloy sheets has been built, tested and evaluated in order to assess its behaviour during axial crushing. The profiles were joined from plane sheets of AZ31 and ZE10, respectively, by laser beam welding and were then tested in compression. Numerical simulations have been conducted to understand the complex interplay between hardening characteristics of the materials under investigation, profile cross-section variation and energy absorption. The results from the compression testing of the profiles show that the welds are not the source of damage initiation and failure. The performance of the magnesium profiles in terms of dissipated specific energy is confirmed for small and intermediate displacements to be comparable to that of aluminium profiles. For large displacements, however, the shear-type failure mode of magnesium causes a sharp drop of the crushing force and thus limits the energy absorption. These findings demonstrate the requirement for an alloy and wrought magnesium process development specifically for crash applications which aims at progressive hardening along with high ductility for improving the bending and shear behaviour.Publicationtype: Conference Paper263 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On energy and entropy infuxes in the Green-Naghdi Type III theory of heat conductionThe energy-influx/entropy-influx relation in the Green-Naghdi Type III theory of heat conduction is examined within a thermodynamical framework à la Müller-Liu, where that relation is not specified a priori irrespectively of the constitutive class under attention. It is shown that the classical assumption, i.e. that the entropy influx and the energy influx are proportional via the absolute temperature, holds true if heat conduction is, in a sense that is made precise, isotropic. In addition, it is proved that influx proportionality cannot be postulated in general, because a counterexample can be given in the case of transversely isotropic conduction. © 2013 The Author(s) Published by the Royal Society.Publicationtype: Journal Article98