Michael Zaiser | Friedrich-Alexander-Universität Erlangen-Nürnberg (original) (raw)

Papers by Michael Zaiser

Journal of The Mechanics and Physics of Solids, May 1, 2019

Nanoindentation is a convenient method to investigate the mechanical properties of materials on s... more Nanoindentation is a convenient method to investigate the mechanical properties of materials on small scales by utilizing low loads and small indentation depths. However, the effect of grain boundaries (GB) on the nanoindentation response remains unclear and needs to be studied by investigating in detail the interactions between dislocations and GBs during nanoindentation. In the present work, we employ a threedimensional multiscale modeling framework, which couples three-dimensional discrete dislocation dynamics (DDD) with the Finite Element method (FEM) to investigate GB effects on the nanoindentation behavior of an aluminum bicrystal. The interaction between dislocations and GB is physically modeled in terms of a penetrable GB, where piled-up dislocations can penetrate through the GB and dislocation debris at GBs can emit full dislocations into grains. In the simulation, we confirmed two experimentally observed phenomena, namely, pop-in events and the dependence of indentation hardness on the distance from GB. Two pop-in events were observed, of which the initial pop-in event is correlated with the activation and multiplication of dislocations, while the GB pop-in event results from dislocation transmission through the GB. By changing the distance between the indenter and GB, the simulation shows that the 2 indentation hardness increases with decreasing GB-indenter distance. A quantitative model has been formulated which relates the dependency of indentation hardness on indentation depth and on GB-indenter distance to the back stress created by piled-up geometrically necessary dislocations in the plastic zone and to the additional constraint imposed by the GB on the plastic zone size.

European Physical Journal B, Aug 1, 2019

$Research paper thumbnail of Atomistic aspects of load transfer and fracture in CNT-reinforced aluminium$

arXiv (Cornell University), Oct 27, 2021

2010 International Snow Science Workshop, Oct 22, 2010

The avalanche environment in Scotland differs from typical alpine environments. A warmer, maritim... more The avalanche environment in Scotland differs from typical alpine environments. A warmer, maritime climate with regular high winds leads to a prevalence of dense, old but mobile snow. Melt freeze cycles and wind transport dominate snow pack evolution, with wet snow and slab avalanches being the dominant avalanche types. SnowMicroPen (SMP) measurements were performed alongside conventional snowpits in order to identify representative samples of SMP data for six typical Scottish snow types. Artificial snow was aged in a cold lab and tested with the SMP. Comparisons are drawn between the artificial snow and natural Scottish snow. It is found that artificial snow can be considered a good model material for systematic studies of interactions between specific snow types and measurement instruments.

Nature Communications, Apr 23, 2021

European Physical Journal B, May 1, 2019

APL Materials, Feb 1, 2021

Designing materials with tailored structural or functional properties is a fundamental goal of ma... more Designing materials with tailored structural or functional properties is a fundamental goal of materials science and engineering. A vast research activity is currently devoted to achieving metamaterials with superior properties and optimized functionalities by carefully fine tuning both the microstructure and geometry of the material. Here, we discuss the impact of digital technologies in this research field by providing fast and cost effective tools to explore a large array of possibilities for materials and metamaterials. We report on recent progress obtained by combining numerical simulations, optimization techniques, artificial intelligence, and additive manufacturing methods and highlight promising research lines. The exploration of the space of possible material microstructures and geometries is reminiscent of the process of biological evolution in which traits are explored and selected according to their fitness. Biomimetic materials have long profited from adapting features of biological systems to the design of new materials and structures. Combining biomimetic approaches with digital simulation and optimization and with high throughput fabrication and characterization techniques may provide a step change in the evolutionary development of new materials.

Geophysical Research Letters, Feb 19, 2020

Nature Communications, May 20, 2022

Metals

This paper investigates the interaction of edge dislocations with voids in concentrated solid sol... more This paper investigates the interaction of edge dislocations with voids in concentrated solid solution alloys (CSAs) using atomistic simulations. The simulation setup consists of edge dislocations with different periodicity lengths and a periodic array of voids as obstacles to dislocation motion. The critical resolved shear stress (CRSS) for dislocation motion is determined by static simulations bracketing the applied shear stress. The results show that shorter dislocation lengths and the presence of voids increase the CRSS for dislocation motion. The dislocation–void interaction is found to follow an Orowan-like mechanism, where partial dislocation arms mutually annihilate each other to overcome the void. Solute strengthening produces a ‘friction stress’ that adds to the Orowan stress. At variance with classical theories of solute pinning, this stress must be considered a function of the dislocation line length, in line with the idea that geometrical constraints synergetically enha...

Nature Communications, 2021

Dislocation glide is a general deformation mode, governing the strength of metals. Via discrete d... more Dislocation glide is a general deformation mode, governing the strength of metals. Via discrete dislocation dynamics and molecular dynamics simulations, we investigate the strain rate and dislocation density dependence of the strength of bulk copper and aluminum single crystals. An analytical relationship between material strength, dislocation density, strain rate and dislocation mobility is proposed, which agrees well with current simulations and published experiments. Results show that material strength displays a decreasing regime (strain rate hardening) and then increasing regime (classical forest hardening) as the dislocation density increases. Accordingly, the strength displays universally, as the strain rate increases, a strain rate-independent regime followed by a strain rate hardening regime. All results are captured by a single scaling function, which relates the scaled strength to a coupling parameter between dislocation density and strain rate. Such coupling parameter al...

Advanced Energy Materials, 2019

Organic electronic devices (OEDs), e.g., organic solar cells, degrade quickly in the presence of ... more Organic electronic devices (OEDs), e.g., organic solar cells, degrade quickly in the presence of ambient gases, such as water vapor and oxygen. Thus, in order to extend the lifetime of flexible OEDs, they have to be protected by encapsulation. A solution‐based encapsulation method is developed, which allows the direct deposition of the diffusion barrier on top of OEDs, thus avoiding lamination of barrier films. The method is based on the deposition of a perhydropolysilazane (PHPS) ink and its subsequent conversion into a silica layer by deep UV irradiation. The resulting barrier films show water vapor transmission rates (WVTRs) of <10−2 g m−2 d−1 (40 °C/85% relative humidity (RH)) and oxygen transmission rates (OTRs) of <10−2 cm3 m−2 d−1 bar−1 at ambient conditions. Flexibility of the resulting barrier films is improved by coating a barrier stack of several thin PHPS layers alternating with organic polymer interlayers. These stacks show an increase of WVTR values by less than ...

arXiv (Cornell University), Jun 9, 2023

Modeling dislocations is an inherently multiscale problem as one needs to simultaneously describe... more Modeling dislocations is an inherently multiscale problem as one needs to simultaneously describe the high stress fields near the dislocation cores, which depend on atomistic length scales, and a surface boundary value problem which depends on boundary conditions on the sample scale. We present a novel approach which is based on a peridynamic dislocation model to deal with the surface boundary value problem. In this model, the singularity of the stress field at the dislocation core is regularized owing to the non-local nature of peridynamics. The effective core radius is defined by the peridynamic horizon which, for reasons of computational cost, must be chosen much larger than the lattice constant. This implies that dislocation stresses in the near-core region are seriously underestimated. By exploiting relationships between peridynamics and Mindlin-type gradient elasticity, we then show that gradient elasticity can be used to construct short-range corrections to the peridynamic stress field that yield a correct description of dislocation stresses from the atomic to the sample scale.

Physical Review Materials

Hierarchical microstructures are often invoked to explain the high resilience and fracture toughn... more Hierarchical microstructures are often invoked to explain the high resilience and fracture toughness of biological materials such as bone and nacre. Biomimetic material models inspired by such hierarchical biomaterials face the obvious challenge of capturing their inherent multiscale complexity, both in experiments and in simulations. To study the influence of hierarchical microstructure on fracture properties, we propose a large-scale three-dimensional hierarchical beam-element simulation framework, in which we generalize the constitutive framework of Timoshenko beam elasticity and maximum distortion energy theory failure criteria to the complex case of hierarchical networks of up to six self-similar hierarchical levels, consisting of approximately 5 million elements. We perform a statistical study of stress-strain relationships and fracture surface morphologies and conclude that hierarchical systems are capable of arresting crack propagation, an ability that reduces their sensitivity to preexisting damage and enhances their fault tolerance compared to reference fibrous materials without microstructural hierarchy.