Michael Zaiser - Profile on Academia.edu (original) (raw)

Papers by Michael Zaiser

arXiv (Cornell University), Sep 12, 2017

Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and ... more Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and connected dislocation lines in terms of density-like field variables. Here we discuss how the processes of dislocation multiplication and annihilation can be described within such a framework. We show that both processes are associated with changes in the volume density of dislocation loops: dislocation annihilation needs to be envisaged in terms of the merging of dislocation loops, while conversely dislocation multiplication is associated with the generation of new loops. Both findings point towards the importance of including the volume density of loops (or 'curvature density') as an additional field variable into continuum models of dislocation density evolution. We explicitly show how this density is affected by loop mergers and loop generation. The equations which result for the lowest order CDD theory allow us, after spatial averaging and under the assumption of unidirectional deformation, to recover the classical theory of Kocks and Mecking for the early stages of work hardening.

arXiv (Cornell University), Sep 12, 2017

Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and ... more Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and connected dislocation lines in terms of density-like field variables. Here we discuss how the processes of dislocation multiplication and annihilation can be described within such a framework. We show that both processes are associated with changes in the volume density of dislocation loops: dislocation annihilation needs to be envisaged in terms of the merging of dislocation loops, while conversely dislocation multiplication is associated with the generation of new loops. Both findings point towards the importance of including the volume density of loops (or 'curvature density') as an additional field variable into continuum models of dislocation density evolution. We explicitly show how this density is affected by loop mergers and loop generation. The equations which result for the lowest order CDD theory allow us, after spatial averaging and under the assumption of unidirectional deformation, to recover the classical theory of Kocks and Mecking for the early stages of work hardening.

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

In this work we use density functional theory (DFT) calculations to benchmark empirical potential... more In this work we use density functional theory (DFT) calculations to benchmark empirical potentials for the interaction between nickel and sp 2 bonded carbon nanoparticles. These potentials are then used in order to investigate how Ni decorated or coated carbon nanotubes (CNT) affect the mechanical properties of Al/CNT composites. In particular we look at the pull-out behaviour of pristine as well as Ni-decorated and Ni-coated CNT from an Al matrix. Our result shows that Ni coating may produce an extended interface (interphase) where a significant amount of energy is dissipated during CNT pull-out, leading to a high pull-out force. We also demonstrate that surface decorated CNT may act as efficient nano-crystallization agents and thus provide a novel strengthening mechanism not previously discussed in the literature. We discuss our results in view of promising approaches for engineering CNT-metal interfaces such as to achieve high strength metal-CNT composites.

arXiv (Cornell University), Oct 27, 2021

This paper describes atomistic simulations of deformation and fracture of Al reinforced with carb... more This paper describes atomistic simulations of deformation and fracture of Al reinforced with carbon nanotubes (CNTs). We use density functional theory (DFT) to understand the energetics of Al-graphene interfaces and gain reference data for the parameterization of Al-C empirical potentials. We then investigate the load transfer between CNTs and Al and its effect on composite strengthening. To this end, we perform uniaxial tensile simulations of an Al crystal reinforced with CNTs of various volume fractions. We also study the interaction of the embedded CNTs with a crack. We show that the interaction between CNTs and Al is weak such that, under tensile loading, CNTs can easily slide inside the Al matrix and get pulled out from the cracked surface. This effect is almost independent of CNT length and volume fraction. Little load transfer and consequently no crack bridging are observed during the simulation of pristine CNTs threading the crack surfaces. CNTs that are geometrically fixated inside Al, on the other hand, can increase the fracture stress and enhance plastic dissipation in the matrix. CNTs located in front of a growing crack blunt the crack and induce plastic deformation of the Al matrix. Depending on the CNT orientation, these processes can either increase or decrease the failure stress of the composite.

Dry snow slab avalanches form when fracture, once initiated in a weak snowpack layer, propagates ... more Dry snow slab avalanches form when fracture, once initiated in a weak snowpack layer, propagates over large distances, progressively debonding a vast area of slab on its passage. Over the past decades the underlying fracture process has mostly been attributed to the growth of a volume-conserving shear crack, but in reality the fracture is most often accompanied by a rearrangement of the grains in the weak layer, causing a reduction in specific volume and a slope-perpendicular settling of the slab during the fracture process. This has important consequences both in theory and in practice. This contribution presents insights into a new theory of fracture propagation in snow based on the principles of mixed-mode anticracking at the fracture front. We find that fracture is propagated by a stable, kink-shaped wave travelling with sub-shear velocity through the snowpack. We also find that the fracture energy, despite being a crucial factor that determines whether fracture can be triggered or not, has negligible influence on the propagation of fracture after a supercritical crack has been formed. We use our model to calculate the deformation profile at the fracture front and compare the mathematical results with measured deformation profiles obtained from field experiments. The accordance between the theoretical and experimental results is very satisfying. *

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.

European Physical Journal B, May 1, 2019

We discuss the relevance of methods of graph theory for the study of damage in simple model mater... more We discuss the relevance of methods of graph theory for the study of damage in simple model materials described by the random fuse model. While such methods are not commonly used when dealing with regular random lattices, which mimic disordered but statistically homogeneous materials, they become relevant in materials with microstructures that exhibit complex multi-scale patterns. We specifically address the case of hierarchical materials, whose failure, due to an uncommon fracture mode, is not well described in terms of either damage percolation or crack nucleation-and-growth. We show that in these systems, incipient failure is accompanied by an increase in eigenvector localization and a drop in topological dimension. We propose these two novel indicators as possible candidates to monitor a system in the approach to failure. As such, they provide alternatives to monitoring changes in the precursory avalanche activity, which is often invoked as a candidate for failure prediction in materials which exhibit critical-like behavior at failure, but may not work in the context of hierarchical materials which exhibit scale-free avalanche statistics even very far from the critical load.

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

The rate-dependent mechanical response of snow is commonly believed to directly inherit the ducti... more The rate-dependent mechanical response of snow is commonly believed to directly inherit the ductile-to-brittle transition (DBT) from the viscoplastic ice matrix. Recent work has however stressed the impact of microstructure evolution by rapid sintering during deformation. To understand these phenomena we have conducted deformation-controlled compression experiments in an X-ray tomography stage. By varying the strain rate over 3 orders of magnitude, we find an intermediate regime where the stress response changes from smooth to serrated behavior. This regime is accompanied by microstructure quakes associated with strain localization bands in the sample interior. To interpret the results we developed a minimal, scalar model with a rate-dependent, elastoplastic constitutive law and healing which falls into the general class of rate-and-state models. The model correctly predicts the range of the instability as well as amplitude and frequency of the serrations and reveals a formal equivalence of the observed (compressive) stick-slip with seismic faults.

Nature Communications, May 20, 2022

Being able to predict the failure of materials based on structural information is a fundamental i... more Being able to predict the failure of materials based on structural information is a fundamental issue with enormous practical and industrial relevance for the monitoring of devices and components. Thanks to recent advances in deep learning, accurate failure predictions are becoming possible even for strongly disordered solids, but the sheer number of parameters used in the process renders a physical interpretation of the results impossible. Here we address this issue and use machine learning methods to predict the failure of simulated two dimensional silica glasses from their initial undeformed structure. We then exploit Gradientweighted Class Activation Mapping (Grad-CAM) to build attention maps associated with the predictions, and we demonstrate that these maps are amenable to physical interpretation in terms of topological defects and local potential energies. We show that our predictions can be transferred to samples with different shape or size than those used in training, as well as to experimental images. Our strategy illustrates how artificial neural networks trained with numerical simulation results can provide interpretable predictions of the behavior of experimentally measured structures.

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...

A Non-Linear Multiple Slip Theory in Continuum Dislocation Dynamics

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...

Acta Materialia, 2019

Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence p... more Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence plastic deformation in a complex manner. They not only act as dislocation obstacles but can also accommodate twinning dislocations (TDs) whose motion enables twin boundary migration as an alternative deformation mechanism. Exploiting this dual effect offers interesting perspectives in view of designing materials that combine high strength and ductility. In the present work, we propose a model for dislocation-TB interactions in the framework of discrete dislocation dynamics (DDD) simulations, which we use to investigate the mechanical properties of bicrystalline copper micropillars containing a TB. We systematically investigate how the compressive response depends on the orientation of the TB with respect to the micropillar crosssection. The simulations show significant strengthening effects for TB orientation angles less than 45º, where the interaction between mixed dislocations and TBs plays an important role. For

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 ...

This PDF contains a full analysis based on plane strain data - as opposed to the plane stress data considered in the published manuscript

ARTICLES-Structure, structural phase transitions, mechanical properties, defects, etc.-Electron localization on dislocations in metals: Real-space first-principles calculations

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.

arXiv (Cornell University), Sep 12, 2017

Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and ... more Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and connected dislocation lines in terms of density-like field variables. Here we discuss how the processes of dislocation multiplication and annihilation can be described within such a framework. We show that both processes are associated with changes in the volume density of dislocation loops: dislocation annihilation needs to be envisaged in terms of the merging of dislocation loops, while conversely dislocation multiplication is associated with the generation of new loops. Both findings point towards the importance of including the volume density of loops (or 'curvature density') as an additional field variable into continuum models of dislocation density evolution. We explicitly show how this density is affected by loop mergers and loop generation. The equations which result for the lowest order CDD theory allow us, after spatial averaging and under the assumption of unidirectional deformation, to recover the classical theory of Kocks and Mecking for the early stages of work hardening.

arXiv (Cornell University), Sep 12, 2017

Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and ... more Continuum dislocation dynamics (CDD) aims at representing the evolution of systems of curved and connected dislocation lines in terms of density-like field variables. Here we discuss how the processes of dislocation multiplication and annihilation can be described within such a framework. We show that both processes are associated with changes in the volume density of dislocation loops: dislocation annihilation needs to be envisaged in terms of the merging of dislocation loops, while conversely dislocation multiplication is associated with the generation of new loops. Both findings point towards the importance of including the volume density of loops (or 'curvature density') as an additional field variable into continuum models of dislocation density evolution. We explicitly show how this density is affected by loop mergers and loop generation. The equations which result for the lowest order CDD theory allow us, after spatial averaging and under the assumption of unidirectional deformation, to recover the classical theory of Kocks and Mecking for the early stages of work hardening.

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

In this work we use density functional theory (DFT) calculations to benchmark empirical potential... more In this work we use density functional theory (DFT) calculations to benchmark empirical potentials for the interaction between nickel and sp 2 bonded carbon nanoparticles. These potentials are then used in order to investigate how Ni decorated or coated carbon nanotubes (CNT) affect the mechanical properties of Al/CNT composites. In particular we look at the pull-out behaviour of pristine as well as Ni-decorated and Ni-coated CNT from an Al matrix. Our result shows that Ni coating may produce an extended interface (interphase) where a significant amount of energy is dissipated during CNT pull-out, leading to a high pull-out force. We also demonstrate that surface decorated CNT may act as efficient nano-crystallization agents and thus provide a novel strengthening mechanism not previously discussed in the literature. We discuss our results in view of promising approaches for engineering CNT-metal interfaces such as to achieve high strength metal-CNT composites.

arXiv (Cornell University), Oct 27, 2021

This paper describes atomistic simulations of deformation and fracture of Al reinforced with carb... more This paper describes atomistic simulations of deformation and fracture of Al reinforced with carbon nanotubes (CNTs). We use density functional theory (DFT) to understand the energetics of Al-graphene interfaces and gain reference data for the parameterization of Al-C empirical potentials. We then investigate the load transfer between CNTs and Al and its effect on composite strengthening. To this end, we perform uniaxial tensile simulations of an Al crystal reinforced with CNTs of various volume fractions. We also study the interaction of the embedded CNTs with a crack. We show that the interaction between CNTs and Al is weak such that, under tensile loading, CNTs can easily slide inside the Al matrix and get pulled out from the cracked surface. This effect is almost independent of CNT length and volume fraction. Little load transfer and consequently no crack bridging are observed during the simulation of pristine CNTs threading the crack surfaces. CNTs that are geometrically fixated inside Al, on the other hand, can increase the fracture stress and enhance plastic dissipation in the matrix. CNTs located in front of a growing crack blunt the crack and induce plastic deformation of the Al matrix. Depending on the CNT orientation, these processes can either increase or decrease the failure stress of the composite.

Dry snow slab avalanches form when fracture, once initiated in a weak snowpack layer, propagates ... more Dry snow slab avalanches form when fracture, once initiated in a weak snowpack layer, propagates over large distances, progressively debonding a vast area of slab on its passage. Over the past decades the underlying fracture process has mostly been attributed to the growth of a volume-conserving shear crack, but in reality the fracture is most often accompanied by a rearrangement of the grains in the weak layer, causing a reduction in specific volume and a slope-perpendicular settling of the slab during the fracture process. This has important consequences both in theory and in practice. This contribution presents insights into a new theory of fracture propagation in snow based on the principles of mixed-mode anticracking at the fracture front. We find that fracture is propagated by a stable, kink-shaped wave travelling with sub-shear velocity through the snowpack. We also find that the fracture energy, despite being a crucial factor that determines whether fracture can be triggered or not, has negligible influence on the propagation of fracture after a supercritical crack has been formed. We use our model to calculate the deformation profile at the fracture front and compare the mathematical results with measured deformation profiles obtained from field experiments. The accordance between the theoretical and experimental results is very satisfying. *

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.

European Physical Journal B, May 1, 2019

We discuss the relevance of methods of graph theory for the study of damage in simple model mater... more We discuss the relevance of methods of graph theory for the study of damage in simple model materials described by the random fuse model. While such methods are not commonly used when dealing with regular random lattices, which mimic disordered but statistically homogeneous materials, they become relevant in materials with microstructures that exhibit complex multi-scale patterns. We specifically address the case of hierarchical materials, whose failure, due to an uncommon fracture mode, is not well described in terms of either damage percolation or crack nucleation-and-growth. We show that in these systems, incipient failure is accompanied by an increase in eigenvector localization and a drop in topological dimension. We propose these two novel indicators as possible candidates to monitor a system in the approach to failure. As such, they provide alternatives to monitoring changes in the precursory avalanche activity, which is often invoked as a candidate for failure prediction in materials which exhibit critical-like behavior at failure, but may not work in the context of hierarchical materials which exhibit scale-free avalanche statistics even very far from the critical load.

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

The rate-dependent mechanical response of snow is commonly believed to directly inherit the ducti... more The rate-dependent mechanical response of snow is commonly believed to directly inherit the ductile-to-brittle transition (DBT) from the viscoplastic ice matrix. Recent work has however stressed the impact of microstructure evolution by rapid sintering during deformation. To understand these phenomena we have conducted deformation-controlled compression experiments in an X-ray tomography stage. By varying the strain rate over 3 orders of magnitude, we find an intermediate regime where the stress response changes from smooth to serrated behavior. This regime is accompanied by microstructure quakes associated with strain localization bands in the sample interior. To interpret the results we developed a minimal, scalar model with a rate-dependent, elastoplastic constitutive law and healing which falls into the general class of rate-and-state models. The model correctly predicts the range of the instability as well as amplitude and frequency of the serrations and reveals a formal equivalence of the observed (compressive) stick-slip with seismic faults.

Nature Communications, May 20, 2022

Being able to predict the failure of materials based on structural information is a fundamental i... more Being able to predict the failure of materials based on structural information is a fundamental issue with enormous practical and industrial relevance for the monitoring of devices and components. Thanks to recent advances in deep learning, accurate failure predictions are becoming possible even for strongly disordered solids, but the sheer number of parameters used in the process renders a physical interpretation of the results impossible. Here we address this issue and use machine learning methods to predict the failure of simulated two dimensional silica glasses from their initial undeformed structure. We then exploit Gradientweighted Class Activation Mapping (Grad-CAM) to build attention maps associated with the predictions, and we demonstrate that these maps are amenable to physical interpretation in terms of topological defects and local potential energies. We show that our predictions can be transferred to samples with different shape or size than those used in training, as well as to experimental images. Our strategy illustrates how artificial neural networks trained with numerical simulation results can provide interpretable predictions of the behavior of experimentally measured structures.

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...

A Non-Linear Multiple Slip Theory in Continuum Dislocation Dynamics

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...

Acta Materialia, 2019

Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence p... more Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence plastic deformation in a complex manner. They not only act as dislocation obstacles but can also accommodate twinning dislocations (TDs) whose motion enables twin boundary migration as an alternative deformation mechanism. Exploiting this dual effect offers interesting perspectives in view of designing materials that combine high strength and ductility. In the present work, we propose a model for dislocation-TB interactions in the framework of discrete dislocation dynamics (DDD) simulations, which we use to investigate the mechanical properties of bicrystalline copper micropillars containing a TB. We systematically investigate how the compressive response depends on the orientation of the TB with respect to the micropillar crosssection. The simulations show significant strengthening effects for TB orientation angles less than 45º, where the interaction between mixed dislocations and TBs plays an important role. For

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 ...

This PDF contains a full analysis based on plane strain data - as opposed to the plane stress data considered in the published manuscript

ARTICLES-Structure, structural phase transitions, mechanical properties, defects, etc.-Electron localization on dislocations in metals: Real-space first-principles calculations

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.