Research

2021

  • Nationale Forschungsdateninfrastruktur für Materialwissenschaft & Werkstofftechnik

    (Third Party Funds Group – Sub project)

    Overall project: Nationale Forschungsdateninfrastruktur für Materialwissenschaft & Werkstofftechnik
    Term: 1 October 2021 - 30 September 2026
    Funding source: Deutsche Forschungsgemeinschaft (DFG)

2019

  • P13 – Modelling of the development of deformation bands in porous rocks and their influence on the permeability evolution of reservoirs

    (Third Party Funds Group – Sub project)

    Overall project: GRK 2423 FRASCAL: Skalenübergreifende Bruchvorgänge: Integration von Mechanik, Materialwissenschaften, Mathematik, Chemie und Physik (FRASCAL)
    Term: 1 April 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    Abstract

    An extended DEM approach with multi-scale aggregates and healing algorithms will be used to study structures on the grain and single-band scale, whereas the reservoir scale flow properties will be determined with continuum models. Codes will be developed and tested simultaneously and natural examples from our rock collection and field examples can be used in the other projects. We will then develop an algorithm for the healing of fractured grains and will finally approach the large scale and look at the influence of deformation bands on the permeability of aquifers. Here we will vary mechanical content in bands, deformation conditions from shear to compaction, compactional and extensional shear, and finite strain.

  • Fracture in Polymer Composites: Nano to Meso

    (Third Party Funds Group – Sub project)

    Overall project: Skalenübergreifende Bruchvorgänge: Integration von Mechanik, Materialwissenschaften, Mathematik, Chemie und Physik (FRASCAL)
    Term: 2 January 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    URL: https://www.frascal.research.fau.eu/home/research/p-3-fracture-in-polymer-composites-nano-to-meso/
    Abstract

    The abrasion and fracture toughness of polymers can considerably be increased by adding hard nanoparticles such as silica. This is mainly caused by the development of localized shear bands, initiated by the stress concentrations stemming from the inhomogeneity of the composites. Other mechanisms responsible for toughening are debonding of the particles and void growth in the polymer matrix. Both phenomena strongly depend on the structure and chemistry of the polymers and shall be explored for branched networks (epoxy) and matrices of nestled fibres (cellulose, aramid).

    The goal of the present project is to develop and apply dynamics simulation approaches to understanding polymer-nanoparticle and polymer-polymer interactions at i) the atomic scale and ii) at larger scales using coarse-graining.

  • Teilprojekt P1 – Chemistry at the Crack Tip

    (Third Party Funds Group – Sub project)

    Overall project: Skalenübergreifende Bruchvorgänge: Integration von Mechanik, Materialwissenschaften, Mathematik, Chemie und Physik (FRASCAL)
    Term: 2 January 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    URL: https://www.frascal.research.fau.eu/home/research/p1-chemistry-at-the-crack-tip/
    Abstract

    The chemical environment can critically affect the fracture processes, leading to subcritical crack growth. The inner surfaces of the cracks are covered by adsorbates from the surrounding liquid or gas phase. When bonds break in the course of crack propagation, these adsorbates strongly react with the newly created surfaces, for example, by saturating the broken bonds. Examples are stress corrosion cracking in metals and semiconductors or the moisture-driven crack growth in silica. In both cases, the crack propagation induces and drives the incorporation of oxygen species, leading to an oxidation/hydroxylation of the inner surfaces, which completely alters the chemistry at the crack tip.

    In this project we propose to study the complex interplay between bond breaking at the crack tip and the adsorption/bond saturation with molecules from the environment by MD simulations. The aim is to obtain mechanistic insights into environmentally-assisted fracture for model ceramic materials.

  • Teilprojekt P12 - Postdoctoral Project: Quantum-to-Continuum Model of Thermoset Fracture

    (Third Party Funds Group – Sub project)

    Overall project: Skalenübergreifende Bruchvorgänge: Integration von Mechanik, Materialwissenschaften, Mathematik, Chemie und Physik (FRASCAL)
    Term: 2 January 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    URL: https://www.frascal.research.fau.eu/home/research/p-12-postdoctoral-project-quantum-to-continuum-model-of-thermoset-fracture/
    Abstract

    Fracture is an inherently multiscale process in which processes at all length- and timescales can contribute to the dissipation of energy and thus determine the fracture toughness. While the individual processes can be studied by specifically adapted simulation methods, the interplay between these processes can only be studied by using concurrent multiscale modelling methods. While such methods already exist for inorganic materials as metals or ceramics, no similar methods have been established for polymers yet.

    The ultimate goal of this postdoc project is to develop a concurrent multiscale modelling approach to study the interplay and coupling of process on different length scales (e.g. breaking of covalent bonds, chain relaxation processes, fibril formation and crazing at heterogeneities,…) during the fracture of an exemplary thermoset and its dependence on the (local) degree of cross-linking. In doing so, this project integrates results as well as the expertise developed in the other subprojects and complements their information-passing approach.

  • Teilprojekt P2 - Atomistics of Crack-Heterogeneity Interactions

    (Third Party Funds Group – Sub project)

    Overall project: Skalenübergreifende Bruchvorgänge: Integration von Mechanik, Materialwissenschaften, Mathematik, Chemie und Physik (FRASCAL)
    Term: 2 January 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    URL: https://www.frascal.research.fau.eu/home/research/p-2-atomistics-of-crack-heterogeneity-interactions/
    Abstract

    The fracture of a brittle solid is crucially determined by material heterogeneities directly at the crack front where the stress field diverges and the usual homogenization strategies are no longer applicable. While this problem has attracted significant interest, currently no consistent theory that relates local changes in properties to the local fracture behavior and macroscopic failure criteria exists. In contrast to the long-range elastic interactions, the direct interaction of the crack front with heterogeneities cannot be described by continuum methods but requires an atomistic treatment.

    The aim of this project is to study the influence of various types of heterogeneities on the energy dissipation mechanisms in different classes of materials.

  • Teilprojekt P4 - Fragmentation in Large Scale DEM Simulations

    (Third Party Funds Group – Sub project)

    Overall project: Skalenübergreifende Bruchvorgänge: Integration von Mechanik, Materialwissenschaften, Mathematik, Chemie und Physik (FRASCAL)
    Term: 2 January 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    URL: https://www.frascal.research.fau.eu/home/research/p-4-fragmentation-in-large-scale-dem-simulations/
    Abstract

    During the past decade, the technique of Discrete Element Simulations (DEM) made great progress and by now it is generally acknowledged as a reliable tool for bulk solids description in a variety of applications. There is a number of models available in the literature to describe fragmentation of particles in DEM simulations, however, by now the predictive power of these models is still poor, especially when dealing with fragmentation probabilities and fragment size distribution. Current approaches use purely spherical models and there is still a gap in predictive fragmentation models for non-spherical particles.

    The aim of the present research project is to develop a particle model which allows for both realistic modelling of fragmentation in DEM simulations and at the same time highly efficient large scale simulations.

  • Teilprojekt P5 - Compressive Failure in Porous Materials

    (Third Party Funds Group – Sub project)

    Overall project: Skalenübergreifende Bruchvorgänge: Integration von Mechanik, Materialwissenschaften, Mathematik, Chemie und Physik (FRASCAL)
    Term: 2 January 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    URL: https://www.frascal.research.fau.eu/home/research/p-5-compressive-failure-in-porous-materials/
    Abstract

    Materials such as solid foams, highly-porous cohesive granulates, for aerogels possess a mode of failure not available to other solids. cracks may form and propagate even under compressive loads (‘anticracks’, ‘compaction bands’). This can lead to counter-intuitive modes of failure – for instance, brittle solid foams under compressive loading may deform in a quasi-plastic manner by gradual accumulation of damage (uncorrelated cell wall failure), but fail catastrophically under the same loading conditions once stress concentrations trigger anticrack propagation which destroys cohesion along a continuous fracture plane. Even more complex failure patterns may be observed in cohesive granulates if cohesion is restored over time by thermodynamically driven processes (sintering, adhesive aging of newly formed contacts), leading to repeated formation and propagation of zones of localized damage and complex spatio-temporal patterns as observed in sandstone, cereal packs, or snow.

    We study failure processes associated with volumetric compaction in porous materials and develop micromechanical models of deformation and failure in the discrete, porous microstructures. We then make a scale transition to a continuum model which we parameterise using the discrete simulation results.

  • Teilprojekt P7 - Collective Phenomena in Failure at Complex Interfaces

    (Third Party Funds Group – Sub project)

    Overall project: Fracture across Scales: Integrating Mechanics, Materials Science, Mathematics, Chemistry, and Physics (FRASCAL)
    Term: 2 January 2019 - 31 December 2027
    Funding source: DFG / Graduiertenkolleg (GRK)
    URL: https://www.frascal.research.fau.eu/home/research/p-7-collective-phenomena-in-failure-at-complex-interfaces/
    Abstract

    Interface failure in both tension and shear is characterized by a dynamic interplay of local processes (breaking of bonds, interface contacts or – in case of frictional interfaces – asperities) and long-range elastic load re-distribution which may occur either quasi-statically or in a dynamic manner associated with wave propagation phenomena and can be mapped onto a network of partly break-able load transferring elements. This interplay may give rise to complex dynamics which are strongly influenced by contact geometry and also the chemical properties of the interface. A particularly simple case is the transition from static to sliding friction between continuous bodies where such dynamic collective phenomena are being discussed under the label of ‘detachment waves’.

    The goal of P7 is to generalize this concept of ‘detachment waves’ to general problems of failure of frictional or adhesive joints, and to interfaces and bodies which possess a complex multi-scale chemical or geometrical structure, including hierarchical geometrical structures as encountered in biosystems.

No projects found.

2021

  • Versagens- und Brucheigenschaften hierarchisch strukturierter Werkstoffe

    (Third Party Funds Single)

    Term: 1 October 2021 - 30 September 2024
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)

2020

  • Atomistic Simulations of Dislocation Processes

    (Third Party Funds Group – Sub project)

    Overall project: Structural and chemical atomic complexity – from defect phase diagrams to material properties
    Term: 1 January 2020 - 31 December 2023
    Funding source: DFG / Sonderforschungsbereich / Transregio (SFB / TRR)
    URL: https://www.sfb1394.rwth-aachen.de/index.php?id=273&L=1

  • Exploiting Artificial Intelligence for Predicting Subcritical Failure of Microstructurally Disordered Materials

    (Third Party Funds Single)

    Term: 1 October 2020 - 30 September 2023
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract
    Subcritical (creep) failure occurs when materials are subjected over extended periods of time to loads below their short-time strength. Time dependent processes such as plastic creep or chemical reactions, which are often thermally activated, may then lead to a gradual accumulation of microstructural damage and ultimately to materials failure. Even for identically manufactured samples, failure times may exhibit a huge scatter. Furthermore, long failure times are difficult to access in experiment. It is therefore desirable to use monitoring data for sample- or component-specific prediction of residual lifetime. This can avoid costs associated with premature replacement of still functional parts as well as mitigate against in-service failure of damaged components. In the past, such predictions were often based on the characteristic U shape of the creep rate vs time curve, where creep rate first decelerates over time (Stage I) then passes a broad minimum with approximately constant creep rate (Stage II) and then accelerates in the run-up to failure 8Stage III). Sample specific lifetime predictions may then be based upon the location of the creep rate minimum, or upon the time dependency of creep rate in the approach to failure which may be described mathematically by a finite-time singularity at the failure time. Other monitoring approaches focus on characteristic increases in the rate of acoustic emissions, or on the localization of damage and deformation activity in the vicinity of the ultimate failure plane. The proposed project investigates whether methods of artificial intelligence / machine learning can be exploited to obtain improved predictions of residual sample lifetime from monitoring data which characterize the spatial and temporal evolution of deformation activity during creep. To this end we will use both computer generated data obtained from simulation of material models of different complexity, and on the other hand experimental data obtained from serial creep tests on paper samples accompanied by acoustic and optical monitoring. In each case we are dealing with large numbers of data sets (typically some 10000 in case of simulated samples and several 100 for experimental samples) which describe the creep history of individual samples. Part of these data sets is used to train so-called Neural Networks in ‘predicting’ failure by relating the spatial and temporal pattern of creep activity to a predicted failure time. The remaining data are then used to assess the quality of these predictions, and the method is benchmarked against other forecasting strategies described in the literature.

  • Coupling Effects in Re-Programmable Micro-Matter

    (Third Party Funds Group – Sub project)

    Overall project: SPP 2206 - Cooperative Multistage Multistable Microactuator Systems (KOMMMA)
    Term: 1 July 2020 - 30 June 2023
    Funding source: DFG / Schwerpunktprogramm (SPP)
    URL: https://www.spp-kommma.de/89.php
    Abstract

    Programmable matter (PM) is a new emerging concept that is based on self-folding origami. Origami refers to a variety of techniques of transforming planar sheets into three-dimensional (3D) structures by folding, which has been introduced in science and engineering for, e.g., assembly and robotics. In principle, 2D pattern consisting of various materials can be transferred into any 3D pattern. The underlying idea of PM is to create a programmable material that can be shaped on demand reversibly and in different ways in order to perform multiple tasks. The initial planar system is composed of interconnected sections (tiles) that self-fold into a set of predetermined shapes using embedded actuators and magnetic latching. Thus, multiple 3D shapes with multiple functions can be realized. Current demonstrators use unidirectional actuators, consisting of a thin foil of the one-way shape memory alloy (SMA) Nitinol. Therefore, resetting to the initial planar state has to be performed manually before folding can be repeated. This concept has been demonstrated at the macro scale and the scalability of the current technology approach is limited to the size of tiles of several mm.
    Here, we propose to transfer this concept into microtechnology by combining state-of-the art methods of micromachining, multifunctional materials as well as coupled simulation. As manual resetting will not be possible at the microscale, cooperative bi-directional actuation will be introduced, allowing for large bending angles up to 180°. Further challenges are the selective multistable latching and release of many tiles at the micro scale. Cooperation of both mechanisms will be needed to transform from flat shape to various specific 3D shapes and back to the flat shape by autonomous unfolding. Therefore, this project intends to develop a multistage multistable system of SMA and magnetic microactuators.  This new concept of re-programmable micro matter will enable formation and multistage adaptation of 3D shape at different length scales as well as reusability by reversible active unfolding. A monolithic fabrication route will be essential to realize many tiles with high integration density.
    The development of the methods and tools for re-programmable micro matter requires an interdisciplinary approach. Therefore, this project combines the expertise from functional films (S. Fähler), microsystems (M. Kohl) and system simulation (F. Wendler).

  • Coupling Effects in Re-Programmable Micro-Matter

    (Third Party Funds Single)

    Term: 1 July 2020 - 30 June 2023
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract
    Programmable matter (PM) is a new emerging concept that is based on self-folding origami. Origami refers to a variety of techniques of transforming planar sheets into three-dimensional (3D) structures by folding, which has been introduced in science and engineering for, e.g., assembly and robotics. In principle, 2D pattern consisting of various materials can be transferred into any 3D pattern. The underlying idea of PM is to create a programmable material that can be shaped on demand reversibly and in different ways in order to perform multiple tasks. The initial planar system is composed of interconnected sections (tiles) that self-fold into a set of predetermined shapes using embedded actuators and magnetic latching. Thus, multiple 3D shapes with multiple functions can be realized. Current demonstrators use unidirectional actuators, consisting of a thin foil of the one-way shape memory alloy (SMA) Nitinol. Therefore, resetting to the initial planar state has to be performed manually before folding can be repeated. This concept has been demonstrated at the macro scale and the scalability of the current technology approach is limited to the size of tiles of several mm. Here, we propose to transfer this concept into microtechnology by combining state-of-the art methods of micromachining, multifunctional materials as well as coupled simulation. As manual resetting will not be possible at the microscale, cooperative bi-directional actuation will be introduced, allowing for large bending angles up to 180°. Further challenges are the selective multistable latching and release of many tiles at the micro scale. Cooperation of both mechanisms will be needed to transform from flat shape to various specific 3D shapes and back to the flat shape by autonomous unfolding. Therefore, this project intends to develop a multistage multistable system of SMA and magnetic microactuators. This new concept of re-programmable micro matter will enable formation and multistage adaptation of 3D shape at different length scales as well as reusability by reversible active unfolding. A monolithic fabrication route will be essential to realize many tiles with high integration density.The development of the methods and tools for re-programmable micro matter requires an interdisciplinary approach. Therefore, this project combines the expertise from functional films (S. Fähler), microsystems (M. Kohl) and system simulation (F. Wendler).

  • Project K – Multi-Scale Modeling of Electromechanical Coupling in Perovskite-Based Ferroelectric Materials and Composites

    (Third Party Funds Group – Sub project)

    Overall project: IGK 2495: Energy Conversion Systems: From Materials to Devices
    Term: 1 January 2020 - 30 June 2024
    Funding source: DFG / Graduiertenkolleg (GRK)
    Abstract

    Ferroelectrics based on disordered perovskites, including relaxor-type materials, are strong alternatives to conventional lead-based PZT. Ferroelectrics exhibit a strict coupling between crystal lattice deformation and the spontaneous electric polarization, which can be used as an order parameter in phase-field models (PFM) to describe the microstructure at the meso-scale. Such PFM are suitable for modeling the coupled evolution of electrical and mechanical fields and microstructural order parameters in a multi-physics setting. However, unlike conventional ferroelectrics, the properties of relaxor ferroelectrics (RFs) depend crucially on the presence of atomic level randomness. This can be expressed as quenched compositional disorder that induces random fields that interact with the ferroelectric domain structure. The question remains how meso-scale modeling approaches can be adequately parameterized within a multiscale framework, based upon parameters characterizing compositional disorder on the atomic scale and quantum mechanical descriptions of the atomic-level interactions. To develop a multiscale framework for RFs, the range of accessible system sizes in molecular dynamics (MD) needs to be expanded beyond the current state-of-the art and reliable MD potentials for describing polarizable multi-component systems need to be developed. Moreover, the dependence of the phase transition in ferroelectrics on (chemical) disorder is of major importance, where especially the elastic constraints imposed by deposition as thin films may offer strongly improved effects in the dielectric response.

    This project aims to establish a consistent multi-scale description of perovskite-based ferroelectrics for both chemically ordered and disordered perovskite materials, including the atomic-scale interactions of structure, disorder, and polarization, and the implementation of atomistic information into a meso-scale model, capable of predicting macro-scale performance. In the focus is the formation and stability of polar nano-regions in RFs and their mutual interaction by simulations on both length scales, explaining their temperature dependent dynamics. Existing methods for large-scale MD systems will be adapted and improved for ferroelectric materials, and an effective numerical implementation of the PFM realized.

  • Untersuchung der Versetzungsdynamik an Oberflächen und Grenzflächen mit der Diskret-Kontinuums-Methode mit Anwendung auf das Verformungsverhalten von metallischen Nanolaminaten

    (Third Party Funds Single)

    Term: 1 June 2020 - 31 May 2023
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract

    Hauptziel des Projekts ist es, das plastische Verformungsverhalten metallischer Nanolaminate durch versetzungsdynamische Simulation zu untersuchen und hierfür verbesserte Simulationsverfahren zu entwickeln. Metallische Nanolaminate entstehen durch Kombination von Schichten aus unterschiedlichen Metallen mit Schichtdicken im Nanometerbereich. Ihre Plastizität wird durch Wechselwirkungen der Kristallversetzungen mit den intermetallischen Grenzflächen der Laminatstruktur bestimmt. Diese Wechselwirkungen finden einerseits auf der atomaren Skala statt. Hier bestimmen, ähnlich wie an inneren Grenzflächen (Korn- und Zwillingsgrenzen), atomare Umlagerungsprozesse die Emission, Absorption oder Transmission von Versetzungen. Zusätzlich führen die elastischen Randbedingungen an Grenzflächen aus elastisch stark unterschiedlichen Materialien zu weitreichenden elastischen Wechselwirkungen in Form von Bildkräften. So werden Versetzungen des elastisch weicheren Materials von der Grenzfläche abgestoßen, Versetzungen des elastisch härteren Materials angezogen (Koehler-Effekt). Zugleich modifizieren die elastischen Grenzflächeneffekte die Spannungsfelder und Wechselwirkungen der Versetzungen untereinander. Eine wesentliche Voraussetzung für die Simulation des Verformungsverhaltens von Nanolaminaten mit der Methode der diskreten Versetzungsdynamik ist daher eine genaue Berechnung der Grenzflächeneinflüsse auf die inneren Spannungsfelder des Versetzungssystems.In dieser Hinsicht sind in den letzten Jahren durch Entwicklung der sogenannten Diskret-Kontinuums-Methode wichtige Fortschritte gemacht worden. Hierbei werden die inneren Spannungsfelder in hybrider Weise durch Überlagerung der singulären Spannungsfelder von Versetzungssegmenten mit einem langsam variierenden Spannungsfeld beschreiben, das durch Lösung eines räumlich vergröberten Eigenspannungsproblems bestimmt wird und elastische Randbedingungen an Grenzflächen und Oberflächen berücksichtigt. Allerdings wurden bislang für die singulären Spannungsfelder stets Ausdrücke verwendet, die streng nur im Inneren eines unendlich ausgedehnten Körpers gelten. Dies führt an Grenz- und Oberflächen zu einer Fehlpassung und damit zu einer ungenauen Beschreibung der Versetzungsdynamik. Wir beheben dieses Defizit, indem wir bei der Berechnung der singulären Spannungsfelder in Grenz- bzw. Oberflächennähe mathematisch exakte Korrekturterme berücksichtigen. Diese wurden in den letzten Jahren von Prof. Yuan hergeleitet, der an dem Projekt als Mercator-Fellow beteiligt ist. Die verbesserte Diskret-Kontinuums-Methode erlaubt es uns, Grenzflächeneffekte und inhomogene Verformungsprozesse (Nanoindentierung) in Nanolaminaten genau zu beschreiben. Wir nutzen dies, um komplexe Beobachtungen wie z.B. Festigkeitsumkehr in Cu-Au Laminaten (das elastisch festere Cu plastifiziert stärker als das elastisch weichere Au) zu verstehen und das Verhalten gradierter Nanolaminate unter verschiedenen Lastfällen zu analysieren und zu optimieren.

2019

  • Forschungspreis Prof. Dr. Stefano Zapperi

    (Third Party Funds Single)

    Term: 1 September 2019 - 31 August 2020
    Funding source: Alexander von Humboldt-Stiftung

2018

  • Dämpfung von intelligenten miniaturisierten Systemen mit Formgedächtnislegierungen

    (Third Party Funds Single)

    Term: 1 February 2018 - 30 September 2019
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)

  • Flexural strength and Deformation Mechanics of BCC Metallic Nanowires under Ben

    (Third Party Funds Single)

    Term: 1 January 2018 - 31 December 2019
    Funding source: Deutscher Akademischer Austauschdienst (DAAD)

  • Influence of topology, interfaces and local chemical compositions on the deformations behavior of nanostructures

    (Third Party Funds Group – Sub project)

    Overall project: In situ Microscopy with Electrons, X-rays and Scanning Probes
    Term: 1 April 2018 - 30 September 2022
    Funding source: DFG / Graduiertenkolleg (GRK)
    Abstract

    Metallic nanostructures and nanostructured materials currently receive much attention due to their often superior mechanical properties compared to bulk materials with larger microstructural features. In Project B6 atomistic simulations are used to study the mechanical properties of individual nano-objects and grain boundaries, as well as their combination (e.g., twinned nanowires, nanocrystalline thin films). The aim is to complement the experimental investigations and provide insights into fundamental deformation mechanisms not readily observable in the experiments, and to derive information for meso- and continuum-scale models of small scale plasticity.

  • Molekulare Simulationen zur Entwicklung nanoskaliger Verbundwerkstoffe aus Metallen und Kohlenstoff-Nanoteilchen

    (Third Party Funds Single)

    Term: 1 April 2018 - 31 March 2021
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)

  • Schädigungstoleranz hierarchisch-modularer Netzwerke: Wie vielskalige Biosysteme trotz Schädigung funktionieren.

    (Third Party Funds Single)

    Term: 1 February 2018 - 31 January 2021
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)

2017

  • Fracture Across Scales and Materials, Processes and Disciplines

    (Third Party Funds Group – Sub project)

    Overall project: Fracture Across Scales and Materials, Processes and Disciplines
    Term: 1 September 2017 - 31 July 2022
    Funding source: EU - 8. Rahmenprogramm - Horizon 2020

  • Microscopic Origins of Fracture Toughness

    (Third Party Funds Single)

    Overall project: Microscopic Origins of Fracture Toughness
    Term: 1 May 2017 - 30 April 2022
    Funding source: ERC Consolidator Grant

2016

  • Fiber Bundle and Fiber Network Models for Failure of Materials with Herachical Microstructure

    (Third Party Funds Single)

    Term: 1 January 2016 - 31 January 2019
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract

    Many natural (biological) materials, but also engineered materials are characterized by hierarchical microstructures where load carrying elements are hierarchically grouped into modules, which in turn are grouped into modules of higher order (for a simple example, think of a wire rope). In biological load-carrying components many hierarchical levels may be nested in this manner (e.g. up to seven in the case of tendon). Hierarchical materials exhibit very high resilience against failure despite consisting of elementary components that may be highly unreliable and defected. It is our main conjecture that this resilience can be attributed to two aspects of the hierarchical architecture -- on the one hand a capability to efficiently re-distribute loads across the microstructure, which mitigates against failure by system-wide damage accumulation and damage percolation, on the other hand a capability to contain failure in single modules which mitigates against growth of localized cracks. This dualism between resilient system connectivity on the one hand and modular localization on the other hand is but one example of a generic type of behavior in systems with hierarchical modular architectures, which also encompass metabolic networks or brain networks as recently investigated by statistical physicists (see e.g. P. Moretti and M. Munoz, Nature Communications 4, 2013). In this project we aim at achieving a basic conceptual understanding of load-driven failure processes in materials with hierarchical microstructure that consist, on the smallest scale, of highly unreliable elements with a large scatter in failure strength. In particular we want to clarify whether failure in such materials occurs by system-wide breakdown (damage percolation) or by nucleation-and growth of localized flaws, and we want to understand the nature of the related precursor dynamics in view of the fundamental question whether failure prediction from precursors is possible or not in such systems. We also want to understand how the damage resilience of systems with hierarchical architecture compares with that of on-hierarchical reference systems, and how a system that is optimized in terms of failure stress or fracture energy might look like. The proposed investigation is posited on a conceptual level, we therefore focus on two types of generic models, namely (hierarchical) fiber bundle models and (hierarchical) random fuse models which can be thought of as concept models for real materials. The project as proposed should be understood as the proof-of-concept stage of a larger project, which in a second stage would include quantitative modelling of load bearing materials with engineered hierarchical microstructures through beam and finite element models, their manufacturing by additive manufacturing methods and subsequent testing, and their structural optimization.

  • Dislocation Motion in Single-Phase High-Entropy Alloys -- Theory and Simulation

    (Third Party Funds Single)

    Term: 1 May 2016 - 30 April 2019
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract

    High-entropy alloys (HEA), loosely defined as metallic solid solutions containing more than four elements in near-equimolar composition, represent an exciting new class of alloys. In particular the combination of high strength and good ductility as well as high hardness, wear and corrosion resistance makes HEAs promising candidate materials for high performance structural applications. The excellent failure resistance of HEAs is commonly attributed to some extreme form of solid solution strengthening. However, as in HEAs "every atom is a solute atom", conventional theories of solid solution hardening cannot be directly applied, and the necessary novel theoretical concepts for analyzing and predicting dislocation motion in HEAs have not yet been established. The aim of this project is to obtain a fundamental understanding of how dislocation glide motion, and hence dislocation plasticity in HEAs, is influenced by their unique underlying atomic structure, and to develop a theoretical framework for predicting the stress and temperature dependence of the dislocation velocity and the concomitant plastic deformation behavior. To this end, we propose a multi-scale modeling approach where atomistic simulations are used to characterize the energy landscape in which dislocations move. The results are analyzed by drawing on methods and theoretical concepts developed in statistical physics to identify the relevant features and spatial correlations of the energy landscape. This information serves as input for the mesoscopic simulation of dislocations using the discrete dislocation dynamics (DDD) method, where atomic-scale information on dislocation-lattice interactions is incorporated in the form of a stochastic pinning field. This combination of atomistic and mesoscale simulations methods allows for the study of thermally activated dislocation motion governed by complex, extended energy barriers, which result from the possibility of the dislocations to adjust their shape to the local pinning energy landscape. In addition, Monte Carlo simulations are used to study how short-range diffusion can lead to ageing by changing the local energy landscape around a dislocation at rest. The changes in energy and the associated length and time scales are then used in a mesoscale framework to investigate the dynamic strain aging and PLC-like phenomena, which have been recently observed in HEAs. Ultimately, this study on model systems for single-phase fcc HEAs will serve to develop a methodological framework which enables the computational prediction of the plastic deformation behavior of HEAs based on their atomic structure and composition. Such a framework is crucial for computational alloy design, which is of particular importance for HEA systems, where the large number of compositional degrees of freedom renders conventional alloy optimization by experimental trial-and-error approaches particularly challenging.

  • Dislocation Motion in Single-Phase High-Entropy Alloys -- Theory and Simulation

    (Third Party Funds Single)

    Term: 1 June 2016 - 31 May 2019
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract
    High-entropy alloys (HEA), loosely defined as metallic solid solutions containing more than four elements in near-equimolar composition, represent an exciting new class of alloys. In particular the combination of high strength and good ductility as well as high hardness, wear and corrosion resistance makes HEAs promising candidate materials for high performance structural applications. The excellent failure resistance of HEAs is commonly attributed to some extreme form of solid solution strengthening. However, as in HEAs "every atom is a solute atom", conventional theories of solid solution hardening cannot be directly applied, and the necessary novel theoretical concepts for analyzing and predicting dislocation motion in HEAs have not yet been established. The aim of this project is to obtain a fundamental understanding of how dislocation glide motion, and hence dislocation plasticity in HEAs, is influenced by their unique underlying atomic structure, and to develop a theoretical framework for predicting the stress and temperature dependence of the dislocation velocity and the concomitant plastic deformation behavior. To this end, we propose a multi-scale modeling approach where atomistic simulations are used to characterize the energy landscape in which dislocations move. The results are analyzed by drawing on methods and theoretical concepts developed in statistical physics to identify the relevant features and spatial correlations of the energy landscape. This information serves as input for the mesoscopic simulation of dislocations using the discrete dislocation dynamics (DDD) method, where atomic-scale information on dislocation-lattice interactions is incorporated in the form of a stochastic pinning field. This combination of atomistic and mesoscale simulations methods allows for the study of thermally activated dislocation motion governed by complex, extended energy barriers, which result from the possibility of the dislocations to adjust their shape to the local pinning energy landscape. In addition, Monte Carlo simulations are used to study how short-range diffusion can lead to ageing by changing the local energy landscape around a dislocation at rest. The changes in energy and the associated length and time scales are then used in a mesoscale framework to investigate the dynamic strain aging and PLC-like phenomena, which have been recently observed in HEAs. Ultimately, this study on model systems for single-phase fcc HEAs will serve to develop a methodological framework which enables the computational prediction of the plastic deformation behavior of HEAs based on their atomic structure and composition. Such a framework is crucial for computational alloy design, which is of particular importance for HEA systems, where the large number of compositional degrees of freedom renders conventional alloy optimization by experimental trial-and-error approaches particularly challenging.

2015

  • Deterministic and Stochastic Continuum Models of Dislocation Patterning

    (Third Party Funds Group – Sub project)

    Overall project: FOR 1650: Dislocation based Plasticity
    Term: 1 October 2015 - 30 September 2018
    Funding source: DFG / Forschungsgruppe (FOR)
    Abstract
    The main goal of this project is to understand how spontaneously emerging spatio-temporal fluctuations of dislocation fluxes give rise to inhomogeneous dislocation arrangements such as cell structures (dislocation patterning), and how in turn the presence of these dislocation patterns alters the dislocation fluxes and introduces new emergent length scales into the dislocation dynamics. To analyze the inter-related dynamics of dislocation patterns and dislocation fluxes/plastic flow, we use the CDD framework developed in DFG-FOR1650 over the past three years. We investigate the following specific questions: (i) can the CDD evolution equations explain the spontaneous formation of heterogeneous dislocation patterns in bulk samples (no boundary constraints) from small initial fluctuations, assuming that the dislocation fluxes to depend on dislocation microstructure in a deterministic manner? (ii) how can large fluctuations of the dislocation fluxes, which are an intrinsic feature of plastic flow, be incorporated into the CDD framework? Do these fluctuations assist the emergence of dislocation patterns, or do they rather create a "noise" which impedes pattern formation? (iii) Which factors control the presence or absence of dislocation patterns in small samples and confined geometries, i.e., how does the emergent length scale of dislocation patterns interact with external scales related to phase microstructure, grain and sample size?

  • Localization and criticality in hierarchical modular networks: Modeling activity patterns in the human brain

    (Third Party Funds Single)

    Term: 1 January 2015 - 31 March 2019
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract
    Patterns of neural activity in the human brain are characterized by persistent localization phenomena: the brain is able to sustain activity in specific regions for long times and in the absence of external stimuli - a feature which may be essential for explaining the functional differentiation of distinct brain modules. At the same time, the brain is known to give rise to critical behavior: at global scales, activity develops in the form of temporal spikes, called neural avalanches, whose size distribution follows a power law. This aspect may seem in contradiction with the localization picture since critical behavior is, in statistical physics, usually associated with correlations that span an entire system. However this seeming contradiction can be resolved by adopting novel concepts regarding the structure of brain networks. It has been found recently that such networks can be envisaged as hierarchical modular networks (HMNs). If we consider this type of network models, not only localization emerges naturally as a consequence of the network structure, but one also finds that such networks are able to sustain robust critical behavior without the need for fine tuning to a critical point, which would be required to explain criticality in less sophisticated modelling frameworks. Recent mappings of the brain - both structure and activity mappings - are consistent with the idea that the brain can be modeled as a HMN. Starting from this idea, this project aims at a deeper understanding of the relationship between structural features and activity patterns in HMNs, an investigation to which extent similar relationships between network structure and activity patterns can be identified in real structural and functional mappings of the human brain, and a study to which extent they are modified by age or disease. To this end we use the concept of network localization in HMNs and its investigation by spectral analysis, as previously introduced by the applicant. We apply this methodology both to computer generated model brain networks and to experimental data regarding patterns of anatomical connectivity and activity correlation of the human brain. We investigate the following questions: What are the spectral fingerprints of localization in real brain networks, common to both structural and functional mappings? Can a spectral theory of localization in HMNs explain the emergence of such fingerprints? What insights regarding localization and persistence of activity in the human brain and regarding neural avalanche phenomena can we obtain from large-scale computer simulations of empirically supported brain network models?

2014

  • Pre-standardisation of incremental FIB micro-milling for intrinsic stress evaluation at the sub-micron scale

    (Third Party Funds Group – Sub project)

    Overall project: Pre-standardisation of incremental FIB micro-milling for intrinsic stress evaluation at the sub-micron scale
    Term: 1 January 2014 - 31 December 2016
    Funding source: EU - 7. RP / Cooperation / Verbundprojekt (CP)
    Abstract

    Intrinsic (or residual) stresses, resulting from manufacturing or processing steps, mostly define the performance and limit the lifetime of nanostructured materials, thin films, coatings and MEMS devices. The established techniques for micron-scale measurement of residual stress still have strong limitations, e.g. in terms of spatial resolution, lack of depth sensing, their applicability on non-crystalline materials or accessibility to industry.In this project, a European consortium is established to develop an innovative, highly reproducible and automated measurement protocol for the analysis of residual stress distribution on a (sub)micron-scale, based on incremental focused ion beam (FIB) milling, combined with high-resolution in situ Scanning Electron Microscopy (SEM) imaging and full field strain analysis by digital image correlation (DIC). The activities will focus on the implementation and pre-standardisation of fully automated FIB-SEM, DIC and inverse stress calculation procedures, together with a quantitative analysis and modelling of FIB induced artefacts and stress-structure-properties relationship for the selected materials and devices. The final aim of the project will be the development of innovative design rules, implemented into simulation and optimization tools under coordination of industry partners, for the production of residual stress-controlled nanostructured and amorphous materials, with specific focus on (i) multi-layered nano-coatings, (ii) micro/nano-crystalline and amorphous materials, (iii) MEMS and 3D metal interconnects. The project is expected to realize a breakthrough in measurement, standardization and modelling ability of the residual stress distribution at the (sub)micron scale. The measurement techniques and the simulation tools will provide SMEs in particular with enabling technologies for the design and efficient production of innovative micro-devices with improved in-service performance and substantially reduce development costs.

2013

  • Atomistic simulation of mechanical properties of nanostructures and interfaces

    (Third Party Funds Group – Sub project)

    Overall project: In-situ Characterization of Nanomaterials with Electrons, X-rays/Neutrons and Scanning Probes
    Term: 1 October 2013 - 30 September 2017
    Funding source: DFG / Graduiertenkolleg (GRK)
    Abstract

    Metallic nanostructures (i.e., objects with at least one dimension in the sub-micron range, like thin films, nanowires or nanoparticles) and nanostructured materials (i.e., materials in which the characteristic internal length scale of the microstructure is below 100 nm, like nanocrystalline metals or multilayers) currently receive much attention due to their often superior mechanical properties compared to bulk materials with larger microstructural features.

    Project B6 uses atomistic simulations to study the mechanical properties of individual nano-objects and grain boundaries as well as their combination (e.g., twinned nanowires, nanocrystalline thin films, nanowire junctions).

    The aim of our work is to complement the experimental investigations (collaboration with A1, B3-5) and provide qualitative insights in the fundamental deformation mechanisms not readily observable in the experiments, and to derive information for meso- and continuum-scale models of small scale plasticity.

  • FASS - Physically based modelling and simulation of the mechanical behaviour of metallic thin film systems and fine grained surfaces under cyclic loading

    (Third Party Funds Single)

    Term: since 1 September 2013
    Funding source: DFG-Einzelförderung / Sachbeihilfe (EIN-SBH)
    Abstract
    In this project we aim at physically based modelling and simulation of the mechanical behaviour of polycrystalline metallic thin film systems and micropillars under cyclic loads. For the first time in a fatigue study we will use a comprehensive multiscale modelling framework which comprises molecular dynamics, discrete and continuum dislocation dynamics. With these methods we investigate the interplay of surfaces and interfaces (grain boundaries) with fatigue-induced dislocation patterns and cracks, in order to understand early stages of fatigue failure in bulk systems as well as in microscale components. Focusing the investigation on small samples, which are amenable to full simulation of microstructural processes, allows us to directly validate the modelling effort by comparing our simulations with specifically tailored experiments which are an important part of the project.The ultimate goal of the project is to provide physical foundations for computational design of fatigue resistant microstructures by establishing a predictive multiscale modelling framework for the early stages of fatigue failure. This will be of benefit for a vast range of technological applications including the enhancement of fatigue resistance by surface treatment and fatigue of microscale components.

  • Structure-property relations of individual nanowires

    (Third Party Funds Group – Sub project)

    Overall project: In situ Microscopy with Electrons, X-rays and Scanning Probes
    Term: 1 October 2013 - 30 September 2022
    Funding source: DFG / Graduiertenkolleg (GRK)
    Abstract

    Silver nanowire networks are very promising as flexible electrode for organic (opto)electronics.  They fulfill the requirement of a low sheet resistance combined with high transmittance and macroscopic bending tests show excellent performance. In order to understand failure mechanisms and prospectively optimize the deformation behavior of AgNW electrodes in situ mechanical testing in the TEM is conducted. In situ tensile tests of single Ag NWs  as well as Ag NW networks are conducted in this project in order to perform scale bridging failure analysis

2012

  • Atomistic simulations of elementary dislocation processes in coherent and semi-coherent y/y'-microstructures (C03)

    (Third Party Funds Group – Sub project)

    Overall project: TRR 103: Vom Atom zur Turbinenschaufel - wissenschaftliche Grundlagen für eine neue Generation einkristalliner Superlegierungen
    Term: 1 January 2012 - 31 December 2019
    Funding source: DFG / Sonderforschungsbereich / Transregio (SFB / TRR)
    URL: http://www.sfb-transregio103.de
    Abstract

    Hauptziel des Projekts C3 ist die Aufklärung der relevanten Verformungsmechanismen auf atomarer Skala während der versetzungsgetragenen Hochtemperaturverformung einkristalliner Superlegierun-gen. Hierzu werden atomistische Simulationen zur Bestimmung der Struktur und der Eigenschaften der Elementardefekte (Versetzungen, Stapelfehler, γ/γ’-Grenzflächen,...) sowie zur Untersuchung der Defekt-Defekt Wechselwirkung durchgeführt. Die Ergebnisse werden in enger Zusammenarbeit mit den Mikroskopiegruppen des SFBs interpretiert. Die atomistisch bestimmten Material- und Defektei-genschaften stellen weiterhin wichtige Eingabegrößen für die Multiskalenmodellierung im Rahmen des SFBs dar.

  • Influence of Topological Anisotropy on the Mechanical Properties of Silicate Glasses

    (Third Party Funds Group – Sub project)

    Overall project: SPP 1594: Topological Engineering of Ultra-Strong Glasses
    Term: since 1 August 2012
    Funding source: DFG / Schwerpunktprogramm (SPP)
    Abstract

    Although glasses are generally viewed as isotropic material, freezing-in the flow structure of a glass under load can easily be used to produce anisotropic glass components. One example of such a process is the drawing of oxide glass fibers. Macroscopically, the anisotropic nature of the glass manifests itself in the phenomenon of optical birefringence. Moreover, mechanical properties of glass may be strongly affected by anisotropy. Indeed, it was recently shown that topological anisotropy, i.e. a direction dependence in the way the silica tetrahedra are connected with each other, is the main cause for the one order of magnitude higher strength of glass fibers compared to bulk glasses of the same composition. Given the technical relevance of these findings, relatively little is known about the nature of the topological changes which lead to anisotropic properties, and on how topological anisotropy influences the various mechanical properties.The aim of this proposed research project is to study how anisotropy develops in silicate glasses, how it can be characterized on a topological level, and how topological anisotropy affects the stress-strain response and toughness of silicate glasses. For this purpose, we will combine experimental investigations on the macro scale with in-situ nanomechanical testing in the transmission electron microscope (TEM) and atomistic computer simulations.In detail, the key objectives for the experimental work are the production of bulk anisotropic oxide glasses, the detailed characterization of their structure by scattering techniques and fluctuation electron microscopy and the determination of their (direction-dependent) mechanical properties by macroscopic and microscopic tensile tests, fracture experiments and indentation studies. In addition, silica nanostructures (nanospheres, nanofibers) will be rendered anisotropic by in-situ mechanical quenching in the TEM exploiting the recently discovered phenomenon of superplasticiy that can be controlled via electron irradiation.The atomistic simulations will focus on characterizing the topological anisotropy and studying the mechanisms which lead to anisotropy, as well as on determining the direction dependent mechanical properties as function of anisotropy.Throughout the project, the experimental and simulations efforts are closely linked, e.g. by the simulation of electron diffraction patterns and fluctuation electron microscopy images of MD generated samples and comparison to experimental results, or by the comparison of MD simulations of nanomechanical tests with corresponding in-situ experiments.Such knowledge will be used to (a) specifically engineer anisotropic crack propagation by generating dedicated topological anisotropy and (b) understand, on a topological basis, crack propagation in a more complex (multiaxial) field of topological anisotropy and stress. The proposed research will strongly profit from collaborations within the priority programme “Topological Engineering of Ultra-Strong Glasses”, e.g. on the influence of topology on mechanical properties of bulk metallic glasses, where recently similar effects of anisotropy on elastic properties were reported, or on micromechanical testing of oxide glasses.

2011

  • CDD as a Mesoscopic Field Theory: Dynamic Closure and Multiphysics Extension

    (Third Party Funds Group – Sub project)

    Overall project: FOR 1650: Dislocation based Plasticity
    Term: since 1 October 2011
    Funding source: DFG / Forschungsgruppe (FOR)
    Abstract
    CDD as developed in the previous reporting period is a mesoscopic field theory which describes dislocation microstructure evolution in terms of density-like field variables. Due to its relations with the classical continuum theory of dislocations, CDD gives natural access to mesoscopic internal stresses above the single-dislocation scale. At the same time, the theory requires dynamic closure relationships which express the local dislocation velocities as nonlinear and in general non-local functionals of the dislocation fields. In physical terms, these closure relationships provide an averaged representation of the interactions of individual dislocations in a continuum setting.The first line of research pursued in the present project uses an entirely novel approach to the problem of dynamic closure, by departing from commonly used phenomenological approximations and using instead a data-driven research paradigm to parameterise generic nonlinear functions on the basis of data extracted from large-scale discrete dislocation dynamics simulations.The second line of research in this project will bridge the gap between CDD and experiments and demonstrate direct technological relevance of our model: coupling CDD with other mesoscopic field theories in the sense of a multiphysics approach allows e.g. to predict and analyze the coupled evolution of defect and phase microstructures in advanced alloy systems. This will be demonstrated for the coupling of CDD with a phase field approach in order to describe the coupled stress-driven dynamics of dislocation creep and directional coarsening in gamma/gamma´ alloys.

2007

  • Plastic deformation, crack nucleation and fracture in lightweight intermetallic composite materials

    (Third Party Funds Group – Sub project)

    Overall project: Exzellenz-Cluster Engineering of Advanced Materials
    Term: 1 November 2007 - 31 October 2017
    Funding source: DFG / Exzellenzcluster (EXC)
    URL: https://www.eam.fau.eu/