Multiscale mechanics

The concurrently coupled Quantum Mechanics (QM) - Continuum Mechanics (CM) approach for electro-elastic problems is considered in this proposal. Despite the fact that efforts have been made to bridge different description of matter, many questions are yet to be answered. First, an efficient Finite Element (FE)-based solution approach to the Kohn-Sham (KS) equations of Density Functional Theory (DFT) will be further developed. The h-adaptivity in the FE-based solution with non-local pseudo-potentials,…

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MOCOPOLY is a careful revision of an AdG2010-proposal that was evaluated above the quality threshold in steps1&2. In the meantime the applicant has made further considerable progress related to the topics of MOCOPOLY. Magneto-sensitive polymers (elastomers) are novel smart materials composed of a rubber-like matrix filled with magneto-active particles. The non-linear elastic characteristics of the matrix combined with the magnetic properties of the particles allow these compounds to deform…

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Classical continuum approaches do not explicitly consider the specific atomistic or molecular structure of materials. Thus, they are not well suited to describe properly highly multiscale phenomena as for instance crack propagation or interphase effects in polymer materials. To integrate the atomistic level of resolution, the “Capriccio” method has been developed as a novel multiscale technique and is employed to study e.g. the impact of nano-scaled filler particles on the mechanical properties of …

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Aussagefähige Bauteilsimulationen erfordern eine quantitativ exakte Kenntnis der Materialeigenschaften. Dabei sind klassische Charakterisierungsmethoden
teilweise aufwendig, in der Variation und Kontrolle der Umgebungsbedingungen anspruchsvoll oder in der räumlichen Auflösung begrenzt. Das Projekt beschäftigt sich
deshalb mit der Ertüchtigung hochauflösender Meßmethoden wie Nanoindentation oder Rastkraftmikroskopie und der komplementierenden Entwicklung…

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Based on the gained knowledge of projects B4 and C5, the aim of this project is to account for the influence of part borders on the resulting material/part-mesostructure for powder- and beam-based additive manufacturing technologies of metals and to model the resulting meso- and macroscopic mechanical properties. The mechanical behavior of these mesostructures and the influence of the inevitable process-based geometrical uncertainties is modelled, verified, quantified and validated especially for cellular grid-based structures.

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In a continuum the tendency of pre-existing cracks to propagate through the ambient material is assessed based on the established concept of configurational forces. In practise crack propagation is however prominently affected by the presence and properties of either surfaces and/or interfaces in the material. Here materials exposed to various surface treatments are mentioned, whereby effects of surface tension and crack extension can compete. Likewise, surface tension in inclusion-matrix interfaces can often not be neglected. In a continuum setting the energetics of surfaces/interfaces is captured by separate thermodynamic potentials. Surface potentials in general result in noticeable additions to configurational mechanics. This is particularly true in the realm of fracture mechanics, however its comprehensive theoretical/computational analysis is still lacking.

The project aims in a systematic account of the pertinent surface/interface thermodynamics within the framework of geometrically nonlinear configurational fracture mechanics. The focus is especially on a finite element treatment, i.e. the Material Force Method [6]. The computational consideration of thermodynamic potentials, such as the free energy, that are distributed within surfaces/interfaces is at the same time scientifically challenging and technologically relevant when cracks and their kinetics are studied.

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The mechanical properties and the fracture toughness of polymers can be increased by adding silica nanoparticles. This increase is mainly caused by the development of localized shear bands, initiated by the stress concentrations due to the silica particles. Other mechanisms responsible for the observed toughening are debonding of the particles and void growth in the matrix material. The particular mechanisms depend strongly on the structure and chemistry of the polymers and will be analysed for two classes of polymer-silica composites, with highly crosslinked thermosets or with biodegradable nestled fibres (cellulose, aramid) as matrix materials.

The aim of the project is to study the influence of different mesoscopic parameters, as particle volume fraction, on the macroscopic fracture properties of nanoparticle reinforced polymers.

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In engineering applications, plastics play an important role and offer new possibilities to achieve and to adjust a specific material behaviour. They consist of long-chained polymers and possess, together with additives, an enormous potential for tailored properties.

Recently, techniques have been established to produce and to disperse filler particles with typical dimensions in the range of nanometers. Even for low volume contents of filler particles, these so-called nanofillers may have significant impact on the properties of plastics. This can be most likely traced back to their very large volume-to-surface ratio. In this context, the polymer-particle interphase is of vital importance: as revealed by experiments, certain nanofillers may e.g. increase the fatigue lifetime of plastics by a factor of 15.

The effective design of such nanocomposites quite frequently requires elaborated mechanical testing, which might - if available - be substituted or supplemented by simulations. For this purpose, however, continuum mechanics together with the Finite Element Method (FE) as the usual tool for engineering applications is not well-suited since it is not able to capture processes at the molecular level. Therefore, particle-based techniques such as molecular dynamics (MD) have to be employed. However, these typically allow only for extremely small system sizes and simulation times. Thus, a multiscale technique that couples both approaches is required to enable the simulation of so-called representative volume elements (RVE) under consideration of atomistic effects.

The goal of this 4-year project is the development of a methodology which yields a continuum-based description of the material behaviour of the polymer-particle interphase of nanocomposites, whereby the required constitutive laws are derived from particle-based simulations. Due to their very small dimensions of some nanometers, the interphases cannot be accessed directly by experiments and particle-based simulations must substitute mechanical testing. The recently developed Capriccio method, designed as a simulation tool to couple MD and FE descriptions for amorphous systems, will be employed and refined accordingly in the course of the project.

In the first step, the mechanical properties of the polymer-particle interphase shall be determined by means of inverse parameter identification for small systems with one and two nanoparticles. In the second step, these properties shall be transferred to large RVEs. With this methodology at hand, various properties as e.g. the particles’ size and shape as well as grafting densities shall be mapped from pure particle-based considerations to continuum-based descriptions. Further consideration will then offer prospects to transfer the material description to applications relevant in engineering and eventually suited for the simulation of parts.

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Nanocomposites have great potential for various applications since their properties may be tailored to particular needs. One of the most challenging fields of research is the investigation of mechanisms in nanocomposites which improve for instance the fracture toughness even at very low filler contents. Several failure processes may occur like crack pinning, bi-furcation, deflections, and separations. Since the nanofiller size is comparable to the typical dimensions of the monomers of the polymer chains, processes at the level of atoms and molecules have to be considered to model the material behaviour properly. In contrast, a pure particle-based description becomes computationally prohibitive for system sizes relevant in engineering. To overcome this, only e.g. the crack tip shall be resolved to the level of atoms or superatoms in a coarse-graining (CG) approach.

Thus, this project aims to extend the recently developed multiscale Capriccio method to adaptive particle-based regions moving within the continuum. With such a tool at hand, only the vicinity of a crack tip propagating through the material has to be described at CG resolution, whereas the remaining parts may be treated continuously with significantly less computational effort.

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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.

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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.

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In previous works, the dependence of failure mechanisms in composite materials like debonding of the matrix-fibre interface or fibre breakage have been discussed.  The underlying model was based on specific cohesive zone elements, whose macroscopic properties could be derived from DFT. It has been shown that the dissipated energy could be increased by appropriate choices of cohesive parameters of the interface as well as aspects of the fibre. However due to the numerical complexity of applied simulation methods the crack path had to be fixed a priori. Only recently models allow computing the full crack properties at macroscopic scale in a quasi-static scenario by the solution of a single nonlinear variational inequality for a given set of material parameters and thus model based optimization of the fracture properties can be approached.

The goal of the project is to develop an optimization method, in the framework of which crack properties (e.g. the crack path) can be optimized in a mathematically rigorous way. Thereby material properties of matrix, fibre and interfaces should serve as optimization variables.

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The current research project aims to develop microstructurallymotivated mechanical models for brain tissue that facilitate early diagnosticsof neurodevelopmental or neurodegenerative diseases and enable the developmentof novel treatment strategies. In a first step, we will experimentallycharacterize the behavior of brain tissue across scales by using versatiletesting techniques on the same sample. Through an accompanying microstructuralanalysis of both cellular and extra-cellular components,…

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This project involves manufacturing biopolymer hydrogels and cataloguing their mechanical properties. They serve as replacement materials in order to understand and model the highly-complex behaviour of soft biological tissue. The aim is to generate a catalogue of replacement materials for various soft tissue that links the specific characteristics of their mechanical responses with the relevant modelling approach. This catalogue could make the process of selecting suitable materials for 3D printing of artificial organs or generating suitable models for prognostic simulations considerably easier in the future.

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