Leibniz Universität Hannover Faculty Faculty of Mechanical Engineering
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Institute of Continuum Mechanics
 
Research
List of all research projects

List of all research projects

  • TSM for dynamic processes
    Modeling and simulation of materials with stochastic properties is typically computationally expensive especially for nonlinear materials or dynamic simulations. The Time-separated stochastic mechanics (TSM) can be extended for the dynamic analysis of stochastic visco-elasic materials by incorporation of the transient terms. In transient time-domain simulations a good approximation of the expectation and variance of the reaction force and the stresses for the dynamic response can be observed. A numerical extra cost of 10% compared with one deterministic finite element simulation is reported. However, the Monte Carlo simulation needs a minimum number of 400 finite element computations to arrive at results, that can be considered converged. Therefore, the TSM provides a fast yet accurate procedure for the dynamic simulation of visco-elastic structures witch stochastic properties.
    Led by: P. Junker, J. Nagel
    Team: H. Geisler
    Year: 2022
  • Virtual element formulation for trusses and beams
    The virtual element method (VEM) was developed not too long ago, starting with the paper Beirao da Veiga et al. (2013) related to elasticity in solid mechanics. The virtual element method allows to revisit the construction of different elements in solid mechanics, however, has so far not been applied to one dimensional structures like trusses and beams. In this project, several VEM elements suitable for trusses and beams are derived. It could be shown that the virtual element methodology produces elements that are equivalent to well know finite elements but also elements that are different, especially for higher order ansatz functions, like 2nd and 3rd order for the truss and 4th order for the beam. It will be shown that these elements can be easily incorporated in classical finite element codes since they have the same nodal degrees of freedom as finite beam elements. Furthermore, the formulation allows to compute nonlinear structural problems undergoing large deflections and rotations.
    Led by: P. Wriggers
    Year: 2021
  • Virtual Kirchhoff-Love plate elements for isotropic and anisotropic materials
    The virtual element method allows to revisit the construction of Kirchhoff-Love elements because the C1-continuity condition is much easier to handle in the VEM framework than in the traditional finite element methodology. Here we study the two most simple VEM elements suitable for Kirchhoff-Love plates as stated in (Brezzi and Marini (2013)). The formulation contains new ideas and different approaches for the stabilization needed in a virtual element, including classic and stabilization. An efficient stabilization is crucial in the case of C1-continuous elements because the rank deficiency of the stiffness matrix associated to the projected part of the ansatz function is larger than for C0-continuous elements. This project aims at providing engineering inside in how to construct simple and efficient virtual plate elements for isotropic and anisotropic materials and at comparing different possibilities for the stabilization. Different examples and convergence studies discuss and demonstrate the accuracy of the resulting VEM elements. Finally, reduction of virtual plate elements to triangular and quadrilateral elements with 3 and 4 nodes, respectively, yields finite element like plate elements. These C1-continuous elements can be easily incorporated in legacy codes and demonstrate an efficiency and accuracy that is much higher than provided by traditional finite elements for thin plates.
    Led by: P. Wriggers, B. Hudobivnik
    Team: P. Wriggers, B. Hudobivnik, O. Allix
    Year: 2021
  • The neighbored element method for damage processes at large deformations
    The developed damage model was extended for the treatment of hyperelastic material subjected to large deformations. Along with the model derivation, a technique for element erosion in the case of severely damaged materials was also developed. Numerical results showed convergence for different mesh sizes and increasing regularization parameter. Efficiency and robustness of the approach is demonstrated by numerical examples including snapback and springback phenomena.
    Led by: P. Junker
    Year: 2021
  • Thermodynamic topology optimization
    For the optimization of the topology, the local material density is defined as design variable within a given design space. The design space describes the geometrical bounds of the structure and to which the (mechanical) boundary value problem is applied. In each point of the design space, the density indicates whether material should be applied in that region or not. For mathematical relaxation, the density variable is continuous allowing intermediate densities during the optimization process, i.e. porous material. Intermediate densities are penalized so that the final topology contains approximately only full and void material (SIMP-approach). The underlying mathematical problem is ill-posed and according regularization techniques have to be applied. A gradient-enhanced regularization is added for the density field and the evolution equation is formulated in its strong form. With the backward Euler scheme and an internal loop for numerical stability, no additional equation systems besides the FEM have to be solved within the optimization process. The second spacial derivatives in the strong form are computed via the neighbored element method. Herein, only the minimum number of neighboring points are used to calculate the required second spatial derivatives to reduce the calculation effort even further. The formulation is independent of the spacial discretization of the design variable: only data on the close neighborhood between points is required. Therefore, the method is suitable for mesh-based as well as for mesh-free methods. The minimum member size, i.e. the minimum cross section width of a structure feature, can be directly controlled by a user-given parameter. Furthermore, the regularization technique can also be applied to regularization in other material models, as for example damage, wherein the width of the damaged zone can be controlled directly.
    Led by: D. R. Jantos, P. Junker
    Year: 2021
  • Large deformations
    With only minor modifications to the optimization, the model is also capable to optimize structures under finite (large) deformations including buckling phenomena. The optimized topology is rather different than for small deformations and with large deformations, the direction of the applied forces influence the final topology (changing the sign of the forces considering small deformations does not change the result). The regularization and therefore the minimum member size are applied from perspective of production, i.e. the undeformed state, and do not require any recalculation of the operator matrices. The calculation effort of the optimization compared to the mechanical analysis is negligible.
    Led by: P. Junker
    Year: 2021
  • Plasticity
    Plastic deformation or plastic zones can weaken the structure drastically or are also planned into the design of structure. Usual approaches for optimization with plastic material require the calculation of a full plasticity analysis with multiple load steps until convergence for each design optimization step, which results in a large number of mechanical analysis steps and therefore large calculation efforts. In the novel approach, a dissipation-free plasticity model is developed, whose evolution is path-independent, so that only one mechanical analysis step is required for each optimization step. In combination with the operator split, the calculation effort for the optimization with plastic material is negligible higher than for an optimization with pure elastic material.
    Led by: P. Junker, D. R. Jantos
    Team: M. Kick
    Year: 2021
  • Anisotropic materials
    High performance materials, as for example carbon fiber reinforced polymers but also structures produced with additive manufacturing inhere anisotropic material properties, which can be influenced during the production process, i.e. the applied direction of fibers or print path within 3D printing. Since the material orientation has a major influence on the structure performance, the local material orientation should also be considered as design variable for the optimization process. With the thermodynamic optimization approach, evolution equations for the optimal material direction described by Euler angles can be found and are combined with a simultaneous topology optimization, which results in significantly different varying optimal typologies in comparison to a topology optimization with isotropic material. For some production processes, as for example reinforcement with long fibers, or simply for a smoother fiber path design, the maximum fiber curvature can be constrained via a filtering technique with the filter radius R given by the user.
    Led by: D. R. Jantos, P. Junker
    Year: 2021
  • Tension and compression affine materials
    Concrete is economical but rather weak under tension load, whereas steel may bear tension and compression very well, but is much less economical. Therefore, an simplified approach for economical steel-concrete structures is to apply concrete only in regions predominant to compression loading and steel under tension loading. By introducing an energetic penalization, this approach can be implemented into an topology optimization with two elastic materials, in which one material is affine to compression (e.g. concrete) and one is affine to tension (e.g. steel). Due to different elastic properties of the both materials, i.e. Young's modulus an Poisson's ratio, the resulting optimization depends strongly on the load direction.
    Led by: D. R. Jantos, P. Junker
    Year: 2021
  • Optimization and additve manufacturing
    The results from the topology optimization are usually very difficult or even impossible to manufacture with conventional methods. However by use of additive manufacturing, as for example 3D printing, the production becomes not only feasible but most optimized structures can be directly produced without modification. However, the material characteristics and also bounds of the additive manufacturing processes, as for example material anisotropy, print directions, overhangs, thermo-mechanical properties should be considered as constraints for the optimization. Those effects strongly depend on the chosen additive manufacturing process and are considered in future projects.
    Led by: D. R. Jantos, P. Junker
    Year: 2021
  • TSM for damage processes
    In industrial applications the reliability of components with high cycle load is of interest. In addition to the TSM a damage model needs to be used. The combination of TSM with a damage model enables to predict the evolution of the reaction forces, which are influenced by the stochastic nature of the materials and the damage processes. The advantages of the Time-Separated Stochastic Mechanics remain. The stochastic properties can be computed in advance of any concrete finite element simulation. This results in a low computational effort in comparison to Monte Carlo Simulations and enables to simulate industrially relevant problems with moderate computational resources.
    Led by: P. Junker, J. Nagel
    Team: H. Geisler
    Year: 2021
  • Physics-Informed Data-Driven Simulation
    This project investigates to what extent simulation with neural networks on the one hand and data-based empirical modeling on the other hand can be combined in a symbiotic manner. The ultimate goal is the generation of reliable models for complex dynamical systems known as digital twins.
    Led by: H. Wessels, P. Wriggers
    Team: H. Wessels
    Year: 2020
  • Water-induced damage mechanisms of cyclic loaded high-performance concretes
    The use of offshore wind energy is expanding and fatigue-loaded concrete structures are built that are submerged in water. This currently already applies to so-called grouted joints, where high-strength fine-grained concrete (grout) is used in the steel support structures of offshore wind turbines. Such constructions are subjected to several hundred million load cycles within their service life. An increased water content in the concrete results from the offshore exposure which is principally different to onshore constructions,. Comparatively few investigations of fatigue-tested concrete specimens immersed in water are documented in the literature. Despite the fact, that considerably scatterings occur in these results a clear tendency can be observed. Specimens that are immersed in water have a significantly lower fatigue resistance compared with specimens tested in air. Some investigations also show that fatigue-loaded concrete specimens immersed in water have a significant change in their fracture behaviour compared with specimens tested in air. This can be seen, in tests, for example, by ascending air bubbles, wash-outs of fine particles and premature crack initiation.Water-induced damage mechanisms in fatigue-loaded concrete have indeed been recognised in the past, but they were not identified and described with sufficient precision. Consequently, they cannot be quantified reliably. Based on the existing knowledge gap, the vast majority of these mechanisms have currently escaped numerical modelling and simulation.The aim of this research project is to understand, analyse and quantify macroscopically water-induced damage mechanisms of fatigue-loaded high-performance concretes in the Experimental-Virtual-Lab (EVL) with complementary very latest state-of-the-art experimental methods. At the same time, models will be created and numerically implemented on a micromechanical basis that enables proving of hypotheses that will be derived from the experimental investigations. The structural data serve for the validation of these models; the data will be determined by µCT scans, NMR measurements and mercury intrusion porosimetry.After a first clarification of the origin of mechanisms by the EVL, modelling at the macroscopic level will be attempted on the basis of the micromechanical investigations. In this way it will be verified, how the water influences the degradation behaviour of fatigue-loaded high-performance concretes and which additional active, water-induced damage mechanisms are decisively involved in the degradation process. It will be possible for the first time to carry out a prediction of the degradation behaviour of fatigue-loaded high-performance concretes immersed in water based on microstructurally orientated parameters.
    Led by: Fadi Aldakheel, Peter Wriggers
    Year: 2020
    Funding: DFG SPP 2020, zweite Förderperiode
    Duration: 3 Jahre
  • Numerical simulation of pile installation in a hypoplastic framework using an SPH based Method
    In this project, a 3D computational tool using smoothed particle hydrodynamics (SPH) is developed which based on a hypoplastic constitutive approach for the mechanical behavior of the soil. The numerical code is firstly validated against a benchmark problem. Then several test cases are simulated including monotonic and vibratory penetration of piles into the soil. A good agreement with the experimental observation is found. Additionally, the impact of pile running in the presence of sheetpiles (retainers) are investigated to see how pile running can alter the applied forces on the sheet piles. The simulation of such complex geotechnical problems which involve large deformation, material nonlinearity and moving boundary conditions demonstrates the applicability and versatility of the proposed numerical tool in this field.
    Led by: Peter Wriggers
    Team: Meisam Soleimani, Christian Weißenfels
    Year: 2020
  • Virtual Element Method for 3D Contact
    Contact plays a very important role in engineering problems, where two or more bodies interact with each other through their surfaces. Many techniques were developed in the past, to formulate the contact constraint at the contact interface between two bodies. Nevertheless, VEM provides efficient and robust properties to enforce the contact constraint through the contact interface. Investigations in 2D have been done so far. This work aims an extensions of VEM to 3D contact problems.
    Led by: F. Aldakheel, B. Hudobivnik, P. Wriggers
    Team: M. Cihan
    Year: 2020
  • Peridynamic Galerkin Methods
    Simulation-driven product development is nowadays an essential part in the industrial digitalization. Notably, there is an increasing interest in realistic high-fidelity simulation methods in the fast-growing field of additive and ablative manufacturing processes. Thanks to their flexibility, meshfree solution methods are particularly suitable for simulating the stated processes, often accompanied by large deformations, variable discontinuities, or phase changes. Furthermore, in the industrial domain, the meshing of complex geometries represents a significant workload, which is usually minor for meshfree methods. Over the years, several meshfree schemes have been developed. Nevertheless, along with their flexibility in discretization, meshfree methods often endure a decrease in accuracy, efficiency and stability or suffer from a significantly increased computation time. Peridynamics is an alternative theory to local continuum mechanics for describing partial differential equations in a non-local integro-differential form. The combination of the so-called peridynamic correspondence formulation with a particle discretization yields a flexible meshfree simulation method, though does not lead to reliable results without further treatment. In order to develop a reliable, robust and still flexible meshfree simulation method, the classical correspondence formulation is generalized into the Peridynamic Galerkin (PG) methods in this project. On this basis, conditions on the meshfree shape functions of virtual and actual displacement are presented, which allow an accurate imposition of force and displacement boundary conditions and lead to stability and optimal convergence rates. Based on Taylor expansions moving with the evaluation point, special shape functions are introduced that satisfy all the previously mentioned requirements employing correction schemes. In addition to displacement-based formulations, a variety of stabilized, mixed and enriched variants are developed, which are tailored in their application to the nearly incompressible and elasto-plastic finite deformation of solids, highlighting the broad design scope within the PG methods. Compared to related Finite Element formulations, the PG methods exhibit similar convergence properties. Furthermore, an increased computation time due to non-locality is counterbalanced by a considerably improved robustness against poorly meshed discretizations.
    Led by: Christian Weißenfels, Peter Wriggers
    Team: M.Sc. Tobias Bode
    Year: 2019
  • Modelling and simulation of the joining zone during the tailored forming process
    [Translate to Englisch:] The topic of project C4 is the multiphysical modelling and simulation of the microstructural behaviour of the joining zone during the tailored forming process. The goal is the determination of the macroscopic, effective, thermomechanical properties of joining zones during and after forming.The model will be able to capture the contact and diffusion processes during joining, the thermal and mechanical properties during forming as well as the material behaviour during heat treatment which is responsible for calibrating material properties.
    Led by: F. Aldakheel, P. Wriggers
    Team: C. Böhm, F. Töller
    Year: 2019
    Funding: DFG im Rahmen des SFB 1153
  • Virtual Element Method for Dynamic Applications
    The Virtual Element Method is a recent developed discretization method, which can be seen as an extension of the classical Galerkin finite element method. It has been applied to various engineering fields, such as elasto-plasticity, multiphysics, damage and fracture mechanics. This project focuses on the extension of VEM towards dynamic applications. In the first part the appropriate computation of the Massmatrix regarding the vitual element ansatzspace will be done. In future works, VEM will be applied to engineering problems, considering the dynamic behavior.
    Led by: F. Aldakheel, B. Hudobivnik, P. Wriggers
    Team: M. Cihan
    Year: 2019
  • Effective Characterization and Modelling of Elastomeric Materials for FE-Applications
    Finite-Element simulations are in wide use for the development and design of industrial products. The calculation of rubber parts faces special problems due to the highly nonlinear and time dependent nature of the material. For good prediction of the final product these effects have to be captured by the underlying material models. Although there is a vast amount of different models, most of them do not offer a sufficient description of the materials behaviour or are numerically inefficient. The latter class, containing more advanced models, suffers from a large amount of parameters. Parameter identification is usually carried out by doing mechanical experiments dependent on the use case of the simulation. There is no guideline regarding the choice, parameters and amount of these experiments. In many cases there has to be a trade-off between numerical cost, effort of characterization and accuracy of the resulting simulation, but the impact of each contribution is not precisely known. The proposed project aims to benchmark both established and emerging models for filled and unfilled elastomers in the context of usability. In more detail, the following indicators shall be investigated -Numerical properties of the models. This includes numerical cost and speed, mesh size dependency and ease of implementation. -Stability and robustness, especially in complex deformation states. -Predictability and plausibility of the models response. Moreover, the relation between fitting parameters and aspects of the materials behaviour is investigated. This question is closely related to the effort of model parameterization. -Physical basis and possibility of physical interpretation of fitting parameters. -Quality of the model fit, which is related to the amount of effects (e.g. relaxation/creep, temperature, volume changes etc.), modelled. -Modularity and extensibility. Based on the benchmarking a scheme for efficient parameter identification shall be drawn, dependent on the specific use case. This includes -Choice of samples and measurement setup. Here, the effort for the different tests is taken into account for different materials. -Measurement protocol including speed and relaxation/creep, if appropriate. -Possibilities to characterize the temperature dependent response.
    Led by: N.H. Kröger
    Team: A. Ricker
    Year: 2019
    Funding: Joint Industry Project
    Duration: 3 years
  • TSM for visco-elastic structures
    The local TSM model for visco-elastic materials can also be employed for finite structures. To this end, it is evaluated for each integration point within a finite element routine. It is also possible to find analytic formulas for the expectation value and the standard deviation of each component of the reaction force. The numerical extra costs are less than 5% needed for a deterministic finite element simulation. Considering a minimum number of 400 finite element computations for a Monte Carlo simulation reveals that TSM provides a fast yet accurate procedure for the modeling of visco-elastic components with stochastic properties.
    Led by: P. Junker, J. Nagel
    Year: 2019
  • The neighbored element method for damage processes
    Damage processes are modeled by a softening behavior in a stress/strain diagram. This reveals that the stiffness loses its ellipticity and the energy is thus not coercive. The underlying partial differental equation wouldn't have a unique solution and the numerical implementation of such an ill-posed problem yields results that are strongly dependent on the chosen spatial discretization. Consequently, regularization strategies have to be employed that render the problem well-posed. A prominent method for regularization is a gradient enhancement of the free energy. This, however, results in field equations that have to be solved in parallel to the Euler-Lagrange equation for the displacement field. Therefore the number of degrees of freedom (unknowns) would increase and the system solution using a finite element approach would be cumbersome and numerically demanding. A gradient-enhanced material model for brittle damage using Hamilton’s principle for nonconservative continua was developed. The model is based on an improved algorithm, combining the finite element with strategies from meshless methods, for a fast update of the damage field function. This numerical treatment is referred to as neighbored element method (NME). The model proves to be numerically stable and fast, with simulation times close to purely elastic problems. In addition, the model provides mesh-independent results.
    Led by: P. Junker, D. R. Jantos
    Year: 2018
  • 2D VEM for crack-propagation
    Led by: F. Aldakheel, B. Hudobivnik, P. Wriggers
    Team: A. Hussein
    Year: 2018
    Funding: IRTG 1627
  • Virtual element method (VEM) for phase-field modeling of brittle and ductile fracture
    Led by: F. Aldakheel, B. Hudobivnik, P. Wriggers
    Year: 2018
    Funding: DFG SPP 1748
  • In-stent restenosis
    Led by: Michele Marino, Peter Wriggers
    Team: Meike Gierig
    Year: 2018
  • Multiscale Modeling of Buckling of Fiber-Reinforced Polymers
    This project is about multiscale modeling of fiber kinking in unidirectional fiber reinforced composites.
    Led by: S. Löhnert, P. Wriggers
    Team: S. Hosseini
    Year: 2018
    Funding: DFG (Graduiertenkolleg 1627)
  • Creep deformation of nickel based superalloys
    Modeling of nickel based superalloys on two scales using crystal plasticity and XFEM methods.
    Led by: P. Wriggers
    Team: Lukas Munk
    Year: 2018
  • Using Machine Learning to Improve the Modelling of Machining and Cutting Processes
    Metal cutting is a fundamental process in industrial production. The fast and accurate on-line prediction of metal cutting processes is crucial for the Intelligent Manufacturing (IM). With the advent of high-speed computing, robust numerical algorithms and machine learning technology, computational modelling serves as a tool for not only accurate but also fast predicting the complex machining processes and understanding the complex physics. In this work, the machine learning based numerical model is developed for simulation of metal cutting processes.
    Led by: C. Weißenfels, P. Wriggers
    Team: M.Sc. Dengpeng Huang
    Year: 2018
    Funding: China Scholarship Council (CSC)
  • The stress and fatigue analysis of the transportation line
    The conveyor of the production line in Salzgitter Flachstahl GmbH which carries the steel coils is going to be subjected to an extra load due to the bigger coil size. The objective is to do a structural analysis of the conveyor to see if the safety factor of structure is still in the allowable region. Since, the loading condition is naturally variable due to continuously feeding the moving conveyor with steel coils, a fatigue analysis is required in addition to an ordinary static analysis.
    Led by: Peter Wriggers
    Team: Meisam Soleimani
    Year: 2018
  • Charakterisierung sowie Modellbildung zur Beschreibung von Kompressionsmoduli technischer Gummiwerkstoffe
    In der Auslegung von Elastomerbauteilen steht zu Beginn ein aufwendiger Entwicklungsprozess. Dieser Prozess wird oftmals mit Finite-Elemente-Analysen (FEA) zur Vorhersage der mechanischen und dynamischen Eigenschaften begleitet. Insbesondere für Dichtungsanwendungen sind vorab Aussagen über die Verformungen im Bauteil sowie Anpressdrücke, somit Aussagen zur Beständigkeit, von großem Interesse. Im Gegensatz zur Reifenindustrie, die fast ausschließlich sehr große Unternehmen umfasst, sind in der Herstellung von technischen Gummiwaren wie z.B. Feder-Dämpfer-Elementen, Dichtungen, Profilen und Schläuchen zahlreiche kleinere und mittlere Unternehmen tätig, für die die Projektergebnisse und deren Umsetzung auf Grund zu meist relativ geringer Entwicklungskapazitäten besonders wichtig sind. Zur Vorabberechnung per FEA sind Materialmodelle für die technischen Gummiwerkstoffe von Nöten. In Vereinfachung wird für die eingesetzten Elastomere Inkompressibilität angenommen. Diese Annahme ist korrekt für einfache Zugzustände. Überlagern sich einfache Zugzustände mit hydrostatischem Druck können deutliche Abweichungen zwischen der Annahme idealer Inkompressibilität sowie Kompressibilität vorkommen. In realen Bauteilbelastungen wird das Elastomer zudem zyklisch belastet und Teilbereiche im Bauteil erfahren unterschiedliche Belastungshistorien. Die aus der Belastung resultierende Inelastizität und Materialerweichung zum einem und Effekte eines nichtlinearen Kompressionsmoduls zum anderen führen in der Validierung bestehender Materialmodelle zu unbefriedigenden Resultaten. Im Rahmen dieses Projektes soll insbesondere ein Augenmerk auf die Quantifizierung des Kompressionsmoduls auf zyklische hydrostatischer Drücke, vgl. Dichtungen, gelegt werden. Diese Betrachtung findet bisher in der Literatur kaum Beachtung. Auf Grundlage der experimentellen Versuche wird ein geeignetes Materialmodell zur Beschreibung der Effekte erweitert und in der FEA validiert. In der Herstellung von technischen Gummiwaren wie z.B. Feder-Dämpfer-Elementen, Dichtungen, Profilen und Schläuchen sind zahlreiche KMU tätig, für die die Projektergebnisse und deren Umsetzung auf Grund zu meist relativ geringer Entwicklungskapazitäten besonders wichtig sind.
    Led by: N.H. Kröger
    Team: A. Ricker
    Year: 2018
    Duration: 2 Jahre
  • TSM for visco-elastic materials
    The TSM has successfully been derived for visco-elastic materials. Here, two internal variables need to be computed which approximate the usual viscous part of the total strain. These internal variables are time-dependent and thus also vary for varying loads; however, they are deterministic. The expectation and standard deviation for both the stochastic viscous part of the strains and the stresses is computed by means of the two internal variables and deterministic coefficients which depend on the stochastic behavior of the material. Once they are computed, they remain constant even if the external load is changing. This results in a computational effort which is doubled as compared to classical (deterministic) material models for visco-elasticity. However, for a Monte Carlo approach, at least 400 realizations need to be performed. This renders the TSM approach to be faster than Monte Carlo simulations by a factor of approximately 200.
    Led by: P. Junker, J. Nagel
    Year: 2018
  • Smart hydrogels for drug-delivery and bioprinting applications
    Led by: Peter Wriggers
    Team: Aidin Hajikhani
    Year: 2017
  • Water-induced damage mechanisms of cyclic loaded high-performance concretes
    Led by: P. Wriggers
    Team: F. Aldakheel
    Year: 2017
    Funding: DFG SPP 2020, erste Förderperiode
    Duration: 3 Jahre
  • Environmentally best practices and optmisation in hydraulic fracturing for shale gas/oil development
    Collaborative Research Project funded in the "MSCA-RISE-2016 - Research and Innovation Staff Exchange" Program and the "Horizon2020-EU.1.3.3. - Stimulating innovation by means of cross-fertilisation of knowledge" Program.
    Led by: Prof. Dr.-Ing. Timon Rabczuk
    Year: 2017
    Funding: : H2020-EU.1.3.3. - Stimulating innovation by means of cross-fertilisation of knowledge
    Duration: 01.01.2017-31.12.2020
  • Computational modeling of in-stent restenosis
    This project aims to develop a computational tool for the modeling of the long-term behaviour of stent deployment, with a special focus on both current and prospective approaches. A multiscale description of arterial constitutive behaviour will be employed, including possible damage due to the stenting procedure. Moreover, the chemo-biological mechanisms underlying the growth and remodelling due to wound healing in tissues will be introduced. Therefore, a multiphysics computational framework will be developed and applied for the analysis of both metal and drug-eluting stents. The project will allow to identify the dominant mechanisms driving in-stent restenosis, deciphering the role of the different molecular species in the pathological remodeling of arteries. The final target is to optimise the clinical outcome of current approaches and propose targeted therapeutic approaches.
    Led by: M. Marino
    Year: 2017
    Funding: Masterplan SmartBiotecs, MWK (Lower Saxony, Germany)
  • In silico morphogenesis of collagen tissues for targeted drugs and bio-printing
    This project targets to understand the mechanisms behind the morphogenesis and the development of living tissues. To this aim, mechanical and biological actions that contribute to confer tissue desired topology and functionality will be modelled and analysed in silico. A computational framework for the modeling of tissue morphogenesis in natural and bio-printed systems will be developed. In particular, the project will address: the multiscale hierarchical and organized arrangement of tissue constituents; the chemo-mechanical interaction among tissue constituents and among cells; the chemo-mechano-biological mechanisms driving growth and remodeling; the extrusion and the curing of multicellular aggregates blended with bio-inks. Obtained results will target to elucidate mechanisms behind tissue functional/dysfunctional structure; conceive targeted drug systems for stimulating optimal molecular pathways promoting tissue healing; develop novel or optimize existing 3D bio-printing technologies.
    Led by: M. Marino
    Year: 2017
    Funding: Masterplan SmartBiotecs, MWK (Lower Saxony, Germany)
  • ISPH-based Simulation of the Selective Laser Melting Process
    Development of a thermo-mechanical model for the simulation of the SLM process.
    Led by: Christian Weißenfels, Peter Wriggers
    Team: M.Sc. Jan-Philipp Fürstenau
    Year: 2017
  • Fatigue lifetime prediction using Wavelet transformation induced multi-time scaling (WATMUS)
    A fast and accurate numerical method for fatigue lifetime prediction using eXtended Finite Element Method(XFEM) and WATMUS
    Led by: S. Löhnert, P. Wriggers
    Team: Tengfei Lyu
    Year: 2017
  • Red blood cell simulation using a coupled shell-fluid analysis purely based on the SPH method
    If the rheological behavior of a Red-Blood-Cell (RBC) changes, for example due to some infection, it is reflected in its deformability when it passes through the microvessels. It can severely affect its proper function which is providing the oxygen and nutrient to the living cells. In this research project, a novel 3D numerical method has been developed to simulate RBCs based on the interaction between a shell-like solid structure and a fluid. RBC is assumed to be a thin shell encapsulating an internal fluid (Cytoplasm) which is submerged in an external fluid (blood plasma). The approach is entirely based on the smoothed particle hydrodynamics (SPH) method for both fluid and the shell structure. The method was motivated by the goal to benefit from the Lagrangian and meshless features of SPH in order to handle several complexities in the problem due to the coupling between the RBC membrane as a deformable elastic shell and interior/exterior fluids.
    Led by: Peter Wriggers
    Team: Meisam Soleimani
    Year: 2017
    Duration: 3 Jahre
  • Improved Frictional Models for Pile Installations
    Team: M.Sc. Ajay Harish
    Year: 2016
  • Patient-Specific FSI Analysis of the Blood Flow in the Thoracic Aorta
    The complexity of numerical modeling and simulation of blood flow in patient-specific thoracic aorta geometries leads to a number of major computational challenging issues. For instance, for an adequate simulation of the flow and pressure field, the incompressible Navier-Stokes equations have to be solved with the assumption that the relatively thin blood vessels suffer large displacements and undergo large elastic or visco-elastic deformations caused by the pulsatile blood flow. Subsequently, inaccurate predictions would be obtained for the hemodynamic quantities with the very simplifying rigid-wall assumption (CFD modeling). Moreover, when applying only the CFD modeling approach the essential phenomena of pressure wave propagation in cardiovascular systems are disregarded, however these phenomena are of major relevance for clinical practice. Accordingly, strongly coupled FSI schemes are inevitable for comprehensive blood flow simulations in arterial systems, and as blood and vascular walls have comparable densities, monolithic or at least partitioned strongly coupled FSI schemes are required for solving the multi-physics problem with its inherent significant added-mass effects.
    Led by: P. Wriggers, B. Avci
    Team: B. Avci
    Year: 2016
  • Process Simulation for Selective Laser Melting
    A phase change model for solution with the meshfree Galerkin OTM method is developed.
    Led by: Christian Weißenfels, Peter Wriggers
    Team: M.Sc. Henning Wessels
    Year: 2016
  • A Novel Design Approach for Safety at Ship Collision
    Team: M.Sc. Mohsin Ali Chaudry
    Year: 2016
  • 3D-Printing of Curing Polymers
    3D-Printing simulations of curing polymers within the concept of Peridynamics are developed.
    Led by: Christian Weißenfels, Peter Wriggers
    Team: M.Sc. Philipp Hartmann
    Year: 2016
    Funding: DFG (Graduiertenkolleg 1627)
  • High Performance Computing of Stereolithography Processes
    Team: M.Sc. Sandeep Kumar
    Year: 2016
  • Micro- and meso-scale modeling of dental composite materials
    Homogenization techniques can help to optimize composite materials. In this special PhD topic dental composite materials will be investigated. The main goal is, to optimize and improve the mechanical properties of these materials that consist of acrylate polymers and nanoparticles as fillers. One possibility is to change shape and geometric distribution of the fillers. This can be investigated at micro-scale by using homogenization to obtain the effective material parameters and direct computations to investigate material damage. The micro structure of the composite will be obtained by up-scaling of results obtained by the group of P. Behrens and M.A. Schneider who investigate the molecular structure composite. The results are then validated by means of experiments performed in the group of Dr. L. Borchers/Prof. M Stiesch.
    Led by: P. Wriggers, P. Behrens
    Team: M. Shahbaz
    Year: 2016
  • Interfacial effects and ingrowing behaviour of magnesium-based foams as bioresorbable bone substitute material
    Led by: P. Wriggers
    Team: A. Krüger
    Year: 2016
    Funding: DFG
  • Topology optimization of nano piezo/flexoelectric structures for energy harvesting applications
    Energy harvesters convert the ambient vibration into useful electrical energy. The energy harvesting ability of a structure is characterised by the energy conversion factor, which is the ratio of electrical energy and mechanical energy under external mechanical vibrations. The developments in nanotechnology has lead to the field of Nano Electro Mechanical Systems (NEMS). Nano scale energy harvesting devices with nano sized piezoelectric layers have also become a possibility, where the surface elastic and surface piezoelectric effects become dominant.
    Led by: X. Zhuang
    Team: Nanthakumar Srivilliputtur Subbiah
    Year: 2016
  • Nanoindentation for material property characterization
    In this work, techniques are developed for nanoindentation of soft polymers and brittle powdery materials and measurement of properties like modulus, hardness and fracture toughness.
    Led by: P. Wriggers, S. Löhnert
    Team: A. B. Harish, V. Kruppernikova
    Year: 2016
  • Hypoplastic material models for soil-structure interaction problems
    In this work, a hypoplastic material model is implemented using AceGen with an eventual goal to model soil-structure interaction.
    Led by: P. Wriggers, C. Weissenfels
    Team: A. B. Harish
    Year: 2016
  • Thermal conductivity study of two – phase nano composite material
    This project focues on the thermal conductivity of polymer-matrix nano composites accounting for the interface conductance. The influence of the scale effect and different fillers, i.e. spherical, cylindrical and plate-like fillers (fullerene, carbon nanotubes and graphene sheets) with different ratios (plate diameter to plate thickness and length to diameter ratios for plate-like and cylindrical fillers, respectively) on the thermal conductivity is studied. The representative volume elements (RVEs) possess periodical fillers. The computational homogenization is performed to extract the thermal conductivity of polymer-matrix composites.
    Led by: X. Zhuang
    Team: Bo He
    Year: 2016
  • A 3D CAD/CAE integration using isogeometric symmetric Galerkin boundary element method
    A seamless communication of computer aided design (CAD) and computer aided engineering (CAE) has always been the ultimate goal in product lifecycle management. The forward in- tegration CAD/CAE, in which the simulation tasks are operated directly on CAD model, can be achieved by the isogeometric analysis (IGA) within the conventional finite element method (FEM). Despite of this successful implementation that covers many engineering aspects, the crucial challenge in this CAD/CAE integration is the incompatible geometric representation, namely the volumetric representation of CAE versus the boundary representation of CAD in three-dimensional problems.
    Led by: X. Zhuang
    Team: Binh H. Nguyen
    Year: 2016
  • Micro-structure Topology Optimization of Auxetic Materials
    [Translate to Englisch:] Auxetic materials with negative Poisson’s ratio can lead to dramatic enhancements in mechanical properties of structures. Such materials are created by modifying periodic unit cells so that the micro-mechanical structure of the unit cells contain hinge-like features. One of the implication of auxetic materials is their resistance to fracture since the lateral expand of material close up potential cracks. Auxetic materials flow toward the point of applied force in the impact problem and result in increase of dense at the impact zone. This show the indentation properties of auxetic materials and are adopted for various needs in military, automotive industries.
    Led by: X. Zhuang
    Team: Thanh Chuong Nguyen
    Year: 2016
  • Higher-order stress-based gradient-enhanced damage model using isogeometric analysis for shell delamination analysis
    The micro-damage associated with diffuse fracture processes in quasi-brittle materials can be described by continuum damage mechanics. In order to overcome the mesh dependence of local damage formulations, non-local and gradient-enhanced approaches are often employed.
    Led by: X. Zhuang
    Team: Thai Q. Tran
    Year: 2016
  • Virtual Element Method for modeling crack propagation
    The virtual element method (VEM) is a very recent numerical technique for solving partial differential equations. It can be seen as a generalization of the Finite Element Method to arbitrary polygons and polyhedra. What makes VEM become special is that the explicit calculation of integral shape functions is not required. It is possible through introduced polynomial functions and defined degrees of freedom. Due to the fact that VEM is able to generate flexible element mesh type even convex elements or concave elements, it allows us to arbitrarily add more nodes to the large stress concentration areas such as crack tips in crack simulation. In this study, we aim to develop an approach utilizing VEM to model crack growth with minimal remeshing or without remeshing. Nevertheless, the final formulation should fulfill the consistency and stability term in our approach to guarantee the accuracy and the convergence.
    Led by: X. Zhuang
    Team: Minh T.V Nguyen
    Year: 2016
  • Durability analysis of composite materials by strong discontinuity embedded multifield framework
    Composite materials experience energy mass transport, potential phase change, chemical degradation and mechanical deformation when subjected to high temperature loading or long term chemical interaction with the surrounding environments, with is a coupled multi-physical-chemical process. Moreover, the evolving micro/macro cracks in the materials will cause extra mass transport and accelerate the degradation of materials, jeopardizing the durability of the structure.
    Led by: X. Zhuang
    Team: Yiming Zhang
    Year: 2016
  • A 3D CAD/CAE integration using isogeometric symmetric Galerkin boundary element method
    Year: 2016
  • Entropic approach to modeling Mullins effect in non-crystallizing filled elastomers
    The work was done in collaboration with Mrs. Aarohi B. Shah and Dr. Julian J Rimoli of School of Aerospace Engineering, Georgia Tech, USA. In this work, we investigate non-crystallizing nanoparticle-reinforced polymers. The effects of the interface rubber between elastomeric matrix and filler particles and its alteration are investigated as a primary cause of Mullins and Payne effect.
    Led by: P. Wriggers, J. J. Rimoli
    Team: A. B. Harish, A. B. Shah
    Year: 2016
  • Advanced multiscale computational mechanics for physiopathological behavior analysis of tissues and organs
    The physiological functionalities of large biological structures are highly affected by the mechanics of living tissues which is, in turn, related to microstructural arrangement of histological constituents and biochemical environment at nanoscale. Present research activity aims to develop a novel tissue multiscale description, including also inelastic mechanisms, coupled with an advanced computational formulation under finite strain and large-displacement assumptions. As a result, an innovative in-silico tool for simulation of organs and large biological structures will be developed, allowing to predict pathology-related damage or pharmacological-related healing and providing novel diagnostic and clinical indications for highly patient-specific medical treatments.
    Led by: P. Wriggers, M. Marino
    Year: 2015
    Funding: Alexander von Humboldt-Stiftung
  • Micro-Mechanically Based Modeling of Degradation of Composite Materials with Random Microstructure
    Materials undergo degradation in different situations that are characterized by the environmental conditions. Here a thermo-chemo-mechanical investigation using knowledge of the microstructure will be performed. However usually the microstructures have a random distribution of the phases. For such microstructures a continuum damage mechanics framework has to be developed at micro-level that depends on thermal but also chemical reactions. The associated coupled problem includes partial differential equations of different type and thus robust numerical schemes have to be designed for the solution of the problem. Validation at micro level will be needed to calibrate the continuum modeling. This will be achieved in collaboration with the Institute for Material Science.
    Led by: Prof. P. Wriggers
    Team: Dipl.-Ing. V. Krupennikova
    Year: 2015
    Funding: DFG (Graduiertenkolleg 1627)
  • Towards multiscale modeling of Abrasive wear
    The work is motivated towards understanding wear as a multiscale-multiphysics approach. A 3D framework is developed to simulate cracks propagation in a microstructure due to contact loading to eventually predict wear trends in filled elastomeric compounds.
    Led by: P. Wriggers
    Team: A. B. Harish
    Year: 2015
  • Mesoscale constitutive modeling of filled elastomers
    In this work, we develop a heterogeneous (or multiphase) constitutive model at the mesoscale model explicitly considering filler particle aggregates, elastomeric matrix and their mechanical interaction through an approximate interface layer. The developed constitutive model is used to demonstrate cluster breakage, also, as one of the possible sources for Mullins observed in non-crystallizing filled elastomers. This work considers the large deformation behavior of the filled elastomers.
    Led by: P. Wriggers
    Team: A. B. Harish
    Year: 2015
  • Numerical Simulation and Experimental Validation of Biofilm Growth
    Biofilms are bacterial colonies growing on solid-fluid interfaces, wherever enough dissolved nutrients are available. Their formation is a complex process in the sense that several Physical phenomena (Reaction-Diffusion-Advection, Sedimentation, Erosion, Fluid-Solid-Interaction) are coupled and consequently different time-scales are involved. In this project, the focus is on the biofilm formation in a flow chamber which resembles the mouth cavity in the vicinity of dental implants. The goal is to develop a computational tool capable of simulating the biofilm growth. Numerical solution of the Navier–Stokes equation in domains with complex boundaries that dynamically change as a result of biological diffusion-reaction, detachment and sedimentation in biofilm growth presents a very serious challenge to grid-based methods. In this project, a fully Lagrangian particle approach(mesh-less method) based on smoothed particle hydrodynamics (SPH) is developed.
    Led by: P. Wriggers , M. Stiesch
    Team: M. Soleimani
    Year: 2015
  • Constitutive modeling of large deformation behavior of filled elastomers
    In this work, a finite thickness cohesive zone model is used to study the large deformation & fracture behavior of filled elastomeric materials at the mesoscale. The study includes and has implications on filler cluster breakage, Mullins softening etc.
    Led by: P.. Wriggers
    Team: A. B. Harish
    Year: 2015
  • Large Deformation Cohesive-Zone Element for Fracture in Rubbery Polymers
    In this work, a 3D cohesive zone element is developed considering material and geometric nonlinearities and suitable for modeling large deformations and rotations.
    Led by: P. Wriggers
    Team: A. B. Harish
    Year: 2015
  • Experimental and Numerical investigation of collision of particle-filled double hull vessel
    A novel design approach for safety of double hull vessel is presently being investigated, which involves usage of granular materials between the hull of ship. This strategy provides a medium between the hull which can absorb impact energy and transfer the load to the inner hull. Therefore, impact energy is shared between two hulls, in contrast to localized impact on outer hull only.
    Led by: P. Wriggers, C. Weißenfels
    Year: 2015
  • Simulation of atherosclerotic plaque impact on the red blood cells dynamics in arteries
    Atherosclerosis, a coronary disease, is one of the main cause of mortality in the many countries. Atherosclerosis emerges as a results of hardening and narrowing of the blood vessels , caused by the gradual accumulation of deposits on the inside of arteries walls. This finally leads to atherosclerotic plaque. In this work, The impact of atherosclerotic plaque on red blood cells (RBC) dynamics in the blood vessels is studied. A fluid-solid interaction (FSI) analysis is developed. For the fluid phase, a mesh-less Lagrangian method is adopted which is called smoothed particle hydrodynamics (SPH). RBCs are considered to be the solid phase and are assumed to behave like deformable solid floated in the blood. A strong two way coupling is developed in the context of immersed boundary method (IBM) to capture the accurate fluid-solid interaction.
    Led by: P. Wriggers
    Team: M.R.Hojjati
    Year: 2015
    Funding: DAAD
  • Gekoppelte Simulation von Aerosolströmungen in asthmatischen Bronchien
    Das Ziel dieses Projektvorhabens ist es, auf Basis eines gekoppelten 3D Mehrfeldmodells die partikelbeladene Luftströmung in gesunden sowie in asthmatisch verengten Bronchien anhand von numerischen Simulationen zu studieren.
    Led by: P. Wriggers, B. Avci
    Team: J. Stasch, J.-P. Fürstenau
    Year: 2014
    Funding: Leibniz Universität Hannover, Wege in die Forschung
  • In many cases, massive parallel large-scale computations are indispensable for solving problems of practical interest. The numerical treatment of such class problems requires not only highly efficient scalable parallel solvers, moreover, efficient parallel algorithms covering the whole simulation pipeline – also including pre- and post-processing, mesh generation and design solvers considering uncertainties – are essential to model and to simulate large-scale or exascale class problems. The specific goal of this project is therefore the development and implementation of new parallel algorithms and methods that will allow to solve large-scale class problems with high efficiency.
    Led by: P. Wriggers, B. Avci
    Team: B. Avci
    Year: 2014
    Funding: FP7 of the EU
  • 3D Dynamic Fracture in Heterogeneous Media
    In this project crack growth in brittle media is being investigated by means of the eXtended Finite Element Method (XFEM) and damage mechanics. XFEM is a numerical method, based on the Finite Element Method (FEM), which is especially designed for treating non-smooth problems such as cracks. An essential advantage of the XFEM is that the finite element mesh does not require updating to be able to track the crack path. Enrichments added to classical FE models take into account the effects of a crack or discontinuity. In fiber reinforced materials a fracture process often starts with the delamination between the matrix material and the fibers. At some point these crack propagation processes may lead to an abrupt rupture of the entire structure. A gradient enhanced damage model is being utilized to evaluate degradation of the material at each point of the domain. In gradient enhanced damage models, a chosen length scale behaves as a localization limiter and describes the influence of the microstructure on the damage process. Moreover, such a model smoothes the deformation of the structure and avoids energy dissipation in a narrow band (surface). Damage values obtained based on this approach are used as the crack propagation criterion.
    Led by: Principal Investigator: Dr.-Ing. Stefan Löhnert - French Co-Advisor in Cachan: Prof. Pierre-Alain Guidault
    Team: Mahmoud Pezeshki
    Year: 2014
    Funding: DFG (Graduiertenkolleg 1627)
  • Application of the Virtual Element Method to Non-Conforming Contact Interfaces
    When using standard Finite Elements the discretization is subject to limitations depending on the element geometry. In contrast to this the Virtual Element Method offers the possibility for elements with an arbitrary number of nodes and special geometries like non-convex polygons or hanging nodes. In this Project the application of the Virtual Elements to different problems is investigated. Here it is used to create an efficient contact discretization.
    Led by: P. Wriggers
    Team: W. Rust
    Year: 2014
  • Modeling 3D crack coalescence and percolation with the XFEM and level sets
    [Translate to Englisch:] In three dimensions the accurate geometrical and mechanical modeling of crack coalescence, crack percolation and the splitting of cracks due to dynamic processes is a severe challenge. Using the XFEM in combination with level sets, new enrichment patterns as well as multiple level set functions need to be defined to account for the complex crack geometries and discontinuities within elements. In addition the definition of accurate fracture criteria for more complex material models remains a challenge. In this project crack coalescence and percolation in three dimensions is investigated in detail and accurate fracture criteria for elastoplastic material behavior within the fracture process zone are developed.
    Led by: S. Löhnert, E. Budyn
    Team: H. Attar
    Year: 2014
  • Multi-Fluid Simulations for High Density Ratios
    Although the contact between fluids or gases of different densities is a common event in nature, like the interaction of water and air, the simulation of multiple fluids often comes along with numerous problems. With the Lagrangian description of continuous fluids in terms of the Smoothed Particle Hydrodynamics (SPH) method multi-fluid interactions within the particle scale can simulated. Nevertheless with the standard SPH algorithms multi-fluid problems can not be solved because of the density jumps at the interfaces. Within the project the state of the art of current multi-fluid approaches are compared and evaluated to develop appropriate methods for the simulation of multi-fluid systems with high density ratios.
    Led by: P. Wriggers, B. Avci
    Team: J.-P. Fürstenau
    Year: 2014
  • Non-convex particle shape and parallelization
    Many materials found in nature or technical processes have a granulated structure. Examples are sand and ores, fruits and grain, (dry) pharmaceutical and chemical products. Compared to other materials, granular materials are difficult to handle: Different particle shapes result in different material behaviour. To have a simulation with realistic particle properties, more complex and more realistic particle shapes are needed. This means, in addition to the often used purely convex particles (spheres and ellipsoids), a description for more complex and non-convex shaped particles is essential. The higher computational costs can be handled by a parallelization.
    Led by: P. Wriggers
    Team: M. Hothan
    Year: 2014
    Funding: DFG (Project: IRTG 1627)
  • Homogenization procedures for coupled thermo-chemo-mechanical problems
    In order to understand processes of continuum damage mechanics it is necessary to investigate thermo-chemo-mechanical coupled processes at micro scale. However for engineering purposes it is still necessary to be able to model these processes at macro scale, especially when large structures have to be designed. Thus a homogenization procedure or a FE^2 framework is necessary to transform the relation, describing the coupled thermo-chemo-mechanical response at micro scale to the macro scale. The related equations and procedures have to be developed and implemented in a finite element software system.
    Led by: P. Wriggers, E. Baranger
    Team: Jin Man Mok
    Year: 2013
  • Modelling the temperature development and crack propagation during sheet-bulk metal forming
    n metal forming processes a large amount of mechanical work is dissipated due to large plastic deformations. The accompanying temperature rise leads to thermal strains and a change in the material behaviour which can influence the mechanical behaviour during the forming process and the final shape of the part. For this reason it is important to consider temperature effects and heat conduction in the material modelling of the polycrystalline microstructure. The resulting thermomechanical problem exhibits a strong coupling since on the one hand through mechanical deformation heat sources are introduced and on the other hand material parameters may depend on the temperature and also large thermal strains can emerge. Experimentally the temperature influence can be analysed by performing experiments at IW, IFUM, LFT or IUL with material specimens at different temperatures. The results can be used to develop a thermomechanical material model for the microstructure. By using homogenization techniques the macroscopic effective material model developed in period 1 of the SFB/TR73 will be extended by temperature effects. Another critical effect occurring during the forming process is the initiation and propagation of microcracks. This effect will lead to a stiffness reduction or even to failure of the entire structure. Therefore it is essential to study the degradation mechanisms of the crystallographic microstructure. A nonlocal damage model will be used to induce microcracks. For propagating the crack existing models have to be extended to nonlinear anisotropic and inelastic materials. Especially a criterion has to be found when cracks collide with grain boundaries. For the case of stable crack growth with a statistical simulation series a representative volume element can be found. This is used to produce a micromechanically motivated stress strain relationship by a homogenization procedure. With this material response the effective material model of period 1 of the SFB/TR73 will be extended. Here it is important to capture the softening effects with a nonlocal damage model which is for example used and developed at the IUL. In a last step the two approaches will be combined for the construction of a material model capturing thermomechanical effects and cracks on the microstructural level.
    Led by: S. Löhnert, P. Wriggers
    Team: S. Beese, S. Zeller
    Year: 2013
  • Numerical simulation and experimental validation of biofilm formation
    In this Reserch , a state-of-the-art 3D computational model has been developed to investigate biofilms in a multi-physics framework using smoothed particle hydrodynamics (SPH) based on a continuum approach. Biofilms are in fact aggregation of microorganisms such as bacteria. Biofilm formation is a complex process in the sense that several physical phenomena are coupled and consequently different time-scales are involved. On one hand, biofilm growth is driven by biological reaction and nutrient diffusion and on the other hand, it is influenced by the fluid flow causing biofilm deformation and interface erosion in the context of fluid and deformable solid interaction (FSI). The geometrical and numerical complexity arising from these phenomena poses serious complications and challenges in grid-based techniques such as finite element (FE). Such issues are generally referred to as mesh distortion. Here the solution is based on SPH as one of the powerful meshless methods. SPH based computational modeling is quite new in the biological community and the method is uniquely robust in capturing the interface-related processes of biofilm formation especially erosion. The fact is that SPH is a versatile tool owing to its adaptive Lagrangian nature in the problems whose geometry is temporarily varying (dynamic). Moreover, its mesh-less feature is considered to be favorable in interpreting the method as a particle based one. Hence, it is quite straight forward to incorporate complex interactions and ad-hoc rules at the particle level into the method. This is the case for the problems with coupled governing equations with different time and length scale. In this thesis all different physics which account for biofilm formation have been implemented in the framework of SPH and one can say that this tool is purely SPH based. Besides the numerical simulation, experiments were conducted by our partners in the medical school of Hannover. The obtained numerical results show a good agreement with experimental and published data which demonstrates that the model is capable of predicting overall spatial and temporal evolution of the biofilms. The developed tool can be employed in either controlling the detrimental biofilms or harnessing the beneficial ones.
    Led by: Peter Wriggers
    Team: Meisam Soleimani, Peter Wriggers, Meike Stiesch
    Year: 2013
  • Numerical simulations of delamination in FRP shell structures using XFEM
    When using Fiber Reinforced Plastics (FRP) delamination can occur leading to significant reduction of the load carrying capacity of a structure. Here the focus is on structural stability (buckling and snap-through). Furthermore, the propagation of delamination in pre- and postbuckling regime is of interest. A standard model for such simulations consists either of two shell elements with nodes at a given location of delamination or of a stack of shell-like solids, one per layer. The former has limits in its application while the latter leads to enormous computational effort. In this project eXtended FEM is applied to structural delamination problems. This allows the description of delamination at arbitrary through-the-thickness locations by means of shape functions enriched for discontinuities. Criteria for starting and propagating delamination should be integrated, if applicable combined with cohesive zone models. Contact in a delaminated zone must be accounted for. The new element must be suitable for large rotations and buckling.
    Led by: Prof. Wilhelm J.H. Rust, Prof. P. Wriggers
    Team: Saleh Yazdani
    Year: 2013
  • Dynamic crack propagation using the XFEM
    Imperfections in composite materials can occur at interfaces but also within the matrix material, particles or fibers. These imperfections often lead to cracks and thus are responsible for degradation and failure of these heterogeneous materials. In this project the attention turns to the dynamic crack propagation. Therefore a numerical approach by using the eXtended Finite Element Method (XFEM) to describe the problem within heterogeneous materials will be developed. The XFEM has proven to be adequate to handle cracks and heterogeneities in a precise geometrical and numerical framework.
    Led by: P. Wriggers, S. Löhnert
    Team: D. Nolte
    Year: 2012
  • SFB 599 TP D1 – Functionalized middle ear prostheses
    The participating institutions in the sub-project D1 of the SFB 599 (Institute of Inorganic Chemistry, LUH, Helmholtz Centre for Infection Research, Ear, Nose and Throat Clinic, Hannover Medical School, IKM) have the aim to develop an optimized middle ear prosthesis. This is done through the use of newly developed biomaterials on the one hand and by using simulation techniques to optimize the design on the other hand. During the last funding period, the focus of the simulation was on the healthy ossicular chain. In the third period the contact between the implant and the tympanic membrane should be simulated. Polymer cushions will be used to create a tissue-friendly interface that enables a uniform loading of the eardrum.
    Led by: P. Wriggers
    Team: S. Besdo, D. Doniga-Crivat
    Year: 2012
    Funding: DFG im Rahmen des SFB 599
  • Investigation of the behavior of the crystalline lens accommodation by introducing femtosecond laser-induced (fs-laser) cut surfaces
    This project is conducted in cooperation with the Laser Zentrum Hannover eV. With age, the ability of the human eye lens to adjust from the distant view of the near vision increases. There is still no satisfactory method of treatment for the so called pres-byopie. However, it was shown that it is possible to influence the flexibility of the lens by introducing micro-cuts with a femtosecond laser (fs-Lentotomie). The aim of this project is to develop a method which makes it possible to predict the changes of accommodation behavior of ophthalmic lenses after cutting using finite element analyses.
    Led by: S. Besdo
    Year: 2012
    Funding: DFG im Normalverfahren
  • SFB 599 TP R6 – Degradable Osteosynthese
    Within the framework of SFB 599 osteosynthesis systems for fracture stabilization out of magnesium alloys are being developed. The advantage of magnesium is that the body needs it for its metabolism and it degrades over time. Due to the mechanical properties and the degradation of magnesium alloys, it is necessary to adjust the implant design. First, the primary stability of implant-bone composite is investigated using finite element analysis, before the implants are tested on animals. In a second step, the degradation has to be integrated into the simulation. Therefore, different simulation methods have been developed and should be adapted to results from in vitro experiments. Furthermore, the magnesium degradation will be considered in the simulation of bone healing.
    Led by: P. Wriggers
    Team: S. Besdo
    Year: 2012
    Funding: DFG im Rahmen des SFB 599
  • Mutiscale FEM approach for rubber friction on rough surfaces
    Understanding the frictional behaviour of elastomers on rough surfaces is of high practical importance in many industrial applications. For example the traction of a tire is directly linked to the material properties of the considered elastomer and the surface conditions of the road track. One goal of our studies is to gain a deeper understanding of the underlying contact physics at all length scales. Another aim is to determine a macroscopic coefficient of friction for varying material and surface properties and to validate the results with experimental data.
    Led by: P. Wriggers
    Team: P. Wagner
    Year: 2012
  • Computational multiscale modelling of localized ductile failure
    Many phenomenological material models capture ductile damage. However, to flexibly incorporate the underlying the governing microstructural properties and damage mechanisms, a multiscale framework is needed. The few recent numerical multiscale works on localized failure are limited to brittle microdamage. Therefore, the goal of this project is to provide a computational multiscale framework for the modelling of localized failure stemming from a microstructure undergoing ductile damage.
    Led by: P. Wriggers
    Team: H. Clasen
    Year: 2012
    Funding: DFG im Normalverfahren
  • Computational homogenisation of elasto plastic material
    Particle-matrix materials are commonly used in different fields(aerospace components, bicycle frames and racing car bodies) for its mechanical and economical advantages,such as high strength, low weight and less expense.However,Because of the microstructural complexities,it is very time and labor consuming to determin the mechanical properties of composite materials. Hence,in order to reduce laboratory expense, numerical simulations via Homogenized techniques are performed on RVE to predict mechanical behavior of composite material.
    Led by: P.Wriggers
    Team: Chao Zhang
    Year: 2012
  • Contact models for soil mechanics
    The installation of foundations influences strongly the load bearing capacity of the soil. The large discrepancy between experimental and numerical results, using Coulomb friction law for modeling the soil structure interaction, points out that new strategies to solve this kind of problems are necessary. Experimental observations show that for rough surfaces of the structure the friction angle at the contact zone corresponds to the friction angle of the soil. This leads to the conclusion that the contact zone lies completely within the soil. A way to improve the friction laws for soil structure interactions is to project the soil models onto the contact surface which is the motivation of this work. </a></p>
    Led by: P. Wriggers
    Team: C. Weißenfels
    Year: 2012
  • Improvement of cardiovascular Implants and a FE-Framework for Degradation of Mg-Alloys
    A surgical option is to replace damaged myocardial tissue, e. g. after a heart attack, with tissue transplants. Problematic is the minor initial strength of these biological grafts to resist loads of the high pressure system. Therefore, scaffolds are developed to mechanically support these grafts. Until now, developed scaffolds do not achieve the required durability. With use of the finite element method, simulations are performed where the scaffolds are deformed according to the heart movement. This allows identifying highly strained regions within the implant that need design changes. Another approach to reduce stresses is preforming scaffolds according to the heart curvature preoperatively. Further, new scaffold designs are developed and tested.&nbsp;<br /> Scaffolds are made from magnesium alloys, which can be resorbed by the body. This degradation affects the mechanical behavior of the implants under load. There are FE simulations developed in which the magnesium degradation is considered.
    Led by: P. Wriggers, J. Lamon, S. Besdo
    Team: M. Weidling
    Year: 2011
    Funding: DFG within IRTG 1627
  • Discrete Element Method
    This project is concerned with the development of a discrete element method (DEM) code for the simulation of large particle systems in 3-D, where also complex moving boundary geometries can be taken into account. The DEM is a well established numerical method to simulate systems consisting of granular matter. Granular mixing, tumbling mills, transport of particles via conveyor belts or screw conveyors are just some examples of important particulate processes in industry sectors like mining, pharmaceutical and food industries. For such systems, the optimization of the design variables as well as the appropriate choice of the operating parameters is still a difficult and a challenging task.
    Led by: P. Wriggers
    Team: B. Avci
    Year: 2011
  • Multiscale Methods for Fracturing Solids
    In this project multiscale methods and homogenization techniques for the numerical simulation of three dimensional fracture processes are developed. These methods are important in aerospace and automotive industries and many other fields of mechanical and civil engineering as well as in bio-mechanics and material science. They will improve the prediction of the failure of structures.
    Led by: P. Wriggers, S. Löhnert
    Team: D. Müller-Hoeppe
    Year: 2011
  • Interaction between tire and vulcanizing mold during extraction
    One sub-step of tire production is the vulcanization of a green tire under pressure inside a mold. Adherence of the rubber to this mold leads to problems during the subsequent extraction phase. Within this project the extraction of a vulcanized tire from a mold will be simulated, to obtain a better understanding of the mechanisms leading to the problems and to develop solution strategies.
    Led by: P. Wriggers
    Team: J.-H. Dobberstein
    Year: 2011
  • Micromechanical Modelling of inelastic grain boundary effects in polycrystalline materials
    The research project focuses on the computational materials modelling of metals microstructure. Within the project, polycrystalline materials shall be micromechanically investigated by means of dislocation-based crystal plasticity. The goal is to gain a clearer understanding in the nonlocal behaviour and to obtain useful models for the prediction of the inelastic response, which become relevant in applications such as micro-manufactured structures.
    Led by: Britta Hirschberger
    Team: H. Clasen
    Year: 2010
    Funding: Leibniz Universität Hannover via programme "Wege in die Forschung II"
    Duration: 1 year
  • Crack propagation and crack coalescence in a multiscale framework
    In this project a numerical framework for propagating and intersecting cracks on micro and macro scales is set up. Modeling cracks using the eXtended Finite Element Method (XFEM) provides an accurate and efficient numerical framework to model propagating and intersecting cracks. Since cracks of different length scales are assumed, a multiscale method is applied in order to be numerical efficient.
    Led by: S. Löhnert, P. Wriggers
    Team: M. Holl
    Year: 2010
  • Multiscale Method for Hydro-Chemo-Thermo-Mechanics Coupling due to Alkali Silica Reaction in Concrete
    Alkali Silica Reaction(ASR) is one of most determinant reasons leading to the deterioration of concrete structures, which can be ascribed to the expansion of gel produced by ASR in the microstructural level of concrete. The challenge in the modeling of ASR is due to the necessity to take into account the presence of heterogeneities and physical processes distributed over multiple length scales. With increasing computation power and the tomography scan technology, it can be implemented by numerical simulation . In this contribution, 3D multiscale hydro-chemo-thermal-mechanical coupling based on a staggered method is demonstrated, which explicitly describes the damage evolution originating from the chemical reaction in the microscale and the dependence on environmental factors.
    Led by: P. Wriggers
    Team: T. Wu
    Year: 2009
  • DEVELOPMENT OF A MATERIAL MODEL FOR METAL SHEETS AT FINITE DEFORMATION
    [Translate to Englisch:] Technical procedures claim an improved material model for the simulation of metal forming processes. An effective, anisotropic elastoplastic material model for the macroscopic material behaviour for sheet metals is developed for further usage in commercial finite element programs.
    Led by: P. Wriggers, S. Löhnert
    Team: E. Lehmann, S. Zeller
    Year: 2009
  • Contiuum Mechanical Modelling of Self-Cleaning Surface Mechanisms
    Some biological surfaces, like several plant leaves, exhibit remarkable self cleaning mechanism. These are called hydrophobic surfaces, where the water does not coat the surface but rather forms small droplets which then roll-off easily on inclined surfaces and sweep foreign pollutants, like dirt or germ particles, away from the surface. A continuum mechanical model suitable for computational multiscale analysis is therefore required for describing the interaction between the water droplet, the pollutant particle and the substrate surface. This model provides better understanding of basic droplet-substrate interaction characteristics such as contact angle, roll off angle and droplet deformation due to the microstructure of the substrate. The surface self-cleaning capabilities can be therefore improved by optimizing the artificial surface microstructure.
    Led by: R. A. Sauer
    Team: M.Osman
    Year: 2009
  • Multiscale modeling and extended finite element analysis of fracture processes in ceramics
    The extended finite element method (XFEM) enables the modelling and calculation of cracks independent of mesh topologies. Due to this advantage it has become the most widely used method for computations of fracture processes. In the vicinity of a macrocrack front the microstructure of the material can have a significant influence and needs to be considered in detail. Especially microcracks can affect crack propagation drastically. This leads to the necessity of multiscale methods like the multiscale projection method to capture microstructural details where necessary. Even though the XFEM is known to be a comparably accurate method to calculate cracks, discretisation errors occur. Therefore, the discretisation error on the finescale is estimated via a stress smoothing technique. The realtive error of stresses enables adaptive mesh refinement on the microscale leading to more accurate results also on the macroscale. The computational model is validated by comparison of the results to experimental data.
    Led by: P. Wriggers, S. Löhnert
    Team: C. Prange
    Year: 2009
  • Coupled Contact of Lubricated Contact
    [Translate to Englisch:] In many engineering applications fluid lubricants are used to separate two solid bodies that rub against one another to minimize friction. Such devices are referred to as hydrodynamic bearings. There exist a large variety of bearings for translational and rotational movement. For engineers designing such bearing, computational methods can deliver useful information on its performance characteristics prior to its manufacturing. Within this project a three-dimensional finite element model is developed, that describes lubricated contact between two bodies with rough surfaces.
    Led by: P. Wriggers
    Team: M. Budt
    Year: 2009
  • MULTIPHYSICS COMPUTATIONAL HOMOGENIZATION METHODOLOGIES
    Computational homogenization techniques that are amenable to a multiscale implementation are being developed for multiphysics problems at the finite deformation regime. Applications include the estimation of the contact conductance for rough interfaces and the modeling of the coupled thermomechanical response of heterogeneous materials.
    Led by: P. Wriggers, I. Temizer
    Year: 2009
  • Direct Numerical Simulation of Multiphase Flows
    Multiphase flows consisting of a continuous fluid phase and a dispersed phase of macroscopic particles are present in many engineering applications. In general, a main task in the study of the particle-laden fluid flow of an application is to make predictions about the system's nature for various boundary conditions, since, depending on the volume fraction and mass concentration of the dispersed phase a fluid-particle system shows quite different flow properties. Unfortunately, often it is impossible to investigate such a system experimentally in detail or even at all. An option to capture and to predict its properties is performing a direct numerical simulation of the particulate fluid. For this purpose, an efficient approach is developed in this project by coupling the discrete element method and finite element method.
    Led by: P. Wriggers
    Team: B. Avci
    Year: 2009
  • Numerical modeling of electrical contacts
    The focus of this work is the investigation of the behavior in electrical contacts, where electrical, thermal and mechanical fields are coupled. Specifically, theoretical constitutive models for the electrical conductance and electrical wear phenomena are developed and implemented in a three dimensional finite element setting. Also a new relation for wear is proposed, where the amount of wear is coupled to the dissipation arising at the contact interface. </a></p>
    Led by: P. Wriggers
    Team: C. Weißenfels
    Year: 2009
  • FAILURE ANALYSIS - ERROR ESTIMATION FOR MULTISCALE METHODS
    This project is concerned with the development of tools for error-estimation based adaptive multiscale failure analysis. In order to enable a more accurate mechanical analysis of composite aircraft substructures, existing discretisation error estimators will be improved to be used as indicators for mesh refinement. Additionally, physical error estimators will be developed to identify regions where higher-order material modelling is required.
    Led by: P. Wriggers
    Team: N. Hajibeik
    Year: 2009
  • MULTISCALE CONTACT HOMOGENIZATION OF GRANULAR INTERFACES
    Dry granular third bodies are frequently encountered at multiple scales of contact interfaces in contexts that range from mechanical problems of tire traction and semiconductor manufacturing to biological problems of wear debris generation and mobility in implant joints. The investigations that are envisaged within this proposal will provide further insight into the modeling and simulation of third body effects in a fully nonlinear three-dimensional virtual setting that accounts for inelastic phenomena.
    Led by: P. Wriggers, I. Temizer
    Team: R. Weidlich
    Year: 2009
  • ADAPTIVE MULTISCALE MODELING AND ANALYSIS OF HETEROGENEOUS MATERIALS
    Accurate computational analyses of a composite structure requires multiple levels of resolution: (i) a region where effective elastic properties are employed, (ii) a region where embedded micro-macro problems are solved, and (iii) a region where explicit microstructural evaluation is required. Development of such computational schemes for the adaptive multiscale analysis of heterogeneous materials is the main purpose of this project.
    Led by: P. Wriggers, I. Temizer
    Year: 2009
  • Analysis of local friction between rubber and dry and wet surfaces
    A prediction of the friction behavior of a rubber compound sliding over a road surface is an important topic in tire industry. Within this project the parameters which mainly influence the friction between a single tread block and a rough surface are investigated. The relevant physical mechanism of contact between a tread block and a rough surface, both wet or dry has to be understood. To validate the model a large number of test rigs is investigated at the Institute of Dynamics and Vibration Research (IDS).
    Led by: P. Wriggers, R. A. Sauer
    Team: J. Dobberstein
    Year: 2009
  • RAMWASS - Integrated Decision Support System for Risk Assessment and Management
    The objective of the EU-project RAMWASS is to develop and validate a new decision support system (DSS) for the risk assessment and management for the prevention and/or reduction of the negative impacts caused by human activities on the water/sediment/soil system at river basin scale in fluvial ecosystems.
    Led by: P. Wriggers
    Team: B. Avci
    Year: 2009
  • Einfluss inelastischer Effekte auf die Reibung in zyklischen Prozessen
    Bei der Modellierung der Polreibung von Reifen ist eine große Anzahl einzelner Effekte zu berücksichtigen, insbesondere, dass sich Gummi mit ursprünglich einheitlichem Ausgangszustand nach einiger Verformung von Ort zu Ort sehr unterschiedlich verhalten kann. Die Ursache ist, dass die so genannte Gummi-Elastizität sehr beträchtliche inelastische Anteile aufweist, die von den vorangegangenen Lastzyklen, insbesondere von früheren Verformungsmaxima signifikant abhängen (Mullins-Effekt). Somit ist bei einer genauen Modellierung des Rollkontaktes unbedingt die Heterogenität des Vor-Verformungszustandes und ihr Einfluss auf die inelastischen Effekte zu beachten.
    Led by: D. Besdo
    Year: 2008
  • Frictional contact of elastomer materials on rough rigid surfaces
    The purpose of this work is the derivation of a friction law for rubber materials on rough tracks by numerical investigations. Rubber friction includes a number of influences like hysteresis due to material damping, adhesional effects or thermomechanical coupling.
    Led by: P. Wriggers
    Team: J. Reinelt
    Year: 2008
  • Simulating the microstructure of cement-based construction materials
    In this thesis three-dimensional computational homogenization of hardened cement paste (HCP) including micro-structural damage due to frost is introduced. Based on a computer-tomography at a resolution of 1µm a finite-element model of HCP is developed with different elastic and inelastic constitutive equations for the three parts unhydrated residual clinker, pores, and hydration products.
    Led by: P. Wriggers, S. Löhnert
    Team: N. Dabagh
    Year: 2008
  • Micromechanics of Nanoindentation
    The properties of many modern metallic materials are obtained through a defined micro structural treatment. The grain size of a steel influences its macroscopic material behavior to a considerable extent. Nanoindentation technics are a possibility to examine the micro mechanical coherences. The goal of this project is a proper simulation of a nanoindentation test into a poly crystalline metal. In order to model the occurring size effects, it is necessary to implement a model for gradient crystal plasticity and grain boundary behavior.
    Led by: P. Wriggers, C.B. Hirschberger
    Team: D. Gottschalk
    Funding: DFG IRTG 1627

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