bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase‐Field Simulations of Cracks under Dynamic Loading
Authors: Bilgen Carola , Wieners Christian , Weinberg Kerstin
Published: November 2019

Phase‐field fracture simulations have been established to simulate crack propagation in fracture mechanics. This contribution sets the focus on different driving forces for crack growth and on the simulation of waves propagating along the fractured interfaces.

Typically, phase‐field methods for brittle fracture employ a variational framework which has been proven to converge to Griffith’ classical model. This approach, however, has limits in the pressure dominated regime. For that reason we propose ad‐hoc driving forces which are motivated physically using general fracture mechanic concepts. Additionally we investigate the effect of the phase‐field modeled cracks and interfaces on the propagation of the arising waves. The accuracy and the robustness of the simulation method will be demonstrated by numerical examples.
Modelling of cellular materials by a microsphere‐based material model
Authors: Jung Anne , Bleistein Thomas , Diebels Stefan
Published: November 2019

Metal foams are a very interesting class of cellular materials which, due to their structure, can be used both for lightweight construction and for the absorption of kinetic energy. They have a microheterogeneous structure, which makes it difficult to simulate these materials efficiently. Although microstructure models are very precise in terms of strut size and pore geometry, they are very computationally intensive due to their high resolution and therefore do not allow the simulation of entire components. While continuum models that do not resolve the specific microstructure are very efficient, they do not allow the influence of variations in strut size, strut geometry or pore size to be modelled directly by the simulation. Therefore, simulation approaches such as microsphere models are necessary, which combine the macroscopic component scale with the microscopic microstructure.
Uncertainty Quantification for Optimal Power Flow Problems
Authors: Mühlpfordt Tillmann , Hagenmeyer Veit , Faulwasser Timm
Published: November 2019

The need to de‐carbonize the current energy infrastructure, and the increasing integration of renewables pose a number of difficult control and optimization problems. Among those, the optimal power flow (OPF) problem—i.e., the task to minimize power system operation costs while maintaining technical and network limitations—is key for operational planning of power systems. The influx of inherently volatile renewable energy sources calls for methods that allow to consider stochasticity directly in the OPF problem. Here, we present recent results on uncertainty quantification for OPF problems. Modeling uncertainties as second‐order continuous random variables, we will show that the OPF problem subject to stochastic uncertainties can be posed as an infinite‐dimensional L2‐problem. A tractable reformulation thereof can be obtained using polynomial chaos expansion (PCE), under mild assumptions. We will show advantageous features of PCE for OPF subject to stochastic uncertainties. For example, multivariate non‐Gaussian uncertainties can be considered easily. Finally, we comment on recent progress on a Julia package for PCE.
Science Thriller: The dramatic destiny of Alexander Mohrenstein‐Ertel and the history of elastohydrodynamics
Authors: Popova Elena , Popov Valentin L.
Published: November 2019

Presentation is devoted to the life and work of Alexander Mikhailovich Ertel, the founder of elastohydrodynamics. He was the first to recognize the importance of elastic deformation of partners in lubricated contacts as well as pressure dependence of lubricants. In his dissertation written in Moscow during the Second World War, he suggested a solution of this complicated problem, which was not changed essentially by the later research. However, due to his escape to West Germany and change of identity, the real authorship remained unknown over many years. In West Germany, Mohrenstzein‐Ertel continued his work by practical implementation of the ideas suggested in his dissertation. He could reveal his identity after the Stalin's death, and could witness the recognition of his contribution through the German Tribological Society and international tribological community.
Modeling of electric field‐induced magnetization switching in multiferroic heterostructures
Authors: Dornisch Wolfgang , Müller Ralf
Published: November 2019

Multiferroic heterostructures consist of materials with either pronounced ferroelectric or ferromagnetic effect. The combination of both types of material, be it in layers, columns or inclusions, potentially yields a significant magneto‐electric coupling effect even at room temperature. The magnetization in the ferromagnetic material can be controlled by the application of electric fields to the ferroelectric material. In this contribution a linear elastic continuum formulation is coupled with a phase field formulation for the polarization and magnetization in the ferroelectric and the ferromagnetic layer, respectively. The strain transfer at the interface of the layers yields a magneto‐electric coupling effect within the heterostructures. The finite element method is used to discretize the arising differential equations. A numerical example provides a proof of concept for the simulation of the magneto‐electric coupling effect in multiferroic heterostructures.
Structural and material optimization based on thermodynamic principles
Authors: Jantos Dustin Roman , Hackl Klaus , Junker Philipp
Published: November 2019

In our previous work, we developed a variational approach for topology optimization based on thermodynamic principles, i.e. Hamilton's principle for dissipative processes. Hamilton's principle yields a closed set of differential equations for a variety of problems in continuum mechanics, which include microstructural processes described by internal variables (e.g. plasticity, damage modeling, crystallographic transformations, etc.). These internal variables can also be used to describe design variables for an optimization, i.e. structural compliance minimization. The resulting differential equations yield evolution equations as known from material modeling that can be used as update scheme for an iterative optimization procedure.

With this method, we derived differential equations for different design variables: 1) the topology, which is described by a continuous density distribution with penalization of intermediate densities (SIMP); 2) the local material orientation of a anisotropic base material, for which we introduce a filtering technique to control the fiber path smoothness; 3) the material distribution for tension and a compression affine materials, e.g. steel and concrete, in which tension affine material is applied in regions predominant to tension and compression affine material in regions predominant to compression.
Mixed‐primal Discontinuous Galerkin approximation of flows in fractured porous media on polygonal and polyhedral grids
Authors: Facciolà Chiara , Antonietti Paola Francesca , Verani Marco
Published: November 2019

We propose a formulation based on discontinuous Galerkin methods on polygonal/polyhedral grids for the simulation of flows in fractured porous media. We adopt a model for single‐phase flows where the fracture is modelled as a (d − 1)‐dimensional interface in a d‐dimensional bulk domain and the flow is governed by the Darcy's law in both the bulk and the fracture. The two problems are then coupled through physically consistent conditions. We focus on the numerical approximation of the coupled bulk‐fracture problem, discretizing the bulk problem in mixed form and the fracture problem in primal form. We present an priori h‐ and p‐version error estimate in a suitable (mesh‐dependent) energy norm and numerical tests assessing it.
On the robustness of stress integration algorithms for finite single crystal (visco)‐plasticity
Authors: Prüger Stefan , Kiefer Björn
Published: November 2019

The accurate determination of the slip activity in single crystal plasticity models is crucial to predict the mechanical response and the reorientation of the crystal. Therefore, a comparison of two rate‐independent and two rate‐dependent formulations of single crystal plasticity is carried out, subject to homogeneous proportional and non‐proportional deformation histories. From the simulation results it is concluded that the robustness of the corresponding stress‐integration algorithm depends on the complexity of the hardening law.
A physically and geometrically nonlinear formulation for isogeometric analysis of solids in boundary representation
Authors: Chasapi Margarita , Klinkel Sven
Published: November 2019

This contribution concerns a physically and geometrically nonlinear formulation for isogeometric analysis of solids in boundary representation. The parametrization of the proposed approach is inspired by the scaled boundary finite element method, where the geometry of the boundary is sufficient to define the entire solid surface. This suits perfectly the boundary representation modeling technique, which is commonly employed for the design of solids in CAD. In order to provide a boundary‐oriented approach, the solid surface is parametrized by a radial scaling parameter emanating from a scaling center and a parameter in the circumferential direction of the boundary. According to the idea of isogeometric analysis, NURBS basis functions define the geometry of the boundary and approximate the solution on the boundary. We also employ NURBS to approximate the solution in the scaling direction in the interior of the domain. This enables a straightforward treatment of nonlinear problems while preserving the exact geometry of the boundary. The approximation in scaling direction is in principle flexible and requires for coupling of adjacent patches inside the domain. The Galerkin method is employed for the solution in both parametric directions and a linearization is derived which is used within an iterative Newton‐Raphson scheme. We consider nonlinear problems in solid mechanics including elasto‐plastic material behavior and large deformations as well as complex geometries commonly employed in engineering applications. In this paper we study a numerical example of a complex geometry in order to evaluate the performance of the proposed approach.
3D Dynamic Crack under Cyclic Loading using XFEM: Numerical Treatment
Authors: Lyu Tengfei , Löhnert Stefan , Wriggers Peter
Published: November 2019

The eXtended Finite Element Method (XFEM) is a special numerical method to handle arbitrary discontinuities in the displacement field independent of the finite element mesh. This is advantageous during crack initiation, growth and propagation processes. In the range of continuum damage mechanics, gradient‐enhanced damage models can be used to model damage and fracture without spurious mesh dependencies. Gradient‐enhanced damage models have been investigated extensively in the context of quasi‐brittle and elasto‐plastic materials. To avoid fracture and failure of materials, modelling the component under cyclic loading is significant for fatigue lifetime prediction. The focus of this contribution is set on algorithmic issues. The numerical treatment of 3d cracks under cyclic loading is investigated. The domain is discretized with ten‐node tetrahedral elements. Discrete cracks are captured using XFEM and updated by level set methods. In oder to take advantage of the explicit time discretization scheme, a modified differential equation for the gradient‐enhanced damage is presented and the central difference explicit time stepping method is employed to obtain 2nd oder accuracy of the solved equations.
Influence of thermal boundary conditions on the parameter identification in thermodynamics
Authors: Rose Lars , Menzel Andreas
Published: November 2019

Finite‐Element based identification schemes, such as the FEMU‐method, are a powerful tool for the (quantitative) adjustment of material models to an observed material behaviour. A relative sparingly explored segment of this field, however, is the identification of thermal material parameters based on full field temperature measurements. Hence, the focus of this contribution lies on the influence of thermal boundary conditions on the result of such an identification. More precisely, the impact of the convection and conduction coefficient is analysed by simply performing several identifications, each with different values prescribed. Results suggest that some parameters are indeed very sensitive to the choice of coefficients.
Topology optimization combined with element‐by‐element solution techniques
Authors: Sala Ramses , Kuhn Charlotte , Sator Christian , Müller Ralf
Published: November 2019

Topology optimization approaches are commonly used for design problems involving physical phenomena related to solid mechanics, acoustics, electromagnetism, fluid mechanics, and combinations thereof. In computational models of these physical phenomena, the field variables are commonly approximated using spatial discretizations within the domain using the Finite Element Method. Even for topology design problems in which the field variables must only satisfy linear state equations, the storage and solution of the resulting global system of equations can become a computational bottleneck. In this contribution, we investigate a topology optimization approach in which the Solid Isotropic Material with Penalization (SIMP) method is combined with element‐wise solution approaches, in order to reduce computational memory requirements.
Two‐Grid hp‐DGFEMs on Agglomerated Coarse Meshes
Authors: Congreve Scott , Houston Paul
Published: November 2019

We generalise the a priori error analysis of two‐grid hp‐version discontinuous Galerkin finite element methods for strongly monotone second‐order quasilinear elliptic partial differential equations to the case when coarse meshes consisting of general agglomerated polytopic elements are employed.
Experiment on wave propagation in soft resins
Authors: Aghayan Sam , Bieler Sören , Reppel Thomas , Weinberg Kerstin
Published: November 2019

The wave propagation in a group of soft resin based (3d printed) materials is investigated by using a Split Hopkinson Bar test. The principal assumptions and the role of pulse shaping, as an approach to achieve constant strain rates and dispersion reduction, is discussed. The experimental stress‐strain curves, gained by aluminium Split Hopkinson bars, are presented and indicates that tough and white standard resins have a dynamic elastic modulus of almost 3 GPa.
Particle Finite Element Simulation of Fresh Cement Paste – Inspired by Additive Manufacturing Techniques
Authors: Reinold Janis , Meschke Günther
Published: November 2019

The properties of hardened concrete structures, fabricated by novel additive manufacturing techniques with fresh concrete, are significantly influenced by the manufacturing process. A numerical framework for the simulation of extrusion processes in additive manufacturing techniques based on layered extrusion is proposed. The flow behavior of fresh concrete is approximated with a regularized Bingham model and in order to deal with large deformation, governing equations are solved via the Particle Finite Element Method.
A comparison between the performance of different Mindlin's theory based strain gradient models in local singularities
Authors: Makvandi Resam , Juhre Daniel
Published: November 2019

The classical continuum mechanics theories are originally supposed to determine deformations in ranges from millimeter to meter, the so‐called macroscopic scale. In fact, these theories are approximations of physical systems neglecting the underlying microstructure. For instance, a Cauchy continuum, i.e. a continuum with an elastic energy determined as a function of the gradients of its macroscopic displacement, can only approximate the behavior of a physical system sufficiently as long as the microstructure has a much smaller length‐scale than the macrostructure [1]. Although these models were exploited in studies for large and small scales, experiments have shown that the classical models are not able to properly cover the smaller scales; in particular, problems in micron‐ and nano‐dimensions are frequently observed. Size‐effects, which cannot be captured exploiting these theories, seem to be the source of this issue. On top of that, the appearance of local singularities at the crack tips (or more broadly, in the presence of point and line loads) is one of the known limitations of the classical continuum mechanics theories. Generalizing these models by introducing additional kinematic terms to consider the underlying microstructure effects at macroscopic levels is one way of overcoming the already mentioned problems. In this contribution, we will focus on the Mindlin's theory of elasticity with microstructures [2] and its different forms. Therein, it is shown that for the first strain gradient theory, five additional parameters must be introduced. However, in practice, due to the complexities of measuring the new parameters, various simplified versions of the theory are being exploited, among them we name Altan et al. [3] and Reiher et al. [4]. Our aim here is to compare the performance of these simplified theories in removing the local singularities of the conventional continuum mechanics theory.
Model Reduction for Second‐Order Dynamical Systems Revisited
Authors: Beddig Rebekka S. , Benner Peter , Dorschky Ines , Reis Timo , Schwerdtner Paul , Voigt Matthias , Werner Steffen W. R.
Published: November 2019

Second‐order control systems are used to describe the dynamics of mechanical and vibrational systems, and in particular, their response to excitations. In this article, we discuss model order reduction (MOR) of such systems. A particular focus is on preserving the second‐order structure and physical properties such as stability and passivity.
On grasp based objectives in human grasping simulation
Authors: Phutane Uday , Roller Michael , Leyendecker Sigrid
Published: November 2019

We solve optimal control problems to perform predictive human grasping simulation for a two‐finger rigid multibody model. The quality of the grasp is strongly influenced by the choice of the objective function. We investigate the quality of the grasp using objectives based on grasp matrix and the hand Jacobian. We compare the performance of optimal control grasping simulations by minimising objectives based on these grasp quality measures for a lateral grasp activity.
Automatic analysis of the stress‐strain response of rheological models
Authors: Schultz Timm , Müller Ralf
Published: November 2019

Viscoelastic material behaviour can be represented by rheological models combining elastic and viscous rheological elements. These combinations can be serial and parallel connections of such elements as well as combinations of both. A method based on transfer functions formulated in the Laplace domain allows the calculation of constitutive equations of serial and parallel connections by a simple summation.

Furthermore the use of a breadth‐first search algorithm for the analysis of complex systems of rheological elements is explained. By using this search algorithm a hierarchy of nested connections in a system can be found. This hierarchy guarentees dependencies are satisfied during the calculation of constitutive equations and signal propagation through the system.
Macroscopic modeling of open cell foams
Authors: Kirchhof Stephan , Ams Alfons
Published: November 2019

Open cell foams are widely used in engineering which includes an application in areas, where vibration excitations can act on the structures. Therefore dynamical properties as eigenfrequencies and natural modes are of great interest. We will discuss the description of the dynamic behavior of open cell foams by stochastic macroscopic models. These are based on a computed tomography scan of the foam, which gives access to parameters and the power spectral density needed for the generation of the stochastic process. The simulation results shall be used to get an estimate of the natural frequencies.
Sensitivity analysis of elastoplastic structures and application to optimal specimen design
Authors: Liedmann Jan , Barthold Franz-Joseph , Gerke Steffen , Brünig Michael
Published: November 2019

This paper is concerned with the shape improvement of novel specimens for biaxial experiments in terms of optimal stress states, characterized by the stress triaxiality. For this, gradient based optimization strategies are utilized and the needed sensitivities are obtained using a variational approach. Considering elastoplastic material behavior, the deformation history as well as its sensitivity has to be taken into account. In a first step, the material parameters have to be identified for the given material (AlCuMgSi). With the identified parameters, a chosen specimen is numerically analyzed and optimized with the aim to achieve a preferably homogeneous stress triaxiality distribution in the relevant part of the specimen.
How Housing Dynamics Affect the Monitoring of Rotor Unbalance: A Case Study
Authors: Krügel Samuel , Maierhofer Johannes , Wagner Christian , Thümmel Thomas , Rixen Daniel J.
Published: November 2019

When monitoring faults in a rotating system, the rotor support and housing have an important effect on the measured dynamic response. Within this contribution, we show the effect of stiffness orthotropy in the supports when monitoring the unbalance of a rotor. The study is illustrated with an industrial case, namely a cooling blower for a drive system. The considered application was evaluated in a new and a defective state by measurements using radially distributed accelerometers to answer two questions:

 1. May a suspected unbalance be detected at different operating conditions by measuring on the housing?

 2. Which problem can appear in estimating the rotor unbalance when the support has orthotropic stiffnesses?

Thereby, we figured out that the suspected unbalance can be well detected only between global resonance frequencies of the application. The correlating modes superimpose the acquired signals, initially evoked by unbalance excitation. Due to these modes, a phase shift of the measured unbalance signals in correlating measurement directions occurs. Thus, forward and backward whirl expresses themselves within the rotational frequency in the sensor signals. Due to these effects, the knowledge of global modes is a requirement for unbalance monitoring. Keywords: Orthotropic‐elastic rotor support, unbalance, vibration‐based monitoring
Coupled physics solvers for analysing the magneto‐thermal behaviour of a permanent magnet synchronous machines
Authors: Lotfi A. , Marcsa D. , Horvath Z. , Prudhomme C. , Chabannes V.
Published: November 2019

The aim of this study is to present a thermal analysis of a permanent magnet synchronous machine based on the finite element method. The developed model can be used to predict temperature distribution inside the studied motor during the rated operation. Electromagnetic computation (EM) is carried out with the aid of two 2D finite‐element (FE) simulations on the cross‐section of the PM motor. To analyse the process of heat transfer (HT) in an electrical machine, empirical correlations are used to describe the convective (HT) from the different surfaces of the PM motor. The (HT) coefficient is determined using dimensionless numbers. The proposed numerical method to solve the coupled problem is based on independently and successively solving the (EM) and thermal problems and then transferring the results between the two sub‐problems. The results obtained by the model are compared with experimental results from testing the prototype electric motor.
Parameter Variation on Nonlinear Energy Sink attached to Multiple Degree of Freedom System
Authors: Jahn Martin , Panning-von Scheidt Lars , Wallaschek Jörg
Published: November 2019

The importance of lightweight structures has increased in the last years motivated by the goals of improving efficiency of vehicles and conserving resources. In most cases the loss of weight by using fewer or lighter materials is accompanied by a reduction of the structures stiffness having a strong effect on occuring vibrations. Even if the damping ratio of the structure remains unchanged, by reducing weight and stiffness the vibration amplitudes might increase if the vibration excitation stays the same. Damping elements might be introduced to the structure mitigating critical resonance gains of the structure to reduce these vibrations. In this work the dynamics of a linear multiple degrees of freedom structure with a nonlinear damping element are investigated. The damping element is chosen to have a nonlinear characteristic to allow for a situation adaptive design. Numerical parameter studies are carried out to optimize the resulting damping of the entire structure. The nonlinear equation of motion is solved by applying the Harmonic Balance Method. The evaluation of the performance of the nonlinear damping element is done by consideration of the nonlinear frequency response functions of the structure.
Numerical investigations of the hydrodynamics and the oxygen mass‐transfer in aerated tanks
Authors: Höffmann Ann Kathrin , Schmidt Johanna , Ehrhard Peter
Published: November 2019

The present work concentrates on numerical investigations of the hydrodynamics, the oxygen mass‐transfer, and the biochemical reactions in a pilot‐plant‐scale aeration tank, engaging the Euler‐Euler method and the Activated Sludge Model No. 1. The oxygen concentration, the ammonium concentration, and the nitrate concentration are discussed using results from the experimental investigations at the pilot‐plant‐scale aeration tank in the waste‐water‐treatment plant Ebersbach carried out by the HZDR.
A damage detection study of a bridge using bypassing vehicles and computational intelligence
Authors: Hesser Daniel Frank , Bamer Franz , Markert Bernd
Published: November 2019

Structural integrity is an ubiquitous topic in research and daily living. Conventional monitoring strategies follow a preventive maintenance plan or require permanent installation of sensors. These steps are time and cost‐inefficient taking into account the high number of bridges to be monitored. In this study, a bridge structure is passively monitored using the acceleration signal of a bypassing vehicle. Thereby, the signal is measured on the vehicle. The health status of the bridge can be evaluated after each pass, which allows a continuous monitoring of the structure. A numerical model of a bridge and a bypassing car is carried out to prove the feasibility of this approach and to create a large database with different damage locations and vehicle velocities. Moreover, an artificial neural network is trained on that database to detect damaged bridge structures. The proposed method proves the feasibility of monitoring the structural integrity of bridges and supports conventional maintenance methods. Therefore, the combination of active and passive monitoring strategies paves the way to efficient and cheap infrastructure monitoring strategies.
Demand‐based coupling of the scales in the finite temperature CADD method
Authors: Wurm Patrick , Ulz Manfred H.
Published: November 2019

Concurrent finite temperature atomistic‐to‐continuum models commonly use a fixed time interval coupling of the atomistic and continuum scale. With special focus on the continuum, this means, that its solution fields are updated in fixed intervals of time. However, in times, when the change in the continuum boundary conditions is negligible, these computations are unnecessary. We present the novel idea of a demand‐based coupling in this work, in which the continuum solution fields are only updated if needed, to save computational effort. To demonstrate the approach, we apply the coupling to a prototypical representative of the mentioned models, namely the coupled atomistic and discrete dislocation (CADD) model. We test the basic functionality of the approach in a 1‐dimensional pulse propagation problem.
Numerical investigation of aerosol deposition on a single 2D fiber
Authors: Mohan Saravanan , Chaudhuri Jayotpaul , Gödeke Lutz , Ehrhard Peter
Published: November 2019

The aerosol filtration technique deserves extensive attention, due to its wide applications found in health, environment, and industry. The simulation of aerosol filtration over fibrous filters involves the numerical investigation of the motion and capture of particles (aerosols) under different fluid operating conditions.

The capture of aerosol in fibrous filters involves the following deposition mechanisms: Impaction, interception, diffusion and gravitation. The motion of the particles is governed by the relevant forces acting on the particles. Due to the dilute concentration of the aerosol, the influence of particle motion onto the fluid and particle‐particle interaction can be neglected. The particles are removed by a fibrous filter when they collide and stick to the surface of the fiber. An Euler‐Lagrangian approach is engaged to simulate the particle trajectories. The air flow field (continuous phase) is simulated in Ansys CFX (Euler approach) and the particle trajectories (dispersed phase) are computed by Lagrangian simulations in MATLAB.

Real fibrous filters consist of a large number of fibers of non‐uniform size and random orientation and position. For simplification, a single 2D fiber is considered and placed normal to the airflow to analyse several deposition mechanisms. The efficiency at which the particles are collected by a single fiber from an aerosol stream is called single‐fiber efficiency EΣ, which is highly dependent on particle size. The particle motion is also affected by diffusion, where the dispersion of particles due to Brownian motion follows a Gaussian distribution. The diffusion coefficient can be inferred from the mean distance travelled by the particle and agrees well with the theoretical diffusion coefficient calculated from the Stokes‐Einstein equation. The diffusion mechanism becomes significant at particle sizes below 0.2 µm. The single‐fiber efficiency is determined considering all deposition mechanisms and compared with existing theoretical models.
A higher order geometrically nonlinear Cosserat‐shell model with initial curvature effects
Authors: Neff Patrizio , Bîrsan Mircea , Ghiba Ionel-Dumitrel
Published: November 2019

Using a geometrically motivated 8‐parameter ansatz through the thickness, we reduce a three‐dimensional shell‐like geometrically nonlinear Cosserat material to a fully two‐dimensional shell model. Curvature effects are fully taken into account. For elastic isotropic Cosserat materials, the integration through the thickness can be performed analytically and a generalized plane stress condition allows for a closed‐form expression of the thickness stretch and the nonsymmetric shift of the midsurface in bending. We obtain an explicit form of the elastic strain energy density for Cosserat shells, including terms up to order O(h5) in the shell thickness h. This energy density is expressed as a quadratic function of the nonlinear elastic shell strain tensor and the bending‐curvature tensor, with coefficients depending on the initial curvature of the shell.
Towards overcoming the bottleneck of optimizing control parameters in finite element active human body models
Authors: Martynenko Oleksandr V. , Stollenmaier Katrin , Endler Carola A. , Neininger Fabian T. , Schmitt Syn , Haeufle Daniel F.B.
Published: November 2019

The number of Finite Element Active Human Body Model (FE aHBM) applications for the design and test of vehicle safety systems is growing. Primarily they are used for simulations of the accident pre‐crash phase where the influence of occupants active movements is significant. Such models are capable of accounting for dynamic human behaviour and reflexes by incorporating bio‐inspired muscle controllers. These controllers need to govern hundreds of active muscle elements during simulation in every time‐step thereby dramatically increasing runtime compared to passive HBMs. As runtime is an essential element of the entire research and development process of a new vehicle, new approaches for its reduction are required. The current contribution presents methods for the tuning of controller and active muscle element parameters using a reduced multibody (MB) model with a subsequent transfer to a fully deformable FE model.
On Osmotic Pressure in Hyperelastic Biphasic Fiber‐Reinforced Articular Cartilage
Authors: Egli Franziska , Ricken Tim
Published: November 2019

Articular cartilage belongs to the group of connective tissues and ensures a low‐friction performance of the human musculoskeletal system. It is a complex multiphase material with two main components: a collagen fiber‐reinforced porous solid and a liquid phase. The mechanical behavior of articular cartilage is decisively determined by the interaction between the highly anisotropic, inhomogeneously distributed collagen fiber network, the isotropically distributed proteoglycans and the interstitial fluid. The electrically negatively charged proteoglycans generate an electrical gradient that creates an osmotic negative pressure on the fluid which causes swelling of the cartilage. This leads to mechanically prestressed collagen fibers resulting in a decrease of total pressure occurring inside the cartilage. An increasingly common joint disease is osteoarthritis (OA), which severely impairs everyday life, especially in the elderly population. It is characterized by a loss of matrix stiffness as well as by a reduced tension stiffness and viscoelasticity of the collagen fibers. The aim of the developed model is to describe the previously mentioned material properties of cartilage as precisely as possible. The biphasic model uses the Theory of Porous Media for homogenization and contains an incompressible poroelastic solid matrix reinforced with collagen fibers and an incompressible pore fluid. By including the occurring osmotic pressure, the correct initial stress state of the imaged configuration is considered.
Mean field surrogate model of a dilute and chaotic particle suspension
Authors: Vibe Alexander , Marheineke Nicole
Published: November 2019

We present a mean field approximation for a dilute suspension of small particles, which accounts for a non‐homogeneous particle distribution and small inertial effects of the particle phase. The model consists of the momentum equation for the fluid coupled to a transport equation for the probability density function on the particle phase space. Apart from the well‐known particle extra stress tensor first presented by Batchelor (1970), the fluid equation contains additional force terms resulting from inertia and hydrodynamic interaction of the particles. A brief sketch of the main derivation steps is presented.
Cabin noise prediction using wave‐resolving aircraft models
Authors: Langer Sabine C. , Blech Christopher
Published: November 2019

Within the Coordinated Research Center (CRC) 880, future civil aircraft with short take‐off and landing capabilities are investigated. A main motivation are efficient point‐to‐point connections within Europe. Environmental friendliness is a key factor in the development of such aircraft systems in order to ensure the acceptance of new technologies. Therefore, the sound immission at the ground and in the passenger cabin is a major focus in the project. In this paper, wave‐resolving models of complex aircraft fuselage structures and solution strategies of these large systems in frequency domain are presented.

On the basis of preliminary aircraft design data available in the project, a mechanical model of all fluid and structural parts is developed. The model comprises the back partition starting from the wing‐box and assumes a linear behavior in deflection, strain and material. The thickness distribution on the outer surface, the stiffeners, the floor, the bulk heads and the inner panels are considered concerning the structural domain. The insulation is modeled by an equivalent fluid approach. The cabin fluid, the structural domain, and the insulation are fully coupled. All domains are discretized automatically applying coincident nodes at the coupling interfaces. Frequency‐dependent pressure fluctuations distributed on the outer surface as a result of Computational Aeroacoustic (CAA) simulations of jet noise are applied to the model. A realistic consideration of such characteristic noise footprints is only possible due to the wave‐resolving approach. The direct result of the aircraft model is the cabin's pressure field in frequency domain.

The finite element discretization leads to a system of equations with 1‐3 mio degrees of freedom to be solved in frequency domain. As one solver run is easily realized, many solutions in the frame of uncertainty analyses or sensitivity studies require a highly efficient solution process. Different strategies applying direct solvers, iterative solvers or its combination are discussed within this contribution as well.
Impact of thin‐walled square aluminum tubes
Authors: Aghdam Nima Jafarzadeh , Tatikonda Uday Bhaskara Chary , Bühring Jannik , Mekala Narasimha Rao , Schröder Kai-Uwe
Published: November 2019

Thin‐walled profiles are widely used as crash energy absorbers. In this paper thin‐walled square aluminum crash boxes under impact load are studied numerically. ABAQUS commercial software is used for simulating the CBes. The simulated model is validated experimentally using a drop tower test. Folding pattern of a CB has a significant effect on the energy absorption of it. In this study the effects of different parameters such as wall thickness and width on the dynamic behavior and folding pattern of the CB under impact load have been investigated.
Structure‐preserving Galerkin approximation for a class of nonlinear port‐Hamiltonian partial differential equations on networks
Authors: Liljegren-Sailer Björn , Marheineke Nicole
Published: November 2019

The development of structure‐preserving approximation methods, which regard fundamental underlying physical principles, is an active field of research. Especially when the application of model reduction is desirable, systematic and rather generic approaches are of great interest. In this contribution we discuss a structure‐preserving Galerkin approach for a prototypical class of nonlinear partial differential equations on networks. Its derivation is guided by port‐Hamiltonian‐type modeling and appropriate variational principles. Also complexity‐reduction schemes can be integrated in a structure‐preserving way, which becomes crucial in the context of model reduction for nonlinear systems.
Modeling electro‐elastic coupling phenomena in electrostrictive polymers in the context of structural mechanics
Authors: Hansy-Staudigl Elisabeth , Krommer Michael , Vetyukov Yury , Humer Alexander
Published: November 2019

Bringing into focus the design aspect of thin film electro‐active polymer actuators justifies the deployment of a structural mechanics framework. We propose a physically consistent constitutive model for such actuators, which is valid for plates and shells as material surfaces within a complete direct formulation. To this end, we use the principle of virtual work to deduce the general form of the constitutive law from an augmented Helmholtz free energy, as a function of the structural Green‐Lagrange type strain measures and of the material electric field, without the need of a‐priori assumptions concerning the state of strain and stress.

Mechanical deformations of thin film devices – e.g. made of polyurethane – under the action of an external electric field, are caused by two different sources. On the one hand, the applied electric field causes a dielectric polarization of the polymer matrix, yielding to corresponding attractive electrostatic forces between the electroded surfaces resulting into a squeezing of the film. On the other hand, crystalline graft units with a certain natural, but arbitrarily directed polarization, embedded in between the polymer chains, have to align in the direction of the applied electric field such that a rotation of the whole crystal unit takes place. This rotation results in an additional macroscopic thickness squeeze– known as the electrostrictive effect.

We treat both electromechanical coupling phenomena separately, where it turns out, that the electrostatic forces can be accounted for by an electrical contribution to the augmented free energy, whereas, the electrostrictive effect is taken into account in the elastic part of the augmented free energy by virtue of a hybrid multiplicative and additive decomposition of the plate/shell deformation measures.

Benefiting from the structural mechanics formulation, we gain a lower – two‐dimensional – formulation, which provides a clear physical insight into the nature of the deformation process initiated by the external electric field. E.g. for the linearised problem, a comparison to the literature on thermoelastic plates and shells uncovers the action of the electric field as a combined source of self‐stresses. In order to solve particular problems, the constitutive relation of the geometrically and physically nonlinear formulation is implemented into our in‐house finite element code. The computed results, which were tested against results from the literature as well as against test problems of our previous work (where numerical integration of the three dimensional plate/shell augmented free energy through the thickness was employed), show a very good agreement.
A counterexample for the inf‐sup stability of the RT0 − P1 ⊂ L2(Ω) × H10(Ω) finite element combination for the mixed Poisson equation
Authors: Bertrand Fleurianne , Boffi Daniele
Published: November 2019

This paper investigates the inf‐sup stability of a dual mixed discretization of the Poisson problem using the lowest‐order (Hdiv‐conforming) Raviart–Thomas space combined with the space of standard linear (continuous) Lagrange elements.
An investigation of the heat generated during cyclic loading of ultra‐high performance concrete
Authors: Tran Ngoc Linh , Deutscher Melchior , Scheerer Silke
Published: November 2019

In this paper, the mechanism of heat generation in concrete due to cyclic loading is studied, in which the development of load‐induced temperature is modelled using a heat transfer equation. The heat generation rate is theoretically determined and the calculation is experimentally validated. The influence of the maximum size of aggregate of concrete on the temperature development is also taken into account. It is shown that the load‐induced temperature is generated by conversion of dissipate energy, which mainly occurs at the interfacial transition zone between binders and concrete aggregates and can be described by the plastic work of concrete material.
Simulation of generalized Newtonian fluids with the Smoothed Particle Hydrodynamics method
Authors: Simeunović Miloš , Steeb Holger
Published: November 2019

In this paper we examine Direct Numerical Simulation DNS of single phase non‐Newtonian fluids described by a generalized Newtonian rheology. Non‐Newtonian flow including large deformations and free surface flow could be observed in a wide range of industrial and environmental applications. For that reason, we choose the Lagrangian Smoothed Particle Hydrodynamics (SPH) as a simulation tool. The non‐Newtonian solver is implemented into the general purpose particle framework HOOMD‐blue [1,2], which allows for massive parallel CPU and GPU simulations. Numerical accuracy of the model is demonstrated by simulating confined Poiseuille flow between parallel plates. The validation has been done for a wide variety of shear strain–shear stress ratios where also the effect of the solid‐fluid interface boundary condition is examined. Furthermore, a comparison with the experimental results for the broken dam problem [3] showed good accuracy of the free‐surface flow prediction. Additionally, we have investigated a slump test (Abrams cone [4]), a relatively simple but in practice often performed experiment for non‐Newtonian yield‐stress fluids. Finally, we have investigated in further detail the cone diameter, height, flow time, the final shape and the influence of the rheological model parameters as well as the boundary conditions related to the stationary cone configuration.
Asynchronous time integration in structural mechanics
Authors: Seibold Andreas S. , Rixen Daniel J.
Published: November 2019

In some cases of structural dynamics problems, that are decomposed into substructures, it might be beneficial to have different time‐step‐sizes in each substructure. This is often referred to as asynchronous time‐integration. Since we want to extend our nonlinear FETI‐framework with such an asynchronous time‐integrator in the future, a Lagrange‐multiplier‐based framework should suit us best. As a first step we apply the PASTA‐DDM, which is based on a three‐field‐approach, to a simple linear heterogeneous multi‐mass‐oscillator and investigate the method's energy‐conservation properties. The integration is stable but slightly more energy is dissipated in the asynchronous case than in the synchronous one. Furthermore we solve the global interface‐problem and the local update of Lagrange‐multipliers with a residual‐based conjugate‐gradient method in order to prepare the PASTA‐DDM for an iterative parallel implementation.
Low‐grade fibromyxoid sarcoma: A report of the Cooperative Weichteilsarkom Studiengruppe (CWS)
Authors: Scheer Monika , Vokuhl Christian , Veit‐Friedrich Iris , Münter Marc , von Kalle Thekla , Greulich Michael , Loff Steffan , Stegmaier Sabine , Sparber‐Sauer Monika , Niggli Felix , Ladenstein Ruth , Kazanowska Bernarda , Ljungman Gustaf , Jahnukainen Kirsi , Fuchs Jörg , Bielack Stefan S. , Koscielniak Ewa , Klingebiel Thomas ,   on behalf of the Cooperative Weichteilsarkom Studiengruppe (CWS)
Published: November 2019

Background

Low‐grade fibromyxoid sarcoma (LGFMS) is a rare soft‐tissue tumor with benign histologic appearance, though fully malignant behavior is possible.

Methods

Patients with LGFMS <21 years registered in Cooperative Weichteilsarkom Studiengruppe trials until 2017 were analyzed. Firstline treatment consisted of complete surgical resection whenever possible.

Results

Median age of 31 patients was 10.9 years (first month to 17.1 years). Twenty‐six tumors were confirmed to the tissue of origin (T1), four invaded contiguous structures (T2), one was TX. Eight were >5 cm. The best surgical result was resection with free margins (R0) in 24 and microscopic residuals (R1) in seven. Five‐year event‐free (EFS), 5‐year local‐relapse‐free (LRFS), and 5‐year overall‐survival were 71 ± 18.6% confidence interval (CI) 95%, 76 ± 17.6% CI 95%, and 100%, respectively. Six patients suffered local relapse in a median of 1 year, one combined within 1.3 year and one metastatic relapse with lesions in the lung, back muscles, and thigh discovered in whole‐body imaging 6 years after the first diagnosis. In univariate analysis, T status correlated with EFS (T1 79.6 ± 18.6%, T2 50.0 ± 49.0%, P = .038). Resection with free margins tends to be associated with better LRFS (R0 82.4 ± 18.6%, R1 53.6 ± 39.4%, P = .053). Among 24 patients with R0 resection, five (21%) suffered relapse, thereof three local, one metastatic, and one combined. Among seven patients with R1‐resection, three (43%) suffered local relapse.

Conclusion

Special caution is advisable in T2 tumors. The metastatic potential with lesions in unusual sites indicates that affected patients need to be informed. If long‐term follow‐up with whole‐body imaging is beneficial, it may be addressed in larger intergroup analyses. Further research in disease biology is essential for optimal treatment and follow‐up care.