bluuebunny/crossref_metadata_2025 huggingface dataset:
Implicit time discretization schemes for least‐squares finite element formulations to model incompressible flows
Authors: Averweg Solveigh , Schwarz Alexander , Nisters Carina , Schröder Jörg
Published: December 2018

In the present contribution different implicit time integration schemes to model incompressible flow problems are investigated. Amongst the tested methods are the fully implicit Newmark, Houbolt and Crank‐Nicolson schemes.

The first‐order system for the proposed formulation is given in terms of stresses and velocities as introduced in [1]. Raviart‐Thomas and standard Lagrange interpolation functions are used to approximate the stresses and velocities, respectively.

For the comparison of all time discretization schemes, the unsteady two‐dimensional laminar flow around a cylinder at Reynolds number Re = 100 is chosen. For a rating on the accuracy, the obtained results for the drag coefficient are benchmarked against reference solutions by [2].
Region‐ and loading‐specific finite viscoelasticity of human brain tissue
Authors: Budday S. , Sommer G. , Paulsen F. , Holzapfel G. A. , Steinmann P. , Kuhl E.
Published: December 2018

Computational simulations are a powerful tool to understand the mechanical behavior of our brain in health and disease, with the ultimate goal to prevent pathological conditions. Accurate numerical predictions, however, require the development of appropriate constitutive models and, equally important, the careful calibration of the corresponding constitutive parameters. This has been exceptionally challenging due to the ultrasoft and heterogeneous nature of brain tissue, resulting in a distinctly nonlinear, rate‐dependent, compression‐tension asymmetric, and region‐dependent behavior. Previous constitutive models have been deduced from a single loading mode, but fail to predict the behavior under various loading conditions. Here, we developed a large strain, nonlinear, viscoelastic constitutive model for brain tissue on the basis of cyclic and relaxation experiments under multiple loading modes, simple shear, compression, and tension. We carefully calibrated individual parameter sets for four different regions of the human brain, the cortex, the basal ganglia, the corona radiata, and the corpus callosum. The model captures effects such as nonlinearity and compression‐tension asymmetry, but also time‐dependent effects with substantial pre‐conditionning during the first loading cycle, only minor conditioning effects during subsequent cycles, and successive softening when the applied strain is stepwise increased. With close consideration of the underlying microstructure, we evaluate the physical meaning of viscoelastic material parameters with rate‐dependent regional trends. Our results help to improve the accuracy of human brain simulations during development and disease or to predict outcomes of neurosurgical procedures.
Fully discrete solution for Bean's critical‐state model in type‐II superconductivity
Authors: Winckler Malte , Yousept Irwin
Published: December 2018

We consider a hyperbolic Maxwell‐type variational inequality arising in Bean's critical state model for type‐II superconductivity. Combining the implicit Euler method in time with a mixed Ritz‐Galerkin approximation in space results in a fully discrete scheme for the concerned variational inequality. We present some numerical experiments, which visualize the Meissner‐Ochsenfeld effect in type‐II superconductivity and verify that the proposed approach is physically reasonable.
Numerical models for evaluating the vibro‐acoustic properties of acoustic metamaterials
Authors: Aumann Quirin , Miksch Matthias , Müller Gerhard
Published: December 2018

The demand for lightweight structures in aerospace and automotive is on the rise and so are the requirements regarding the vibro‐acoustic behavior. Materials with a high stiffness‐to‐mass ratio are often used in this field, but these materials typically have an unfavorable vibrational behavior regarding structure‐borne noise. Acoustic metamaterials offer a solution for improving the vibro‐acoustic performance of the structure, while the stiffness‐to‐mass ratio remains nearly constant. Such a metamaterial consists of periodic patterns of microstructures placed on the host structure. They modify the vibrational behavior of the host by adding local resonances and thus minimize vibrations at specific frequencies. Distinct resonances can have a major contribution to the acoustic behavior of a structure, so it is of interest to tailor microstructures, that generate stop bands at certain frequencies. This can be computationally expensive, so efficient numerical models are required for the design of acoustic metamaterials. Initial investigations show, that a parametric model order reduction (PMOR) method based on the Loewner framework can generate such models.
Configurational forces in fracture mechanics: A comprehensive concept of crack initiation and growth
Authors: Judt Paul , Ricoeur Andreas
Published: December 2018

In engineering structures cracks typically initiate at stress raisers such as holes and notches. In terms of a fracture mechanical assessment, it is important to identify the position xk, the angle ϕc and the length Δac of initiating cracks. Classically a stress based criterion is applied to obtain the position of crack initiation. Recently, the finite fracture mechanics approach [1] was presented, combining a stress and energy based criterion to obtain the positions and initiation lengths of cracks. In this approach, the energy states of the uncracked and the cracked specimens are compared, providing the reduction of the elastic potential ΔΠ due to crack initiation. In general, the global solution ΔΠ(xk, ϕ, Δa) is required for accurately predicting new cracks. In this paper, the material or configurational forces [2] at stress raisers are investigated with respect to the initiation of cracks. The material forces are strongly related to path‐independent integrals, e.g. the J‐integral [3] as crack driving quantity or loading quantity at notches [4]. A new criterion based on configurational forces is suggested, efficiently providing all data of the onset of cracks at stress concentrators.
Homogeneous Cauchy stress induced by non‐homogeneous deformations
Authors: Schweickert Eva , Mihai L. Angela , Neff Patrizio
Published: December 2018

We discuss whether homogeneous Cauchy stress implies homogeneous strain in isotropic nonlinear elasticity. While for linear elasticity the positive answer is clear, we exhibit an example with inhomogeneous continuous deformation but constant Cauchy stress. The example is derived from a non rank‐one convex elastic energy.
A numerically efficient phase‐field model for fatigue fracture – 1D analysis
Authors: Seiler Martha , Hantschke Peter , Brosius Alexander , Kästner Markus
Published: December 2018

Fatigue loads and fatigue failure come along with high numbers of load cycles. This makes the simulation of fatigue fracture computationally very demanding, if each load cycle is simulated comprehensively with its loading and unloading phase. We present a numerically efficient method for the simulation of fatigue fracture, which avoids resolving the loading path and instead requires only one increment per load cycle at most.

We combine the phase‐field method for brittle fracture with a classic fatigue concept for lifetime calculation of components, which is applied to the material point here. A local lifetime variable is obtained, which we use to modify the material resistance incrementally in order to consider the progressive weakening of the material. Since we make use of an elasto‐plastic revaluation technique, a linear‐elastic material model is sufficient in the simulations. Furthermore, the phase‐field method allows to describe fatigue crack initiation as well as propagation.

In this contribution we describe the method's framework including the mentioned lifetime concept, followed by a proposal for a simulation scheme. Finally, results – for now limited to the one‐dimensional case and constant load amplitudes – are discussed.
A finite strain elasto‐plastic material model incorporating kinematic and isotropic hardening for thermoplastic polymers
Authors: Felder Sebastian , Simon Jaan‐Willem , Reese Stefanie
Published: December 2018

Semicrystalline thermoplastic polymers have superior mechanical properties and are thus very important in numerous technically relevant applications. During the thermoforming process of parts made of thermoplastic polymers the “springback effect”, i.e. the undesired distortion of a part after unloading and removal from tooling, depicts a major problem. These shape distortions occur due to the formation of residual stresses. To counteract this effect, time‐consuming and cost‐intensive trial and error approaches are typically conducted to carefully determine the optimal set of process parameters. Hence, a strong demand for computational models, which accurately predict the material and structural response of the part during forming, arises. Therefore, a phenomenological model formulation is proposed in this work, which is valid for large deformations and large deformation rates in the context of isothermal processes. To capture the biphasic nature of the polymer, i.e. to account for the amorphous and semicrystalline regions of the underlying microstructure, a parallel arrangement of both phases is applied. The overall material response is obtained by a rule of mixture. The qualitative validation of the hyperelastic, viscoplastic material model shows the promising potential to predict the formation and relaxation of residual stresses during the forming process of thermoplastic polymers.
Model‐based identification of injector aging in common rail fuel systems
Authors: Hofmann Oliver , Rixen Daniel
Published: December 2018

This paper discusses a strategy for identifying aging phenomena such as coking or erosion damage in the nozzle area of common rail injectors. These phenomena occur when the systems are operated over a longer time and they are known to impair the engine behavior. The method is based on a cycle‐to‐cycle state estimator of the Kalman filter type, which combines characteristic features of both simulated data and measured feedback from either a high‐frequency pressure sensor or an eddy current displacement sensor. The results show the great potential in accurately predicting the injection behavior and the method presented can thus be used to improve performance and emissions over an engine's lifetime.
Modelling and Finite Element Analysis of Viscoelastic Adhesive Joints
Authors: Rehbein Thomas , Matzenmiller Anton
Published: December 2018

Besides large nonlinear deformations, polyurethane adhesives exhibit elastic and viscous material behaviour simultaneously. Although the concept of rheological elements is a quite simple method to describe rate effects, the identification of model parameters is a challenging task. Dynamic mechanical analysis provides the experimental database. By using time‐temperature superposition, the experimental frequency range can be extended in order to identify the relaxation times and dimensionless stiffnesses of the rheological network. The simulation results are in good agreement with stress‐strain curves from tests with different strain rates.
Bridging protein rigidity theory and normal modes using kino‐geometric analysis with hierarchical constraint relaxation
Authors: Budday Dominik , Leyendecker Sigrid , van den Bedem Henry
Published: December 2018

Elastic‐Network Models (ENM) and pebble game based rigidity analysis are two distinct coarse‐grained approaches that have provided tremendous insight into conformational flexibility of proteins. However, the topological nature of the pebble game, and thereby the absence of motion modes have limited its applicability and a detailed comparison to ENM. Here, we present an alternative approach to rigidity analysis which eliminates these drawbacks through detailed kinematic analysis of dihedral degrees of freedom and non‐covalent hydrogen bond constraints, collected in the constraint Jacobian matrix J. Our new procedure reveals a spatial hierarchy of protein motions intrinsic to the hydrogen bonding network, ranked by the singular values of J. This spectrum of J yields a striking, fold‐specific signature, differentiating stiffer α‐helical from more collective β‐sheet proteins, which often goes undetected in similarly coarse‐grained methods. Overall, our results agree with experimental data and more detailed simulations, indicating that hydrogen bond networks have evolved with different protein folds to modulate structural dynamics and molecular mechanisms, with broad implications for protein and drug design.
Residual‐type a posteriori estimator for a viscoelastic contact problem with velocity constraints
Authors: Walloth Mirjam
Published: December 2018

We consider a linear viscoelastic contact problem in which the non‐penetration condition is enforced by constraints in the velocities. We present a residual‐type a posteriori estimator and sketch how upper and lower bounds are derived based on the works [1,4].
A continuum damage based approach for cavitation and shear failure of hyperelastic adhesives for finite element applications
Authors: Nelson Alexander , Matzenmiller Anton
Published: December 2018

Ship‐ and rail‐vehicle building as well as the automotive industry use rubber‐like adhesives for glazing and joining of structural components. These adhesives are suitable to compensate relative motions of adherends due to their ability to withstand large elastic strains. In service, the adhesive layer exhibits distinct amounts of isochoric and volumetric deformations that decisively effect the occurence of two different failure mechanisms. In this contribution, a material model of a rubber‐like adhesive is proposed, which accounts for cavitation and shear fracture. A polynomial strain energy density function is applied to describe the hyperelastic response. The failure is modelled with a continuum damage based approach using an energy based failure function. Computations of a component‐like specimen are performed to validate the model.
On an optimal control problem for a class of elliptic VIs of the second kind involving unbounded operators
Authors: Betz Livia , Yousept Irwin
Published: December 2018

In this note, we consider an optimal control problem governed by an elliptic variational inequality (VI) of the second kind involving an unbounded operator. We present our main ideas for deriving optimality conditions.
A Numerical Comparison of Consensus‐Based Global Optimization to other Particle‐based Global Optimization Schemes
Authors: Totzeck Claudia , Pinnau René , Blauth Sebastian , Schotthöfer Steffen
Published: December 2018

We compare a first‐order stochastic swarm intelligence model called consensus‐based optimization (CBO), which may be used for the global optimization of a function in multiple dimensions, to other particle swarm algorithms for global optimization. CBO allows for passage to the mean‐field limit resulting in a nonlocal, degenerate, parabolic PDE. Exploiting tools from PDE analysis, it is possible to rigorously prove convergence results for the algorithm (see [3]). In the present article we discuss numerical results obtained with the Particle Swarm Optimization (PSO) [4], Wind‐Driven Optimization (WDO) [6] and CBO and show that CBO leads to very competitive results.
Improving constitutive equations in multiscale modelling by means of the sufficiency criterion using the example of nano wire contraction
Authors: Kurzeja Patrick , Sievers Christian , Mosler Jörn
Published: December 2018

We demonstrate how two concepts from statistical modelling, namely the sufficiency criterion and the Rao‐Blackwell‐Kolmogorov theorem, can improve mechanical models. Applied to the contraction of relaxed nano wires, we start with a rough and simple equation. A set of sufficient parameters is then used to derive an improved equation that resembles the real solution in shape and quantity. The analytical solution and molecular simulations will finally highlight the functionality and the advantages of the scheme.
Objective modeling of multixial softening of concrete
Authors: Kaliske Michael , Zreid Imadeddin
Published: December 2018

Concrete in tension and low confined compression is characterized by strain‐softening, which leads to highly localized cracks or shear bands. Attempting to incorporate this response in standard continuum descriptions causes numerical instabilities and mesh sensitive results. This work presents a regularized plasticity‐damage model, which ensures mesh objectivity for general multixial strain softening. Combining plasticity and damage is necessary to describe the cyclic response of concrete. The damage part contains a split to consider the transition between compression and tension. Regularization of the model is achieved by an over‐nonlocal implicit gradient formulation. The model is implemented into a 3D finite element code and its capabilities are illustrated by numerical examples.
Simulation of bone ingrowth into bone substitutes on implant level length scale
Authors: Krüger Ann‐Kathrin , Julmi Stefan , Klose Christian , Besdo Silke , Maier Hans Jürgen , Wriggers Peter
Published: December 2018

Computational modelling is a promising and ethical reasonable method in implant development. A computational model for bone ingrowth on implant level length scale has been implemented and applied for an open‐pored space‐filling bone substitute. First numerical simulations showed reasonable results for the average bone density and the effective Young's modulus of the bone‐implant‐compound.
Numerical Simulation of Crack Propagation in an Anisotropic Medium
Authors: Bilgen Carola , Hennig Paul , Kästner Markus , Weinberg Kerstin
Published: December 2018

Phase‐field methods have been proven to address the main challenges in fracture mechanics – the identification of crack initiation and the simulation of the unknown crack paths – in an elegant way. This approach has therefore become very popular recently. Our contribution sets the focus on different ways to capture anisotropy in the phase‐field model. In order to deal with the tension‐compression asymmetry in fracture problems, a suitable operator split has to be deduced to take only the tensile deformations, which lead to crack growth, into account. In general, the strain energy function can be written in terms of principal stretches or principal invariants. A comparison of different decompositions is demonstrated in the context of both, finite and linearized strains. Furthermore, energetic and stress based fracture criteria are considered and checked against each other in more detail. Additionally, material anisotropy is examined within the phase‐field approach using an operator‐scaling anisotropic random field to consider the microstructure of the material implicitly.
Remarks on Fluid‐Structure Interaction Simulations in Realistic Arterial Geometries with regard to the Transmural Stress Distribution
Authors: Fausten Simon , Balzani Daniel , Heinlein Alexander , Klawonn Axel , Rheinbach Oliver , Schröder Jörg
Published: December 2018

In this contribution, Fluid‐Structure‐Interaction (FSI) in blood vessels, in detail the simulation of realistic arterial geometries, where the interaction of the blood flow and the vessel wall is of special interest, is considered. Based on pervious research, cf. [1], our existing framework for FSI‐simulations is extended towards realistic arterial geometries. The inflow and outflow boundary conditions for the fluid, as well as the boundary conditions for the structure are enhanced and adjusted to the chosen patient‐specific geometry. In detail, an inflow profile for arbitrary shaped inflow cross‐sections and a zero pressure boundary condition at the outflow are applied. Furthermore, the vessel wall is discretized using realistic material parameters of the media layer. The geometry and material parameters are adopted from [2]. In order to deal with the increasing complexity of the boundary value problem parallel computing and a two‐level overlapping Schwarz method with energy‐minimizing coarse space are applied; cf. [3]. The numerical simulations are performed using the Open‐Source software LifeV, in particular a code which has been developed in cooperation with the group of Prof. Quarteroni from the EPF Lausanne.
Modelling of lung‐metastases apoptosis within brain tissue
Authors: Schröder Patrick , Wagner Arndt , Stöhr Daniela , Rehm Markus , Jensch Antje , Radde Nicole , Ehlers Wolfgang
Published: December 2018

Apoptosis, a form of programmed cell death, can be induced in lung‐cancer cells by treatment with death‐receptor ligands. In this contribution, the coupled multiphasic process is described using a continuum‐mechanical model based on the Theory of Porous Media. Furthermore, the data‐enriched model incorporates the crucial apoptosis parameters, which are estimated via the maximum likelihood estimation based on cell‐culture experiments.
A mass‐lumped mixed FEM for acoustic wave propagation with inhomogeneous coefficients
Authors: Egger Herbert , Radu Bogdan
Published: December 2018

We consider the numerical approximation of acoustic wave propagation in the time domain by a mixed finite element method based on the BDM1–P0 spaces. In [2], we considered a mass‐lumping strategy for the BDM1 element that allows for efficient time integration. Moreover, we managed to show that while the accuracy of the method is reduced to first order by the quadrature error, the method does still manage to carry second order global information. This allowed us to devise post‐processing strategies for both variables. Here, we consider the case of inhomogeneous coefficients a and b, and show that the same results hold as for the homogeneous case. In addition, we illustrate the results by a numerical test.
Evolution of structures with homogeneous equivalent stress
Authors: Muench Ingo , Keller Alexander
Published: December 2018

A phase field model is used to evolve structures with almost homogeneous equivalent stress σv. The model combines the sensitivity of a stress‐based and a couple‐stress‐based objective function. Since couple‐stress typically increases at stress‐singularities, it is an efficient parameter to influence the topology of load‐bearing structures facing that problem. Further, it will be shown that this combination yields an increased solution space concerning the topology of the evolving structures. The effect is demonstrated for the topology of a bridge. Whereas traffic load q onto the driving surface prefers to evolve an arch as topology, the self‐weight g of the bridge prefers to evolve a topology similar to a suspension bridge. The couple‐stress‐based objective function evolves the combination of both topologies such that a smooth transition appears between the topology for q/g = 0 and the topology for q/g → ∞.
A comparison of methods for approximating periodic limit cycles in nonlinear systems with joint friction
Authors: Kappauf Jonas , Hetzler Hartmut
Published: December 2018

In this contribution, approximation methods for calculating periodic limit cycles of self‐excited dynamical systems with frictional damping are studied. In industrial application, e.g. the analysis of brake squeal, the so‐called Energy‐Balance has attracted a lot of interest [3], while rather classical Harmonic Balance methods are commonly used in non‐linear analysis. Focussing on the effect of both smooth and non‐smooth non‐linearities, limit cycles were calculated for two minimal models, which are commonly used when dealing with NVH or brake squeal analysis. Moreover these two methods are compared in terms of accuracy and numerical effort.
A variational framework for distance‐minimizing data‐driven method for linear elasticity
Authors: Nguyen Lu Trong Khiem , Rambausek Matthias , Keip Marc‐André
Published: December 2018

A variational framework for the distance‐minimizing data‐driven computing method is proposed based on the method of Lagrange multipliers. The data‐driven solution is defined as an optimization solution of a double‐minimization problem. A staggered scheme is employed to decompose the original problem into a sequence of single‐minimization problems. As an advantage, the variational formulation renders straightforward implementation of the high‐order polynomial interpolation. The non‐uniqueness of the data‐driven solution is the consequence of the use of the current solution strategy and non‐convexity of the data function space. A convergence study of the solution with respect to the data size is presented. The decrease of mean values and standard deviations of errors with respect to the reference solution justify the variational formulation.
A consistent multi‐scale derivation of a micro‐plane model within the framework of RVE homogenisation
Authors: Storm Johannes , Kaliske Michael
Published: December 2018

The micro‐plane model is an approach to describe complex anisotropic material behaviour by a set of uniaxial deformation states. The model is widely applied in the context of concrete materials and has various formulations for anisotropic damage, creep and plastic material behaviour. The paper contributes to this class of material models by a consequent multi‐scale derivation of a micro‐plane‐like model within the framework of RVE homogenisation. The new approach preserves the elegant and efficient model structure and yields a thermodynamically consistent scale transition. The model derivation allows to clearly state the conditions on the micro‐structure and thereby to classify the micro‐plane models and the known extensions with respect to their micro‐structural motivations.
A Stochastic Model for Strong Wind Events in Central Europe based on Historical Wind Data
Authors: Niekamp Rainer , Brands Dominik , Götzen Carina , Kiseleva Maria , Niekamp Jana , Schröder Jörg , Siems Onnen
Published: December 2018

For the generation of synthetic storms in order to estimate claims due to storm hazards there exist a couple of commercial tools. They are based on expert's knowledge and physical models of wind appearance or work with distortions of historical hazards. In contrast to these approaches we present a purely data driven model. From the historical data we extracted the hourly wind peaks over the last two decades and derived by algebraic methods a stochastic storm model in form of a polynomial chaos expansion.
Investigation of austenitic TRIP steels by means of a phase field model
Authors: Schmidt Simon , Klein Matthias W. , Boemke Annika , Smaga Marek , Beck Tilmann , Müller Ralf
Published: December 2018

Austenitic TRansformation Induced Plasticity (TRIP) steels offer an outstanding combination of formability and strength. Using Electron Backscatter Diffraction (EBSD) technique, the grain orientation and morphology of f.c.c. and b.c.c. phases can be clearly detected in initial state and at definite strains [2]. In order to qualify the driving mechanisms of phase transformations occuring during deformation of metastable austenites, e.g. TRIP steels, a phase field model is used. For the modelling, we follow [6]. The field equations are solved using the finite element method with bi‐linear shape functions and 4‐node elements. Features of this model are demonstrated by illustrative numerical examples.
Micro‐structure related modelling of ultra‐high‐performance fibre reinforced concrete (UHPFRC) subjected to cyclic tensile loading
Authors: Höper Svenja , Kowalsky Ursula , Dinkler Dieter
Published: December 2018

Ultra‐high performance concrete with a compressive strength of more than 140 N/mm2 reacts very brittle without the addition of fibres. At designing economical and resource‐efficient building components as well as performance‐optimized materials, the supporting effects of the additional microfibres must be considered. Prospectively, cyclic loading and the fatigue behaviour of high performance materials and slim components will come to the fore. Therefore, the influence of cyclic deterioration on the load bearing capacity of the interface transition zone connecting microfibres and the surrounding concrete surface must be investigated. As part of the DFG Priority Programme 2020, the damage mechanisms and processes regarding the deterioration of UHPFRC subjected to cyclic tensile loading are explored in an Experimental‐Virtual Lab. Using the concepts of elasto‐plasticity and continuum damage mechanics, the material models for the components of the composite material UHPFRC are improved and enhanced to be combined on a mesoscale with the help of a non‐linear bond model. This bond model is discretized by interface elements with curved surfaces in 3D space and describes the interaction of fibres and concrete in the bond zone in terms of adhesion and friction effects. For validation and verification, single and multiple fibre pullout tests are simulated in 3D‐finite element analyses. These FE‐simulations and preliminary findings are supposed to provide the essential basis for future numerical degradation prediction.
Oscillations of a visco‐elastic belt drive
Authors: Steindl Alois
Published: December 2018

We investigate the loss of stability of the steady configuration of a planar visco‐elastic belt drive. The belt is considered as a linearly visco‐elastic slender beam with small bending stiffness, which is driven by a steadily rotating drum. For a belt clamped on both ends at different heights no Hopf bifurcation is found, but a perturbed pitchfork bifurcation is observed. For a belt on a drum different Hopf bifurcation points are detected, which depend strongly on the viscous damping.
Micromorphic and micropolar continua: A comparative study based on homogenisation of particle simulations
Authors: Bidier Sami , Ehlers Wolfgang
Published: December 2018

Extended continuum‐mechanical theories, such as the micromorphic, the micropolar or the microstrain continuum, are suitable formulations when additional characteristic microstructural deformation modes should be included into macroscopic modelling techniques. This is especially the case when the microscopic view of the considered material reveals a granular or particulate structure. Then, by means of a particle‐centre‐based homogenisation technique, it is possible to link the microstructural information that can be gained, for example, from Discrete‐Element (DE) simulations, to the extended deformation and stress states of the above mentioned extended continua. In this context the present contribution focuses on two main aspects: at first, different granular microstructures, in particular bonded and unbonded granular systems, are investigated using a discrete‐element formulation with the emphasis on the evolution of strain localisation phenomena. Secondly, the obtained results of these initial‐boundary‐value problems are homogenised towards micropolar and micromorphic stress measures. Depending on the microstructural characteristics, the activation and evolution of the extended stress states are compared and evaluated with special emphasis on the size of the chosen homogenisation domain.
An Adam‐Gibbs based model for the temperature behavior of polymers near glass transition
Authors: Wicht Daniel , Kehrer Loredana , Wood Jeffrey T. , Böhlke Thomas
Published: December 2018

The mechanical behavior of polymers and other glass forming materials is strongly time‐ and temperature‐dependent. Based on the rheological Poynting‐Thomson Model and the Adam‐Gibbs equation, a shift function is developed which relates the time constants of a material to temperature. The behavior of this function is compared to the established Williams‐Landel‐Ferry and Arrhenius equations in the glassy and rubbery temperature regimes.
Modeling of anisotropic boundary layer effects in plate theory
Authors: Schneider Patrick , Kienzler Reinhold
Published: December 2018

The modeling of a Reissner‐type plate theory for elastic, monoclinic material is presented. The basic theory is derived using the a‐priori‐assumption free uniform‐approximation approach, which is based upon series expansions of the displacement field and a structured truncation of the elastic potential that gives rise to a hierarchy of approximating theories. An a‐priori estimate for the approximation error shows that higher‐order theories have indeed a higher rate of convergence with respect to the relative slenderness of the plate. Using a pseudo‐reduction approach, the number of PDEs to be solved is reduced significantly. The resulting first‐order theory is the classical monoclinic plate theory, whereas, the second order theory is not determined uniquely by the approach. Uniqueness is achieved by introduction of an orthogonal decomposition of higher‐order gradients of the in‐plane displacements. The final second‐order theory coincides with the Reissner‐Mindlin theory for the special case of isotropic material.
Iterative spectral identification of bone macroscopic properties described by a probability box
Authors: Rosić Bojana , Kumar Shivanand Sharana , Vinh Hoang Truong , G. Matthies Hermann
Published: December 2018

This paper considers Bayesian identification of macroscopic bone material characteristics given digital image correlation (DIC) data. As the evaluation of the full Bayesian posterior distribution is known to be computationally intense, here we consider the approximate estimation in a Newton‐like manner by using the theory of conditional expectation. The approach is extended to include the epistemic uncertainties in the process of modelling the prior.
Fatigue Reliability Analysis and Design for Structural Components in Quasi‐One‐Shot Device
Authors: Wang Haiqiao , Chen Min , Yao Xiaofei , Liu Zhiyuan , Wang Xiang , Tate Derrick
Published: December 2018

The reliability of the quasi‐one‐shot device is a crucial index and hard to predict due to its complexity and cost‐consuming of testing. In this paper, a bi‐directional approach of reliability optimization design (ROD), integrating stress‐strength interference model and fatigue analysis, was proposed to evaluate and optimize the weakest component of the quasi‐one‐shot device. High‐voltage vacuum circuit breaker was used as a numerical validation example.
Semi‐active vibration reduction based on a variable structure controlled serial‐stiffness‐switch‐system
Authors: Min Chaoqing , Dahlmann Martin , Sattel Thomas
Published: December 2018

A concept for semi‐active vibration control with a serial‐stiffness‐switch‐system based on vibration energy harvesting was previously presented. This contribution investigates a variable structure control‐based switching law to reduce the settling time of the system response under a nonzero velocity initial condition. The proposed switching law is able to fasten the transformation of initial vibration energy of the system into potential energy stored equally in its two springs. It is characterized, simulated, and proven to be more effective than the old switching law.
Influence of Hemorheology on Nanoparticle Transport in Vessel Bifurcations
Authors: Bavandi Mahrokh , Wünsch Olaf
Published: December 2018

This study presents the effect of hemorheology on nanoparticle transport in the left coronary artery. The flow is considered to be pulsating and particles are modeled by concentration equation. The influence of Newtonian and non‐Newtonian modeling approaches on particle transport is investigated and compared. Three bifurcations are investigated to determine the effect of bifurcation angles. The results show that considering blood as non‐Newtonian in certain configurations is necessary.
Polymorphic Uncertainty Modeling for Optimization of Timber Structures
Authors: Schietzold Friedemann Niklas , Graf Wolfgang , Kaliske Michael
Published: December 2018

The uncertainty characteristics of wood are mainly affected by natural variation. Out of this, the traditional approach of stochastic variables can be expanded to a polymorphic uncertainty model. Therefore, e.g. fuzzy probability based randomness is used by extending stochastic variables with fuzzy variables in parameterization concerning the distribution functions, see [1] and [2]. The coupling of both aleatoric and epistemic uncertainty is involved in the uncertainty analysis.

The FEM is applied as basic solution of particular load situations on the focused timber structure. A local orthotropic formulation is used and the properties are evaluated on each integration point with respect to the tree trunk axis.

In this contribution, an approach to polymorphic uncertainty modeling for timber structures is introduced. According to [3], models representing the spatial variation and interdependencies of material parameters are necessary for a realistic representation in numerical simulation. For this purpose, on the one hand interactions between fuzzy variables, on the other hand correlations among random variables are considered. Random fields are utilized to capture spatially varying material properties in context with the discretization of FE. Approaches to both spatially and structurally depending autocorrelations along with crosscorrelations based on [4] are presented.

The preliminary steps aim at an optimization in design of timber structures, provided that polymorphic uncertain design as well as a priori parameters are considered. The developed tools for uncertainty analysis and the basic FEM solution are prepared as a basis for an automated optimization processing, whereas they are preferably parallelized, incorporating methods for reducing the numerical effort. Results of the uncertainty analysis of a timber structure are shown exemplary.
Low latency output feedback predictive control based on optimality conditions
Authors: Kögel Markus , Findeisen Rolf
Published: December 2018

In output feedback predictive control the control input is calculated by solving an optimization problem at each time instant based on a state estimate obtained by an observer. To efficiently counteract disturbances it is important to reduce the delay/latency due to the solution of the optimization problem. To this end we present an approach splitting the optimization into two phases: In the inter‐sample period a set of control laws parameterized by future measurements is determined exploiting the optimality conditions. Once the new measurement is available, the controller selects the corresponding control law and determines the optimal input, which can be achieved fast and with a low computational demand.
Gradient Approximation in Flexible Multibody Systems Using a Multigrid Approach
Authors: Moghadasi Ali , Held Alexander , Seifried Robert
Published: December 2018

The approximation of gradients with a multigrid approach is presented in the framework of flexible multibody systems, in which the flexible bodies are modeled using finite element method. For the cases where the number of design variables are high, for example, in topology optimization where the density of each element is chosen as a design variable, the gradient computation becomes computationally infeasible. The proposed approximation, in which the gradients are computed simultaneously on a fine grid and a coarse grid, can significantly reduce the computation time without hindering the accuracy of the gradients.
The Influence of Cooling Nozzle Positions on the Transient Temperature Field during Cryogenic Turning of Metastable Austenitic Steel AISI 347
Authors: Becker Steven , Hotz Hendrik , Kirsch Benjamin , Aurich Jan C. , von Harbou Erik , Müller Ralf
Published: December 2018

Metastable austenitic steels offer the opportunity of a surface layer hardening integrated in the machining process. The hardening effect is achieved by a deformation induced austenite‐martensite phase transformation, for which high mechanical loads and low temperatures are necessary, typically below room temperature. These conditions can be accomplished during cryogenic turning, allowing a phase transformation in the surface layer of the workpiece. To study the austenite‐martensite transformation behavior, information about the temperatures in the contact zone between tool and workpiece during machining is necessary, which can be hardly measured. Therefore, FE simulations of the process are utilized to calculate the transient process temperatures. In this paper predictive temperature field simulations for different settings of the cooling system, i.e. varying cooling nozzle positions, are performed in order to investigate the influence of the cooling nozzle positions on the temperature distribution during cryogenic turning and to correlate the results with measurements of the martensite content resulting for these cooling setups.
Leakage current and polarization domains in ferroelectric nanogenerators for energy harvesting
Authors: Wöhler Franziska , Muench Ingo , Wagner Werner
Published: December 2018

Energy harvesting based on the transformation of mechanical into electrical energy is under investigation. Mechanical deformations from ambient parasitic vibrations serve as energy source for our nanogenerator concept with barium titanate as ferroelectric material. The energy harvesting process is enabled by the designed polarization domain topology. Due to the deformation in the ferroelectric film, the polarization domains reorder resulting alternating charge density within electrodes. Defects and other impurities in the single crystal turn the ferroelectric film into a semiconductor. We account for leakage current mechanisms in our finite element phase field model phenomenologically. Ohm's law (OL) and space‐charge‐limited‐ current (SCLC) are presented by linear or quadratic functions for the current density, respectively. The distance between the electrodes as well as the ferroelectric film thickness have an immense influence on the leakage current density. Further investigations focus on the influence of polarization domain topology on the leakage current.
Desingularization in boundary element analysis of three‐dimensional Stokes flow
Authors: Harmel Maximilian , Rajski Michał P. , Sauer Roger A.
Published: December 2018

The boundary element method (BEM) is able to solve partial differential equations without volumetric discretization and integration. Therefore, the BEM is able to reduce the compuational as well as the meshing effort compared to volumetric methods like classical finite elements. In this work, a conventional and a nonsingular BEM formulation for Stokes flow are presented and investigated in three‐dimensions, considering rotating spheres within a viscous fluid.
Effect of graphene quantum dots on photoluminescence property of polyvinyl butyral nanocomposite
Authors: Arthisree Devendran Lakshmi , Sumathi Rajappan Radhakrishnan , Joshi Girish
Published: December 2018

Design and development of new photoluminescence system are much in demand for various engineering and technological applications. The present investigation focused on the influence of graphene quantum dots (GQDs) dispersion in the polyvinyl butyral (PVB) matrix. The structural and chemical interaction of GQD‐dispersed PVB composites was confirmed by X‐ray diffraction (XRD), Fourier transform infrared (FTIR), micro‐Raman spectroscopy, ultraviolet and visible (UV‐Vis), and photoluminescence (PL) techniques. Chemical interaction between the functional groups leads to PL quenching at 455 nm. Changes on crystallite size and interplanar spacing hinders on the structural properties of the nanocomposite. Raman spectroscopy reveals the decrease in D/G intensity ratio influenced by GQD loading wt% in the polymer system. The dispersion and occupied network of GQD in the PVB matrix was confirmed by optical polarizing microscopy (OPM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Effect of electrical conductivity of composites as a function of temperature has been verified. Decrease in direct bandgap as a function of GQD loading confirms the promising PL properties of the prepared composite system. Thus GQD‐derived composites may further be developed as a membrane for improved PL property.
Copolymerization modification: improving the processability and thermal properties of phthalonitrile resins with novel comonomers
Authors: Liu Yang , Liu Zheng‐zhou , Peng Wei‐feng , Lu Zheng , Hu Jiang‐huai , Zeng Ke , Yang Gang
Published: December 2018

Two novel tetrahydrophthalic anhydride end‐capped imide compounds (THAN and THBN) with high thermal stability were synthesized to promote the curing reaction of 1,3‐bis(3,4‐dicyanophenoxy)benzene (3BOCN), and to study the effects of comonomer structure on the curing behavior and thermal performance of phthalonitrile resins. The curing behaviors of THAN/3BOCN and THBN/3BOCN blends with various molar ratios were investigated using rheological analysis and differential scanning calorimetry, suggesting a wide processing window. Dynamic mechanical analysis and thermogravimetric analysis showed that the cured resins possessed high glass transition temperatures (> 500 °C), and superior thermal and long‐term thermo‐oxidative stabilities with weight retention of 95% ranging from about 544 to 558 °C in both nitrogen and air. All these results indicated that the processability and thermal properties of phthalonitrile resins could be improved further by modifying the structure of comonomer in this kind of curing system. © 2018 Society of Chemical Industry
Proteomics in the Diagnosis of Endometriosis: Opportunities and Challenges
Authors: Ferrero Simone
Published: December 2018

The non‐surgical diagnosis of endometriosis is still challenging for the clinician. Ultrasonography and magnetic resonance imaging can be used to diagnose ovarian endometriotic cysts and deep infiltrating endometriosis; but their performance is poor in the diagnosis of initial stages of endometriosis. CA‐125 and other serum markers (such as CA 19‐9, serum protein PP14, interleukins, and angiogenetic factors) have been measured in women with endometriosis but they are not reliable for the diagnosis of the disease. Although several studies used proteomics technologies to identify plasmatic markers of endometriosis, the non‐invasive diagnosis of endometriosis is far from being achieved. In this issue, Manousopoulou et al. compare the integrated quantitative proteomic profile of eutopic endometrium and serum of women with endometriosis and controls. 1214 proteins are differentially expressed in the eutopic endometrium and 404 proteins in the serum of the two study groups. 21 proteins are aberrantly expressed in both eutopic endometrium and serum of women with endometriosis. More work is needed to assess if the differentially expressed proteins identified in this study can be used as clinical markers of endometriosis.
Developing a revised definition of the Bobath concept
Authors: Vaughan‐Graham Julie , Cott Cheryl , Holland Ann , Michielsen Marc , Magri Alba , Suzuki Mitsuo , Brooks Dina
Published: December 2018

Objective

This study was developed as a consensus‐building exercise within the International Bobath Instructors Training Association (IBITA) to develop a revised definition of the Bobath concept.

Methods

A three‐phase design utilizing (a) focus groups, (b) survey methods, and, (c) real‐time Delphi. This paper details Phase 1 and 2.

Results

Forty IBITA members participated in five focus groups. Eight broad themes were developed from the focus groups from which the survey statements were developed. There was a high level of agreement on all nine survey statements identifying overarching constructs and on 12 of the 13 statements identifying unique aspects of Bobath clinical practice. Lower scores were attributed to lack of understanding of the term humanistic, Bobath clinical practice addressing multiple domains such as impairments, activities, and participation and limited agreement on the description of the term “placing.”

Conclusion

Focus groups and a web‐based survey were successful in soliciting the opinions of IBITA members on themes and statements of importance for the development of a revised Bobath definition. The results of Phase 1 and 2 will inform Phase 3, a real‐time Delphi, to gain consensus within IBITA on statements on which a revised Bobath definition is to be based.