bluuebunny/crossref_metadata_2025 huggingface dataset:
Cardiometabolic risk factors in treatment‐seeking youth versus population youth with obesity
Authors: Fox C. K. , Kaizer A. M. , Ryder J. R. , Rudser K. D. , Kelly A. S. , Kumar S. , Gross A. C. ,   on behalf of POWER Work Group
Published: March 2018

Background

Although obesity affects approximately one in five youths, only a fraction is treated in pediatric weight management clinics. Characteristics distinguishing youth with obesity who seek weight management treatment from those who do not are largely unknown. Yet identification of specific health characteristics which differentiate treatment‐seeking from non‐treatment seeking youth with obesity may shed light on underlying motivations for pursuing treatment.

Objectives

Compare the cardiometabolic profiles of an obesity treatment‐seeking sample of youth to a population‐based sample of youth with obesity, while controlling for body mass index (BMI).

Methods

This cross‐sectional study included participants, ages 12–17 years, with obesity from the Pediatric Obesity and Weight Evaluation Registry (POWER) and National Health and Nutrition Examination Survey, representing the treatment‐seeking and population samples, respectively. Mean differences were calculated for systolic and diastolic blood pressure percentiles, total cholesterol, low‐density and high‐density lipoprotein‐cholesterol, triglycerides, fasting glucose, glycated hemoglobin and alanine aminotransferase, while adjusting for age, sex, race/ethnicity, insurance status, and multiple of the 95th BMI percentile.

Results

The POWER and National Health and Nutrition Examination Survey cohorts included 1,823 and 617 participants, respectively. The POWER cohort had higher systolic blood pressure percentile (mean difference 17.4, 95% confidence interval [14.6, 20.1], p < 0.001), diastolic blood pressure percentile (21.8 [19, 24.5], p < 0.001), triglycerides (42.3 [28, 56.5], p < 0.001) and alanine aminotransferase (7.5 [5.1, 9.8], p < 0.001) and lower fasting glucose (−6.9 [−8.2, −5.6], p < 0.001) and high‐density lipoprotein‐cholesterol (−2.3 [−3.8, −0.9], p < 0.002). There were no differences in total cholesterol or low‐density lipoprotein‐cholesterol or clinical differences in glycated hemoglobin.

Conclusion

For a given BMI, obesity treatment‐seeking youth are more adversely affected by cardiometabolic risk factors than the general population of youth with obesity. This suggests that treatment‐seeking youth may represent a distinct group that is at particularly high risk for the development of future cardiometabolic disease.
Model‐reduction in micromechanics of polycrystalline materials
Authors: Michel Jean‐Claude , Suquet Pierre
Published: March 2018

The present study is devoted to the generalization of the Nonuniform Transformation Field Analysis (NTFA), a model‐reduction approach introduced by the authors. First, the local fields of internal variables are decomposed on a reduced basis of modes. Second, the effective (average) dissipation potential of the phases is replaced by accurate approximations. The reduced evolution equations of the models, in other words the homogenized constitutive relations, can be entirely expressed explicitly in terms of quantities which are pre‐computed “off‐line”. The example of creep of polycrystalline ice is used to assess the accuracy of the models. Their predictions, both the overall response and the local response, are shown to be in good agreement with full‐field simulations with a significant speed‐up. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Turbulent superstructures in Rayleigh‐Bénard convection for varying Prandtl numbers
Authors: Pandey Ambrish , Schumacher Jörg
Published: March 2018

Using direct numerical simulations, we study the properties of turbulent superstructures in thermal convection in a large aspect ratio square cell. We estimate the characteristic length scale of superstructures using spatial auto‐correlation functions and two‐dimensional power spectra, and observe that the typical length scale increases weakly with increasing Prandtl number. We also find that the Prandtl number dependence of heat and momentum transport are similar to those observed in small aspect ratio systems. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The combination of POD model reduction with adaptive finite element methods in the context of phase field models
Authors: Gräßle Carmen , Hinze Michael
Published: March 2018

In the present paper we derive a reduced order model utilizing proper orthogonal decomposition (POD‐ROM), for which we utilize adaptively obtained spatial snapshots. In a fully discrete setting this contains the challenge that the snapshots are vectors of different lengths. In order to handle this issue, we interprete the snapshots as elements of a common Hilbert space and consider the POD method from an infinite‐dimensional perspective. Thus, the inner product of pairs of snapshots can be computed explicitely, which enables us to build the reduced order model. This approach is applied to a phase field model which is described by a Cahn‐Hilliard equation. In the numerical examples we illustrate our appoach and compare a nonsmooth with a smooth free energy concerning the influence on the quality of the solution to the ROM. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A fuzzy finite element method for sparse experimental data based on a possibilistic approach
Authors: Dridger Alex , Caylak Ismail , Mahnken Rolf
Published: March 2018

This work considers the possibilistic interpretation of the fuzzy set theory for the fuzzy finite element method (FFEM). The underlying uncertainty type of design variables arises from sparse experimental data and is referred to as imprecise probability. In this context, a linear elastic body with given sparse experimental results for the design variables Young's modulus and Poisson's ratio is studied. To this end, the fuzzy input parameters are represented by possibility distributions generated by a probability‐possibility transformation. A computational approach involving the α‐level discretization technique [4] is applied in order to calculate the possibility of a quantity of interest. Finally, our method is applied in a numerical example for a plate with a ring hole. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A multivariate stochastic material model with correlated material parameters
Authors: Penner Eduard , Caylak Ismail , Mahnken Rolf
Published: March 2018

This paper concentrates on the aleatoric uncertainty of hyperelastic material models caused by material parameters which are characterized by random variables. To this end, the Polynomial Chaos Expansion (PCE) is applied to represent the random variables by a series of deterministic coefficients and random polynomials. To determine the corresponding PC coefficients and the polynomials of correlated stochastic material parameters we use the Cholesky decomposition and the Gram‐Schmidt algorithm. Based on experimental results and artificial data parameter identification of an Ogden material model for rubber is applied. As a numerical example we consider a static problem for uniaxial tension of the rectangular plate. This structure is investigated in order to represent the conditions for the experimental setup. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A primal discontinuous Petrov‐Galerkin Finite Element Method for linear elasticity
Authors: Steiner Tobias , Wriggers Peter
Published: March 2018

Volume‐locking is a significant problem when using the standard Finite Element Method (FEM) since this phenomenon can lead to failure of a numerical method. To circumvent constraints as related in (nearly) incompressible materials the novel discontinuous Petrov‐Galerkin (dPG) FEM is introduced for linear elasticity. A first result of the implemented primal dPG FEM is shown for a patch test. Further tests on the performance of the proposed method are work in progress and are not part of this contribution. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of a locking‐free hybrid discontinuous Galerkin element that is very easy to implement into FE‐codes
Authors: Wulfinghoff Stephan , Bayat Hamid Reza , Alipour Atefeh , Reese Stefanie
Published: March 2018

In this work, a hybrid discontinuous Galerkin (dG) quadrilateral element formulation is investigated. In hybrid formulations, the elements' interior is treated independently from the skeleton (the element boundaries). The global degrees of freedom (dofs) are located on the element corners. This makes the implementation into existing finite element codes simple. The dofs in the element interior are condensed out. The deformation gradient is taken homogeneous within the element instead of using bilinear shape funcitons. The result is a very simple formulation and numerical implementation. The element has been shown to be free of volumetric locking and shear locking. Moreover, the mesh convergence is close to well‐known formulations like the Q1E4‐, Q1P0‐ or Q1SP‐element. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modelling of hydraulic fracturing and fluid flow change in saturated porous domains
Authors: Heider Yousef , Markert Bernd
Published: March 2018

The underlying research work aims to develop a numerical model of pressure‐driven fracturing of saturated porous media. This is based on the combination of the phase‐field modelling (PFM) scheme together with a continuum‐mechanical approach of multi‐phase materials. The proposed modelling framework accounts for the crack nucleation and propagation in the solid matrix of the porous material, as well as the fluid flow change in the cracked region. The macroscopic description of the saturated porous material is based on the theory of porous media (TPM), where the proposed scheme assumes a steady‐state behaviour (quasi‐static) and neglects all thermal and chemical effects. Additionally, it assumes an open system with possible fluid mass production from external source. Special focus is laid on the description of the interface and change of the volume fractions and the permeability parameter between the porous domain and the crack. Finally, a numerical example using the finite element method is presented and compared with experimental data to show the ability of the proposed modelling strategy in capturing the basic features of hydraulic fracturing. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Heated vertical duct flow of liquid metal with and without expansion
Authors: Niemann Martin , Saini Vishal , Fröhlich Jochen
Published: March 2018

The effects of buoyancy on vertical duct flows with one heated side wall are investigated by means of Direct Numerical Simulations. In particular, the flow through square ducts and behind the expansion of a backward‐facing step with side walls are presented for the forced and mixed convection regime. These configurations allow studying the interaction of buoyancy forces with secondary flow of second kind in the square duct and with the shear layer and recirculation zone in case of the expansion. In both configurations, buoyancy substantially alters the mean flow field, the turbulent stresses, and the heat transfer compared to the corresponding forced convection cases. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rotorcraft Aeromechanics Simulation — When applied mathematics hits real engineering
Authors: Keßler Manuel
Published: March 2018

Aerodynamic simulation has reached a level of sufficient maturity, that not only benchmark problems are solved easily, but real application data can be delivered even for such complex configurations like rotorcraft. Leaving behind generic cases like driven cavity or isotropic turbulence and moving on to actual engineering, several fields more or less remotely related to aerodynamics have to be taken into account to provide accurate and reliable value. This is less a matter of ingenious invention, but much more so a careful selection of usable procedures for problems identified to matter, and mostly hard work, putting the pieces together and shaking all the wrinkles out. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Higher‐Order Index‐1 Co‐Simulation Approach: Solver Coupling for Multibody Systems
Authors: Meyer Tobias , Li Pu , Lu Daixing , Schweizer Bernhard
Published: March 2018

This paper attends to a co‐simulation approach for solver coupling in time domain. A general multibody system is divided into several subsystems, which are coupled by algebraic constraints. The coupling technique analyzed here is a linear‐implicit predictor/corrector approach, i.e. coupling variables for the corrector step are calculated by one step of a Newton‐iteration. Within the presented approach, the coupling conditions together with its first and second derivatives are enforced simultaneously at the communication‐time points. This index‐1 approach uses cubic polynomials to approximate the coupling variables. The space of polynomials of degree ≤ 3 is a four‐dimensional vector space. One of the four degrees of freedom is used for a continuous approximation of the coupling variables at the communication‐time points. The three remaining degrees of freedom are used in order to enforce the coupling conditions on position, velocity, and acceleration level. Due to the higher order approximation, the numerical errors are very small and a good convergence behavior is achieved. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The long History of Impact Mechanics, Rolling Contact and Multibody System Dynamics
Authors: Schiehlen Werner
Published: March 2018

Contact mechanics is a branch of solid mechanics where two or more bodies are involved. The contacting points are characterized by a common tangential plane and a common normal vector given by the surface curvature of the bodies. Additional parameters of contact problems are the material of the bodies, the forces and torques acting on the bodies as well as their time history, location of the centers of mass and the roughness of the surface around the contact point. The classical contact problem is based on spherical bodies surfaces, linear‐elastic material, constant static loading in normal direction and smooth surfaces. The rigorous solution [1] is often used for benchmarking. More details can be found in the monographs [2, 3]. The bodies of mechanical systems in engineering applications get in contact to each other mainly by impacts, rolling or sliding, or by joints. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical realisation of freezing processes in frost‐resistant plant tissues
Authors: Eurich Lukas , Wagner Arndt , Ehlers Wolfgang
Published: March 2018

Understanding functional principles of frost‐resistant plant tissues under frost conditions is considered as an important milestone with regard to frost damage prevention in construction materials, as plant tissues are capable to withstand many freezing and thawing cycles without any damage. This contribution introduces a modelling approach for the biological role model based on the Theory of Porous Media (TPM) with an emphasis on structural properties and the phase transition of extracellular water. The presented numerical examples show the ice formation, represented by a frost front, where also structural effects of the plant's microstructure, such as inhomogeneity, can be considered in a numerical investigation. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical modeling of active response of arteries
Authors: Gilbert Rose Rogin , Hartmann Stefan , Kudela László , Rank Ernst , Yossef Ofry , Yosibash Zohar
Published: March 2018

Even though many studies have been conducted to understand and model the passive response of arteries, very few studies have been performed to develop constitutive equation for the active response of arteries and to perform numerical simulations. In this paper, the diffusion of the neurotransmitter, for example noradrenaline, is combined with the anisotropic constitutive model to describe the temporal contraction of the smooth muscle cells in the artery wall. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Adaptive Algorithms for Optimal Hip Implant Positioning
Authors: Moldenhauer Marian , Weiser Martin , Zachow Stefan
Published: March 2018

In an aging society where the number of joint replacements rises, it is important to also increase the longevity of implants. In particular hip implants have a lifetime of at most 15 years. This derives primarily from pain due to implant migration, wear, inflammation, and dislocation, which is affected by the positioning of the implant during the surgery. Current joint replacement practice uses 2D software tools and relies on the experience of surgeons. Especially the 2D tools fail to take the patients' natural range of motion as well as stress distribution in the 3D joint induced by different daily motions into account. Optimizing the hip joint implant position for all possible parametrized motions under the constraint of a contact problem is prohibitively expensive as there are too many motions and every position change demands a recalculation of the contact problem. For the reduction of the computational effort, we use adaptive refinement on the parameter domain coupled with the interpolation method of Kriging. A coarse initial grid is to be locally refined using goal‐oriented error estimation, reducing locally high variances. This approach will be combined with multi‐grid optimization such that numerical errors are reduced. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Development of patient‐specific computational models of the human aortic valve under healthy and diseased conditions
Authors: Morales Ortuno Sergio , Röehrle Oliver
Published: March 2018

A 3‐D finite element modelling (FEM) approach of the human aortic valve (AV) is developed to study its extra‐cellular matrix (ECM) destructive remodelling caused presumably by ventricular diseases, assuming that the function, deformation and performance of the valve is strongly dependent on its geometry. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Influence of Vibrissa Morphology on Artificial Tactile Sensors for Surface Texture Detection
Authors: Scharff Moritz , Steigenberger Joachim , Behn Carsten , Alencastre Jorge
Published: March 2018

Animals, e.g. rats and cats, have different types of tactile hairs. One type are the mystacial vibrissae. They are located around the snout and in combination with the follicle‐sinus complex they are powerful tactile sensors. With them, animals are able to detect the distance to an object, recognize the object shape and determine the surface texture. Adapting the natural example, the goal is to design an artificial tactile sensor with a similar functionality. In the course of this, a model for surface texture detection is developed. The vibrissa is modeled as an Euler‐Bernoulli bending beam, incorporating large deflections. The contact, between vibrissa and surface, is modeled using Coulomb's law of friction. The mechanical modeling results in a nonlinear, system of first order differential equations. Solving the boundary‐value problem by applying a shooting method, a quasi‐static simulation is performed. Some first relations between the vibrissa morphology and the surface contact are analyzed. With view to an artificial sensor, the change of the reaction forces and moments at the base of the vibrissa due to the surface contact is the point of focus. Out of the reaction forces, the coefficient of static friction between vibrissa and surface is determined. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Vibration of the Basilar Membrane and Fluid Pressure Distribution in the Human Cochlea
Authors: Ziegler Pascal , Wahl Philipp , Eberhard Peter
Published: March 2018

The human inner ear or cochlea is a bone structure of spiral shape and is composed of mainly two conical chambers which are filled with fluid and separated by a soft membrane, referred to as the basilar membrane. At the apical end, both chambers are connected through a small opening, the helicotrema. At the base, the chambers are closed by the stapes footplate and the round window membrane. In case of a normal ear, sound is received by the eardrum, transmitted through the middle ear ossicles and finally excites the inner ear fluid through the vibration of the stapes footplate. According to present hearing theory, this leads to pressure waves in the cochlear fluid which in turn results in characteristic vibration behavior of the basilar membrane. Related to the sound frequency, hair cells in certain areas of the basilar membrane are stimulated and cause hearing nerve stimulation. In order to predict the effect of inner ear diseases on hearing impression as well as to develop new hearing implants, a deeper understanding of the cochlear dynamics is needed. However, since the cochlea represents a closed hydraulic system with a complex geometry, the motion of the basilar membrane as well as the fluid pressure can hardly be measured. Therefore, a numerical model of the uncoiled cochlea is developed representing the fundamental physical effects occurring in the cochlea. Taking the fluid‐structure interactions into account, the transfer behavior of the cochlear system is investigated for different excitation frequencies within the auditory frequency range of humans. The simulations show the passive vibration of the basilar membrane resulting in the characteristic traveling wave. These results allow to study the mapping of the excitation frequency to its characteristic pattern along the basilar membrane, called tonotopy. Further, the spatial fluid pressure distribution along the cochlear chambers is evaluated and allows new insights into the cochlear physics. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Determination of the diameter of columnar jointed volcanics using bifurcation analysis
Authors: Anderssohn Robert , Hofmann Martin
Published: March 2018

The development of the typical hexagonal pattern of solidifying volcanic material can be described by fracture mechanics. While following the moving inhomogeneous temperature field, these columns reach a steady growth with a certain diameter. This spacing increases sometimes instantaneously. Using a bifurcation analysis based on the free Helmholtz energy this phenomenon is investigated and presented with this contribution. In order to handle the complex three‐dimensional geometry, regularity in the pattern and a Fourier series expansion of the crack contour are used. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimental investigation and numerical simulation of fatigue crack formation in polycrystalline materials
Authors: Hohe Jörg , Beckmann Carla , Kennerknecht Tobias , Preußner Johannes , Farajian Majid , Luke Michael
Published: March 2018

The present contribution is concerned with an experimental investigation of fatigue crack initiation in polycrystalline metals using a micro scale specimen testing technique. Based on the results of the experimental investigation and a fractographic investigation, a probabilistic microscale simulation procedure for prediction of the fatigue crack formation phase is proposed. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Crack path predictions in anisotropic structures considering residual stresses
Authors: Judt Paul , Ricoeur Andreas
Published: March 2018

Different mechanical processes induce a directional microstructure (texture) during the production of a material, e.g. during the hot rolling of aluminum plates or the injection molding process of natural fiber reinforced bio‐polymers. In such materials the anisotropic mechanical features are related to the texture and have a decisive impact on the fracture process. Rolling or shot peening processes, on the other hand, induce an irreversible plastic deformation and residual compressive stresses for the purpose of a surface toughening. Similarly, residual stresses are induced if a circular core with interference fit is inserted into a drilled hole. The measurement of crack paths in specimens with residual stresses due to a core reveals an influence of the core's interference with respect to the crack deflection and the resulting paths [1,2]. Numerical investigations of fatigue crack growth show that conventional crack deflection criteria are not valid in such specimens, as the deflection angle depends on the magnitude of the external loading, going along with a rotation of the principal stress axes and thus the Jk‐vector [2]. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Calibration of a gradient‐enhanced damage model for viscoplastic shell structures subjected to shock‐wave loadings
Authors: Nguyen An Danh , Stoffel Marcus
Published: March 2018

In this study, we proposed both a numerical and experimental methodology for material parameter identification to validate the gradient‐damage model in [1]. A nonlocal anisotropic damage model for viscoplastic thin‐walled shells was proposed by introducing a gradient‐enhanced free energy function. For the experimental methodology, cyclic micro‐bending and ‐tensile tests were conducted to identify the damage size effects to calibrate the proposed FE model. Numerical results were obtained and compared with displacement‐force curves from shock‐tube tests. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A new approach in material modeling towards shear cutting of CFRP
Authors: Poggenpohl Lukas , Wulfinghoff Stephan
Published: March 2018

In this project, the punching process of carbon fiber reinforced plastics (CFRP) is investigated numerically and experimentally. Punching presents a low cost and potentially high quality manufacturing technology for piercing, but has not been investigated sufficiently so far for CFRP. The project aims on the key mechanical processes from a damaging point of view. The goal of the project is to identify the principle influence parameters on the quality of the cutting face from a simulational and an experimental point of view. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Application and parameter identification of cohesive zone elements for intergranular fracture in thin layer
Authors: Rezaei Shahed , Wulfinghoff Stephan , Reese Stefanie
Published: March 2018

Coating layers are usually applied on different manufacturing tools to improve their wear resistance and surface properties of the final parts. In this work, fracture and damage behavior of hard coatings such as (Cr,Al)N is studied. The coatings are produced by means of direct current magnetron sputtering (dcMS) and high power pulsed magnetron sputtering (HPPMS) method. The fracture properties of the coating are measured by investigating the force‐displacement curve obtained from a bending test [1]. The cohesive zone element parameters are then fitted to the experimental results to simulate the same behavior. The intergranular fracture within the coating system can then be modeled by employing the cohesive zone (CZ) regions at grain boundaries [2]. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Bending of Viscoplastic Cables
Authors: Dörlich Vanessa , Linn Joachim , Diebels Stefan
Published: March 2018

This contribution deals with the experimental investigation of cables showing inelastic behavior under bending deformation. A new experimental device, which enables the direct measurement of the quantities entering the constitutive law for bending, i.e. the bending moment and bending curvature, is introduced. Results from advanced cyclic experiments executed on this new pure bending device show inelastic effects. It is remarkable, that this procedure allows for the distinction of intrinsic inelastic effects, e.g. due to friction between the constituents, from global inelastic effects such as damage of the specimen. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The finite cell method: Polygonal and tetrahedral cells
Authors: Duczek Sascha , Duvigneau Fabian , Würkner Mathias , Gabbert Ulrich
Published: March 2018

Partial differential equations arising in many physical problems are most commonly solved by using the finite element method (FEM). Despite being very versatile, the FEM has one crucial drawback when heterogeneous material systems are considered, i.e. it relies on geometry‐conforming discretization. This meshing process often constitutes a bottleneck in the simulation pipeline and therefore needs to be alleviated. One idea is to exploit the benefits of the fictitious domain concept. Here, Cartesian grids can be deployed to straightforwardly discretize an extended domain. In conjunction with higher order shape functions the recently introduced method is referred to as the finite cell method (FCM).

The main objective of the contribution at hand is to extend the FCM to arbitrary unstructured meshes. In two‐dimensional application polygonal finite elements based on generalized barycentric coordinates are deployed, while the three‐dimensional implementation is based on tetrahedral finite elements. A further extension to pyramidal and pentahedral (wedge) elements is, however, straightforward.

There are two distinct advantages of using unstructured meshes: (i) a local mesh refinement can easily be conducted and (ii) existing computational models can be re‐used for parametric studies. The latter point is important if virtual defects need to be introduced in the ideal model to study their effect on the material behaviour. These possibilities make the unstructured FCM a powerful numerical tool for the investigation of complex highly heterogeneous materials. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A polyconvex phase‐field approach to fracture with application to finite‐deformation contact problems
Authors: Franke Marlon , Dittmann Maik , Hesch Christian , Betsch Peter
Published: March 2018

Variationally consistent phase‐field methods allow for an efficient investigation of complex three‐dimensional fracture problems (see [1, 2]). However, formulations for large deformation problems often exhibit a lack of numerical stability for different loading scenarios. In the underlying contribution a novel formulation for finite strain polyconvex elasticity is adapted to phase‐field fracture problems. In particular we introduce a new anisotropic split based on the principal invariants of the right Cauchy‐Green strain tensor for a proper treatment of fracture within the polyconvex framework (see [4]). This polyconvex phase‐field fracture formulation can be implemented in a straightforward manner and improves the numerical stability. Furthermore, a fourth order crack density functional is considered to improve accuracy and convergence. To account for the C1 requirement the system is embedded in a sophisticated isogeometric framework with the ability of local refinement. Eventually, a variationally consistent Mortar contact algorithm is applied (see [3]) to handle contact boundaries. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Automatic quadtree‐based modeling using a coupled SBFEM/SCM approach
Authors: Gravenkamp Hauke , Duczek Sascha , Birk Carolin
Published: March 2018

We present an approach for the automatic image‐based modeling in computational mechanics. The geometry is divided into subdomains by employing a quadtree‐decomposition. The scaled boundary finite element method (SBFEM) can directly be applied to quadtree meshes without introducing hanging nodes, while the spectral cell method (SCM) allows for an accurate representation of curved boundaries. Hence, a combination of both methods can lead to highly accurate and reliable results in a fully automatic simulation environment. The performance of this coupled SBFEM/SCM approach is demonstrated by means of a modal analysis of a violin soundboard. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of strain‐rate effects in Al foams and Ni/Al hybrid foams on different scales
Authors: Jung Anne , Luksch Jutta , Felten Markus , Reis Martin , Sory David R. , Pullen Andy D. , Proud William G. , Larcher Martin , Valsamos Georgios , Solomos George
Published: March 2018

Open‐cell metal foams are a new class of cellular materials with structural features resembling those of lightweight load‐bearing materials such as cancellous bones and wood. Their high stiffness‐to‐weight ratio coupled with their typical long, flat stress‐strain response make them ideal candidates as cost‐effective shock energy absorbers in crashworthiness, impact loading and blast mitigation strategies. The macroscopic mechanical properties of foams are strongly influenced by both the mechanical behaviour of single pores at the mesoscopic level and the struts and their structure at the microscopic length‐scale, based on a strong structure‐property relationship. This is shown in the present contribution where an experimental‐numerical investigation has been conducted demonstrating the existence of strain‐rate effects at different hierarchical scales. Micro inertia effects arising due to the pore geometry as well as further strain‐rate effects stemming from the rate‐sensitivity of the Ni coating in Ni/Al hybrid foams are also outlined. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Application of Multipreconditioned Iterative Algorithms in Dual Domain Decomposition Methods for Structural Dynamics
Authors: Leistner Michael , Rixen Daniel , Gosselet Pierre
Published: March 2018

We apply a multipreconditioned domain decomposition method based on Finite Element Tearing and Interconnecting to linear structural dynamics with highly heterogeneous material properties. A recently published method to build and select the multiple directions is presented and applied. We hint at possible problems when localized phenomena are considered and present a new simple measure that extends the selection process in existing algorithms. We show numerical results and conclude how to effectively prevent a possible degeneration of the minimization space basis. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigations on a FEM‐sub‐model for “Die‐Less‐Hydroforming” weld seams
Authors: Metzger Andreas , Ruff Daniel C. , Ummenhofer Thomas
Published: March 2018

A global forming model for the special and unconventional forming process called “Die‐Less‐Hydroforming” that can be used for the inflation of arbitrary double‐layered or multi‐layered seal‐welded blanks was already presented by the authors in recent publications. While looking on the modeling of the specific seal‐weld in this global model, as a special modeling step the elements at the edges of the blanks are folded towards each other and after that the seal‐weld is modeled by merging the resulting duplicated nodes in the welded joining zone. This paper focuses on the investigation whether this modeling strategy for the seal‐weld is appropriate, i.e. whether the load‐displacement‐performance and stiffness behavior correspond to reality. This was investigated with a FEM‐sub‐model within LS‐DYNA in combination with a specially developed experimental benchmark‐test for the investigation of “Die‐Less‐Hydroforming” weld seams. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Local buckling of laminated composite beams based on different plate theories
Authors: Herrmann Johannes , Kühn Torsten , Müllenstedt Tjorven , Mittelstedt Siham , Mittelstedt Christian
Published: March 2018

This paper presents an approximate approach for the local buckling analysis of prismatic composite laminated beams under uniaxial compression wherein the segments of the beams (i.e. flanges and webs) are assumed to be moderately thick so that advanced plate theories that transcend the restrictions of classical laminated plate theory (CLPT) need to be employed. We present a novel approximate analysis method based on the so‐called discrete plate approach during which the segment of interest is separated from the beam and is subjected to rotational restraints at the cutting edges. The analysis itself employs rather simple shape functions for the local buckling modes in conjunction with a Rayleigh‐type approach using the principle of minimum elastic potential of the buckled segment wherein we use the kinematic assumptions of Reddy's third‐order shear deformation theory (TSDT). The results are compared to comparative computations based on CLPT and first‐order shear deformation theory (FSDT), and a good agreement is found especially between FSDT and TSDT which lends credibility to the present approach, however neither employing the strict limitations of CLPT imposed by Kirchhoff's classical kinematic assumptions, nor requiring shear correction factors as they are generally required in the framework of FSDT and which is still a topic of on‐going research. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigation of a walking soft robot
Authors: de Payrebrune Kristin M. , O'Reilly Oliver M.
Published: March 2018

Interest in soft robots is motivated by their enormous potential in applications with close contact to humans or unpredictable situations for which their soft structure and versatility is beneficial. New mathematical models and control strategies are required to optimize the geometry and performance of the soft components of these robots. The authors use a nonlinear rod model to describe a pneumatically actuated quadruped soft robot and to study the influences of different parameters on its performance. Although the model does not accommodate contact situations perfectly, it is suitable to study general interrelations. These resulting studies serve to enhance the development of soft robots. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Stress solutions for joints with nonlinear adhesives
Authors: Rosendahl Philipp L. , Stein Nicolas , Becker Wilfried
Published: March 2018

A simple predictive stress analysis model for adhesive joints with nonlinear adhesives is presented. The model allows for the treatment of various joint designs with shear‐flexible laminate or isotropic adherends. Both elastic‐plastic and hyperelastic adhesives are covered. Using a general sandwich‐type model of the adhesive bondline, the governing differential equations are derived analytically and solved numerically. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An explicit reduced order integration scheme for contact problems in structural dynamics
Authors: Shi Jianye , Bamer Franz , Markert Bernd
Published: March 2018

The present contribution deals with the improvement of the computational efficiency of explicit integrators for dynamical contact problems by modal truncation. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of the original and the spectral system of an elastic rotor in two‐lobe bearings with an active geometry variation
Authors: Becker Kai , Seemann Wolfgang
Published: March 2018

The improvement of the dynamic behaviour of an elastic rotor which is supported by journal bearings represents an ongoing field of research, whereby various modifications of the corresponding bearings and their geometry have been proposed in literature. In order to suppress or at least to decrease unwanted oscillations due to negative effects of instability phenomena of the ’oil‐whirl’‐ and/or ’oil‐whip’‐type, a two‐lobe bearing with an active geometry variation is suggested as an alternative approach. A systematic investigation of the dynamic behaviour of a Jeffcott rotor, which is supported by the mentioned bearing type, is performed by means of the associated spectral system. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of Vibrations Induced by Dry Friction in a Pin‐on‐Disk Experimental Set‐up
Authors: Kapelke Simon , Oestringer Lukas J. , Seemann Wolfgang
Published: March 2018

High‐frequency vibrations have been shown to smooth the effective characteristics of dry friction and may be used in order to quench undesired phenomena such as friction induced vibrations. As the agreement between classical modelling and related experimental results appears to be insufficient, the effect of contact compliance on vibrational smoothing of dry friction has been discussed. Recently, a pin‐on‐disk experimental set‐up has been presented in order to validate the corresponding results. Within this contribution, selected properties of the experimental set‐up are analysed. Due to a negative slope in the friction‐velocity characteristics, friction induced vibrations can be observed at low sliding velocities. The oscillatory behaviour of a corresponding minimal model is investigated and compared to the experimental results. When applying appropriate high‐frequency excitation, the friction‐velocity characteristics are smoothed and, consequently, friction induced vibrations can be quenched. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Considering Nonlinearities to Design Micro System's Vibrational Sensitivity
Authors: Roeser Dennis , Gutschmidt Stefanie , Sattel Thomas
Published: March 2018

This contribution analyses the impact of nonlinearities on the sensitivity of micro‐electro‐mechanical sensors measuring quantities based on a change in resonance frequency. The considered sensor is an array of cantilevers which have an integrated actuator and sensor. The mathematical model presented to describe the motion of this array is based on a coupled thermoelastic equation of motion which has a forced and a parametric excitation. Finally, a concept is presented to utilize a two‐to‐one internal resonance to increase a micro system's vibrational sensitivity. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Determination of material parameters for a sheet‐layered lamination stack
Authors: Baloglu Maximilian Volkan , Willner Kai
Published: March 2018

The numerical simulation of sheet‐layered lamination stacks is a challenging task in structural mechanics due to the layout of these components. Depending on the manufacturing process, these sheets are either linked together with the help of a bonding varnish or are just packed up and basically free to slip inside of the stack, which can result in a nonlinear deformation behavior. To avoid a full FE‐simulation incorporating every layer, homogenization can be applied to identify a surrogate material model to achieve the desired accuracy by gaining performance. In this paper, a transversely isotropic material model is analytically formulated for a lamination stack with isotropic sheets in contact, which is captured with the help of a linear penalty method. The resulting stress‐strain relation is compared to results obtained by a numerical homogenization. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigations of CMC nozzle structures: constitutive modelling and finite element technology
Authors: Barfusz Oliver , Schwarze Marco , Reese Stefanie
Published: March 2018

This work deals with the development of numerical tools predicting the lifetime of nozzle structures made of ceramic matrix composites (CMC). These materials are favorable for thermostructural components due to their excellent specific mechanical properties, high thermal conductivity and thermoshock resistance. The anisotropic constitutive behaviour of the CMC is reflected by a continuum model which is micro‐mechanically motivated. The model is based on structural tensors representing different fibre orientations. Furthermore, the thin structure of the nozzle requires an appropriate finite element technology in order to overcome locking phenomena. For this reason, a solid‐shell formulation with reduced integration is utilized, for which the implementation of the fibre orientation is crucial. The enhanced assumed strain (EAS) concept is used to avoid volumetric locking as well as Poisson thickness locking. The transverse shear and curvature thickness locking are cured by the assumed natural strain (ANS) concept. Using reduced integration together with hourglass stabilization leads to high computational efficiency. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Thermodynamically consistent modelling of recrystallization and grain coarsening in precipitation‐hardened alloys
Authors: Kertsch Lukas , Helm Dirk
Published: March 2018

The strength of technically relevant alloys is mainly determined by the grain size distribution, the amount and size of precipitates, solid‐solution hardening and work hardening. A reliable material model to capture these effects should be formulated within a comprehensive thermodynamic framework. The strategy of rational extended thermodynamics is applied to derive a thermodynamically consistent model representing the coupling between elastoplastic deformation, the evolution of the grain structure and recrystallization. In addition, the model takes the dragging effect of precipitates on grain boundaries and dislocations into account, which leads to significant strengthening. For the microstructure description, a mean‐field approach is used. Due to the thermodynamic framework, the model is able to consistently predict the interplay between deformation, microstructure evolution, dynamic hardening and softening and the related temperature change. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A slip gradient crystal plasticity theory based on an extended energy flux
Authors: Prahs Andreas , Böhlke Thomas
Published: March 2018

Based on a slip gradient crystal plasticity theory, cf. [1], an analytical, one dimensional solution for the plastic slip is discussed with respect to a homogeneous stress state. Multislip is considered in a bicrystal with two slip systems in each grain. The orientation of the grains is accounted for by the choice of a free energy based on the grain boundary Burgers tensor, cf. [2]. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimental and Micro‐Macro Integrated FE Investigation of Texture and Microstructure Evolution in Polycrystalline Ag‐SnO2 Metal Matrix Composites
Authors: Schneider Yanling , Wasserbäch Werner , Schmauder Siegfried
Published: March 2018

Our work is aimed to investigate the deformation behavior of Ag‐17vol.%SnO2 metal matrix composites under a hot extrusion process which causes extraordinarily large plastic deformations and recrystallization. The micro‐macro integrated axisymmetric finite element simulation is preferred for the first study, since it costs less calculation effort compared to 3D calculations, presents the mechanical variables in all 3D dimensions and the measured microstructure can be directly applied as the cross section. Due to that the volume (area) fraction of the particle phase in the selected 2D real microstructures often deviate from the real one, we show a numerical method to modify the EBSD‐images in order to match the 3D composition. This modification should be useful for FE simulations with a geometry based on a real microstructure. Considering that the tomograms are actually 2D images with a given distance in the 3rd dimension, such a modification with a further extension in the algorithm can also be applied for 3D microstructures based on tomograms to match the real composition. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Statistical characterization of granular material applied as crash absorber in ship building
Authors: Woitzik Christian , Chaudry Mohsin Ali , Wriggers Peter , Düster Alexander
Published: March 2018

This contribution deals with the determination of material parameters of lightweight granules, which will be used to increase the crashworthiness in ship construction. Using expanded glass or clay granules as a crash absorber requires a detailed knowledge about the single grain properties, such as Young's modulus or crushing strength. Therefore, a uniaxial compression test was performed and the results are covered with a statistical model consisting of nine parameters. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)