bluuebunny/crossref_metadata_2025_split huggingface dataset:
Efficiency increasing reconstruction of 3D‐Vessel scan data using a NURBS surface
Authors: Maurer Lukas , Makvandi Resam , Juhre Daniel 
Published: May 2023

AbstractIn this paper, a new reconstruction method using a mathematical description of the surface with non‐uniform rational B‐splines (so‐called NURBS) of 3D vessel scans is presented. With the advantage of a file size of less than 1% of the original STL file, the developed reconstruction method can be used in numerical simulations such as the placement or extension of a flow diverter. Along a generated centerline, various geometric functions can also be calculated for diagnostic purposes, such as the cross‐sectional area of the vessel, which can be used to locate a vessel stenosis. As for isogeometric analysis, this method can generally be used to reconstruct any tubular geometry.
Direct stress minimization in electro‐mechanical metamaterials
Authors: Blaszczyk Mischa , Hackl Klaus 
Published: May 2023

AbstractMetamaterials are artificially created multiscale materials with many different applications [1]. We assume periodic microstructure. No specific length scale is demanded for the differentiation between the scales. Instead, we consider the microscale as consisting of repeating unit cells, whereas the macroscale is the scale, where desired and sometimes unusual physical effects occur. The precise arrangement and geometry at the microscale level is responsible for the macroscopic material behavior, which can substantially differ from the original components it is made from. This way, unique material properties otherwise not found in nature (e.g. negative refractive materials [2]) are possible. Most previous research regarding metamaterials concentrated only on a single physical branch in each case, e.g. electromagnetic or acoustic metamaterials [3,4].In this contribution we present a class of theoretical electro‐mechanical metamaterials by combining insulating and conducting materials. Our aim is to directly control and reduce the resulting total stress of insulating materials by counteracting the mechanical stress through the application of an electric field, which is created by a conducting material. The solution of the resulting minimization problem is related to the eigenvalues of the mechanical stress tensor. Additionally, we discuss the constrained cases of tension and compression and cover the plane stress case. We show numerical results for all cases and discuss the limits of such a material.
On the Consistency of Arnoldi‐Based Krylov Methods for Conservation Laws
Authors: Linders Viktor , Birken Philipp 
Published: May 2023

AbstractConservation and consistency are fundamental properties of discretizations of systems of hyperbolic conservation laws. Re‐ cently, these concepts have been extended to the realm of iterative methods by defining locally conservative and flux consistent iterations. In this note, the current status of such iterative methods is summarized. In particular, it has been shown that Krylov subspace methods are locally conservative, but that they are not flux consistent. Here, we approach the problem of quantifying the flux inconsistency of Krylov subspace methods. Krylov methods introduce a time retardation factor into discretizations of linear conservation laws. It has thusfar been unknown how to compute the precise value of this factor. This issue is resolved herein for Arnoldi‐based Krylov subspace methods.
On the use of mixed basis function degrees within a convective isogeometric element formulation
Authors: Stammen Lisa , Dornisch Wolfgang 
Published: May 2023

AbstractSolving linear elasticity problems using standard finite element methods, different locking phenomena can occur. In order to counteract these effects, mixed methods or formulations using higher approximation orders can be employed, for instance. As a consequence, this leads to an increased computational effort. Hence, a selective elevation of orders in decisive directions within a purely displacement‐based element formulation is proposed in this contribution. Within isogeometric analysis (IGA), the geometry is discretized using non‐uniform rational B‐splines (NURBS), which simultaneously represent the basis functions for the analysis. Due to the fact that the geometry can be preserved exactly during analysis, this can increase the accuracy of results. In this contribution, convective basis systems that are aligned with the local geometry are employed combined with selective order elevation. The required convective basis systems are interpolated from those determined in each control point.
Semi‐crystalline polymers at finite strains: A thermo‐coupled constitutive model for varying degrees of crystallinity and temperatures
Authors: Reuvers Marie-Christine , Boes Birte , Felder Sebastian , Brepols Tim , Kulkarni Sameer , Loos Klara , Johlitz Michael , Lion Alexander , Reese Stefanie 
Published: May 2023

AbstractThermoplastics are gaining interest for various industrial applications, since they can be widely used for thermoforming and injection moulding processes due to their thermostable material behavior. In combination with the material's low density and high strength to mass ratio, they are especially of interest in times where an improved environmental balance is more and more important. Hence, why they are for example frequently used in the automotive industry to reduce the weight of automotive components.Semi‐crystalline polymers as a subcategory of thermoplastics, partly crystallize after cool‐down from the molten state. During the thermoforming process, they are subjected to large deformations as well as thermal loads and show strong thermo‐mechanical coupling effects in addition to the influence of the evolution of the crystalline phase on the macroscopic material behavior. Therefore, computational models are needed to predict the complex material response reliably and minimize production errors.This work presents a thermomechanically consistent material formulation at finite strains. In order to account for the highly nonlinear material behavior, elasto‐plastic and visco‐elastic contributions are combined in the Helmholtz free energy and a dependency on temperature as well as the degree of cristallinity (DOC) is incorporated. Special attention is devoted to the choice of yield function and hardening behavior.A comparison of the simulation results to experiments at varying degrees of crystallinity and temperatures is presented to review the changes in the formulation. Therefore a special blending technique is used to ensure stable crystallinity conditions in the test samples.
Remarks on wave propagation in an acoustic metamaterial modeled as a relaxed micromorphic continuum
Authors: Voss Jendrik , Rizzi Gianluca , Demetriou Plastiras , Neff Patrizio , Madeo Angela 
Published: May 2023

AbstractIn order to describe elastic waves propagation in metamaterials, i.e. solids with heterogeneities or microstructure, it is necessary to consider non‐local or higher‐order models. The relaxed micromorphic model (RMM) proposed here can describe these effects as a continuous material with enriched kinematics. We present a new unit cell giving rise to a metamaterial for acoustic application. The microstructure is engineered to show a band‐gap in the low acoustic regime (600‐2000 Hz) for which waves cannot propagate through the material. We concentrate on the size effects to make full advantage of the particularly beneficial structure that the model provides. The RMM material parameters are fitted using a new algorithm relying on cutoffs and asymptotes (obtained via a Bloch‐Floquet analysis). In particular, by enhancing the kinetic energy of the model with a new inertia term, we enable decreasing curves (modes with negative group velocity).
Computational Modelling of Nanoporous Materials
Authors: Chandrasekaran Rajesh , Hillgärtner Markus , Itskov Mikhail , Rege Ameya 
Published: May 2023

AbstractRecently proposed constitutive models that describe the mechanical behavior of open‐porous cellular materials take into account the random pore sizes of the materials but not the randomness in the pore shapes [1]. In the present contribution, a computational model to reconstruct the highly irregular structure of porous materials is developed using sphere packing and Laguerre‐Voronoi tessellation [2]. The mechanical structure‐property relationship is then investigated by means of finite element methods. Under compression, porous materials demonstrate linear‐elastic behaviour followed by an irreversible deformation resulting in a plateau region followed by densification [3]. In this contribution, such mechanical responses of porous materials under cyclic loading are investigated using elasto‐plastic material models. Furthermore, effects of tensile loading on the model are also discussed.
Flat input based canonical form observers for non‐integrable nonlinear discrete‐time systems
Authors: Fritzsche Klemens , Röbenack Klaus 
Published: May 2023

AbstractWe define flat inputs for discrete‐time systems and carry over a flat input based observer design strategy for non‐integrable nonlinear continuous‐time systems to the discrete‐time case. This approach allows to design observers for nonlinear discrete‐time systems that cannot be transformed into observer canonical form, i.e., conventional observer design strategies fail. This approach is illustrated by two examples.
Modeling Minimum Cost Network Flows With Port‐Hamiltonian Systems
Authors: Doganay Onur Tanil , Klamroth Kathrin , Lang Bruno , Stiglmayr Michael , Totzeck Claudia 
Published: May 2023

AbstractWe give a short overview of advantages and drawbacks of the classical formulation of minimum cost network flow problems and solution techniques, to motivate a reformulation of classical static minimum cost network flow problems as optimal control problems constrained by port‐Hamiltonian systems (pHS). The first‐order optimality system for the port‐Hamiltonian system‐constrained optimal control problem is formally derived. Then we propose a gradient‐based algorithm to find optimal controls. The port‐Hamiltonian system formulation naturally conserves flow and supports a wide array of further modeling options as, for example, node reservoirs, flow dependent costs, leaking pipes (dissipation) and coupled sub‐networks (ports). They thus provide a versatile alternative to state‐of‐the art approaches towards dynamic network flow problems, which are often based on computationally costly time‐expanded networks. We argue that this opens the door for a plethora of modeling options and solution approaches for network flow problems.
Geometric and material sensitivities for elasto‐plasticity including non‐local damage regularisation
Authors: Guhr Fabian , Barthold Franz-Joseph 
Published: May 2023

AbstractSensitivity analysis is applied to a regularised non‐local ductile damage model. A variational approach is utilised to derive the analytical gradients of different objectives with respect to either geometrical of material parameters. Due to the definition of the material model, enhanced algorithmic treatments are necessary to capture its history dependent nature within the sensitivity computation. The gradient information with respect to the geometrical parameters are used to derive damage tolerant geometries in shape optimisation using Sequential Quadratic Programming (SQP). The sensitivities with respect to the material parameters are used to analyse the response and impact of certain material parameters of the model during loading and unloading of a specimen.
Dislocation Motion Induced by Thermally Driven Phase Transformations
Authors: Strobl Rachel , Budnitzki Michael , Sandfeld Stefan 
Published: May 2023

AbstractThe interaction of dislocations with phase boundaries is an interesting aspect of the interplay between phase transformation and plasticity at the nano‐scale. We capture this interaction within a phase field framework coupled to discrete dislocation dynamics. In order to regularize the stress and driving force for phase evolution at the dislocation core, a first strain‐gradient elasticity approach is used, which leads to more physical, discretization‐independent numerical solutions.From a mathematical point of view, this results in a system of coupled partial differential equations (PDEs) and ordinary differential equations (ODEs). The PDEs include an equation analogous to the balance of linear momentum, a second‐order tensor‐valued Helmholtz‐type equation for the true stress as well as a time‐dependent Ginzburg‐Landau equation for the evolution of the phase field. The ODEs are the equations of motion of the dislocations. The dislocations are modeled as lamellae with eigenstrain that can evolve with time; the resulting stress field is an outcome of the numerical solution. A parallel framework was developed in order to solve these coupled dynamics problems using the finite element library FEniCS. We show the effect of dislocations on phase microstructure as well as the influence of phase microstructure on the motion of dislocations using an illustrative example of a thermally‐driven planar phase boundary, and its interaction with a single edge dislocation.
Reduced‐order modeling for parametrized time‐dependent Navier‐Stokes equations
Authors: Fischer Hendrik , Fau Amelie , Wick Thomas 
Published: May 2023

AbstractIn this work, we apply reduced‐order modeling to the parametrized, time‐dependent, incompressible, laminar Navier‐Stokes equations. The major goal is to reduce the computational costs by replacing the high‐fidelity system by a low‐rank approximation, which preserves the solution behavior. We utilize projection‐based reduced basis methods and carry out the basis generation by POD‐greedy sampling. Both a velocity‐only and a velocity‐pressure reduced‐order model are considered, with the latter stabilized by means of supremizer enrichment. Here, we investigate further reduction possibilities. We present numerical results of the method applied to the benchmark problem of a two‐dimensional flow around a cylinder with physical parametrization.
FEM simulation of thixo‐viscoplastic flow problems: Error analysis
Authors: Begum Naheed , Ouazzi Abderrahim , Turek Stefan 
Published: May 2023

AbstractThis note is concerned with the essential part of Finite Element Methods (FEM) approximation of error analysis for quasi‐Newtonian modelling of thixo‐viscoplastic (TVP) flow problems. The developed FEM settings for thixotropic generalized Navier‐Stokes equations is based on a constrained monotonicity and continuity for the coupled system, which is a cornerstone for an efficient monolithic Newton‐multigrid solver. The manifested coarseness in the energy inequality by means of proportional dependency of its constants on regularization, nonoptimal estimate for microstructure, and extra regularity requirement for velocity, is due to the weak coercivity of microstructure operator on one hand and the modelling approach on the other hand, which we dealt with stabilized higher order FEM. Furthermore, we show the importance of taking into consideration the thixotropy inhabited in material by presenting the numerical solutions of TVP flow problems in a 4:1 contraction configuration.
Analysing the bone cement flow in the injection apparatus during vertebroplasty
Authors: Trivedi Zubin , Gehweiler Dominic , Wychowaniec Jacek K. , Ricken Tim , Gueorguiev-Rüegg Boyko , Wagner Arndt , Röhrle Oliver 
Published: May 2023

AbstractVertebroplasty, a medical procedure for treating vertebral fractures, requires medical practitioners to inject bone cement inside the vertebra using a cannula attached to a syringe. The required injection force must be small enough for the practitioner to apply it by hand while remaining stable for a controlled injection. Several factors could make the injection force unintuitive for the practitioners, one of them being the non‐Newtonian nature of the bone cement. The viscosity of the bone cement varies as it flows through the different parts of the injection apparatus and the porous cancellous interior of the vertebra. Therefore, it is important to study the flow of bone cement through these parts. This work is a preliminary study on the flow of bone cement through the injection apparatus. Firstly, we obtained the rheological parameters for the power law model of bone cement using experiments using standard clinical equipment. These parameters were then used to obtain the shear rate, viscosity, and velocity profiles of the bone cement flow through the cannula. Lastly, an analysis was carried out to understand the influence of various geometrical parameters of the injection apparatus, in which the radius of the cannula was found to be the most influential parameter.
Lifetime Prediction with Combined Hygro‐Thermo‐Mechanical Influences on Thick Layer Adhesives
Authors: Kötz Fabian , Matzenmiller Anton 
Published: May 2023

AbstractSemi‐structural bonded joints of steel components are exposed to long‐term stresses in addition to water diffusion, temperature changes and varying external forces during operation. These hygro‐thermomechanical loadings lead to successive degradation of stiffness and strength due to damage resulting from chemical aging and fatigue processes, causing failure of the bonded joint when its service life is reached. In the following contribution, a methodology is presented to predict the service life of hygro‐thermo‐mechanically loaded semistructural adhesive joints with transient FE simulation. The lifetime prediction is based on a constitutive model with a viscoelastic and a damage part. The first part for viscoelasticity consists of the generalized Maxwell model, in which the effects of temperature changes and varying humidity on the viscoelastic properties of the adhesive bond are captured by the time‐temperature and time‐water concentration shifts. The second part for the material damage is based on an ordinary differential equation for the damage evolution. It consists of a creep and a moisture damage part for the void developments caused by the local water concentration due to mechanical stress and chemical aging. Both damage parts are multiplied by an Arrhenius‐type function to account for the effect of temperature change on the defect growth. The interaction of the creep and the moisture damage part is studied by applying the analytical and numerical solution of the damage differential equation. All parameters of the damage model are determined by using fracture times from creep tests under different climatic conditions and loading levels. The local water concentration in the adhesive is calculated by Fick's model and a concentration boundary condition. The diffusion parameters are determined by gravimetric measurements. A numerical example is setup to demonstrate the lifetime prediction methodology and to show the influence of viscoelasticity on the predicted lifetimes for different hygro‐thermo‐mechanical loading cases.
Computational Fluid‐Structure Interaction towards Simulating Large Wind Turbines with openFOAM and deal.II Coupled via preCICE
Authors: Mang Katrin , Ahrens Jan Dominik , Seume Joerg R. , Rolfes Raimund 
Published: May 2023

AbstractWind energy is an essential part of the Green Deal. The trend to increase the size of wind turbines, especially offshore, introduces additional dynamic effects at the long and flexible blades. Embedded in the CRC 1463, DFG, we are working on the fluid‐structure interaction to avoid dynamic stall and investigate flutter effects and blade breathing of ultra‐slim blades [1,2]. This requires an accurate numerical setup that reliably captures the fluid‐structure interactions due to the highly turbulent flow and large deformations of the blades. In preliminary work, the Unsteady Reynolds‐averaged Navier‐Stokes method (URANS) in openFOAM [3] was used to simulate the flow around rotating helicopter blades with a changing angle of attack. [4] successfully predicted the distinct dynamic stall hysteresis with moderate computational effort and captured extreme values (load peaks) within the experimental uncertainties. This aerodynamic solver is to be coupled with a structural solution, for which deal.II [5] provides the linear elastic blade model. The fluid and the structure solvers are coupled via the software preCICE [6] and solved with a staggered approach. Numerical results are presented for a simplified 2D cross‐section of a rectangular solid of carbon‐fiber‐reinforced polymers and a steady inflow velocity. Key challenges for the coupling of the solvers are discussed and the future work is outlined.
Dispersion‐type Anisotropic Viscoelasticity: Model Validation for Myocardium
Authors: Açan Alp Kağan , Altun Cem , Dal Hüsnü 
Published: May 2023

AbstractThis contribution presents a novel constitutive model for rate‐dependent response of the passive myocardium. As a first step, we performed a comparative study on dispersion‐type anisotropic hyperelastic constitutive models [1–3] and assessed performance of various density distribution functions by fitting to experiments conducted on three distinct tissues [4]. Next, we proposed an angular integration type anisotropic viscoelastic constitutive model that uses bivariate von‐Mises distribution function to capture fiber dispersion in passive myocardium. The baseline hyperelasticity is described by a generalized structure tensor formulation proposed by GASSER ET AL. [1]. The non‐equilibrium part of the model utilizes a quadratic free energy function in the logarithmic strain space and a power‐type nonlinear evolution equation in orientation directions. The overstress response is then obtained by the numerical integration over the unit sphere by making use of 21 quadrature points. The proposed model parameters are obtained from cyclic triaxial shear and triaxial shear relaxation experiments on human passive myocardium [5].
Synthesis and properties of UV‐curable star‐shaped polyester
Authors: Liu Yiyang , Ming Shujie , Zhou Yumei , Qu Jinqing 
Published: May 2023

AbstractUV‐curable coatings mainly include vinyl resin, active thinner, and photoinitiator, among which UV‐curable resin accounts for a large proportion and is the main factor that determines the film performance. Existing vinyl resins have the disadvantages of high viscosity, high cost, and low curable function. Herein, the polyol as the core, first reacted with anhydride to produce half ester, and then with glycolyl methacrylate ring opening to get vinyl star polyester. The molecular structure and molecular weight of UV‐curable star polyesters were analyzed using FTIR, 1H NMR, and GPC. The impact of the type and ratio of anhydride and small molecular polyols on the viscosity and coating properties of vinyl star polyester were investigated. The results indicated that the resin viscosity and film hardness increased as the hydroxyl function of small molecular polyols increased. It was also discovered that the inclusion of polyether polyol N210 in the reaction system reduced the viscosity of the resin to 2025 mPa s and improved the hardness of the film to 2H, as well as its impact toughness.