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Item type:Publication, Access status: with files , On the scaling of strength of flat and mushroom-shaped ends of microstructured adhesivesConsiderable research has been conducted on shape of pillar ends for optimal adhesion. In experiments with elastomers, it has been found that mushrooms-ended ones are superior to flat-ended ones, but early experiments have suggested an extremely strong scaling in strength with pillar radius (del Campo et al. Langmuir 23:10235–10243, 2007). We discuss various theories and experimental results on scaling of strength, and in particular, we elaborate recent experiments on single pillars with mushroom ends finding that the scaling on strength is much less surprising. When the ratio of tip to shaft radii increases toward an optimal value, the mode of failure changes from edge to center defects, and the scaling of strength changes from that associated with the presence of the classical linear elastic fracture mechanics stress singularity at the corner of the flat-ended punches caused by large friction at the interface, with that caused by the population of defects immersed on a nearly uniform stress field in the mushroom center. Strong scaling of strength therefore is also accompanied with large scatter of strength.Publicationtype: Journal Article13 16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A short note on minor and major symmetries in linear elasticitySimply applying the directional derivative either twice to the strain-energy density function (hyperelasticity) or once to the stress–strain state (Cauchy elasticity) does not lead to the symmetries of the fourth-order elasticity tensor specified in the literature. Moreover, there are many justifications and arguments for the desired symmetries, which are summarized in this contribution. Thus, a symmetrization operator has to be introduced to guarantee minor symmetry, since the symmetry of the strain tensor is frequently neglected but is needed to obtain results required for particular elasticity relations. A thorough investigation is provided for both Cauchy elasticity and hyperelasticity, and what conclusion can be drawn on by various assumptions.Publicationtype: Journal Article15 20 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , 13 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Remeshing and eigenvalue stabilization in the finite cell method for structures undergoing large elastoplastic deformationsLarge strain analysis is a challenging task, especially in fictitious or immersed boundary domain methods, since badly broken elements/cells can lead to an ill-conditioned global tangent stiffness matrix, resulting in convergence problems of the incremental/iterative solution approach. In this work, the finite cell method is employed as a fictitious domain approach, in conjunction with an eigenvalue stabilization technique, to ensure the stability of the solution procedure. Additionally, a remeshing strategy is applied to accommodate highly deformed configurations of the geometry. Radial basis functions and inverse distance weighting interpolation schemes are utilized to map the displacement gradient and internal variables between the old and new meshes during the remeshing process. For the first time, we demonstrate the effectiveness of the remeshing approach using various numerical examples in the context of finite strain elastoplasticity.Publicationtype: Journal Article14 101 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Full-field validation of finite cell method computations on wire arc additive manufactured components(Springer, 2024-06-05); ; ; Wire arc additive manufacturing enables the production of components with high deposition rates and the incorporation of multiple materials. However, the manufactured components possess a wavy surface, which is a major difficulty when it comes to simulating the mechanical behavior of wire arc additively manufactured components and evaluation of experimental full-field measurements. In this work, the wavy surface of a thick-walled tube is measured with a portable 3D scanning technique first. Then, the surface contour is considered numerically using the finite cell method. There, hierarchic shape functions based on integrated Legendre polynomials are combined with a fictitious domain approach to simplify the discretization process. This enables a hierarchic p-refinement process to study the convergence of the reaction quantities and the surface strains under tension–torsion load. Throughout all considerations, uncertainties arising from multiple sources are assessed. This includes the material parameter identification, the geometry measurement, and the experimental analysis. When comparing experiment and numerical simulation, the in-plane surface strains are computed based on displacement data using radial basis functions as ansatz for global surface interpolation. It turns out that the finite cell method is a suitable numerical technique to consider the wavy surface encountered for additively manufactured components. The numerical results of the mechanical response of thick-walled tubes subjected to tension–torsion load demonstrate good agreement with real experimental data, particularly when employing higher-order polynomials. This agreement persists even under the consideration of the inherent uncertainties stemming from multiple sources, which are determined by Gaussian error propagation.Publicationtype: Journal Article47 24 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adaptive impact analysis in flexible multibody systems based on hierarchically refined IGA modelsUsually, detailed impact simulation models within flexible multibody systems have to be set up manually rather than being generated automatically. This is because the process requires prior knowledge of the time and location of the impact, as well as the element resolution within the contact area. If the penalty method is used to determine the occurring contact forces, the corresponding penalty factor also needs to be determined manually. This work, however, presents an adaptive algorithm to simulate impacts within flexible multibody systems fully automatically using reduced isogeometric analysis models, the floating frame of reference formulation, and quasistatic contact models for an efficient but still accurate simulation. The adaptive algorithm detects impacts in the system, determines the contact locations on the bodies, refines the contact area, and determines the penalty factor, and therefore automatically simulates impacts. The work shows how to automatically simulate impacts in flexible multibody systems without user action or prior knowledge of impact location and size. The first application example simulates significant elastodynamic effects within a long flexible rod. The goal is to validate the algorithm by preserving the wave propagation and energy of the system. The second application example simulates the impacts of two flexible double pendulums. This setup is a suitable benchmark for the complete adaptive impact analysis procedure as the flexible double pendulums undergo large rigid body motions.Publicationtype: Journal Article47 48 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Detection and correction of synchronization-induced errors in operational modal analysisOperational modal analysis (OMA) methods are frequently applied for obtaining information on structural dynamic parameters, such as natural frequencies and mode shapes. While synchronization between acceleration response data sets used in OMA is not a prerequisite for estimating natural frequencies, synchronization discrepancies may lead to non-negligible errors in the estimation of mode shapes. Synchronization discrepancies are predominantly associated with wireless structural health monitoring systems, where each wireless sensor node functions as a separate data acquisition unit (DAQ), and clock synchronization is therefore not intrinsic. However, synchronization discrepancies may also occur in cable-based systems, particularly when more than one DAQ is used. In this paper, a synchronization method for detecting and correcting synchronization-induced errors in OMA is proposed. Unlike existing approaches on removing synchronization-induced errors at the output stage, the method proposed herein yields synchronized structural response data at an intermediate stage of OMA. Specifically, time lags between acceleration response data sets are detected based on estimates of the mode shapes obtained either from preliminary structural analysis or from engineering judgment assuming classical damping. The applicability of the proposed method is verified through simulations of a multi-degree-of-freedom oscillator and validated through ambient vibration field tests on a pedestrian overpass bridge.Publicationtype: Journal Article88 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Servo-constraint realization for underactuated mechanical systemsThe paper deals with underactuated mechanical systems, featured by less control inputs m than degrees of freedom f, m < f, subject to m servo-constraints (specified in time outputs) on the system. The arising servo-constraint problem (inverse dynamics analysis) is discussed with an emphasis on the way the servo-constraints are realized, varying from orthogonal to tangential, and a geometrical illustration of the different realization types is provided. Depending on the way the servo-constraints are realized, the governing equations are formulated either as ordinary differential equations (ODEs) or differential-algebraic equations (DAEs), and some computational issues for the ODEs and DAEs are discussed. The existence or non-existence of an explicit solution to the governing equations is further discussed, related to so-called differentially flat problems (without internal dynamics) and non-flat problems (with internal dynamics). It is shown that in case of non-flat problems with orthogonal realization of servo-constraints, stability of the internal dynamics must be assured. Simple case studies are reported to illustrate the proposed formulations and methodologies.Publicationtype: Journal ArticleScopus© Citations 14 115 162 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A partitioned solution approach for electro-thermo-mechanical problemsThe purposes of this article are to present new aspects of modeling multi-physically coupled fields, focusing particularly on the partitioned treatment of electro-thermo-mechanical problems. Coupled problems of this kind occur in many industrial applications, such as micro-electrical devices, field-assisted sintering processes or electrical fuses. In this paper, we restrict ourselves to the case of nonlinear thermo-elasticity at finite strains and a heat source resulting from the electrical field. Plasticity effects are not taken into consideration. The objective is to ascertain the coupling of the algorithm relating to the fields involved individually and to demonstrate a global partitioned solution strategy. We also introduce several methods that increase algorithmic stability and accelerate the iterative coupling process. This article aims to present an efficient partitioned coupling strategy for different coupling levels between the fields. To this end, we study the proposed algorithm with the help of several numerical examples ranging from linear to highly nonlinear problems, involving substantial geometric changes and finite strains.Publicationtype: Journal Article138 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Reduktion analytischer Impedanzfunktionen auf lineare Matrizenpolynome am Beispiel der dynamischen SeilsteifigkeitFor the dynamic stiffness of a sagging cable subject to harmonic boundary displacements, frequency-dependent closed-form analytic functions can be derived from the corresponding continuum equations. By consideration of such functions in stiffness matrices of composed systems, however, these matrices become frequency-dependent, too a troublesome fact, especially with regard to the eigenvalue problem which becomes nonlinear. In this paper a method for avoiding such difficulties is described: A complex analytic impedance function is reduced to two constant matrices of any desired dimension. This reduction corresponds to a mathematically performed transition from a continuum to a discrete-coordinate vibrating system. In structural dynamics applications such as for dynamic cable stiffness the two resultant matrices correspond to a static stiffness matrix and a mass matrix. In every case, these matrices can easily be considered within the scope of a linear eigenvalue problem.Publicationtype: Journal Article530 413