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A Graphical User Interface for Simulating Resin-Transfer-Molding Combining LS-DYNA and OpenFOAM
(2015)
The paper describes parts of the joint research project Swim-RTM including several industrial and academic partners. Its goal is to combine LS-DYNA and the open-source CFD solver OpenFOAM to simulate the production process of continuous fiber-reinforced plastics, particularly the resin-transfer-molding (RTM) process, in which the layers of dry fabric (unidirectional or woven) are formed in the mold (draping) and then filled with liquid resin with high pressure at injection points. Through a combined analysis of both the structural mechanical and the fluid dynamical phases, a better prediction and thereby optimization of the textile components properties as well as injection points can be achieved, improving the manufacturing process. The draping simulation of the fabric layers is carried out with LS-DYNA, while the injection simulation of the matrix material is performed in full 3D with OpenFOAM. A key question in this research project is how local porosities can be derived from the structural computation in the draping step. The purpose of the presented subproject is to develop a graphical user interface (GUI) to enable the simulation of the entire RTM process of long-fiber-reinforced components including the transfer of results between the draping and injection phases. The complete simulation task is relatively complex and involves several software packages, meaning a high effort for the user to get familiarized with. To circumvent this, the GUI aims at requiring from the user only the minimum necessary input data, creating and running the simulation and mapping tasks in the background, and showing graphically all demanded intermediate and final results. For the draping step several current fabric materials such as *MAT_034, *MAT_234, *MAT_235, *MAT_249 are available. Several modelling techniques for the composite setup are also conceivable, including a workflow similar to metal forming applications. In the injection step the fabric is modelled as a porous medium and different transport models and liquid resin types are at hand. For the data transfer between the draping and injection models, i.e. the mapping of data between shell and volume meshes within the developed GUI, first the OpenFOAM volume mesh is converted to LS-DYNA format and the necessary passing parameters are extracted from the output files, then the mapping tool DYNAmap [3] from DYNAmore GmbH is invoked, and finally the OpenFOAM command files are created. After the injection simulation is started and successfully terminated, information, such as the distribution of air inclusions or the shear stress distribution to analyze the reorientation of component fibers, is available and can be transferred from the 3D fluid mesh to an LS-DYNA shell mesh for further computations, for instance a crash simulation. This backward data mapping between volume and shell meshes can then be performed inside the GUI.
Forschung 2018
(2018)
In deep-drawing dies for steel sheet parts of car bodies huge masses are moved. To prevent vibrations, which occur by sudden acceleration or stopping of those masses, elastomeric tubular dampers [1] are used. The dampers are made out of carbon filled elastomers. A good knowledge about the material behaviour of metals is available. But for the numerical investigation of complete deep-drawing dies the elastomeric dampers must be taken into account, too. To characterize the material behaviour of the elastomers tensile tests and pressure tests were carried out. The received material data from the tests were read into LS-DYNA [2]. Simulation models of the tensile test and the pressure test were created for LS-DYNA according to the real dimensions and boundary conditions. For validation purposes, calculations of loading cycles were done to enable a comparison between test data and simulation results. For the calculations the implemented material model *MAT_SIMPLIFIED_RUBBER_WITH_DAMAGE (*MAT_183) was used. The comparison shows a good fitting between the test data and the calculation results with respect to the mechanical material behaviour by using this material model in single loading cases. The settings from the simulations of material tests were transferred to simulations of dampers, which are used in deep-drawing dies.
Application of a Krylov subspace method for an efficient solution of acoustic transfer functions
(2021)
Solving acoustic radiation problems, arising from systems including fluid–structure interaction, is of interest in many engineering applications. Computing frequency response functions over a large frequency range is a concern in such applications. A method which solves the Helmholtz equation for multiple frequencies in one step is the matrix-Padé-via-Lanczos connection for unsymmetric systems, as presented by Wagner et al. [1]. The present work is based on Ref. [1] and presents a method for efficiently computing frequency responses over a frequency range for coupled structural-acoustic problems, where the structure and the acoustic near field are discretized with finite elements and an analytical Dirichlet-to-Neumann map approximates the far field. The method is based on a Krylov-subspace projection technique which derives a matrix-valued Padé approximation for a restricted area in the near field and the pressure field on a spherical boundary. On the spherical boundary, where the finite domain is truncated, the non-local modified Dirichlet-to-Neumann operator is applied as a low-rank update matrix. The present contribution extends this method and incorporates new techniques for a more stable model reduction through the Lanczos algorithm and a novel weighted adaptive windowing technique. Further, structural damping is incorporated, for computing the acoustic radiation of a harmonically excited plate. These computed results are compared with acoustic measurements in an anechoic chamber and verified with computational results obtained with a commercial code that uses the perfectly matched layer method.
Finite element simulation of blankholder’s lift-off in a deep drawing tool using Abaqus/Standard
(2015)
In the deep drawing tools for forming car body parts, heavy blankholders are used to prevent buckling and wrinkling of the blank. During each press cycle, those large masses need to be lifted, raising thereby the structural dynamic load on the deep drawing tool and on the press. Therefore a detailed knowledge about the blankholder’s lift-off event is essential for an accurate and robust design of forming tools. In this paper, a dynamic finite element method (FEM) simulation of a blankholder’s lift-off in a selected automotive deep drawing tool is presented enabling identification of regions of critical stresses. The FEM model is built within the Abaqus/CAE environment and solved with Abaqus/Standard. Each dynamic analysis is preceded by a static analysis where the gravity load is applied and the lifting bolts are pre-stressed. A special emphasis is put on modeling the elastomer dampers, which are installed between lifting bolts and the blankholder to avoid hard impacts during the lift-off event. Those dampers are modelled using a hyperelastic material with hysteresis. In addition, an experimental validation of a blankholder’s vibration under operating loading was carried out. The simulation results are in good agreement with the measurements.
This paper presents experimental fatigue tests on specially shaped specimens made from quenched and tempered steel 42CrMoS4 and nodular cast iron EN-GJS-600-3. The specimens are geometrically designed to resemble actual subcomponents of forming and cutting dies for sheet metal manufacturing. These subcomponents have failed in the past.Experiments are carried out using a fictive load-time function, which is a modification of an actually measured load curve and has variable mean and amplitude values. This first test series serves as a general validation of a subsequent fatigue analysis. A second test series is carried out with constant mean and amplitude values with a load ratio 𝑅≈0. Here, the load frequencies in the individual test runs are varied between f = 5 Hz and f = 45 Hz in order to investigate the influence of the frequency. Both series are performed on a servo-hydraulic testing machine.The experimental data is then statistically evaluated. The values for the scatter correlate with data from the literature. The data from the test series with the fictive load-time function can thus be used for subsequent validation of a fatigue calculation, which will be addressed in a future publication. Most significant are the findings from the second series of experiments with varied load frequencies, as a significant frequency influence could be detected. Furthermore, the hardening and softening behavior under cyclic loading and the fracture behavior of the materials are analyzed and compared with data from the literature.
In order to achieve the energy and efficiency goals in modern automotive press shops, press systems with increasingly high stroke rates are being implemented (Meinhardt in proceedings of ACI forming in car body engineering. Bad Nauheim, Germany 2012). As a side effect, the structural dynamic loads on the press and especially on the forming tool increase. Hence, to design reliable and withstanding forming tools, a detailed knowledge of the vibrations and resulting critical loads is essential. In this paper, the main focus is put on the vibration of the blankholder—the heaviest moving component in the forming tool. To predict those vibrations, a coupled multibody-finite element simulation (MBS-FEM) is conducted, which combines rigid and elastic modeling approaches. Also, an experimental validation of the blankholder vibration under operational load is carried out. To compare the numerical and experimental results—both in time and frequency domain—an 1/3-octave analysis of a blankholder’s vibrational speed is performed. The test measurements agree well with the MBS-FEM simulation.
Because of increased stroke rates the loads on forming tools increase too. To ensure a save design of components, durability analyses are intended. For this, simulation results from FE analyses are necessary. Therefore, it is desirable to use elements with quadratic function, because of a good stress approximation.The goal of the described investigation is to show if calculation results created by LS-DYNA [1] can be used for durability analyses. Especially the use of quadratic elements is investigated. For the evaluation, on the one hand explicit FE analyses of a special durability test are carried out. These analyses are validated by available test data. To create results for later durability analyses further FE analyses with implicit time discretisation are carried out. In this paper results of the investigations are presented and evaluated critically.
Effiziente Methode zur Bestimmung von dynamischen strukturellen Belastungen in Großpresswerkzeugen
(2016)
To improve efficiency in automotive press shops, press systems with increasingly high stroke rates are beingimplemented, raising thereby the structural dynamic load on the press and especially on the forming tool. Adetailed knowledge of the vibrations and resulting critical loads is thus essential for accurate and reliable de-signs of forming tools. In this paper, multibody simulation (MBS) of a selected automotive tool is presentedenabling the identification of the vibration of its components. Starting from a pure rigid modelling approach,the MBS model is extended by adding finite element (FE) component to allow extra flexibility in the sys-tem. As an example, a detailed vibrational analysis - both in time and frequency domain - is performed ona blankholder. Also an experimental validation of a blankholder vibration under operational load is carriedout, with test signal data gained by piezoelectric accelerometers.
In the forming tools for pressing car body parts, heavy blankholders are used to prevent buckling and wrinkling of the blank. During each press cycle, those large masses need to be lifted, raising thereby the structural dynamic load on the forming tool and on the press. Therefore a detailed knowledge about the forming tool dynamics is essential for an accurate and robust design of forming tools. In this paper, a dynamic finite element method (FEM) simulation of selected automotive forming tool is presented enabling identification of regions of critical stresses. For validation of the simulation results, the kinematic responses of the tool's components are compared with the measurements obtained in an experiment.
Lineare und nichtlineare FEM
(2017)
Dieses Lehrbuch bringt im ersten Teil leicht verständlich die Merkmale und Eigenschaften linearer finiter Elemente aus dem Bereich der Elastostatik und Dynamik und erläutert Vorgehensweisen bei der Erstellung von Simulationsmodellen. Im zweiten Teil liegt der Schwerpunkt auf nichtlinearen zeitabhängigen Vorgängen und Lösungsverfahren. Praxisbeispiele aus der Umformtechnik werden anschaulich mit der Software LS-DYNA® gelöst. Der Fokus liegt auf einer Erklärung der Zusammenhänge, die für Anwender kommerzieller Software notwendig sind. Theoretische Grundlagen werden in dem Umfang dargestellt, der für das Verständnis der Benutzung eines Programms notwendig ist.