Luise Kärger

A method for validation of finite element forming simulation on basis of a pointwise comparison of distance and curvature

Thermoforming of continuously fiber reinforced thermoplastics (CFRTP) is ideally suited to thin walled and complex shaped products. By means of forming simulation, an initial validation of the producibility of a specific geometry, an optimization of the forming process and the prediction of fiber-reorientation due to forming is possible. Nevertheless, applied methods need to be validated. Therefor a method is presented, which enables the calculation of error measures for the mismatch between simulation results and experimental tests, based on measurements with a conventional coordinate measuring device. As a quantitative measure, describing the curvature is provided, the presented method is also suitable for numerical or experimental sensitivity studies on wrinkling behavior. The applied methods for forming simulation, implemented in Abaqus explicit, are presented and applied to a generic geometry. The same geometry is tested experimentally and simulation and test results are compared by the proposed vali...

Efficient prediction of damage resistance and tolerance of composite aerospace structures

Abstract The present work introduces efficient methodologies based on the finite-element method for a quick evaluation of damage resistance and damage tolerance of composite aerospace structures. Monolithic, stringer-stiffened structures, and sandwich structures are considered. The presented methodologies cover the simulation of the dynamic response of a structure during a low velocity impact event including the prediction of the internal non-visible or barely visible damage that develops during the impact. Additionally, methods for the prediction of the compression-after-impact strength are presented. In order to permit an accurate and efficient calculation of deformations and stresses in sandwich structures, special finite-element formulations have been developed. A comparison of simulation results with experimental data is presented for a two-stringer monolithic panel and for a honeycomb sandwich plate. The examples demonstrate that the presented methodologies can be used to quickly assess the damage tolerance of composite structures.

Accurate Cure Modeling for Isothermal Processing of Fast Curing Epoxy Resins

In this work a holistic approach for the characterization and mathematical modeling of the reaction kinetics of a fast epoxy resin is shown. Major composite manufacturing processes like resin transfer molding involve isothermal curing at temperatures far below the ultimate glass transition temperature. Hence, premature vitrification occurs during curing and consequently has to be taken into account by the kinetic model. In order to show the benefit of using a complex kinetic model, the Kamal-Malkin kinetic model is compared to the Grindling kinetic model in terms of prediction quality for isothermal processing. From the selected models, only the Grindling kinetic is capable of taking into account vitrification. Non-isothermal, isothermal and combined differential scanning calorimetry (DSC) measurements are conducted and processed for subsequent use for model parametrization. In order to demonstrate which DSC measurements are vital for proper cure modeling, both models are fitted to varying sets of measurements. Special attention is given to the evaluation of isothermal DSC measurements which are subject to deviations arising from unrecorded cross-linking prior to the beginning of the measurement as well as from physical aging effects. It is found that isothermal measurements are vital for accurate modeling of isothermal cure and cannot be neglected. Accurate cure predictions are achieved using the Grindling kinetic model.

Composite Process Chain Towards As-Built Design

The relation between design and manufacture is of particular importance within the composite structure development process. Therefore, a continuous composite process chain beyond state-of-the-art is described in this section. Such an all-embracing process chain realizes a concurrent engineering, where iteration loops are enabled and, thus, product improvements and higher process efficiency are achieved. Concurrent engineering comprises the various interdisciplinary working phases and provides the necessary connectivity. In contrast to the traditional one-way relation from design to manufacture, the improved process chain also deals with the feedback from manufacture to design, based on numerical simulations. This is demonstrated by the example of composite parts made by Tailored Fiber Placement (TFP), including effects of the feedback on load bearing capacity.

Simulation of the Influence of Embedded Inserts on the RTM Filling Behavior Considering Local Fiber Structure

Resin Transfer Molding (RTM) enables an intrinsic manufacturing of fiber reinforced composite parts containing integrated metallic inserts. The inserts are embedded into the fiber layers in the preforming stage of the process and therefore influence the following mold filling. The fiber structure around the embedded insert is strongly influenced by the insert resulting in high local variations of fiber volume fraction which changes the local permeability. This leads to an inhomogenic flow front and can even result in dry spots of the cured part. To predict the formation of air bubbles, a two-phase mold filling simulation is used under consideration of local fiber volume fraction. Local fiber structure is determined using CT-scans of manufactured parts with different orientations of the insert in relation to the preform and to the filling direction. The mold filling simulations allow the evaluation of different filling strategies and show a strong influence of the insert on the local flow front propagation.

Rheological In-Mold Measurements and Characterizations of Sheet-Molding-Compound (SMC) Formulations with Different Constitution Properties by Using a Compressible Shell Model

Abstract The rheological characterization of Sheet Molding Compound (SMC) and its modelling is crucial for reliable process simulations. In the past, characterization and material modelling were mainly focusing on SMC with low glass fiber content and a high filler fraction. Due to new application areas, SMC without fillers and with high glass fiber contents, and SMC with carbon fibers become more important. Therefore, these three types of SMCs are characterized in this work, using an inline rheological tool. Differences regarding their compressibility and their flow dependency are identified and considered in an analytical shell modelling. The comparison of the different materials leads to a better understanding of the phenomenological parameters related to the viscosity and friction in the models. Furthermore, the importance to properly consider all relevant material-specific effects becomes evident.

Evaluation of Different Hybrid Material Systems and Systematic Analysis of their Physical Mechanisms in Terms of Fatigue

Hybrid material systems are designed by the specific combination of different materials. As a result, expanded property profiles can be achieved, which would not be possible with monolithic material solutions. For lightweight, high strength and high rigidity, complex shaped structural components, which are used in the automotive industry and in aerospace, hybrid material systems offer an outstanding potential. A comprehensive understanding regarding the interaction of the individual components of the hybrid material is of great importance for a more efficient design of future structures. In this work, existing hybrid solutions for industrial applications and those, which are subject of current research, are analyzed and categorized first. Intrinsic and extrinsic material combinations are considered at different levels, ranging from hybrid laminates on shell level to complex hybrid structures on component level. Based on the situation analysis, different hybrid solutions are evaluated and compared considering the requirements of the automotive industry. Furthermore, the associated physical mechanisms which are responsible for the respective property profile are considered and explained systematically. The long-term objective of the work is to establish a methodology to derive the necessary physical mechanisms and, based on that, to derive optimal hybrid solutions for desired property profiles.

Two Fe Formulations for a Rapid 3D Stress Analysis of Composite Sandwich Structures

A rapid 3D stress analysis of sandwich structures made from composite face sheets and a lightweight core is needed for an efficient simulation of impact damage tolerance and resistance. For that reason, two finite shell element formulations based on layer-wise theories are developed using pure displacement approaches. The number of layers is confined to three, one for each skin laminate and one for the core, accounting for the very different stiffness of skin and core material. In order to obtain reasonable transverse shear stiffness properties and also improved transverse shear and normal stresses the equilibrium approach by Rolfes and Rohwer [1] is extended to a three-layered sandwich structure.

A 2D modeling approach for fluid propagation during FE-forming simulation of continuously reinforced composites in wet compression moulding

Wet compression moulding (WCM) provides large-scale production potential for continuously fiber reinforced components as a promising alternative to resin transfer moulding (RTM). Lower cycle times are possible due to parallelization of the process steps draping, infiltration and curing during moulding (viscous draping). Experimental and theoretical investigations indicate a strong mutual dependency between the physical mechanisms, which occur during draping and mould filling (fluid-structure-interaction). Thus, key process parameters, like fiber orientation, fiber volume fraction, cavity pressure and the amount and viscosity of the resin are physically coupled. To enable time and cost efficient product and process development throughout all design stages, accurate process simulation tools are desirable. Separated draping and mould filling simulation models, as appropriate for the sequential RTM-process, cannot be applied for the WCM process due to the above outlined physical couplings. Within this study, a two-dimensional Darcy-Propagation-Element (DPE-2D) based on a finite element formulation with additional control volumes (FE/CV) is presented, verified and applied to forming simulation of a generic geometry, as a first step towards a fluid-structure-interaction model taking into account simultaneous resin infiltration and draping. The model is implemented in the commercial FE-Solver Abaqus by means of several user subroutines considering simultaneous draping and 2D-infiltration mechanisms. Darcy’s equation is solved with respect to a local fiber orientation. Furthermore, the material model can access the local fluid domain properties to update the mechanical forming material parameter, which enables further investigations on the coupled physical mechanisms.Wet compression moulding (WCM) provides large-scale production potential for continuously fiber reinforced components as a promising alternative to resin transfer moulding (RTM). Lower cycle times are possible due to parallelization of the process steps draping, infiltration and curing during moulding (viscous draping). Experimental and theoretical investigations indicate a strong mutual dependency between the physical mechanisms, which occur during draping and mould filling (fluid-structure-interaction). Thus, key process parameters, like fiber orientation, fiber volume fraction, cavity pressure and the amount and viscosity of the resin are physically coupled. To enable time and cost efficient product and process development throughout all design stages, accurate process simulation tools are desirable. Separated draping and mould filling simulation models, as appropriate for the sequential RTM-process, cannot be applied for the WCM process due to the above outlined physical couplings. Within this study, ...

Modelling approach for anisotropic inter-ply slippage in finite element forming simulation of thermoplastic UD-tapes

Finite Element (FE) forming simulation offers the possibility of a detailed analysis of thermoforming processes by means of constitutive modelling of intra- and inter-ply deformation mechanisms, which makes manufacturing defects predictable. Inter-ply slippage is a deformation mechanism, which influences the forming behaviour and which is usually assumed to be isotropic in FE forming simulation so far. Thus, the relative (fibre) orientation between the slipping plies is neglected for modelling of frictional behaviour. Characterization results, however, reveal a dependency of frictional behaviour on the relative orientation of the slipping plies. In this work, an anisotropic model for inter-ply slippage is presented, which is based on an FE forming simulation approach implemented within several user subroutines of the commercially available FE solver Abaqus. This approach accounts for the relative orientation between the slipping plies for modelling frictional behaviour. For this purpose, relative orientation of the slipping plies is consecutively evaluated, since it changes during forming due to inter-ply slipping and intra-ply shearing. The presented approach is parametrized based on characterization results with and without relative orientation for a thermoplastic UD-tape (PA6-CF) and applied to forming simulation of a generic geometry. Forming simulation results reveal an influence of the consideration of relative fibre orientation on the simulation results. This influence, however, is small for the considered geometry.Finite Element (FE) forming simulation offers the possibility of a detailed analysis of thermoforming processes by means of constitutive modelling of intra- and inter-ply deformation mechanisms, which makes manufacturing defects predictable. Inter-ply slippage is a deformation mechanism, which influences the forming behaviour and which is usually assumed to be isotropic in FE forming simulation so far. Thus, the relative (fibre) orientation between the slipping plies is neglected for modelling of frictional behaviour. Characterization results, however, reveal a dependency of frictional behaviour on the relative orientation of the slipping plies. In this work, an anisotropic model for inter-ply slippage is presented, which is based on an FE forming simulation approach implemented within several user subroutines of the commercially available FE solver Abaqus. This approach accounts for the relative orientation between the slipping plies for modelling frictional behaviour. For this purpose, relative orientat...