Thomas Gereke

Decoupling the bending behavior and the membrane properties of finite shell elements for a correct description of the mechanical behavior of textiles with a laminate formulation

Drape simulation of textiles is a field of research, which is known in the clothing sector for a long time. The ongoing development of high-performance composites made of textile reinforcements and matrix materials focus the interests on a serial production in many industrial sectors, such as aviation and automotive industries. Challenges occur mainly in the serial production technologies and in supplying concepts for the preform architecture and shape. Research aims on the acceleration of preform manufacturing and the reduction of expensive pretests. Numerical simulation models can help to improve the composite development chain with structure and process simulation. A special challenge in drape modeling is the bending behavior of textiles. This study introduces a novel approach for modeling single textile layers as laminates to gain a correct mechanical behavior, where all deformation mechanisms are uncoupled. The implementation in the finite element software LS-DYNA® is described. An algorithm is introduced which provides the membrane stiffness for each layer of a laminate to fit the measured cantilever bending stiffness of textiles in every bending direction and bending side. The calculated parameters for the laminate formulation result in the requested bending stiffness for the textile layer. The cantilever bending stiffness can be used directly for dimensioning the model.

Modelling of textile composite reinforcements on the micro-scale

Abstract Numerical simulation tools are increasingly used for developing novel composites and composite reinforcements. The aim of this paper is the application of digital elements for the simulation of the mechanical behaviour of textile reinforcement structures by means of a finite element analysis. The beneficial computational cost of these elements makes them applicable for the use in large models with a solution on near micro-scale. The representation of multifilament yarn models by a large number of element-chains is highly suitable for the analysis of structural and geometrical effects. In this paper, a unit cell generating method for technical reinforcement textiles, using digital elements for the discretization, is introduced.

Multiscale Stochastic Modeling of the Elastic Properties of Strand-Based Wood Composites

AbstractThis paper introduces a novel modeling approach for wood composites using concepts of numerical homogenization employed in synthetic composites. It describes a multiscale model based on a unit cell that incorporates both the wood and resin phases for simulating structural composite lumber made of strands. In this approach, constant resin thickness and strand geometry, elastic properties of constituents, and perfect bonding between wood and resin are assumed. The multiscale modeling is composed of two steps. The first step estimates the effective elastic properties of a unit cell based on the numerical homogenization with periodic boundary conditions. The second step consists of a macroscopic finite element structural analysis of a beam (assembly of several unit cells) under three-point bending. Random distribution of strand orientation that may be encountered in an actual composite beam is introduced at this stage. Results indicate a significant influence of the resin. The first step of the approa...

Analysis and finite element simulation of the draping process of multilayer knit structures and the effects of a localized fixation

Fiber reinforced composites (FRC) are an interesting alternative for numerous applications due to their lightweight character. However, there are currently several challenges for a serial production. The manufacturing process still requires a high percentage of manual labor which greatly restricts the reproducibility. Additionally, high-quality standards necessary for many applications cannot be met due to the low displacement resistance of the textiles. Structural fixation could greatly improve the displacement resistance and therefore the handling of the material layers. This paper reports on a model used for draping simulations of nonfixed and fixed multilayer knits using the commercial finite element software LS-DYNA®. The aim of this model is to improve the development process of FRCs. With a standardized specification, the basic macromechanical properties can be modeled with finite shell elements. A material model is introduced that accounts to the characteristic mechanisms of the deformation of biaxial fibrous structures. A fixation of the fabrics is achieved by melting the thermoplastic hybrid yarns embedded in the textile structure with infrared radiation. This process improves the handling of the textiles. It is of great benefit when such a structural fixation is applied locally. The process of choosing local fixation zones is described in this paper and the applicability of this process is illustrated.

Prediction of Blended Yarn Evenness and Tensile Properties by Using Artificial Neural Network and Multiple Linear Regression

Abstract The present research work was carried out to develop the prediction models for blended ring spun yarn evenness and tensile parameters using artificial neural networks (ANNs) and multiple linear regression (MLR). Polyester/cotton blend ratio, twist multiplier, back roller hardness and break draft ratio were used as input parameters to predict yarn evenness in terms of CVm% and yarn tensile properties in terms of tenacity and elongation. Feed forward neural networks with Bayesian regularisation support were successfully trained and tested using the available experimental data. The coefficients of determination of ANN and regression models indicate that there is a strong correlation between the measured and predicted yarn characteristics with an acceptable mean absolute error values. The comparative analysis of two modelling techniques shows that the ANNs perform better than the MLR models. The relative importance of input variables was determined using rank analysis through input saliency test on optimised ANN models and standardised coefficients of regression models. These models are suitable for yarn manufacturers and can be used within the investigated knowledge domain.

Geometrical design and forming analysis of three-dimensional woven node structures

Structural frames have been established in many technical applications and typically consist of interconnected profiles. The profiles are commonly joined with node elements. For lightweight structures, the use of composite node elements is expedient. Due to the anisotropic mechanical properties of the fibers, high demands are placed on the orientation of the fibers in the textile reinforcement structure. A continuous fiber course around the circumference and at the junctions is necessary for an excellent force transmission. A special binding and forming process was developed based on the weaving technology. It allows the production of near-net-shaped node elements with branches in any spatial direction, which meet the requirements of load-adjusted fiber orientation. The principles by which these three-dimensional (3D) node elements are converted into a suitable geometry for weaving as a net shape multilayer fabric are reported. The intersections of the branches are described mathematically and flattened to a plane. This is the basis for the weave pattern development. Forming simulations on the macro- and meso-scales complement the analyses. A macro-scale model based on the finite element method (FEM) is used to verify the general formability and the accuracy of the flattenings. Since yarns are pulled through the textile structure in the novel forming process, the required tensile forces and the pulling lengths of the individual yarns are analyzed with a meso-scale FEM model. The flattening for two different node structures is realized successfully, and the simulation proves formability. Furthermore, the necessary forming forces are determined. Finally, the developed method for flattening the 3D geometry is suitable for the design of a variety of spatial node structures and the simulation supports the design of automated forming processes.

A comprehensive multi-scale analytical modelling framework for predicting the mechanical properties of strand-based composites

A multi-scale modelling framework was developed for predicting the mechanical properties of strand-based wood composites. This framework is based on closed-form analytical models at three different resolution levels; micro-, meso- and macro-mechanical. A preprocessing step was performed to provide the input data for the three main modelling steps in this framework. Finite element-based mechanical analyses were employed to verify the accuracy of the analytical models developed for the first two steps. The predictive capability of the entire framework was validated using a set of experimental data reported in the literature. Although the methodology presented is general, it has specifically been applied here to predict an important structural property (modulus of elasticity, MOE) of a special strand-based wood composite product, namely, oriented strand board (OSB). The MOE predictions of OSB panels showed reasonable agreement with the available experimental data, thus providing confidence in the practical utility of this easy-to-use and efficient analytical modelling tool for predicting the properties of wood composites employed in structural members.

Approaches for process and structural finite element simulations of braided ligament replacements

To prevent the renewed rupture of ligaments and tendons prior to the completed healing process, which frequently occurs in treated ruptured tendons, a temporary support structure is envisaged. The limitations of current grafts have motivated the investigation of tissue-engineered ligament replacements based on the braiding technology. This technology offers a wide range of flexibility and adjustable geometrical and structural parameters. The presented work demonstrates the possible range for tailoring the mechanical properties of polyester braids and a variation of the braiding process parameters. A finite element simulation model of the braiding process was developed, which allows the optimization of production parameters without the performance of further experimental trials. In a second modelling and simulation step, mechanical properties of the braided structures were virtually determined and compared with actual tests. The digital element approach was used for the yarns in the numerical model. The results show very good agreement for the process model in terms of braiding angles and good agreement for the structural model in terms of force-strain behaviour. With a few adaptions, the models can, thus, be applied to actual ligament replacements made of resorbable polymers.

Analysis and prediction of air permeability of woven barrier fabrics with respect to material, fabric construction and process parameters

Air permeability is one of the important properties of conventional as well as technical fabrics such as protective garments, filters, and fabrics for airbags and parachutes. In case of surgical textiles, air permeability is an effective measure of thermo-physiological comfort. This study is aimed to analyze PES barrier fabrics and to develop correlation between permeability and influential material, construction and process parameters. Not only the individual effects of yarn, fabric and loom parameters but also the underlying complex interactions between fewer or several of these influencing factors exert significant influence on fabric porosity and permeability. Artificial neural network (ANN) is the suitable tool to map such complex input-output relationships, since a direct analytical solution is not possible. Feedforward neural network models were trained with combination of Levenberg-Marquardt algorithm and Bayesian regularization support incorporated in backpropagation. Based on the number of input variables, three ANN models were optimized. It was observed that the model which was trained with all selected inputs delivered promising results on test data, i.e., R2=0.985 and mean absolute error of 1.8%. To eliminate any doubt of overfitting, 10 % cross-validation was also performed for selected final model. Furthermore, to investigate the relative importance of input variables in the optimized ANN model, the rank analysis was also carried out. Research outcomes reveal that ANN can be used to tailor barrier fabric permeability depending on the requirements quickly without trials and error by adjusting loom, fabric and yarn parameters.

Modelling and Simulation of the Coupling of Normal Pressure and Shear Force in Plain Woven Fabrics

In textile engineering, simulative methods are used more frequently due to their advantages in material and process design. Finite element models were developed for simulating the mechanical and the draping behaviour of fabrics. For large deformation analysis of textile forming, macro mechanical models are employed that use continuum mechanical approaches for matters of reduced computation time. The material data that is required as model input, such as tension and shear properties, can either be obtained by experimental or virtual tests. In such virtual tests the deformation behaviour of fabrics can be determined by deforming the structure on the meso level.