Helga Krieger

Experimental Setup to Validate Textile Material Models for Drape Simulation

The simulation of the draping process of dry textiles allows one to predict the occurrence of folds and the local fibre orientations and fibre positions after draping. In this paper the experiments to determine the mechanical material properties of textile structures are discussed. The mechanical material parameters are used as input for the drape simulation on the macro-scale. The numeric material models can be validated by comparing the numeric results with the experimental draping results of a drapeability test with standardized geometries. The further developed drapeability test to validate the material models for textile structures will be presented.

AutoHD—Automated Handling and Draping of Reinforcing Textiles

In almost all industrial sectors handling processes are automated through the use of robotic systems. However, in the manufacture of fiber-reinforced structures with complex geometries, the handling of dry, pre-impregnated semi-finished textiles is still performed mainly manually resulting in long processing times, low reproducibility and high manufacturing costs. A previous AiF research project “AutoPreforms” aimed at the automation of the entire production process of components with uniaxial curvature. The scope of this AiF research project “AutoHD” is to fully automate the draping and handling process of complex, three-dimensional fiber composite structures with high degrees of deformation and multiaxial curvature (e.g. car wings). Based on a draping simulation wrinkles can already be recognized during the draping process and counteracted by the developed mechanical structure. This is achieved by the utilization of the bending stiffness of textile semi-finished products, a flexible end-effector and a built-in optical quality assurance process. In this paper the main aspects of preforming processes are described revealing the challenges of the project. With examples of currently existing systems, the objective and innovative contribution of the project are described. The paper serves as initial presentation of the project and its solution approaches.

Design of Tailored Non-Crimp Fabrics Based on Stitching Geometry

Automation of the preforming process brings up two opposing requirements for the used engineering fabric. On the one hand, the fabric requires a sufficient drapeability, or low shear stiffness, for forming into double-curved geometries; but on the other hand, the fabric requires a high form stability, or high shear stiffness, for automated handling. To meet both requirements tailored non-crimp fabrics (TNCFs) are proposed. While the stitching has little structural influence on the final part, it virtually dictates the TNCFs local capability to shear and drape over a mold during preforming. The shear stiffness of TNCFs is designed by defining the local stitching geometry. NCFs with chain stitch have a comparatively high shear stiffness and NCFs with a stitch angle close to the symmetry stitch angle have a very low shear stiffness. A method to design the component specific local stitching parameters of TNCFs is discussed. For validation of the method, NCFs with designed tailored stitching parameters were manufactured and compared to benchmark NCFs with uniform stitching parameters. The designed TNCFs showed both, generally a high form stability and in locally required zones a good drapeability, in drape experiments over an elongated hemisphere.

Geometrical analysis of woven fabric microstructure based on micron-resolution computed tomography data

The global mechanical properties of textiles such as elasticity and strength, as well as transport properties such as permeability depend strongly on the microstructure of the textile. Textiles are heterogeneous structures with highly anisotropic material properties, including local fiber orientation and local fiber volume fraction. In this paper, an algorithm is presented to generate a virtual 3D–model of a woven fabric architecture with information about the local fiber orientation and the local fiber volume fraction. The geometric data of the woven fabric impregnated with resin was obtained by micron-resolution computed tomography (μCT). The volumetric μCT-scan was discretized into cells and the microstructure of each cell was analyzed and homogenized. Furthermore, the discretized data was used to calculate the local permeability tensors of each cell. An example application of the analyzed data is the simulation of the resin flow through a woven fabric based on the determined local permeability tensors and on Darcy’s law. The presented algorithm is an automated and robust method of going from μCT-scans to structural or flow models.

Micro-Mechanical Modelling of the Entanglement of Carbon Fiber Tows

Carbon fiber reinforced plastics (CFRPs) are the focus of many research efforts for light weight, high stiffness applications. While CFRPs generally yield more desirable properties than their metal counterparts, variations in the microstructure can lead to problems such as early failure. For example, voids in the matrix provide points of failure initiation in the matrix. The existence of voids is often attributed to nonaxialities in the fiber packing structure, which block the matrix from passing through certain regions of a tow. These non-axialities arise through the processing and handling of tows during manufacturing, where compression of a tow or friction on the outer fibers leads to fibers becoming entangled into one another. It is evident that composite materials cannot be fully utilized without a sound understanding of the manufacturing

Kinematic Drape Algorithm and Experimental Approach for the Design of Tailored Non-Crimp Fabrics

In the preforming process, the textile is draped into the geometry of the structural part and afterwards consolidated with resin via injection. For preforming processes non-crimp fabrics (NCFs) have become increasingly popular for cost effective applications. For the realization of automated draping of two-dimensional textiles into three-dimensional complex geometries during the preforming process there is a high advantage for the use of tailored textiles compared to textiles with uniform material properties. Large flat surfaces require a high bending stiffness and low shear stiffness due to high structural stability of the textile and small radii of curvature require a low bending stiffness due to good drapeability of the textile. The bending and the shear stiffness of NCFs with a given layup can be influenced by the manufacturing parameters of the knitting yarn. With tailored NCFs it is possible to adapt the manufacturing parameters of the knitting yarn locally in the production direction to improve drapeability and handling of the textile in the preforming process. To use the high potential of tailored NCFs, the development of the new textile structure has to go hand in hand with the characterization and with the simulation of the draping process. An experimental approach and a modelling approach using a kinematic drape algorithm have been developed to define the local stitching parameters for tailored NCFs dependent on the geometry of the component part.