Olaf Diestel

Development of Flat Knitted Spacer Fabrics for Composites using Hybrid Yarns and Investigation of Two-dimensional Mechanical Properties:

Flat knitted spacer fabrics offer a strong potential for complex shape preforms, which could be used to manufacture composites with reduced waste and shorter production times. A reinforced spacer fabric made of individual surface layers and joined with connecting layers shows improved mechanical properties for lightweight applications such as textile-based sandwich preforms. We report the development of different flat knitted spacer fabrics from hybrid yarns consisting of glass filament (GF) and polypropylene (PP) filaments. Moreover, the mechanical properties of these reinforcement yarns and two-dimensional composites manufactured using various knit structures and knitting parameters were also studied. The composite structure with reinforcement yarn in weft direction is found to be the best in this regard.

Thermoplastic composites from curvilinear 3D multi-layer spacer fabrics

This article reports on the development of thermoplastic composites using innovative curvilinear 3D multi-layer flat knitted spacer fabrics produced by single stage manufacturing. Thermoplastic composites from textile-based, complex-shaped sandwich preforms, for instance, curvilinear-shaped 3D multi-layer spacer fabrics show great potential for lightweight applications. Faster production time and reduction of waste can be achieved by single stage manufacturing of such 3D textile preforms. Spacer-shaped 3D textile preforms were developed using commingled hybrid yarns made of GF and PP filaments. This 3D spacer fabric was consolidated to 3D composite using the developed mechanical tools. In order to predict the mechanical performance of 3D composites, mechanical properties of reinforcement yarns unraveled from 3D spacer fabrics, 2D knit fabrics, and 2D composites using the 2D knit fabrics produced in the same manner as the individual layers of 3D spacer fabrics were studied. The results are promising for applications in high-performance composites.

Development of a process chain for the realization of multilayer weft knitted fabrics showing complex 2D/3D geometries for composite applications:

The use of thermoplastic components with a complex three-dimensional (3D) shape, manufactured efficiently with thermo-presses, has been increased steadily. Flat knitting technology using reinforcing hybrid yarns in the horizontal and vertical direction is especially suited for producing near-net-shape or fully-fashion multilayer weft knitted fabrics – MLGs (abbreviated from the German word Mehrlagengestrick, meaning multilayer weft knitted fabric). The other advantages of manufacturing such MLGs, using flat knitting technology, are reduced waste and desired reinforcing fibre alignment to obtain improved mechanical properties for high-performance applications. Before knitting 3D shaped MLGs, it is necessary to transfer the 3D component geometry into a suitable two-dimensional (2D) pattern cut by implementing parting lines. The use of computer-aided design (CAD) programs enables an effective development of complex components preforms. The generated 2D pattern cuts are analyzed with the consideration of net-shape preforming processes on V-bed flat knitting machines. The development of a segmented take-down system for effective production of 3D MLG preforms is also discussed.

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.

Electro-mechanical properties of friction spun conductive hybrid yarns made of carbon filaments for composites

Textile-reinforced composites are a leading trend in lightweight structure design. The demand for additional function integration in lightweight structures made from textile-reinforced composites is growing rapidly. This article reports on the production of functional yarns made with friction spinning using carbon filament yarn as the core component and polypropylene fibres as the sheath component. The suitability of these yarns was tested with regards to mechanical properties for use in textile-reinforced thermoplastic composites. Their ability to perform additional functions such as structural health monitoring and signal transfer was also tested. Differences in the electro-mechanical and mechanical properties of hybrid yarns made from different types of carbon filament yarn core and polypropylene sheath were investigated with respect to different spinning parameters used to produce the yarns. Optimum processing parameters have been suggested that improve the functional properties of these yarns.

Vorimprägnierte textile Halbzeuge (Prepregs)

Vorimpragnierte textile Halbzeuge, sogenannte Prepregs, sind ein wichtiges Ausgangsmaterial fur die Herstellung von duroplastischen und thermoplastischen Verbundwerkstoffen. Es handelt sich hierbei um vorgefertigte, meist ebene, flachige Halbzeuge, die eine Verstarkungsstruktur aus endlichen bzw. endlosen Fasern aufweisen, die bereits mit der fur die Bauteilfertigung benotigten duro- bzw. thermoplastischen Matrix kombiniert ist. Als Ausgangsprodukt konnen sowohl Kurzfasern oder Endlosfilamentgarne als auch textile Flachengebilde wie Gewebe und Multiaxial-Kettengewirke zum Einsatz kommen. Grundprinzip der Verwendung dieser speziellen Form der textilen Halbzeuge ist die Trennung des Trankungsvorgangs bei der Verbundwerkstoffherstellung vom eigentlichen Herstellen der Bauteilform.

Weiterverarbeitungsaspekte und Anwendungsbeispiele

Die weltweite Energie- und Klimasituation erfordert, dass zukunftig alle Moglichkeiten zur Senkung des Energieverbrauchs, nicht nur in der Verkehrstechnik und im Bauwesen, sondern auch in allen Wirtschaftszweigen ausgeschopft werden. Der Leichtbau mit textilverstarkten Verbundwerkstoffen bietet bei der Entwicklung energieeffizienter und funktionsintegrierender Strukturbauteile faszinierende Moglichkeiten gegenuber konventionellen metallischen Bauweisen. Aus der Kombination von zwei oder mehreren unterschiedlichenWerkstoffen resultieren neuartige Verbundwerkstoffe, deren Leistungsfahigkeit die Summe der Eigenschaften der Einzelkomponenten ubersteigt.

Processing Aspects and Application Examples

The global energy and climate situation requires future activities to reduce energy consumption as much as possible, not only in transport technology and the construction sector, but also in all other industries. Lightweight construction with textile-reinforced composite materials offers fascinating possibilities in developing energy-efficient and function-integrated structural components, especially when compared to their conventional metallic counterparts. The combination of two or more different types of material results in novel composites, whose performance exceeds the sum of properties of the individual components.

Pre-impregnated Textile Semi-finished Products (Prepregs)

Pre-impregnated semi-finished textile products (prepregs), are an important base material for the manufacture of thermoset and thermoplastic composite materials. They consist of a usually flat and planar textile reinforcement structure and are combined with thermoset or thermoplastic matrices required for the production of the final component. Both short fibers or continuous filaments and textile fabrics, such as woven fabrics or multiaxial warp-knitted fabrics, can be used as base material. The basic principle of using this special textile semi-finished product is to separate the step of impregnating the reinforcement structure with the matrix during composite material production from the final step of producing the component form.

PPS-Polymer-Composites for High Performance Rubber Components

Based on their properties, PPS fibers are a promising material for reinforcing elastomeric components that are subjected to high mechanical and thermal loads. The use of this material is at present hindered because of the low adhesion between the fiber and matrix. Atmospheric pressure plasma treatments based on the dielectric barrier discharge were performed on PPS fibers using air as reactive gas for different treatment durations in order to improve the adhesion. The effects of these treatments have been characterized by determining the surface energy, and the residual tensile strength as well as by analyzing the surface chemistry. Required conditions for an improved wetting behavior and a significant increase in the polar component of the surface energy could then be identified.