Gerald Hoffmann

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.

New solutions for the manufacturing of spacer preforms for thermoplastic textile-reinforced lightweight structures

The novel woven spacer fabrics consist of upper and lower layers which are connected through internal crosslinks. The aim is to weave spacer fabrics with woven crosslinks in a single production step as near net shape sandwich preforms. By this way reproducible and automated manufacturing of sandwich preforms for composites are realized. The paper deals at first with the conception of a new weaving and take-up technology of complete spacer fabrics without subsequent textile assembly processes. Afterwards, the special technology is described, and finally the paper deals with a simulation model for the prediction of dynamic warp thread forces in order to minimize fiber damage during weaving process.

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 Novel Scaffolds for Tissue Engineering by Flock Technology

Flock technology is a method well known in the textile industry. Short fibers are applied almost vertically on a substrate, coated with a flocking adhesive. Until now, this technology has not been used in the field of biomaterials. Our aim was to use electrostatic flocking for fabricating a novel type of scaffolds for tissue engineering. This method offers the possibility to create matrices with anisotropic properties that have a high compressive strength despite high porosity. First, experiments were performed using a membrane made of mineralized collagen as substrate, gelatine as adhesive and polyamide flock fibers. Different kinds of cells were cultured in the scaffolds for up to six weeks. Using microscopic methods and biochemical analyses, we could demonstrate that cells adhered and proliferated well in this new type of scaffold. Therefore, we can summarize that flocking is a technology suitable for fabrication of scaffolds for cell cultivation and tissue engineering.

Mechanical characterization of hybrid yarn thermoplastic composites from multi-layer woven fabrics with function integration

Three dimensional (3D) spacer fabrics are constructed from two dimensional (2D) woven fabrics connected by 2D woven crosslink fabrics using thermoplastic high performance hybrid yarns. Spacer fabrics present great potential in the efficient production of fiber reinforced plastic composites with adjustable mechanical properties targeted for specific applications in Lightweight engineering. The structure of the weave and the stresses placed on the fibers during the weaving process impact the mechanical properties of the finished composite. This study shows that the mechanical properties can be significantly controlled and adjusted by pattern development of the individual 2D composite structures from the 3D spacer fabric preforms constructed of orthogonal multi-layer weaves (non-crimped fiber arrangement and a gentle weaving process). Electrically conductive fibers are woven directly into various layers of the 2D woven fabrics and are therefore successfully integrated during production. This forms the basis for function integration in the composite. The weaving technology ensures a reliable contact of the conductive fibers allowing sensor networks to be integrated into the fabric for online monitoring of the fiber reinforced plastic composite component.

Development of weaving technology for manufacturing three-dimensional spacer fabrics with high-performance yarns for thermoplastic composite applications: An analysis of two-dimensional mechanical properties

This article focuses on the improved strength and stiffness of woven three-dimensional (3D) spacer fabrics used in fiber-reinforced plastics. The spacer fabrics were manufactured using E-glass/polypropylene (GF/PP) hybrid yarns and are intended for lightweight engineering applications. The two-dimensional (2D) woven fabrics used for the outer layers of 3D spacer fabrics were produced from GF/PP hybrid yarns with different yarn alignments and weft densities. Furthermore, the implemented hybrid yarns and the 2D composites constructed of multi-layer fabrics were tested for mechanical properties. The processing of the GF/PP hybrid yarn, as well as the geometry of the woven fabrics, is proven to strongly influence the mechanical properties of the end composites. Multi-layer fabrics can be used for manufacturing woven 3D spacer fabrics with minimal yarn damage and high mechanical properties.

Development of multilayered woven panels with integrated stiffeners in the transverse and longitudinal directions for thermoplastic lightweight applications

Weaving technique offers a large potential for the highly productive manufacture of woven 3D-preform structures in lightweight applications. This research work reports on the technological modification to the construction of a double-rapier weaving machine for the low damage production of double-walled shell structures made from glass/polypropylene hybrid yarn multilayered woven fabrics. On this basis, 3D-woven preform structures with integrated stiffeners in the longitudinal and transverse directions for high composite strength and rigidity within a single weaving process were developed and implemented. Furthermore, textile engineering solutions were acquired for the required reinforcement of the gusset area between the stiffener and the skin. This broadens the range of uses for large-scale fiber-reinforced plastics with integrated stiffeners made from 3D-woven preform structures.

Simulation of the yarn transportation dynamics in a warp knitting machine

To date, in the textile manufacturing process of warp knitting, trouble-shooting and process optimization mainly rely on empirical knowledge and experiments. This factor limits the achievable increase in productivity and quality. On the other hand, using simulations, different phenomena that affect the quality of the knitted fabric and the knitting process can be clarified in the run-up of the experiments. Consequently, an increase in quality and flexibility can be reached with reduced experimental effort. This paper presents a process simulation of the warp thread dynamics in the thread feeding system of a warp knitting machine. For this purpose, a continuum model of the warp thread that includes the spatial dynamics of the thread and the axial transport movement has been developed.

Production Principles and Technological Development of Novel Woven Spacer Preforms and Integrated Stiffener Structures

Abstract3D textile preforms offer a high potential to increase mechanical properties of composites and/or decrease manufacturing costs. Within the scope of this study, production principles were developed for complex spacer preforms and integrated stiffeners. These principles were applied through technological further development of the well-known face-to-face and terry weaving techniques. Various woven preforms were produced with Glass fibre/Polypropylene (GF/PP) Commingled yarns, however, the technology is suitable for any type of reinforcement yarns. U-shaped woven spacer preform was consolidated into a sandwich composite component for lightweight applications.

Development of three-dimensional profiled woven fabrics on narrow fabric looms

Three-dimensional (3D) profiled woven fabrics with varying cross-sections along the component parts are needed in a number of industrial applications. One of the main advantages of the ribbon loom weaving technique is the ability to produce highly diverse structures with open or closed edges. The realization of 3D profiled woven fabrics that satisfy the requirements is directly connected to the ability to process high-performance fibers in the weft direction. The processing of high-performance yarns in the weft direction with low fiber damage will open new application areas for shuttle weaving machines. By employing modified mechanical loom elements, the variety of producible structures can be increased significantly.