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 textilebased sandwich preforms. We report the development of flat-knitted multi-layered innovative three-dimensional (3D) spacer fabrics from hybrid yarns consisting of glass and polypropylene filaments. Moreover, for structural health monitoring of composites, sensor networks could be created into a 3D spacer fabric structure in a single processing step through innovative integration of functional yarns.Keywords composites, flat knitting, glass-polypropylene filament hybrid yarn, knitting, lightweight composite, manufacture, sensor network, spacer fabric Spacer fabrics are complex three-dimensional (3D) constructions made of two separate fabric layers connected vertically with pile yarns or fabric layers. The conventional spacer fabrics composed of two surface layers bound with pile yarns are generally manufactured using weaving and knitting technologies. However, due to inferior mechanical properties, such as elasticity and deformability under applied loads, conventional spacer fabrics are not suitable for high-performance composite applications. Moreover, the restricted distance between the plane layers contributes to the drawbacks of such spacer fabrics. One solution is to connect the planes by means of vertical fabric layers instead of pile yarns. This type of 3D spacer fabric with multi-layer reinforcements in the fabric structures is expected to show superior mechanical properties and be especially suitable as textile preforms for lightweight composite applications.1-14 Future applications of composites made from 3D multi-layer spacer fabrics involve the replacement of conventional panel structures that are being used for aircraft, transport vehicles, marine applications and infrastructures, lift cabins, and ballistic protection for buildings and combat vehicles, etc.However, in order to develop such innovative 3D spacer fabrics, modern electronic flat-knitting machines would be the best solution, since they are capable of manufacturing 3D complex-shaped engineering structures. Unique technical features that allow rapid and complex production include individual needle selection capability, the presence of holding down sinkers, presser-foots, racking, transfer, adapted feeding devices combined with a computer-aided design (CAD) system, and modern programming installations. Furthermore, the flexibility of the knitting process in combination with the possibility of integration of reinforcement yarns into fabric structures is capturing the attention of many researchers.1-17 Moreover, knitted preforms
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.
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