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Fiber-reinforced composites

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Fiber-reinforced composites (http://en.wikipedia.org/wiki/Composite_material) (or fibrous(http://en.wikipedia.org/wiki/Fiber) composites) are the most commonly used form of theconstituent combinations. The fibers of such composites are generally strong and stiff andtherefore serve as the primary load-carrying constituent. The matrix holds the fibers togetherand serves as an agent to redistribute the loads from a broken fiber to the adjacent fibers in thematerial when fibers start failing under excessive loads. This property of the matrix constituentcontributes to one of the most important characteristics of the fibrous composites, namely,improved strength compared to the individual constituent.

Woven (http://en.wikipedia.org/wiki/Woven_fabric) fabrics that are used in composites canbe grouped as two-dimensional (http://en.wikipedia.org/wiki/Dimension) (2-D) and threedimensional (3-D) structures. 2D-weaving (http://en.wikipedia.org/wiki/Weaving) is arelatively high-speed economical process. However, woven fabrics have an inherent crimp orwaviness in the interlaced yarns (http://en.wikipedia.org/wiki/Yarn), and this is undesirablefor maximum composite properties.

In 2D-structures, yarns are laid in a plane and the thickness of the fabric is small compared toits in-plane dimensions. Single layer designs include plain, basket, twill and satin weaveswhich are used in laminates. Two-dimensional woven fabrics are generally anisotropic, havepoor in-plane shear resistance and have less modulus than the fiber materials due to existenceof crimp and crimp interchange. Reducing yarn crimp in the loading direction or using highmodulus yarns improves fabric modulus. To increase isotropy, in-plane shear rigidity andother properties in bias or diagonal direction, triaxially woven fabrics are developed in whichthree yarn systems interlace at 60 angles as shown in Fig. 2. Othermechanical properties required in relation to different loading conditions are: throughthickness stiffness and strength properties, enhanced impact resistance, fatigue resistance,dimensional stability, fraction thickness, damage tolerance, and subtle conformability.



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In 3D-fabric structures (http://en.wikipedia.org/wiki/Fabric_structure), the thickness or Z-direction dimension is considerable relative to X and Y dimensions. Fibers or yarns areintertwined, interlaced or intermeshed in the X (longitudinal), Y (cross), and Z (vertical)directions. For 3D-structures, there may be an endless number of possibilities for yarn spacingin a 3-D space.


Fig. 2: Triaxial weaving

3-D fabrics are woven on special looms with multiple warp and/or weft layers. Fig. 3 showsvarious 3D-Woven structures. In polar weave structure, fibers or yarns are placed equally incircumferential, radial and axial directions. The fiber volume fraction is around 50%. Polar



weaves are suitable to make cylindrical walls, cylinders, cones and convergent-divergentsections. To form such a shape, prepreg yarns are inserted into a mandrel in the radialdirection.


5-Direction construction Polar weave Orthogonal(http://en.wikipedia.org/wiki/Orthogonality) weave

Fig.3: Schematics of various 3D-woven fabric structures for composites

Circumferential yarns are wound in a helix and axial yarns are laid parallel to the mandrel axis.Since the preform lacks the structural integrity(http://en.wikipedia.org/wiki/Structural_engineering), the rest of the yarns are impregnatedwith resin and the structure is cured on the mandrel. Polar weaves can be woven into nearnetshapes. A near-net shape (http://en.wikipedia.org/wiki/Near_net_shape) is a structure thatdoes not require much machining to eachthe final product size and shape. Since fibers are not broken due to machining, net shapesgenerally perform better than machined parts.

In orthogonal weave, reinforcement yarns are arranged perpendicular to each other in X, Y andZ directions. No interlacing or crimp exists between yarns. Fiber volume fraction isbetween 45 and 55 percent. By arranging the amount of yarn in each direction, isotropic oranisotropic preform can be obtained.

Except for the components that are fundamentally Cartesian(http://en.wikipedia.org/wiki/Cartesian_coordinate_system) in nature, orthogonal weavesare usually less suitable for net shape manufacturing than the polar weaves. Unit cell size canbe smaller than polar weaves which results in superior mechanical properties. Since no yarninterlacing takes place in polar and orthogonal structures, they are also referred to asnonwoven 3-D structures in the composites industry. However, it is more proper to labelthese structures as woven structures with zero level of crimp.

In angle interlock type of structures, warp (or weft) yarns are used to bind several layers ofweft (or warp) yarns together as shown in Fig. 4. In place of warp or weft yarns, an additionalthird yarn may also be used as binder. Stuffer yarns, which are straight, can be used to increasefiber volume fraction and in-plane strength. If the binder yarns interlace vertically betweenfabric layers, the structure is called orthogonal weave.





Fig. 4: Angle interlock fabric; (A) with and (B) without added stuffer yarns.Fig. 5: Schematic of Kings 3-D machine

Angle interlock or multi-layer fabrics for flat panel reinforcement can be woven on traditionallooms, mostly on shuttle looms. The warp yarns are usually taken directly from a creel. Thisallows mixing of different yarns in the warp direction. Other more complex 3D-Fabrics such aspolar and orthogonal weaves require specialized weaving machines. Several weavingmachines were developed to weave complex 3D-structures as illustrated in Fig. 5. Multilayerweaving into a three-dimensional preform consists of interlocking warp yarns in many layers.Whereas in conventional weaving all of the warp yarns are oriented essentially in one plane, inthe structure.

A typical step for weaving a multilayer preform includes two, three, or more systems of warpyarns and special shedding mechanism that allows lifting the harnesses to a many levels as thenumber of layers of warp yarns. By this weaving method, various fiber architectures can beproduced, including solid orthogonal panels, variable thickness solid panel, and corestructures simulating a box beam or truss-like structure.



The most widely used materials in 2D- or 3D-weaving are carbon/graphite, glass, and aramid.Any material that can be shaped as a fiber can be woven into preforms, more orless complicated. Woven preforms can be made of a single type of fiber material or differentfiber and yarn materials can be used as a hybrid structure. Due to the nature of wovenstructure geometry and weaving process, when selecting a fiber for weaving or for any othertextile manufacturing process, fiber brittleness and bending rigidity need to be considered. orexample, carbon and graphite fibers, which account for 90% of all 3D-woven preforms, areprone to break and fracture during weaving. Fig. 2.6 shows preform and composite samplesmade of carbon fibers.


Fig. 6: Woven 3D-preform and composite samples made of carbon fibers

Ref:-

Development of the Weaving Machine and 3D Woven Spacer Fabric Structures for LightweightComposites Materials- Book

By:-

Von der Fakultt MaschinenwesenderTechnischen Universitt DresdenzurErlangung des akademischen GradesDoktoringenieur (Dr.-Ing.)angenommene Dissertation



3d fabrics , Cartesian coordinate system , Composite material , Fabric Structure ,Fiber , fiber reinforcement , Near net shape , News , Orthogonality ,Technical Textiles , Textile , Weaving , Weft , Woven fabric , Yarn

This entry was posted on November 7, 2011, 11:38 AM and is filed under Composites, News.You can follow any responses to this entry through RSS 2.0. You can leave a response, ortrackback from your own site.

COMMENTS (1)

#1 by compositebuild on July 8, 2013 - 7:56 PM

Reblogged this on CompositeBuild.com and commented:Fabrics Used in Composite Materials

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