Fibers

Textile machines have been adapted to handle most of the fibers commonly used in structural composites, including glass, quartz, aramid, and carbon. The main limitation is that most textile processes subject yarns to bending and abrasion. Although machines have been modified to minimize fiber damage, in many processes, exceptionally brittle or stiff fibers will suffer significant strength degradation. In general, the higher the modulus of the fiber, the harder it will be to process, and the more prone it is to damage. Strength reductions can vary, depending upon the property being measured, and the textile process used to fabricate the preform. Polymeric sizings are usually applied to fibers to improve their handling characteristics and minimize strength degradation during processing. The sizings may be removed after processing, or left on the fibers for the lamination process. If the sizing remains on the fibers, it is important that it be compatible with the matrix resin. A surface treatment is also generally used to improve the adhesion between the fibers and the matrix.

In traditional textile processes, yarns are usually twisted to improve handling, structural integrity, and their ability to hold shape. However, twist reduces the axial strength and stiffness of the fibers, which is paramount in structural applications. Therefore, yarns with minimal or nominally zero twist (strands and tows) are preferred. Different processes and weaves require different strand or tow sizes. In general, the smaller the tow size, the more expensive the material will be on a per pound basis, particularly for carbon fiber.

7.11.2 Woven Fabrics

Woven fabrics are available as two-dimensional (2-D) reinforcements (x- and y-directions) or three-dimensional (3-D) reinforcements (x-, y-, and z-directions). When high in-plane stiffness and strength are required, 2-D woven reinforcements are used. As pointed out earlier, 2-D woven products can be supplied as either a prepreg or as a dry cloth for either hand lay-up, preforming, or repair applications. Two-dimensional weaves have the following advantages: (1) they can be accurately cut using automated ply cutters; (2) complicated lay-ups with ply drop-offs are possible; (3) there are a wide variety of fibers, tow sizes and weaves that are commercially available; and (4) 2-D weaves are more amendable to thinner structures than 3-D weaves.

Three-dimensional reinforced fabrics are normally used to (1) improve the handleability of the preform, (2) improve the delamination resistance of the composite structure, or (3) carry a significant portion of the load in the composite structure, such as a composite fitting that would be subject to complex load paths and major out-of-plane loading. If improved handlablity is the objective, usually z-direction fiber volumes as low as 1-2% will suffice. Improvements in the delamination resistance of composite structures can be obtained with as little as 3-5% z-direction fiber; however, as the amount of z-directional fiber is increased, the delamination resistance and durability increases.26 If the application calls for major out-of-plane loading, as much as 33% z-direction fiber reinforcement may be required. The fibers will then be arranged with roughly equal load-bearing capacity along all three axes of a Cartesian coordinate system. However, it should be recognized that when the volume of reinforcement in the z-direction is increased, the volume percentages in the x- and y-directions will be decreased.

Historically, composite designs have been restricted to structure that experiences primarily in-plane loading, such as fuselages or wing skins. One of the key reasons for the lack of composite structures in complex substructure, such as bulkheads or fittings, is the inability of 2-D composites to handle complex, out-of-plane loads effectively. The planar load-carrying capability of composites is primarily a fiber-dominated property, requiring stiff and well-defined load paths. Unfortunately, the planar loads must ultimately be transferred through a 3-D joint into adjacent structure (e.g., a skin attached to a bulkhead). These 3-D

joints are subject to high shear, out-of-plane tension, and out-of-plane bending loads, all of which are matrix property dominated properties in a traditional composite design. Since loading the matrix with large primary loads is a totally unacceptable design practice, metallic fittings are used to attach composite structure to metallic bulkheads with mechanical fasteners. With the introduction of high performance 3-D textiles, both woven and braided, this barrier to composite designs has the potential to be eliminated. While 3-D woven preforms show potential for being able to fabricate complex net-shaped preforms, the set-up time is extensive and the weaving process is slow. In addition, small tow sizes that generally increase cost must be used to achieve high fiber volume percents, and eliminate large resin pockets, that are susceptible to matrix microcracking during cure, or later when the part is placed in-service.

7.11.3 Multiaxial Warp Knits

Knitting can be effectively used to produce multiaxial warp knits (MWKs), also called stitch bonding, that combines the mechanical property advantages of unidirectional tape with the handling advantages and low cost fabrication advantages of fabrics. MWKs, as shown in Fig. 7.44, consist of unidirectional tows of strong, stiff fibers woven together with fine yarns of glass or polyester thread. The glass or polyester threads, which normally amounts to only 2% of the total weight, serve mainly to hold the unidirectional tows together during subsequent handling. An advantage of this process is that the x- and y-tows remain straight and do not suffer as much strength degradation as woven materials, in which the tows are crimped during the weaving process.

The multiaxial warp knit process is used to tie tows of unidirectional fibers together in layers with 0°, 90°, and ±0° orientations. During knitting, the polyester threads are passed around the primary yarns, and one another, in interpenetrating loops. Selecting the tow percentage in each of the orientations can be used to tailor the mechanical properties of the resulting stack. The MWK stacks form building blocks 2-9 layers thick that can be laminated to form the thickness desired the structure. Multiple layers of MWKs are often stitched together in a secondary operation to form stacks of any desired thickness and can be stacked, folded, and stitched into net shapes. The stitching operation also greatly improves the durability and damage tolerance of the cured composite. MWK has the advantages of being fairly low cost, having uniform thicknesses, can be capable of being ordered in prefabricated blanket-like preforms, and being very amendable to gentle or no contour parts, such as large skins.

7.11.4 Stitching

Stitching has been used for more than 20 years to provide through-the-thickness reinforcement in composite structures, primarily to improve damage tolerance.

Fig. 7.44. Multi-axial Warp Knitting Machine and Typical Product Form Produced24

The major manufacturing advancement in recent years has been the introduction of liquid molding processes which allows stitching of dry preforms (Fig. 7.45), rather than prepreg material. This enhances speed, allows stitching through thicker material, and greatly reduces damage to the in-plane fibers. As well as enhancing damage tolerance, stitching also aids fabrication. Stitching provides a mechanical connection between the preform elements before the resin is introduced, allowing the completed preform to be handled without shifting or damage. In addition, stitching compacts (debulks) the fiber preform closer to the final desired thickness. Therefore, less mechanical compaction needs to be applied to the preform in the tool. Various stitching materials have been successfully used, including carbon, glass, and aramid, with Kevlar 29 (aramid) being the most popular. Yarn weights for Kevlar of between 800 and 2000 denier have been used. However, one disadvantage of aramid is that it absorbs moisture and can sometimes exhibit leaks through the skin at the stitch locations.

7.11.5 Braiding

Braiding is a commercial textile process dating from the early 1800s. In braiding, shown in Fig. 7.46, a mandrel is fed through the center of the machine at a uniform rate, and fiber yarns from moving carriers on the machine braid over the mandrel at a controlled rate. The carriers work in pairs to accomplish an

over/under braiding sequence. Two or more systems of yarns are intertwined in the bias direction to form an integrated structure. Braided preforms are known for their high level of conformity, torsional stability, and damage resistance. Either dry yarns or prepregged tows can be braided, with typical fibers including glass, aramid, and carbon. Braiding normally produces parts with lower fiber volume fractions than filament winding but is much more amendable to intricate shapes. The mandrel can vary in cross-section with the braided fabric conforming to the mandrel shape. The total thickness of a braided part can be controlled by overbraiding, in which multiple passes of the mandrel are made through the braiding machine, laying down a series of nearly identical layers, similar to a lay-up. Possible fiber orientations are ±6° or 0°/ ± 6° with no 90° layers unless the braider is fitted with filament winding capability. State-of-the-art braiding equipment provides full control over all of the braiding parameters, including translational and rotational control of the mandrel, vision systems for in-process inspection, laser projection systems to check braid accuracy, and even integrated circumferential filament winding.

Due to the material conformity inherent in a braided product form, braided "socks" can be removed from the braiding mandrel and formed over a mandrel of a different shape for curing. Cutting the cylindrical sheet from the mandrel and stretching it out flat can form a flat braided sheet. In other situations, the braided part is cured directly on the mandrel. Permanent, water soluble, or breakout mandrels can be used. Fixed, straight axial yarns (0°) can also be introduced at the center of orbit of the braider yarn carriers. The braider yarns lock the axial yarns into the fabric, forming a triaxial braid, i.e. a braid reinforced in three in-plane directions.

Three-dimensional braiding can produce thick, net section preforms, in which the yarns are so intertwined that there may be no distinct layers. 3-D braided socks can also be shaped into a preform suitable for use in joints and stiffeners. Due to the nature of braiding, a bias or 45° fiber orientation is inherent in the braided preform and, theoretically, allows the preform to carry high shear loads without the necessity of 45° hand layed-up overwrap plies. A 3-D braiding machine can be set-up to produce a near net shape to the cross-section of the final part. The disadvantages of 3-D braiding are similar to 3-D weaving, i.e. complicated set-ups and slow throughput. Again, resin microcracking can be a problem with maximum fiber volumes of 45-50% obtainable.

7.11.6 Preform Handling

Since the stiffness and strength of polymeric composites are dominated by the reinforcing fibers, maintaining accurate positioning of the fibers during all steps of manufacturing process is paramount. Poor handling and processing after preforming can destroy fiber uniformity. Uncontrolled material handling, draping the material over curved tools, debulking, and tool closure can spread or distort the fibers. Manufacturing prove-out parts should be examined to establish that the minimum fiber volume fractions have been obtained, with particular attention paid to geometric details such as joints. The problem of maintaining the desired fiber content is most challenging when fabrics are draped. The draping characteristics of a fabric over a singly curved surface are a direct function of the shear flexibility of the weave. Satin weaves have fewer crossover points than plain weaves, and have lower shear rigidity, and are therefore more easily draped. Draping over a complex compound contour also depends on the inplane extensibility and compressibility. This is difficult for fabrics containing high volume fractions of more or less straight in-plane fibers, as required for most structural applications. For these products, only mild double curvature can be accommodated by draping without a significant loss of fiber regularity. However, compound contours can be achieved through net shape processes, such as braiding onto a mandrel, thus avoiding the problems of draping.

There are several reasons that preforming is conducted prior to the injection process. First, preforming does not tie-up the expensive matched die tool, i.e. the tool can be used to cure parts while the preforming operations are done ahead of time and off-line. Second, a well-constructed preform will be rather stiff and rigid as opposed to laying-up loose fabric directly into the mold. Therefore, preforming improves the fiber alignment of the resultant part and reduces part-to-part variability.

Planar fabric preforms can be stitched together or held together with a tackifier. A tackifier is usually an uncatalyzed thermoset resin that is applied as a thin veil, a solvent spray, or a powder. Veils can be placed between adjacent plies of fabric, followed by fusing the ply stacks with heat and pressure, to form the preform. Tackifiers can also be thinned with a solvent and then sprayed on the fabric plies. A third method is to apply powders to the surface followed by heating to melt the powder and allowing it to impregnate the fabric. Tackified fabric can be thought of as a low resin content prepreg (usually in the range of 4-6%) that can be made into ply kits using conventional automated broadgoods cutting equipment. It is important to keep the tackifier content as low as possible, because it reduces the permeability of the preform and makes resin filling more difficult.27 It is also important that the tackifier and the resin to be injected are chemically compatible, preferably the same base resin system.28 Once the tackifier has been applied to the fabric layers, they are formed to the desired shape on a low cost preforming tool, and then heat-set by heating to approximately 200° F for 30-60 s. The compaction behavior of a preform depends on the preform method used, the type of reinforcement, the tackifier used, the compaction pressure, and the compaction temperature. A tackifier can act as a lubricant and increase compaction, but this will also decrease the preform permeability and make injection more difficult. For any preform construction, it is important that the preform be dried prior to resin injection to remove all surface moisture that may have condensed on the surface from the atmosphere.

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