280 Chapter Four ■ Development of computer programs supported by test results to predict the physical changes that will occur during cure ■ Analytical techniques, such as rheometric dynamic scanning and differential scanning calorimetry, that give information on changes in viscosity and the amount of heat ab- sorbed or liberated during cure 4.7.4 Resin Transfer Molding Previous discussions have centered on moving resin out of the laminate to reduce voids. Resin transfer involves the placement of dry fiber reinforcement into a closed mold and then injecting a catalyzed resin into the mold to encapsulate the reinforcement and form a com- posite. The impetus for the use of this process comes from the large cost reductions that can be realized in raw materials and layup. The process can utilize low injection pressures, i.e., 55 MPa (80 psig); therefore, the tooling can be lower-cost plastic rather than metal. The process is most appropriately used for non-aerospace composites but has been extended to many advanced applications. RTM manufacturing considerations are shown in Table 4.27. The advantages of RTM include the possibility of producing very large (Fig. 4.17) and com- plex shapes efficiently and inexpensively, and reducing production times with the ability to include inserts in the composite. Effective for large structures such as boats, the SCRIMP™ process uses a vacuum bag on one side of the laminate instead of the two plates of a typical mold. Advantages quoted for this technique are one sided mold, better control of and higher fiber volume, and lower porosity in the composite. It has been used for structures that need higher quality than can be obtained by other RTM processes. Table 4.28 shows the range of applications for the RTM technique. RTM is also a way of preparing a composite structure from a knitted preform. Knitting and braiding and sewn tridimensionally reinforced pre- forms offer complex shapes that are not attainable by other techniques. The techniques can possibly lower costs due to reduction of labor. The product may also gain increased impact resistance due to the multiple, interlocked directions of fiber. 4.7.5 Fiber Placement Fiber placement was invented by Hercules Aerospace Co. Now, machines are marketed by Cincinnati Machines and Ingersoll in the U.S.A. The process is a cross between filament Figure 4.16 Time-temperature cure cycle for autoclave cure to prevent moisture-induced void growth. 28 Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Composite Materials and Processes 281 winding and automatic tape laydown, retaining many of the advantages of both processes. Fiber placement, the natural outgrowth of adding multiple axes of control to filament winding machines, results in control of the fiber laydown so that nonaxisymmetric sur- faces can be wound. This involves the addition of a modified tape laydown head to the fil- ament winding machine and other sub-machine additions that include in-process compaction, individual tow cut/start capabilities, a resin tack control system, differential tow payout, low tension on fiber, and enhanced off-line programming. Several manufac- turers now use slit prepreg tape rather than tow. The machines are capable of winding the shapes shown in Fig. 4.18 and can change fiber paths such as shown on Fig. 4.19. Table 4.29 32 shows the advantages quoted for the technique. The present disadvantages are the cost of the machines (very high when compared to some filament winding machines), the dependence on computers and electronics rather than mechanical means of directing fiber laydown, and the cost and complexity of mandrels. Table 4.27 RTM Manufacturing Considerations Materials Tooling Technology Fiber type Preform complexity Preform cost Inserts Mold material Mold surface finish Resin pump type Integral or oven heating Clamping method Tool durability Resin viscosity Flow modeling Gating and vent design Composite strength, stiffness, fiber volume, transverse properties Vacuum assist or not Figure 4.17 RTM (SCRIMP™) process for injecting a boat hull. (Copyright Billy B lack, courtesy of Seemann Composites) Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 282 Chapter Four 4.7.6 Filament Winding Filament winding is a process by which continuous reinforcements in the form of rovings or tows (gathered strands of fiber) are wound over a rotating mandrel. The mandrel can be cylindrical, round, or any other shape as long as it does not have re-entrant curvature. Special machines (Fig. 4.20) traversing a wind eye at speeds synchronized with the man- drel rotation, control winding angle of the reinforcement and the fiber lay-down rate. The reinforcement may be wrapped in adjacent bands or in repeating bands that are stepped the width of the band and that eventually cover the mandrel surface. Local reinforcement can be added to the structure using circumferential windings or local helical bands, or by the use of woven or unidirectional cloth. The wrap angle can be varied from low-angle helical to high-angle circumferential or hoop, which allows winding from about 4–90° relative to the mandrel axis for older mechanical machines; newer machines can place fi- ber at 0°. There are advantages and disadvantages to filament winding as compared to other methods. The most obvious advantages, summarized in Table 4.30, are cost savings (both capital and recurring labor) and the ability to build a structure that is larger than autoclave capacity. The disadvantages of filament winding, in most cases, can be worked around by innovative engineering and manufacture. Fabricators of large rocket motors have used plaster mandrels that can be stripped, reduced in size, and passed out through the relatively small port. Reverse curvature can be formed into a positive curvature by the addition of oriented fibers or mats or, if the curvature is necessary to the design, such as on an airfoil, it can be accomplished by removing the uncured structure from the mandrel and using al- ternate means of compaction to form the composite. This is the fabrication of a continuous multisided preform. Table 4.28 RTM Composite End Uses Composite use Part Industrial Solar collectors Electrostatic precipitator plates Fan blades Business machine cabinetry Water tanks Recreational Canoe paddles Large yachts Television antennas Snowmobile bodies Construction Seating Baths and showers Roofing Aerospace Airplane wing ribs Cockpit hatch covers Speed brakes Escape doors Automotive Crash members Leaf springs Car bodies Bus shelters Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Composite Materials and Processes 283 Newer filament winding machines have the capacity to change the wind angle at any point over the part surface. This gives the option of actually winding the fiber into a re- verse curvature by selecting the wind angle that will follow a hyperboliodal path into a smooth recess, without bridging or slipping. This technique has been used to wind in an in-situ metallic end ring for a composite-to-metal joint, eliminating the need for further bolting or pinning and providing a measure of fail-safe operation The fiber path can be al- tered by pins or sawtooth to avoid slipping or bridging. Mandrels are less expensive than two-sided molds, and their reusability contributes to the cost-effectiveness of filament winding. The expansion of the mandrel during the cure cycle provides the compaction necessary to result in a fully compacted, dense laminate, without external compacting measures such as autoclave. Filament wound laminates, without special curing conditions, can have void contents on the order of 3 to 7%. Because there is no external mold, there is a poor external or bumpy surface that can be improved by mild compaction as exerted by shrink tape. The poor external surface can be smoothed somewhat by proper selection of resin and fiber, use of surfacing mat or filled smoothing compounds at some weight pen- alty, or by compaction and cure in a female die mold using vacuum bag or autoclave pres- sure. Figure 4.21 shows the Beech Starship carbon/graphite epoxy fuselage that was filament wound then expanded into tooling during cure to form a smooth outer skin. Thermoset resins generally have been used as the binders for the reinforcements. These resins can be applied to the dry roving at the time of winding (wet winding) or applied pre- viously and gelled to a “B” stage as prepreg. The fiber can be impregnated and rerolled without B staging and used promptly or refrigerated. Prepreg and wet rerolled materials are useful because of the opportunity to perform quality control checks early. The cure of the filament wound composite is generally conducted at elevated tempera- tures without the addition of any process for composite compaction. The filament winding Figure 4.18 Versatility of shapes fabricated by fiber placement. (Courtesy of Hercules Aerospace Co., now Alliant Techsystems) Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 284 Chapter Four process, like pultrusion, can employ wet resin systems to result in potentially lower cost composite structures. Handling guidelines that are unique to wet filament winding for a wet resin system are: ■ Viscosity should be 2 PaS or lower. ■ Pot life should be as long as possible (preferably >6 hr). ■ Toxicity should be low. The other approach is to use a wet-rerolled system, essentially a wet resin that is applied to the fiber beforehand and then kept in a freezer until use. Table 4.29 Fiber Placement Processing Advantages (adapted from Ref. 32) Flexibility Compaction Material usage Full range of fiber orientations Non-geodesic path generation Constant ply thickness over com- plex shapes Localized reinforcements Continuous fibers over three- dimensional shapes (without joints) Large structures Continuous in-process debulking Complex surface fabrications: ■ Concave/convex ■ Nonaxisymmetric and axi- symmetric shapes Wide use of advanced thermoset material sys- tems Near prepreg tape equiv- alent Figure 4.19 Versatility of shapes fabricated by fiber placement. (Courtesy of Hercules Aero- space Co., now Alliant Techsystems) Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Composite Materials and Processes 285 Fibers are used in tow (carbon/graphite) or (roving) glass and Kevlar. The two terms de- fine a gathered parallel bunch of fibers with essentially no twist. The fibers that are made in individual processes (e.g., boron) have not been used extensively in filament winding applications due to their stiffness. 4.7.7 Pultrusion Braiding and Weaving 4.7.7.1 Pultrusion. Pultrusion is an automated process for the manufacture of con- stant-volume/shape profiles from composite materials. The composite reinforcement is continuously pulled through a heated die and shaped (Fig. 4.22) and cured simulta- neously. If the cross-sectional shape is conducive to the process, it is the fastest and most economical method of composite production. Straight and cured configurations can be fabricated with square, round, hat-shaped, angled I, or T-shaped cross sections from viny- lester, polyester, or epoxy matrices with E and S-glass, Kevlar and carbon/graphite rein- forcements. Some of the available cross-sectional shapes and limitations are shown in Table 4.31. 33 The curing is effected by combinations of dielectric preheating and micro- wave or induction (with conductive reinforcements like carbon graphite) while the shape traverses the die. The resin systems, predominantly polyester, can be wet or prepreg, but the cure rates will be much more rapid than for processes that use thermal conduction for heat transfer into the laminate. The process lends itself to long component lengths with the need for reinforcement in the 0° direction. Many uses have been found for the process; pultrusion is the primary fabrication technique for reinforced plastic booms, ladder com- ponents, light poles, boundary stakes for snow roads, and conduits. The primary reasons for the use of the technique are design considerations driven by commercial uses, such as cost, weight, electrical properties, and environmental resistance, but the process has also been used to produce some components from advanced composite materials. An applica- tion that uses the cost-effective technique for high-volume production is the pultruded car- Figure 4.20 Filament winding. (Courtesy of Plastrex) Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 286 Chapter Four bon/graphite-reinforced drive shaft that replaces two metal shafts, universal joints, and hangers and results in a quieter product because of inherent composite damping (Fig. 4.23). The process has some limitations: ■ The part must be of constant cross section over its length; it cannot be tapered. ■ Transverse strength will be somewhat lower than for other manufacturing methods. ■ Curved shapes require special machines. ■ Thick sections are difficult because of exotherm with rapidly curing resins. ■ Cure shrinkage may cause dimensional and mechanical problems. ■ There generally will be a cut edge, and this must be cut carefully to reduce delamination and then sealed to prevent moisture or other attack to fiber or interface. ■ Joints are more difficult because of fiber orientation and lack of section changes that would allow geometric locking. Table 4.30 Comparison of Filament Winding with Other Fiber Deposition, Compacting, and Curing Processes Advantages Filament winding is highly repetitive and accurate in fiber placement (from part to part and from layer to layer). It can use continuous fibers over the whole component area (without joints); can orient fibers eas- ily in load direction. This simplifies the fabrication of aircraft fuselages and reduces the joints. It avoids capital expense and the recurring expense for inert gas of autoclave. Large and thick-walled structures can be built—larger than any autoclave. Mandrel costs can be lower than other tooling costs. There is only one tool, the male mandrel, which sets the inside diameter and the inner surface finish. The outer surface is uncontrolled and may be rough. The cost is lower for large numbers of components, since there is less labor than in many other processes. Material costs are relatively low, since fiber and resin can be used in their lowest-cost form rather than as prepreg, and no preforming is necessary. (Preforming is necessary for RTM and may be a significant recurring expense.) Disadvantages The shape of the component must permit mandrel removal. Long, tubular shapes will generally have a taper. Different mandrel materials, because of differing thermal expansion and differing laminate layup percentages of hoops versus helical plies, will demonstrate varying amounts of difficulty in removal of the part from the mandrel. One generally cannot wind reverse curvature. To wind a reverse curvature, wind the exact shape on a positive dummy mandrel insert and then remove the insert and place the fiber. One cannot change fiber path easily (in one lamina). It can be done via the use of pins or slip of the tow. Fiber placement is the only fabrication method capable of “steering” the fiber. The process requires a mandrel, which sometimes can be complex or expensive. Usually, the mandrel is less costly than the dies or molds for forming methods other than pultrusion or RTM. Generally poor external surfaces are produced, which may hamper aerodynamics. A better outside surface can be obtained by ■ Use of outer clamshell molds ■ External hoop plies or thinner tows on last ply ■ Shrink tape or porous TFE-glass tape overwrap Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 287 Figure 4.21 Filament wound aircraft fuselage with smooth skin. (Courtesy of Fibertec Div. of Alcoa) Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 288 Chapter Four Figure 4.22 Schematic of pultrusion elements. (From Ref. 34) Figure 4.23 Carbon/graphite reinforced pultruded driveshaft (right) for GMT-400 trucks. (Courtesy of Spicer Div. of Dana Corp.) Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Composite Materials and Processes 289 Pultrusion has also been combined with filament winding to achieve high transverse prop- erties with advanced composite starting materials as shown in Fig. 4.24. 4.7.7.2 Braiding, weaving, and other preform techniques. Braiding, weav- ing, knitting, and stitching (with low- or high-modulus fibers) represent methods of form- ing a shape, generally referred to as a preform (a complete fiber layup representing the total laminate thickness with a small amount of resin or transverse fiber to hold it in place before resin infusion). The shape may be the final product or some intermediate form such as a woven fabric. New techniques allow prepregs to be used, and the introduction of three-dimensional braids has extended braiding to airborne structural components that meet high fracture toughness requirements with high damage tolerance. The braiding pro- cess is continuous and is amenable to round or rectangular shapes or smooth curved sur- faces, and it can transition easily from one shape to another. Resin systems are generally epoxies or polyester, and the fiber options are similar to those for filament winding; the single stiff fibers such as boron and ceramics cannot endure the tight bend radii. Composite Materials and Processes Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. [...]... Unidirectional graphite-epoxy Carbide PCD 4 .85 –7.92 4 .85 –7.92 0–12.7 0–12.7 42.7 61.0 0.0254–0.05 08 0.05 08 0. 088 9 Multidirectional graphite-epoxy PCD Carbide 4 .85 –7.924 85 –7.92 0–12.7 0–12.7 61.0 68. 6 0.0254–0.05 08 0.05 08 0. 088 9 Glass-epoxy HSS 3 10 33.0 0.05 Boron-epoxy PCD 6.35 6.35 2.0 25.4 91– 182 91– 182 25.4* Boron-epoxy PCD 6.35 10.4 79 41.91* Kevlar-epoxy Carbide 5.6 — 1 58 0.05 *Units = mm/min The precautions... Parameters for Sawing Polymeric Composites3 7 Material Graphite-epoxy Boron-epoxy Thickness, mm Cutting speed, m/s Feed rate, mm/min 4 0–25.4 2–12 15.24 50 .8 50 .8 Circular (HSS) Band saw (PCD) 2.0 25.4 30. 48 15.24 254 50 .8 Circular* (PCD) Band saw* (PCD) Type of saw *With coolant 4 .8 Analysis 4 .8. 1 Overview of Mechanics of Composite Materials The 1, 2, and 3 axes in Fig 4.29 are special and are called the ply... 4. 38 shows typical degradation techniques for fiberglass and Kevlar.47 Most of the effects of moisture are reversible, and the affected property will be restored when the composite is dried It may take a long time to dry, since the moisture must diffuse out Many users of composites require a systematic evaluation of the reaction of each composite (fiber and resin) to a broad list of environments Many of. .. to the Terms of Use as given at the website Composite Materials and Processes Composite Materials and Processes 293 Figure 4. 28 Typical composite drilling problem: pushout delamina- tion at exit (Abrate, S., in P.K Mallick, ed., Composites Engineering Handbook, Marcel Dekker, New York, 1997, p 783 ) degradation of the remaining composite A summary of some relevant parameters for drilling of several composite... Four Figure 4.26 Relationship between short fiber length and composite strength (After Hancock, P., and Cuthbertson, R.C., J Mat Sci., 5, 76–7 68, 1970) Figure 4.27 Typical composite drilling problem: peelup delamination at entrance (Kohkonen, K.E., and Potdar, N., in S.T Peters, ed., Handbook of Composites, 2nd ed., Chapman & Hall, p 5 98, London, 19 98) Downloaded from Digital Engineering Library @ McGraw-Hill... quasi-isotropic laminate For large structures, the relative cost of the filament winding is less than onefourth that of hand layup and less than one-half that of the best tape laying machine. 38 Use the least expensive form of composite raw materials Use a wet resin and dry fiber, if appropriate to the process This method reduces the cost of filament winding In Ref 38, the cost for the large MX launch canister wet filament... allowables Because of the possible numbers of permutations of resins, fibers, and curing techniques, it will be some time before standardized strength and modulii values can be published as is done now for most metallic structural materials 4.9 Design of Composite Structures 4.9.1 Composite Laminate Design The design process for composites involves both laminate design and component design and must also... use is subject to the Terms of Use as given at the website Composite Materials and Processes Composite Materials and Processes Table 4. 38 ■ ■ ■ ■ ■ ■ ■ 311 Possible Effects of Absorbed Moisture on Polymeric Composites Plasticization of epoxy matrix Reduction of glass transition temperature reduction of usable range Change in dimensions due to matrix swelling Enhanced creep and stress relaxation Increased... suppliers for composite raw materials, the numbers of permutations of resins, fibers, and manufacturers prevents the kind of standardization necessary to be able to buy composite raw materials as if they were alloys The fabricators of composites will rely on specifications for control of fiber, resin and/ or the prepreg as shown in Table 4.40 Many prepreg resin and fiber vendors will certify only to their own... subject to the Terms of Use as given at the website Composite Materials and Processes Composite Materials and Processes 309 Figure 4.36 Failure modes for bolted joints (From Nelson, W.D., Bunin, B.L., and Hart-Smith, L.J., in Proc 4th Conf Fibrous Composites in Structural Design, Army Materials and Mechanics Research Center, Manuscript Report AMMRC MS 83 -2, 1 983 , pp U-2 through II- 38) Figure 4.37 Recommended . material Hole dia., mm Material thickness, mm Cutting speed, m/min Feed rate, mm/rev Unidirectional graphite-epoxy Carbide PCD 4 .85 –7.92 4 .85 –7.92 0–12.7 0–12.7 42.7 61.0 0.0254–0.05 08 0.05 08 0. 088 9 Multidirectional graphite-epoxy PCD Carbide 4 .85 –7.924 85 –7.92 0–12.7 0–12.7 61.0 68. 6 0.0254–0.05 08 0.05 08 0. 088 9 Glass-epoxy. at entrance. (Kohkonen, K.E., and Potdar, N., in S.T. Peters, ed., Handbook of Composites, 2nd ed., Chapman & Hall, p. 5 98, Lon- don, 19 98) Composite Materials and Processes Downloaded from. data in handbooks and computer material property files as “preliminary” and should verify the necessary constants and failure properties of the composite materials and processes by subscale and full-scale