Volume 21 - Composites Part 8 doc

unidirectional laminates since the latter have low resistance to delamination crack growth during and after impact. The lay-up attributes of woven prepregs are: • Thicker (therefore fewer) layers and faster lay-up rate • Much higher curvature conformability and hence lower susceptibility to wrinkling • Greater material width of 1.25 or 1.7 m (4.1 or 5.6 ft) compared to 0.3 or 0.6 m (1 or 2 ft) for tape prepreg. (Tape prepreg is narrow since it has low conformability, and materials waste is high for wide tape.) • Lay-up rates are therefore approximately 3 to 5 times higher than for unidirectional tape. • No requirement to butt strip edges since fabrics are wider than the parts • Less-precise ply orientation is required since the lay-up is less optimized; lay-up can therefore be faster. Manufacturing disadvantages of woven prepregs are: • Higher proportion of waste from the wider material • Higher cost of low-thickness fabric prepreg since the weaving process preceding prepregging is an added cost. Thicker woven prepreg with a fiber areal weight (FAW) of 370 g/m 2 has become standard since the weaving cost is around half that of the conventional 285 g/ m 2 fabric. These thick prepregs confer reduced stiffness to the resultant components. As a result of the manufacturing-cost benefits of woven prepregs, they are used predominantly for hand lay-up, apart from very lightweight- niche applications. Unidirectional tape lay-up is better suited to automated tape layers that can rapidly cut and deposit material, provided the lay-up is flat enough (see the article “Automated Tape Laying” in this Volume). Recently, thicker unidirectional tape prepreg has been qualified for aircraft use so as to increase laminating rate of thick structures. However, the resulting restriction on thickness tailoring prevents the use of thick prepreg in many structures. The other lay-up characteristics are resin tack and conformability of fabric style. These both determine the difficulty of manipulation of prepreg into tool recesses. For parts with shape complexity, a highly drapable, high-tack resin is preferred to produce a fully consolidated lay-up. For flat or single curvature parts, a less drapable fabric such as plain weave with a low tack (stiff) resin is better suited. Placement Tolerance. Since hand lay-up is a craft skill using floppy materials, the placement tolerance cannot be specified very closely. The acceptable tolerance differs for woven and tape materials. For tapes, which are much stiffer and applied in strips of typically between 150 and 600 mm (6 and 24 in.), a positional tolerance of ±1 mm (±0.04 in.) and a straightness tolerance of ±2° can be realistically achieved. For woven prepreg, tolerances of ±2 mm (±0.08 in.) for position and ±3° for straightness are realistically achievable. Application Suitability. A great range of unidirectional and woven prepreg types have been developed to suit diverse applications. The original prepregs were developed for very highly optimized components in aerospace engines, and similar styles of very thin (0.125 mm, or 0.005 in., ply) prepregs are in use today in large volumes. The fighter aircraft and racing car markets use tape and woven prepregs made from very high-cost narrow tow fiber that provides laminate moduli up to 240 GPa (35 × 10 6 psi) for tape and up to 130 GPa (19 × 10 6 psi) for woven fabrics. Resins to suit these high- performance fibers have complex formulations tailored either for toughness or temperature resistance but have similar lay-up attributes to long established low-cost resins. These thin materials naturally have a low hand-deposition rate, but the labor cost represents a small proportion of the overall manufacturing cost. For low-volume production of thin structures, the manufacturing cost is dominated by mold tooling and assembly costs. Over the past ten years there has been a rapid growth in the use of standard high-strength carbon tapes and fabrics. For performance cars, commercial aircraft, and sporting goods use, two standard prepregs have been established: thick unidirectional tape with a fiber weight of 270 g/ m 2 and five-harness satin woven fabric with a fiber weight of 370 g/m 2 . The use of prepreg thickness above these levels is not normally considered to be worthwhile, since the lay-up sequences needed to achieve balanced and therefore unwarped laminates result in a low level of thickness optimization. Non-weight-critical applications such as wind turbines and lower cost sporting goods generally use glass fiber prepreg at as high a thickness as can be readily handled. For this reason thick unidirectional prepregs of up to 500 g/m 2 FAW and woven (and now multiaxial) fabric prepregs of up to 1000 g/m 2 FAW are being produced. The resin-content and void-level specifications are looser for such materials, which, combined with the high fiber weight, enable prepreg manufacture at up to 16 kg/min. The prepreg production cost is therefore very much lower than that for traditional thin prepregs. Prepreg hand lay-up is well suited to all applications for structures where a stiffness of greater than around 15 GPa (2.2 × 10 6 psi) is required. Below this stiffness, components can be manufactured with far lower labor cost by low fiber volume fraction processes such as chopped fiber, spray up and wet lay-up with heavy (>1 mm, or 0.04 in., thick) fabrics. The process is also uneconomic for simple- shape components of greater that several millimeters thick where more than one component per week is required. For components that have these factors, automated lay-up becomes attractive. However in lower economies, hand lay-up is still preferred for large, thick simple parts. Manual Prepreg Lay-Up Andrew Mills, Composites Manufacturing Research Centre, Cranfield University, United Kingdom Technique Description The process of lay-up definition through to bagging for resin-curing comprises the following five stages: lay-up definition, ply-kit cutting, lay- up, debulking, and preparation for curing. Lay-Up Definition. The lay-up of a component is defined by the: • Overall shape produced by the mold tool curvature • Thickness in terms of the number of layers over the surface • Ply outlines (drop offs) if the thickness is varying • Orientation to suit the load paths For most lightweight components, the lay-up instructions will be produced from a finite-element-analysis model of the component. The model will have the simulated design limit load introduced to the lay-up. The thickness and ply orientations are then modified until all regions of the component are shown to have less than maximum allowable strain in each ply. For structures with complex shape and/or loading, the specified lay-up is generally quasi-isotropic, meaning that there is an equivalent number of 0, 90, 45, and 135° plies. This is also preferred since it removes any complication of resin shrinkage symmetry. A so-called balanced lay- up will have a balanced or symmetric proportion of fibers at each angle about a midplane. This is critical for unidirectional tape materials but also important for satin and twill-weave fabrics; plain-weave fabrics are immune to lay-up imbalance but have lower drapability and stiffness than the former types. The next step is to decide the size and shape of each prepreg piece. To minimize the number of pieces, software tools such as FiberSIM (VISTAGY Inc., Waltham, MA) were developed. These are used to assess the tool shape where prepreg pieces are to be positioned and, using data on the material drapability (ability to be sheared to conform to double curvature), indicate whether prepreg pieces are likely to wrinkle. After one or more iterations, a kit of pieces and their orientations are defined (Fig. 1 and 2). Fig. 1 FiberSIM model of woven-ply draping into fairing tool. Red zones (indicated by arrows) are areas of predicted fiber wrinkling. Courtesy of VISTAGY Inc. Fig. 2 FiberSIM model of woven ply draping into fairing tool after applied ply cuts. The shape on the right is the predicted flattened ply shape to be cut. Courtesy of VISTAGY Inc. Ply-Kit Cutting. The target for the cutting operation is to minimize waste as much as possible. Purchase cost and disposal cost is extremely high, even for low-glass prepregs. Off- cuts can represent a hidden cost, which can result in the manufacturing process being unjustifiable. Software applications such as FiberSIM have been developed to minimize cutting waste from the prepreg roll. The software is used to match the total kit of plies to the material-roll width and to define the cutter paths. Users of large quantities of prepreg use an automated device that cuts the material and, in some models, stamps a bar code or number on it to identify the piece from CAD data. Ultrasonic machines (Fig. 3) using a vibrating knife are able to leave the lower surface backing film uncut, which reduces lay-up time during laminating. Manually, the pieces are stacked in order for lay-up. These kits may be sealed and stored in a freezer if a delay is incurred before use. Fig. 3 Ultrasonic prepreg ply cutting machine. Courtesy of GFM (United Kingdom) Lay-Up. The difficult part of the process is applying the reinforcement, any stiffening cores, and attachment inserts to the mold tool so as to confer the inherent stiffness or strength of the fibers to the molded component. The kit of prepreg pieces is transferred to the mold tool by laminators, who use their fingers and spreading tools to force the tacky, stiff material into the corners of the tool and then smooth it over the flat or gently curving areas. For complex-shape parts such as racing car chassis, the backing film is peeled away progressively to prevent too much of the surface of the pieces from adhering too soon. Hot air blowers are sometimes used locally to soften the prepreg such that it can be conformed into tight recesses. Even with a fully precut kit, the laminator has to trim plies with a blade at the component edges since, for double curvature components, each layer of prepreg is unique in terms of how the plies shear (Fig. 4) (Ref 1). Sandwich structures, which include tapered edge rigid foam or honeycomb core pieces and any attachment inserts, can be placed directly into the lay-up, or placed into the lay-up with uncured film adhesive; in both instances the sandwich structures are cocured with both inner and outer skin. For accurate location of the core and attachment point inserts, the lay- up is cured three times; once for the outer (tool face) skin, once to bond the core and inserts with film adhesives, and once for the inner (bag face) skin. Fig. 4 Lola BMS-Ferrari Formula 1 car monococque, manufactured by hand lay-up of woven carbon fiber prepreg. Courtesy of Nigel Macknight, Motorbooks International Listed are some essential stages or features of the lay-up process to achieve acceptable quality moldings: • The mold tool must be suitably treated with a release agent to prevent bonding during cure. A solvent or (now increasingly for health and safety reasons) a water-based formulation is wiped onto the tool with a cloth. One coating is applied to each molding and three or more layers to a new or repaired tool. • The prepreg must be neither too tacky to be “unrepositionable” (since complex-shape pieces need to be applied in stages) nor too dry such that it will not adhere to the tool or the lay-up. The tack level is dependent on the resin formulation itself, its out-life (the resin becomes harder with time at room temperature), and the lay-up room temperature. • No bridging of prepreg can occur across tool corners such that during cure, the bagging materials fully compress the prepreg to the complete surface of the tool with no air pockets or resin filled corners. • No air pockets can be trapped between layers since these may remain throughout the lay-up and cure resulting in cracking between layers. • No wrinkling or folds can be introduced since the stiffness and strength of the component is dependent on the fibers being as straight as possible along the main load paths. Wrinkles will also act as stress concentrations and may cause failure below design-limit strain. • Nothing can be allowed to contaminate the lay-up such as backing films, grease, insects, and litter. Any inclusion may prevent bonding, cause wrinkling, or produce gas during cure. It is exceptionally easy to leave pieces of thin polythene-backing film between layers. They are frequently brightly colored to help avoid this. Many inclusions are undetectable by nondestructive examination and may become partly bonded. Evidence of an inclusion can possibly only be detected through catastrophic disbonding in service. Such mistakes may be expensive, particularly with aircraft primary structure or space programs. Ply Orientation and Position. In spite of the tacky nature of the prepreg and the complexity of many tool shapes, a laminator has to maintain the ply orientation and edge position. The criticality of this depends on the maximum working strain of the component, the area of structure, and the tooling approach used. Fortunately, there is usually an obvious inverse correlation between shape complexity and normal working strain. Highly loaded parts or areas of components are usually close to being flat and straight. The most complicated parts do not normally work at very high strain. The tooling approach is important because some critical components, such as wing skins, match ply edge positions (ply drops) to steps in tooling. This ensures that there is no resin- rich bead or possible void at ply edges. To allow the laminator to reach an acceptable deposition rate, two visual techniques are used to show where the prepreg piece edges should be positioned: foil templates and laser projection. Before the introduction of laser projection, for components with critical lay-up, ply-drop positions, the laminator needed to apply a foil template over the tool and then over each applied layer and then mark the next ply-edge positions using a noncontaminating marker pen. The laminator starts lamination by laying each ply following the marked most critical edge and working outward to the component edge, trimming any excess. Laser projection is a clever, yet essential and most effective innovation that greatly reduces lay-up time and improves quality. Instead of a laminator following a drawn outline, a laser and mirror device causes very rapid precession of a laser point around the ply outline, which produces a static, bright red line. The line is produced by a suspended laser projector connected to a personal computer, which converts ply outline data with data on the tool curvature to provide the true ply edge (Fig. 5). Fig. 5 Laser ply outline projection system in use on aircraft wing and fuselage fairing tool. Courtesy of Assembly Guidance Systems Debulking. An unfortunate result of the nature of high-quality prepreg is the inevitability of air entrapment between layers. Even after visible air pockets have been forced out, very thin pockets of air can remain. If these are not removed before the curing process, the resulting laminates can have entrapped air bubbles. If the concentration of bubbles or voids is high enough, the laminate is vulnerable to matrix cracking and delamination. A process known as debulking is used to remove entrapped air. A reusable nylon, natural rubber, or silicone- rubber membrane is sealed around the tool periphery over the lay-up with a fabric breather cloth placed in between and a vacuum applied to the lay-up. The lay-up becomes compressed and, during a period of around 30 min, the layers are squeezed more tightly together and air removed. This process is carried out between every 0.5 and 2 mm (0.02 and 0.08 in.) of lay-up thickness. Although this step detracts from process efficiency, the laminator can use the interruption to organize documentation and materials. The debulking process has a secondary benefit resulting from the additional compaction. After the debulking stage, the lay-up is consolidated to a thickness very close to that of the finished laminate. Consequently, when the fully laminated component is cured in an oven or autoclave, the outer plies should remain unwrinkled. Without debulking stages, the outer plies tend to wrinkle as the lay-up underneath compresses (Fig. 6). Fig. 6 Debulking of racing car monococque lay-up. Courtesy of Nigel Macknight, Motorbooks International Preparation for Curing. When the lay-up is complete and checked, it needs to be sealed such that it can be compressed and cured by the specified pressure and temperature cycle. This varies from vacuum only (oven) cure with 120 °C (250 °F) temperature applied for 1 or 2 h for non- weight-critical parts to autoclave cure with typically 5 bar (500 kPa) pressure with a carefully determined temperature-profile application lasting for 5 h or longer for critical parts such as airframe structure. Prepregs for vacuum (oven) cure have a slightly higher resin content than for high- (autoclave-) pressure cure; the laminate fiber volume fraction for woven-prepreg oven- cured laminates is approximately 54%. For applications that can tolerate the high cost of the consumable materials, four layers of material are applied to the lay-up: • Peel ply (woven polyester fabric, sometimes with a corona-discharge electrical treatment to ease removal): to provide a uniform surface that protects the surface during subsequent operations prior to bonding • Release film with small holes (“pin pricked” thin film): to allow air and volatiles to escape from the lay- up upper surface. Release films with perforations encourage resin removal (bleeding), whereas types without holes prevent bleeding. • Breather cloth (polyester fiber wadding): to carry air and volatiles to be expelled through a vacuum pump • Vacuum bag (nylon film) with tacky rubber sealant gasket: to seal the lay-up from the oven or autoclave hot air This is a most difficult and costly process for both labor and materials. The total consumable cost varies from around $15/m 2 to $60/m 2 , depending on the temperature and pressure applied. The vacuum-bag application is particularly difficult since for double curvature parts or those with raised details or tooling flanges, the bag needs to be folded with sealant tucks applied. Bag failures are common with less experienced operators. Consequently, where tooling budgets allow, custom silicone rubber bags are manufactured. These bags are made from 3 to 5 mm (0.12 to 0.20 in.) thick tough rubber that is bonded to a frame; the rubber can be stretched over the component surface by the applied vacuum. Their cost is in the order of $145/m 2 to $715/m 2 of tool surface, depending on the size and complexity. To reduce cure preparation time and the risk of puncture, very tough and “high elongation” consumable bagging films have recently been introduced. Although preparation for cure appears to be a very complex and costly process, it improved with the introduction of nil-bleed prepregs in the 1980s. Prior to these, specific volumes of excess resin would be bled out of the lay-up into glass fabrics. These had to be applied in one or several layers between the peel-ply and release-film layers. Prepregs are now reliably produced with a highly controlled resin content of typically 34 ± 1% by weight. Figure 7 (Ref 2) shows a cured, demolded, and trimmed Formula 1 car chassis, upper half. Figure 8 shows the completed car of which all of the structure apart from the engine and gearbox is composite, predominantly manufactured using 120 °C (250 °F) curing epoxy- resin and woven intermediate-modulus (IM) fiber prepreg. Fig. 7 Autoclave molded Lola Formula 1 car chassis upper half. Courtesy of Nigel Macknight, Motorbooks International Fig. 8 Lola BMS Ferrari Formula 1 car. All structure, including wings, fairings, and monococque, is molded by hand lay-up of woven prepreg and autoclave cured. Courtesy of Nigel Macknight, Motorbooks International References cited in this section 1. Nigel Macknight, The Modern Formula 1 Race Car, Motorbooks International, 1993, p 88–100 2. T.G. Gutowski, Ed., Advanced Composites Manufacturing, Wiley-Interscience, 1997, p 207–239 Manual Prepreg Lay-Up Andrew Mills, Composites Manufacturing Research Centre, Cranfield University, United Kingdom Component Properties Over the history of composite structures, prepreg hand lay-up has been used to mold a great diversity of parts. Extremes of sewage tanks to satellite solar array supports and truck leaf springs to Formula 1 engine air inlet trumpets and fuel injector tubes are examples. These diverse applications have had materials specifically tailored to provide extremes of performance. For instance aramid fibers in conjunction with resins with low-fiber adhesion can provide laminates that are impenetrable to low- velocity bullets. Space satellite structures are optimized for extreme low weight and just enough robustness to reliably survive launch vibrations; such structures can have laminate stiffnesses of up to 280 GPa (41 × 10 6 psi), more than double that of standard carbon fiber unidirectional laminates. One limitation with current polymer prepreg matrix resins is a maximum service temperature of around 270 °C (520 °F). Non- polymer-matrix materials are not amenable to prepreg hand lay-up since they are not tacky. The prepreg hand lay-up process can use all types of reinforcement fiber in tape or fabric form. Fibers range in stiffness from E-glass, providing laminates with tensile moduli up to 42 GPa (6 × 10 6 psi), to ultrahigh-modulus pitch- based carbon, providing laminates with tensile moduli up to 490 GPa (71 × 10 6 psi) (Fig. 9). Any resin can be used that is capable of being formulated to provide a high viscosity such that the prepreg has tacky [...]... Engineered Materials Handbook, Composites, Composites, x x y- z zc- a- ac- a- a- uv q d d- e v- ed- x- y- qe- v- d- q-e- Advanced Composites III: Expanding the Technology, x-, y-, z-,c-, a-, d-, e-, q-, v-, q-, v-, d-, u- ed e • • • • • • • High-Performance Composites Engineered Materials Handbook, Engineered Materials Handbook, Composites, Composites, High Perform Compos., Advanced Composites III: Expanding... reduction compared to prepreg hand lay-up, prepreg lay- up is not being replaced for low -volume applications except for low-surface-area parts with extreme lay-up complexity Very successful examples where resin injection molding of dry preformed fabrics have been developed for propeller-blade molding by Dowty aerospace propellers in England; for sine wave spars and engine-intake ducts for fighter aircraft... advance fiber placement by implementing it on a F/A- 18 E/F fuselage skin (Fig 3) The program implemented at Northrop Grumman realized a labor savings of 38% when compared to hand lay-up Northrop Grumman is also using fiber placement for inlet duct skins, side skins, and covers for the F/A- 18 E/F Fig 3 Fiber placement of the Northrop Grumman F/A- 18 E/F fuselage skin Fiber placement is also being used... 4.1 1.0 Maximum lay-up rate kg/h lb/h 2 1 10 5 10 5 15 7 … … 2 1 2 1 6 3 9 4 1 0.5 Approximate cost $/kg $/lb 30 14 25 11 15 7 5 2 … … 40 18 40 18 50 23 35 16 70 32 Damage resistance Medium Very high Medium High Extremely high Low Very low Medium Medium Very low Woven Epoxy 120 250 0.29 0.011 1.7 5.6 4 2 105 48 Medium High-modulus carbon UD Epoxy 180 360 0.13 0.005 0.3 1.0 1 0.5 85 39 Extremely low... Helicopters A U.S government- funded program was conducted by Boeing and Hercules to develop the design and process for fiber placing the aft fuselage for the Bell/Boeing V-22 Osprey This part was designed to take advantage of the unique capabilities of fiber placement The first four prototype V-22 aft fuselages were made from nine hand-laid sections Switching to single-fiber-placed monolithic structure... coupled with the very low manufacturing cost of the fabric Manual Prepreg Lay-Up Andrew Mills, Composites Manufacturing Research Centre, Cranfield University, United Kingdom References 1 Nigel Macknight, The Modern Formula 1 Race Car, Motorbooks International, 1993, p 88 –100 2 T.G Gutowski, Ed., Advanced Composites Manufacturing, Wiley-Interscience, 1997, p 207–239 3 D.H Middleton, Ed., Composite Materials... this is E- glass fiber/epoxy resin wind turbine blades The thick (typically 600 g/m2) stitched multiaxial fabric in a very tacky resin is able to conform to the blade curvature and to the root section where it joins the hub For low -volume, high-performance applications such as rocket launchers with skin features for attachment points, the lay-ups are complex and are provided by unidirectional-tape prepreg... component curvature Table 1 Prepreg types and lay-up characteristics Fiber type Form(a) Resin type E-glass UD Woven Woven Multiaxial Woven UD UD Woven Multiaxial UD Epoxy Epoxy Phenolic Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Aramid Carbon Intermediate-modulus carbon Cure temperature °C °F 120 250 120 250 135 275 120 250 120 250 120 250 180 360 120 250 120 250 180 360 Typical thickness mm in 0.25 0.010... and Boeing will become further established (see the article “Fiber Placement” in this Volume) There are two apparent trends for prepreg manufacture to further reduce the cost of hand lay-up: • • Scale up of production with low-cost carbon fiber to enter new markets such as low volume production cars, trains, larger wind-turbine blades, and infrastructure repair Companies such as Zoltech and Hexcel are... very repeatable and maintains a tighter tolerance than the hand lay-up process Because of this, QA personnel closely scrutinize the first production part to make sure that it meets all of the design requirements If the part program builds a part that meets all of the design requirements, it is considered “bought off.” As long as the part program is not changed, QA personnel needs to do only periodic . reduction compared to prepreg hand lay-up, prepreg lay- up is not being replaced for low -volume applications except for low-surface-area parts with extreme lay-up complexity. Very successful examples. Motorbooks International, 1993, p 88 –100 2. T.G. Gutowski, Ed., Advanced Composites Manufacturing, Wiley-Interscience, 1997, p 207–239 Manual Prepreg Lay-Up Andrew Mills, Composites Manufacturing. process of lay-up definition through to bagging for resin-curing comprises the following five stages: lay-up definition, ply-kit cutting, lay- up, debulking, and preparation for curing. Lay-Up Definition.