Volume 21 - Composites Part 8 doc

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 •

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/m2 has become standard since the weaving cost is around half that of the conventional 285 g/ m2 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 × 106 psi) for tape and up to 130 GPa (19 × 106 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/ m2 and five-harness satin woven fabric with

a fiber weight of 370 g/m2 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/m2 FAW and woven (and now multiaxial) fabric prepregs of up to 1000 g/m2 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 × 106 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 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

silicone-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 lay-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/m2 to $60/m2, 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/m2 to $715/m2 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 × 106 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 × 106 psi), to ultrahigh-modulus pitch- based carbon, providing laminates with tensile moduli up to 490 GPa (71 × 106 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

surfaces The most common matrix resins are epoxies as a result of their strength, fiber adhesion strength, and slow curing, which provides a freezer out life (i.e., lay- up period) of up to many weeks Their upper service-temperature-limit is around 150 °C (300 °F) in a hot, wet environment and hence for higher service temperatures, bismaleimide resins with a limit of around 200 °C (390 °F) and then polyimides with a limit of around 270 °C (520 °F) were developed (Ref 3) (see the articles “Bismaleimide Resins” and “Polyimide Resins” in this Volume) Figure 9 shows the range of laminate stiffnesses provided by a wide range of prepreg reinforcement types

Fig 9 Laminate stiffness provided by a range of prepreg reinforcement types Range shows the effect of lay-up UD, unidirectional; HS, high strength; IM, intermediate modulus; HM, high modulus

Reference cited in this section

3 D.H Middleton, Ed., Composite Materials in Aircraft Structures, Longman Scientific and Technical,

1990, p 17–38

Manual Prepreg Lay-Up

Andrew Mills, Composites Manufacturing Research Centre, Cranfield University, United Kingdom

Design Guidelines

Materials and process selection for lightweight composite materials is primarily driven by how much an industry will pay for weight reduction of components and how many parts will be made For one-off structures and those with production levels of one or a few per month, the cost of the prepreg is usually greatly outweighed by the design, project engineering, and tooling cost

Structures for which there is little incentive for weight reduction are rarely manufactured from prepreg, since wet lay techniques use much lower-cost composite materials

Hand lay-up of prepregs is applied to a very wide field of industries and applications At the industrial end with higher volume production in the order of five to ten parts per day, a thick prepreg, often using a multiaxial stitched (multilayer, multiangular, nonwoven) fabric with a lower-cost, faster-cure resin, will be used The maximum service-strain levels will be low, and, hence, bubbles and the occasional wrinkle can be tolerated This also allows the prepregs to be manufactured fast, having wider resin content tolerance bands A good example of 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 of very low thickness (typically

125 g/m2 FAW) The fiber choice is high-modulus grade, and the resin is formulated to resist higher temperatures than epoxies can sustain The bismaleimide carbon prepregs are up to 200 times the cost of the glass-epoxy wind-turbine materials

Table 1 gives a comparison of prepreg types and their manufacturing attributes, costs, and laminate-damage resistance Figure 10 indicates the level of curvature conformability that can be provided by prepreg reinforcement forms and its relationship with prepreg width and thickness

Fig 10 Relationship between prepreg form and conformability to component curvature

Table 1 Prepreg types and lay-up characteristics

Cure temperature

Typical thickness

Width Maximum lay-up

rate

Approximate cost

Damage resistance

Fiber type Form (a) Resin

type

Intermediate-modulus

carbon

(a) UD, unidirectional tape with multi-angular lay-up Multiaxial, fabric with 2 to 7 layers of tows at varying angles knitted into a drapable fabric (also called noncrimp fabric)

Manual Prepreg Lay-Up

Andrew Mills, Composites Manufacturing Research Centre, Cranfield University, United Kingdom

Outlook

Despite many impressive research projects and facility investment in the United States and Europe, which have investigated and implemented production of molding processes considered to have the potential to offer cost 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 by Dow–UT (now GKN Westland Aerospace) in the United States and for regional aircraft control surfaces by Bombardier Shorts in Northern Ireland These applications make use of the higher drapability of dry fabrics compared to prepreg and the use of matched metal tools to provide net-shape parts, which do not require shimming during assembly The process-engineering simplicity of prepreg lay-up and cure using a vacuum bag and/ or pressure

is undeniably preferable to liquid molding processes The downside of freezer storage and high prepreg cost if procuring small quantity continues to be overcome by the ability to make components simply and reliably For applications with low curvature, such as aircraft-wing and tailplane panels, prepreg tape laying by large machine tools will continue to replace hand lay-up For applications with highly weight optimized, closed sections that can be rotated, such as small aircraft and helicopter fuselages, the fiber placement process as developed by Alliant Techsystems, Cincinnati Milacron, 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 installing wider, faster prepreg lines

• Introduction of thick multiaxial fabric prepregging is being made to capitalize on the stiffness of these laminates 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 in Aircraft Structures, Longman Scientific and Technical,

1990, p 17–38

Fig 1 Fiber placement head

When starting a fiber band or course, the individual tows are fed through the head and compacted onto a surface As the course is being laid down, the processing head can cut or restart any of the individual tows This permits the width of the fiber band to be increased or decreased in increments equal to one tow width Adjusting the width of the fiber band eliminates excessive gaps or overlaps between adjacent courses At the end of the course, the remaining tows are cut to match the shape of the ply boundary The head is then positioned to the beginning of the next course

During the placement of a course, each tow is dispensed at its own speed, allowing each tow to independently conform to the surface of the part Because of this, the fibers are not restricted to geodesic paths They can be steered to meet specified design goals

A rolling compaction device, combined with heat for tack enhancement, laminates the tows onto the lay-up surface This action of pressing tows onto the work surface (or a previously laid ply) adheres the tows to the lay-up surface and removes trapped air, minimizing the need for vacuum debulking It also allows the fiber to

be laid onto concave surfaces

Figure 2 is a diagram of a fiber placement system This system has seven axes of motion and is computer numeric controlled The machine consists of three position axes (carriage, tilt, crossfeed), three orientation axes (yaw, pitch, roll), and an axis to rotate the work mandrel All of these axes are necessary to make sure the processing head is normal to the surface as the machine is laminating tows The machine also has up to 32 programmable bidirectional electronic tensioners, which are mounted in an air-conditioned creel These tensioners provide individual tow payout and maintain a precise tension The fiber placement head is mounted

on the end of the wrist The head precisely dispenses, cuts, clamps, and restarts individual prepreg tows

Fig 2 Fiber placement system

Another military aircraft that is taking advantage of the unique capabilities of the fiber placements is the F/A-18 E/F Super Hornet The U.S Navy funded a program to further 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 in commercial aircraft Raytheon Aircraft in Wichita, Kansas, is using fiber placement to fabricate fuselage sections for the Premier I and Hawker Horizon business jets (Fig 4) The fuselage is a honeycomb sandwich construction Graphite facesheets inclose a Nomex (DuPont) honeycomb core for a total thickness of 20.6 mm (0.81 in.) This design creates a fuselage shell free of frames and stiffeners The shells are also free of rivets and skin joints Because the shells do not contain frames, there is more usable space for passengers or cargo

Fig 4 Raytheon Premier I fuselage manufactured by fiber placement

By using the fiber placement process to fabricate the fuselages, Raytheon has realized weight savings, material savings, reduced part count, reduced tool count, reduced shop flow time, and increased part quality The Premier I fuselage consists of only two cured parts The forward shell extends from the radome bulkhead to the aft pressure bulkhead and is 8 m (26 ft) long It includes the baggage area, cockpit, and cabin areas The aft shell extends from the aft pressure bulkhead to the tailcone, and is about 5 m (16 ft) long The Premier I shells weigh less than 273 kg (600 lb), whereas an equivalent metal aircraft would weigh at least 454 kg (1000 lb) This is a 40% weight savings If the same two composite fuselage sections were made in a comparable metal design, they would contain more than 3000 pieces It would be made up of stringers, stiffeners, bulkheads, clips, and external skins This reduction in part count significantly reduces part fabrication time and the number

of tools required to make and assemble the parts (Ref 2)

Material scrap for hand lay-up can be as high as 30 to 50% Fiber placement has a typical material scrap of 2 to 7% On a 273 kg (600 lb) fuselage, this material savings becomes very significant

On the Premier I fuselage, quality assurance (QA) review found that the machine is 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 inspections, instead of checking every ply as it is laid

References cited in this section

1 C.G Grant, Fiber Placement Process Utilization Within the Worldwide Aerospace Industry, SAMPE J.,

Slit tape is fabricated by running a 7.6 cm (3 in.) wide tape through a slitter, creating smaller widths of slit tape These narrow slit tapes are then wound onto a number of cores to form spools When the slit tape is wound onto the core, a backing film, which is wider than the slit tape, must be added If the backing film is not used, the slit tape cannot be removed from the spool, because of stringers that will occur during the despooling operation A stringer occurs when the edge of the slit tape separates and stays on the spool while the rest of the slit tape is despooled This will cause the slit tape to eventually break During part fabrication, this backing film is removed before the fiber reaches the fiber placement head

The tow width of the material is very important in controlling the gap between the prepregged tows For example, if the fiber placement head is designed to lay down tows that are 3.2 ± 0.38 mm (0.125 ± 0.015 in.) wide, the tows will be compacted onto the surface in 3.2 mm (0.125 in.) spacings If the tow is exactly 3.2 mm (0.125 in.) wide, there will be no gap between the tows If the tows are 2.5 mm (0.100 in.) wide, there will be a 0.63 mm (0.025 in.) wide gap between the tows If the tows are 3.8 mm (0.150 in.) wide, there will be a 0.63

mm (0.025 in.) overlap

The ideal fiber placement material has no tack at 21 ºC (70 ºF) and high tack at 27 to 32 ºC (80 to 90 ºF) Low tack is needed when the material is being pulled off the spool and guided through a fiber delivery system and head, but high tack is needed when it is being compacted onto the surface

Materials that have a low tack can be despooled with a fiber tension of 0.23 kg (0.5 lb) or less These low tensions are achieved because the resin does not stick to the spool or the components of the fiber delivery system This lower fiber tension is needed while fiber placing concave areas A higher tension will cause the fiber to bridge over concave areas Materials with low tack levels also have less tendency to deform or rope while being pulled through the fiber delivery system They also transfer less resin to the components of the fiber delivery system and head This reduces the number of times that these components need to be cleaned because of a resin buildup Resin buildup in the head can cause it to malfunction

Fiber Placement

Don O Evans, Cincinnati Machine

Part Design Considerations

Fiber placement has the capability of reducing composite material and labor cost To take advantage of these cost savings, the designer must take into consideration the unique capabilities and limitations of fiber placement Some of the items that the designer must consider are ply shapes, tow steering, dropping and adding tows, and surface geometry By optimizing ply shapes, the designer can eliminate the need to hand lay a piece

of the ply that cannot be laid by the machine The designer can also take advantage of the ability of fiber placement to steer tows so they can follow applied stresses, but the tows must not be steered less than 635 mm (25 in.) or they will buckle The designer needs to take into consideration where the tows are added and dropped, making sure there are not too many gaps and overlaps in a small area The surface geometry must be such that there are no head collisions and that the concave radii are not too small for the compaction roller to fit into

When generating ply shapes, the designer must consider the shortest tow length the machine can lay down This length is the distance from the start of the lay-down point to where the tow is cut in the head This is called the minimum cut length It varies from 63.5 to 152 mm (2.5 to 6 in.), depending on the head size and configuration

If the area that is to be filled with tows is less than the minimum course length, the machine cannot lay tow in these locations These areas could be laid in by hand, or the ply shapes could be adjusted to overcome this limitation Three techniques can be used to eliminate areas of missing tows (Ref 3):

• In the problem areas, the exterior ply boundaries can be extended past the required part shape, such as tabs on the corner of 45º plies These extended areas are later removed

• Curved interior plies can be reshaped to match the fiber angles

• Some of the holes can be distributed to full- coverage plies having the same fiber angles

Designers specify the fiber angles that are required to meet mechanical property requirements Steering of the fibers is required to maintain these angles on a complex shaped tool A typical fiber placement machine using 3.2 mm (0.125 in.) wide materials can steer a fiber band along a 63.5 cm (25 in.) radius without buckling the individual tows The buckling occurs because the fibers on the outside steering radius are in tension and the fibers on the inside steering radius are in compression When steering a radius smaller than 63.5 cm (25 in.), the tows will begin to buckle if laid on a flat or a convex surface, or "Venetian blind" if laid on a concave surface Venetian blinding occurs when the fibers on the inside steering radius of the individual tows are adhered to the surface and the fibers on the outside steering radius are in the air

The designer needs to pay special attention to two surface geometry issues when designing a part that is to be manufactured by fiber placement The first is concave surfaces, and the second is areas with small radii of curvature When considering a part with a concave surface area, the designer must make sure the fiber placement head can fit into the concave area without hitting the surface of the part There are some techniques that can be used to overcome some of these limitations To help the head fit into a concave area, the off-line software has a feature known as collision avoidance The part and head geometry are programmed into the software The software constantly checks to see if the two are colliding If they come close to colliding, the software will rock the head off the surface normal away from the collision There are limits to how much the head can be rocked off the surface normal If the head hits on both the front and back sides, the software cannot avoid the collision, and the area should be redesigned Rocking the head to the front or back slightly affects the effective applied compaction force and the minimum cut length Rocking the head sideways also affects the effective applied compaction force and requires extra compactor compliance

Reference cited in this section

3 D.O Evans, “Design Considerations for Fiber Placement,” 38th International SAMPE Symposium, 10–

Automated Tape Laying

Michael N Grimshaw, Cincinnati Machine, A UNOVA Company

Introduction

AUTOMATED TAPE LAYING is a mature process and is currently being used in both commercial and military aircraft applications This article provides a brief history of the process and describes the use of

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