The use of core materials became widespread in the 1970s, when composite sandwich construction was first developed to provide a stiffer, stronger and lighter alternative to solid laminates. In space and aircraft applications, aluminum honeycomb became the core of choice. Its ultralight, open network of thin-walled hexagonal cells was favored especially for its high strength-to-weight ratio and good crush resistance and the fact that it worked well with aerospace-standard autoclaved prepreg.
Today, aluminum honeycomb shares the market with aramid and carbon honeycombs in prepreg constructions, and honeycomb is no longer the only core in town. Advanced composite sandwich constructions are employed in a broad range of applications that involve perform- ance demands, molding methods and constraints on labor, time and cost that make honeycomb's open cell structure impractical.
Rethinking core in aircraft exteriors
Although honeycomb has been the core of choice for composite aircraft structures, condensation and accumulation of moisture within its open cell structure has been a perennial problem in these particularly weight-sensitive applications. Aircraft manufacturers, therefore, are exploring the use of several closed-cell foam products that not only prevent water ingress, but improve sandwich structure performance as well.
Albany Engineered Composites (Rochester, N.H.) has ramped up production of X-COR, its innovative structural core material, which has been selected for use in the new all-composite tailcone, part of a major upgrade of the U.S. Army's UH60M Black Hawk helicopter. X-COR is made by reinforcing lightweight polymethacrylimide (PMI) foam with very small-diameter pultruded carbon fiber/epoxy rods arranged in a tetragonal truss network (see photo, this page). X-COR rods (0.28-mm to 0.5-mm/0.011-inch to 0.020-inch diameter) protrude from the foam, and are embedded in the facesheets during sandwich panel manufacture. X-COR is targeted to cocured structures with gentle contours and/or thick (>1.27 mm/0.050 inch) facesheets. The truss network carries both shear and compressive loads, while the foam base provides buckling support - a combination that ensures high structural efficiency and inherent damage tolerance. The resultant product has mechanical properties that reportedly match and/or exceed properties achieved in high-performance honeycomb. According to John Tauriello, Albany's director of marketing and sales, X-COR construction can be tailored to customer performance criteria. "By changing the rod placement angle and insertion density, we can precisely engineer X-COR to handle unusual and specific loading patterns. In addition, high compression loads can be handled in precisely defined areas for hard point and fastener installation or for high point loading scenarios - without adding undesirable weight and labor cost associated with potting compound."X-COR can be thermoformed and supplied as net-shape details and can be processed using the same cure techniques employed on honeycomb-cored structures.
The venerable Black Hawk helicopter remains a major resource for U.S. Army aviation. Produced by Sikorsky Aircraft (Stratford, Conn.), the UH-60M Upgrade will improve lift, range, maneuverability and safety as well as extend the aircraft's service life. The benefit of using composites in the new all-composite tailcone is a significant decrease in structural weight (47 lb/21.3 kg), which will enable increased performance and lower Operation and Support costs. "An important technology enabler for the composite tailcone is X-COR, said Michele' Ozier, the Army Composite Tailcone Project manager. "Without it we would never have been able to achieve the results and benefits we are getting today."The first of six ship-sets of X-COR part details have been shipped to GKN Aerospace for processing into tailcone parts and the first all composite tailcone is scheduled for delivery and use as a static test article in the fourth quarter of 2006.
In a cost-reduction move, Northrup Grumman (Los Angeles, Calif.) recently transitioned from autoclaved cured prepreg and honeycomb core to vacuum infusion of dry reinforcements with vinyl ester resin to manufacture the dorsal cover on the U.S. Air Force's T-38 Talon jet trainer.
Honeycomb is impractical for infusion because under vacuum, it is difficult to prevent resin flow into the honeycomb cell structure. Northrup Grumman replaced the honeycomb with Soric XF3, a combined core material and flow medium produced by Lantor Composites (Veenendaal, The Netherlands). Soric is a pressure-stable (that is, it resists compression during vacuum bag consolidation) nonwoven polyester mat with a honeycomb-like cell construction designed to enable resin flow in infusion processes. The cells are filled with microspheres to reduce resin absorption and resist compression. The nonwoven channels between cells enable resin transport and provide adhesive bonding between the sandwich faceskins and Soric core. Normally, impregnation during resin infusion occurs from the top down as resin is transported across the surface of the laminate stack. Soric enables resin transport through the laminate stack thickness as well. The material is compatible with most typical resins and closed molding processes.
A quite different resin-infusible core, from WebCore Technologies (Miamisburg, Ohio), is enabling fabrication of what will be the first jet engine fan case made with a composite sandwich construction. A critical structural component, the fan case encircles the jet's turbine blade structure and, in the event of a blade failure, must contain the blade within the engine housing. Fan cases feature one of two types of blade containment rings. The "hard wall"type is designed to reflect the blade back into the engine. The "soft wall"design, however, permits the blade to penetrate the case, relying on a ballistic aramid fiber overwrap to act as a "catcher's mitt,"stopping the blade (see HPC May 2005, p. 76). Soft wall cases are lighter, but conventional metal designs still weigh more than 1,200 lb/544 kg for containment rings as large as 10-ft/3m in diameter. Solid laminates have been used for jet engine fan cases in smaller aircraft, typically with braided reinforcements, but large commercial jet engines, such as the GEnx engine from GE Aircraft Engines (Cincinnati, Ohio), selected for Boeing's new 787 aircraft, usually feature solid aluminum or titanium cases. The GEnx will be the first to use an all-composite solid laminate fan case.
WebCore began working with GE in 2002 to develop a lightweight composite sandwich design for the newest GE90 and GEnx engines, using WebCore's fiber-reinforced foam core materials, trademarked TYCOR. TYCOR is made by WebCore's proprietary manufacturing process, in which fiber reinforcement is stitched through the thickness of closed-cell foam sheets to form a vertical web or rib-stiffened core structure. The dry fiber web and faceskin fabric are resin-infused to form a sandwich panel with robust z-directional reinforcement.
WebCore is currently working on a soft wall design that promises a more than 50 percent weight reduction vs. current metal designs. "We're using a combination of TYCOR core and braided fabric skin, which is integrally stitched through the thickness of the entire sandwich structure,"says Rob Banerjee, WebCore's VP of business development. To ensure that the case maintains its integrity, WebCore is using custom-designed TYCOR, with carbon fiber reinforcement and rigid 100 percent PMI closed cell foam that provides high performance at lighter weight than honeycomb.
"After impact,"Banerjee explains, "you don't see any delaminations, just a clean hole. This ability to prevent delaminations helps us to maintain the strength and stiffness that is required after an engine blade impact."Impact testing at NASA Glenn Research Center (Cleveland, Ohio) has consisted of high-speed photography recording a titanium blade traveling at 500 ft/sec to 1,000 ft/sec as it impacts the fan case. To date, WebCore's sandwich fan case has shown excellent performance in ballistic impact and post-impact structural tests. Further testing is planned with a KEVLAR overwrap and other features. Banerjee projects that this technology will be ready to go into production by 2010. "We are taking the slow and steady approach,"he explains. "Our composite fan case is ... well within the cost target, which is based on metal fan cases.â€
WebCore has successfully completed two Small Business Innovation Research (SBIR) programs, funded by NASA and the U.S. Air Force, to demonstrate building such a large composite sandwich fan case and scale-up of manufacturing. The company has finished all of the extensive design work, built four fan cases for manufacturing demonstrations and completed several rounds of ballistic impact testing at NASA Glenn Research Center.
Banerjee adds that WebCore is looking into designs for smaller business jets, with 3 ft to 4 ft (0.91m to 1.22m) diameter fan cases, and embedding "smart"circuitry into fan cases to provide structural monitoring so that operators can see if there has been a blade impact, and if so, where it occurred and how much damage resulted.
WebCore's TYCOR also is finding use in construction of lightweight, portable aircraft landing strips. WebCore has worked with the U.S. Air Force since 2001 to develop a lightweight alternative to AM-2, the U.S. military's workhorse airfield mat system, in use for more than forty years. The AM-2 system is currently comprised of 2-ft by 12-ft (0.61m by 3.66m) aluminum panels, which are 1.5-inch/38-mm thick aluminum extrusions with extruded end connectors along all four edges to provide a locking mechanism. WebCore's AMX composite mat system will offer lighter weight, improved joint design and easier assembly. A typical airfield large enough to support a fighter squadron requires 480 pallets of AM-2 matting, weighing around 1.3 million lb (about 590 metric tonnes). The AM-2 system weighs 6.4 lb/ft2 - roughly 150 lb/68 kg per 2-ft by 12-ft panel. The target weight is 3.2 lb/ft2, a 50 percent reduction. "We can offer significant weight savings with a composite structure,"states Banerjee, VP of business development for WebCore. "Our current design is a 4-ft by 7-ft [1.2m by 2.1m] panel with aluminum extrusion joints on four sides."WebCore is also working on new joint designs that will ease assembly, including some made from composites, for further weight savings. WebCore's new AMX airfield mat system has a simpler connection design, enabling efficient transfer of loads and quicker installation. WebCore's airfield mat solution uses a carbon noncrimp fabric/epoxy skin with a custom-designed TYCOR core made with carbon fiber reinforcement and high-performance PMI foam. The panels are molded via RTM or infusion. Large-scale testing by the Air Force began this August, involving more than 1,000 ft2/93m2 of panels, installed over soft soil to maximize structural stress and subjected to simulated landings by the F-15 fighter jet and the C-17 Globemaster III cargo transport. The F-15 produces a 35,000 lb (15,875 kg) single wheel load while the C-17's combined wheel loads total more than 500,000 lb (about 226,800 kg). Airfield mat panels must withstand 1,000 passes from each aircraft. If the new composite AMX system passes the test, the next step will be an Engineering and Manufacturing Development (EMD) phase, leading to a 10-year production contract, starting in 2008.
New cores for aircraft interior lightweightingIn aircraft passenger cabins, the strength, stiffness and low weight of composite sidewalls, overhead bins, flooring, air ducts, and other parts traditionally have been optimized with honeycomb. Recently, manufacturers of business and commercial aircraft have saved weight and reduced fabrication time and labor by using several alternatives. “One of the biggest problems in aircraft interiors is not the cost of the raw materials, but the finishing,” explains Nick Tiffin, global business development manager for aerospace interiors for Ten Cate Advanced Composites Group (Nijverdal, The Netherlands). “Surface porosity and telegraphing of the honeycomb core through the skin require applying filler and primer, sanding, and then re-applying filler and primer — repeating the cycle until a cosmetically smooth, paintable surface is achieved.”
To address weight and read-through issues, a premanufactured, thermoplastic-cored composite sandwich panel, CETEX System 3, has been qualified for a variety of interior structures on the Airbus A380 aircraft and as an “ultralight bin” option that airlines can select when ordering Airbus A340-500/600 aircraft. The name “System 3” denotes its three thermoplastic components: polyetherimide (PEI) skins, PEI tubular-celled core, and thermoplastic adhesive. The CETEX skins are manufactured by Ten Cate using ULTEM PEI supplied by GE Plastics (Pittsfield, Mass.), and the PEI tubular core is produced by Tubus Bauer (Bad Säckingen, Germany). (Note that Plascore Inc. of Zeeland, Mich. is a U.S. manufacturer of similar thermoplastic tubular core).
AIM Aviation (Southampton, U.K.) designed the bins and manufactures them in the company’s Renton, Wash. production facility. “CETEX System 3 allows manufacturing of very low-rework structures for overhead baggage bins, galleys and lavatories,” says Tiffin, noting that even when using single-ply skins as thin as 5 mils/0.005 inch, there is no porosity, no paint needed and no fire/smoke toxicity issues. CETEX System 3 is used in the sides and backs of overhead bins, where the material’s pigmentation eliminates the need for paint or other finishes. This not only saves weight and cost, but also extends bin service life, because painted surfaces quickly deteriorate under heavy wear. According to Ten Cate, CETEX System 3’s thermoplastic surface offers better wear resistance than phenolic resins used in sandwich construction for aircraft interiors.
The product also can be adapted for painting. “If the bins are going to be painted dark, we can provide a dark-tinted laminate,” Tiffin explains. “If the bins are going to be painted white, we can supply an off-white tinted laminate.” Tiffin claims that PEI can be painted directly, and bonds well.
Recent analysis of an aircraft cabin screen divider showed typical production times of seven hours for a traditional cored structure (1.5 hours molding; sand sweep, primer; sand sweep, primer; and finishes) compared with two to three hours total production time for the same part made with CETEX System 3 (15 minutes thermoforming, plus primer and finishes). Thermoforming is done at temperatures of 140°C to 150°C (284°F to 302°F).
CETEX System 3 also is being used in commercial aircraft flooring. Three team members have worked together to develop a structural thermoplastic composite pallet for the new fully reclining bed-type seats featured in premium class cabins. W & J Tods Ltd. (Somerset, U.K.) designed and developed the flooring, working with Ten Cate and Tubus Bauer to obtain certification in the Airbus A340 and Boeing’s 747 and 777 aircraft. Fully reclining seats are one of the hottest new trends in commercial aircraft interiors. However, they present a design challenge, because they are placed at an angle to the aircraft wall but must attach to existing aluminum seat tracks, which run fore and aft, according to traditional aircraft seating patterns. In contrast to the customary rectangular honeycomb-cored sandwich flooring panels, the CETEX System 3 thermoplastic plinth is a complex-shaped oblong panel, roughly matching the outline of a fully reclined upper-class seat. The seat is attached to the plinth and the plinth is fastened into the aluminum seat track in place of the standard flooring, solving the angular attachment problem. According to Tom Hitchings, director of aerospace business for W & J Tods, “The pallet incorporates a carbon fiber structural beam to help support the seat, with its equipment and passenger, as well as the 16-G crash load it must withstand in accord with FAA regulations. These pallets have passed stringent dynamic testing, including seats being installed as in a real aircraft and subjected to real crash loads.” Hitchings explains that the thermoplastic solution also reduces the amount of aluminum required to meet structural requirements vs. traditional sandwich panel materials. Another benefit is that PEI is basically impervious to moisture uptake, an issue with honeycomb-cored flooring made from NOMEX paper.
Airlines report that the biggest benefit of CETEX is the damage resistance of the its thermoplastic skins. For example, Virgin Atlantic Airways — one of the first big end-users — changes the carpets in high-wear, heavy traffic areas every two weeks. Carpet is attached with industrial double-stick tape, and when it is pulled up, some of the top faceskin on traditional prepreg/honeycomb flooring can be pulled away with it. Virgin Atlantic has had CETEX System 3 floor panels flying for two to three years with zero damage. There are more than 4,000 aircraft seats currently flying with the CETEX System 3 thermoplastic flooring plinth, and programs now under way will add another 3,000 to 4,000 seats within the next few years.
Elsewhere, the interiors of various business and VIP aircraft now feature sandwich panels with new Atvantage Composite Structural Core (CSC) and Composite Insulating Core (CIC) from National Nonwovens (Easthampton, Mass.). These nonwoven materials are manufactured from discontinuous (3 inch to 6 inch or 76 mm to 152 mm) aramid fibers or blends of aramid and other high-performance fibers, which are “interlocked” and “chemically enhanced” to meet customer specifications. Interlocking refers to the company’s practice of manipulating fiber orientation in the x/y plane to achieve desired compressive strength, flexural modulus and splitting resistance and then needling the fiber stack to generate a high degree of z-directional fiber in the stack to optimize interlaminar sheer strength and vibration damping. The company claims it can orient fibers within 5° of multiple requested angles. Chemical enhancement can include a variety of “back coatings” that provide flame- and moisture resistance, anti-microbial protection or enhanced stiffness.
While CIC nonwovens are heavier than honeycombs, overall part weight reportedly can be decreased because the weight associated with edge-potting of honeycomb is eliminated. CIC edges are sealed much more simply by hand-applying a relatively small amount of resin. Further, the core’s resin-impregnated fiber architecture improves the panel’s ability to support fasteners and attachments and eliminates local potting. In addition, the material’s inherent damping characteristics reduce the need for additional damping materials.
CSC products are compatible with most prepreg resins, and National Nonwovens claims that the increased number of bond sites between a prepreg and CSC increases its resistance to delamination, an issue common with conventional prepreg/honeycomb structures.
Core for rigorous environmentsDuring missions to space, launch vehicles and their payloads must withstand the extreme shock and vibration loads encountered during lift off and, once outside earth’s atmosphere, must endure extreme temperatures. The superior strength- and stiffness-to-weight and low coefficient of thermal expansion (CTE) of composites offer the best-case materials options.
For the GOCE (Gravity-field and Steady-state Ocean Circulation Explorer) satellite program, Ultracor (Livermore, Calif.) is providing materials for two satellites and an engineering module to the prime contractor, Alcatel Alenia Space (Cannes, France). A manufacturer of specialty high-performance honeycomb cores for advanced composites applications, Ultracor has developed honeycomb from quartz/cyanate ester, carbon/carbon, mica/epoxy, Spectra fiber, continuous KEVLAR fiber, phenylene benzobisoxazole (PBO) and more than 50 types of carbon fiber constructions, including ultralow-density carbon and triax (a carbon fabric for which “triax” describes the fabric architecture) carbon honeycomb as well as corrugated carbon core materials. More than half of Ultracor’s business is spacecraft, where the negative CTE, high thermal conductivity, stiffness and very light weight of pitch-based carbon honeycombs meet the rigorous structural demands.
The GOCE’s first launch, by the European Space Agency (ESA) in 2006, will involve a satellite with an optical bench made from carbon fiber skins and Ultracor’s Ultraflex “carbon/carbon” honeycomb measuring 1.5m by 1.5m and 0.1m in thickness (4.92 ft by 4.92 ft by 0.33 ft). (Ultracor produces what it calls its “carbon/carbon” honeycomb by impregnating a carbon fiber paper honeycomb with epoxy resin, then curing and completely carbonizing the result at between 1100°C and 1650°C or about 2000°F to 3000°F. Currently, Ultracor is the only producer of this type of honeycomb.) Ultraflex is one of Ultracor’s newest products, a flexible honeycomb cell shape that can be made with any nonmetallic honeycomb material in any cell size. Since many satellite antennas need to be shaped, Ultraflex offers the ability to easily form the honeycomb to shape, enabling construction of sandwich panels with complex contours. Ultraflex also is enabling fabrication of multiple-contoured radomes and structures with double curvature for stealth aircraft.
Elsewhere, a variant of Albany’s X-COR, known as K-COR, is being qualified by Boeing Integrated Defense Systems (St. Louis, Mo.) for use on the payload shroud of Boeing’s Delta IV launch vehicles, due to its superior damage tolerance and ability to handle temperature extremes. K-COR uses reinforcing rods of 0.50mm/0.020-inch diameter or greater, with rods set flush to the foam surface; facesheets are either cocured or secondarily bonded to the core. Thermoformable K-COR is suitable for more complex contours. According to Albany’s Tauriello, “Whereas unreinforced foam core cracks and disbonds, the carbon rods used in K-COR have a more closely matched coefficient of thermal expansion to the carbon/epoxy faceskins and can withstand the extreme temperature differentials near the liquid hydrogen fuel tank.”
Like space exploration, Formula 1 auto racing has historically pushed boundaries. Racing teams began building monocoque body/chassis structures from carbon composites in the early 1980s. Now carbon is working its way under the skin, replacing aluminum honeycomb in monocoque structures. Ultracor is working with several Formula 1 teams to replace aluminum honeycomb with carbon honeycomb in separate nose structures and integral side impact structures on Formula 1 monocoques. According to Michael Fellman, president of Ultracor, “The theory is that carbon honeycomb should absorb more energy because, as it undergoes impact, it not only folds up but also fails catastrophically, which should consume a significant amount of energy.” The nose and side impact structures consist of carbon fiber skins with carbon fiber honeycomb. Initial testing looks promising.
America’s Cup syndicates are using KEVLAR honeycomb core in place of aluminum honeycomb in deck structures in response to a new race rule that prohibits the use of the latter. America’s Cup yachtbuilders have historically used aluminum honeycomb to meet minimum America’s Cup core density specifications of 2.7 lb/ft³ in the decks of racing vessels. In the past, this was easily achieved with aluminum honeycomb by under- or over-expanding the honeycomb core. Aluminum honeycomb was barred because after the races, boats typically are sold. When new owners change the deck hardware configuration, the potting of through-holes is often inexact. As a result, many of the boats were showing significant corrosion of the aluminum honeycomb in a very short time, with resulting safety issues.
Standard NOMEX honeycomb, however, is sold in a limited number of different densities, forcing many teams to use 3 lb/ft² core in order to meet the density requirement, but that means they are taking a 0.3 lb/ft² weight hit for the entire deck volume. To address this problem, Plascore Inc. has developed a special version of its PK2 honeycomb. To make the core, Plascore begins with KEVLAR 636 honeycomb paper from DuPont Advanced Fiber Systems (Richmond Va.), dips it in phenolic resin, impregnating it to the rule density minimum of 2.7 lb/ft². According to the company, the resulting core is four times stiffer than NOMEX on an equal density basis. Although the custom PK2 core comes at three times the cost of standard 3 lb/ft² NOMEX core, several America’s Cup racing teams have considered the investment a good value.
Pre-engineering to minimize pre-mold modificationCore manufacturers are increasingly taking on the task of forming and/or machining core products for customers, in order to deliver a product that can confidently be used with little or no additional adaptation.
Gurit (formerly SP, Magog, Quebec, Canada) maintains an in-house technical staff that works closely with customers to optimize the use of Gurit materials in specific composite processes and applications, including its Corecell linear, styrene acrylonitrile (SAN) co-polymer foam. Formerly manufactured by ATC Chemicals, Corecell was originally developed for marine applications, as a tougher alternative to polyvinyl chloride (PVC) foam core. For curved parts, Gurit staff can precut or pre-score foam so that it can conform to the desired shape. As composites manufacturers have gravitated to vacuum infusion processes, however, additional surface cuts or grooves are required to aid in resin flow. However, the additional resin cured in the numerous channels can significantly increase weight and make a noticeable impact on mechanical properties and impact performance. “Too many cuts also introduce too much resin at one time,” adds Al Horsmon, chief naval architect for Gurit. “The ideal process is what we call ‘staged infusion,’ where the appropriate number of cuts are used to impel the resin front in combination with multiple ports across the part in order to re-introduce resin as the impregnating front passes through.” Horsmon says the company has investigated optimum Corecell cut configurations and completed extensive testing on over 30 different types of core cuts for infusion processing, and adds that Gurit can work with customers to select the best configuration for specific applications.
Ultracor optimizes its honeycomb core materials for adhesion, to save customers the cost in time and labor for prepping core to enhance core/facesheet bond. The company plasma treats core materials to enhance the surface energy on the contact surfaces. An increase in surface energy optimizes its potential to bond with faceskin materials and adhesives. Ultracor president Michael Fellman explains, “Our products are expensive, so we don’t want the customer to have to do anything else to it.” Nothing further is required on the customers part. “Don’t prime it, vapor degrease it or use solvents, don’t mechanically roughen the surface,” he says, ”Just open the box and use the core.”
Ultracor also perforates honeycomb cell walls so that no pressure can build up inside the individual cells and cause faceskin disbonding. The holes allow vapors to move throughout the core, enabling it to equalize pressure under the severe temperature differentials of the extreme processing and service environments often seen by these high-tech composite cores.