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Structural composites.

Structural composites have proven themselves admirably in aircraft and in numerous selected industrial and consumer uses. The technology is now itching to broaden its application base to take advantage of the materials' light weight, high strength, fatigue life, and corrosion resistance. But there is still work to be done to advance processing techniques, to provide a more comprehensive database, and to further reduce costs.

smart composites

Structural composites may be getting smarter as well as stronger. One significant direction is a broad-based effort in the armed forces and industrial research laboratories to give the structures an ability to inherently sense and react predictably to functional and environmental conditions. At Wright-Patterson Air Force Base (WPAFB) in Ohio, for example, work has been targeted toward the development of first-generation "smart structures," where sensors and/or actuators are embedded within the composite's laminate elements to monitor and react to the system's structural "health." According to Tia Benson Tolle, materials engineer, Materials Directorate, Wright Laboratory, the program aims at ultimately developing multifunctional composites that would be not only load-bearing but also intrinsically capable of active and adaptive responses to a structure's functional condition.

As the R&D proceeds, progressively higher levels of integration will be sought. Thus, instead of the current need to embed separate sensors in the laminate, reinforcing fibers that might double as sensing elements, for example, could also give the composite structure an intrinsic ability to convey signals of its condition.

At this point, the work at Wright Laboratory on smart structures involves generic shapes such as plates and beams. Benson Tolle suggests that newly initiated work at Wright Laboratory could produce advancement of more inherent sensing capability to such structures as aircraft bulkheads, vertical tails, and smart skins, or intrinsic avionics, including antenna systems. She points out, however, that while smart structure development obviously has great potential, with varied systems being investigated in Canada, Europe, and Japan, a more comprehensive benefits assessment of the emerging technology is an important parallel requirement.

higher-temperature organics

Among other R&D activities at WPAFB, development of higher-temperature organic-matrix structural composites for aircraft engine components is a major effort. Ken Johnson, materials engineer, Structural Materials Branch, Non-Metallic Materials Division, Wright Laboratory, says the drive to extend the capability of high-temperature resins in excess of 700|degrees~F now includes a focus on AFR-700, a fluorinated, continuous-fiber-reinforced polyimide. In a reactant method, all the monomers that make up the polymer are initially reacted in a methanol solution to produce the AFR-700 material. Crosslinking, induced by subsequent heating, results in a material with a high glass transition temperature, with the fluorination providing the needed oxidative stability.

Johnson says that the polymer-monomer reactant approach enhances flow during the early stages of prepregging and in-situ polymerization in the mold, thus facilitating penetration of the reinforcing fiber into interstitial areas, rather than "bunching up." Possible applications for AFR-700--a "next-generation" composite--include the compressor shroud on the Pratt & Whitney F100-229 engine and the outer flaps on the F-100 General Electric engine. The new material's antecedents include PMR-15, which is structurally functional to about 550|degrees~F and is used, for example, as filament-wound outer ductwork in the U.S. Navy's F-404 engine; and Avimid-N, a 600|degrees~F to 650|degrees~F fluorinated polyimide material.

Johnson also cites continued interest in higher-temperature organic materials as possible replacements for titanium in jet engine components. A concurrent interest at Wright Laboratory, he adds, is to advance the ability to make complex structural composite shapes "in one shot," such as with closed-die resin transfer molding or filament winding to achieve weight saving and dramatic cost reduction.

lower-cost processing

Two contracts recently awarded by WPAFB, described by Michael Holl, mechanical engineer, Structural Materials Branch, Non-Metallic Materials, are targeted towards the continuing goal of substantially lowering overall structural composite costs through improved processing technology. One contract, awarded to McDonnell Douglas Aircraft, totaling $2.7 million over four years, takes aim at the conventional making of prototypes by labor-intensive hand-lay-up procedures for low-volume production. The objective is a more flexible, less expensive prototype system that would avoid high-cost capital equipment and would involve alternative material selections. These materials then would be better capable of producing quality structural composite parts with more cost-effective processing techniques. One route could be choice of materials with better room-temperature flow and that could contribute to eliminating the need for expensive autoclave tooling to cure the parts.

The second contract, awarded to Pratt & Whitney Aircraft Corp., for $900,000 over four years, is specifically keyed to producing filament-wound engine parts, with minimum scrap, using the high-performance AFR-700 polyimide material. Conventional hand-lay-up prepreg techniques to produce the AFR-700 engine parts generate excessive scrap of the material, which costs more than $500/lb.

The aim is to advance the technology and productivity of automated filament winding of more complex shapes, compared to the typical relatively simple cylindrical constructions. Increased compatibility of design with enhanced filament winding would include automated capability of incorporating such features as integral flanges for attachments and "buildups," for added strength and/or stiffness in selected areas of higher stress, to produce integrated parts without expensive secondary operations. The demonstration part called for in the contract will be a composite grid-reinforced duct, with the goal of efficient production by an advanced filament winding method.

engine development

The Integrated High Performance Turbine Engine Technology (IHPTET) initiative is a unique approach to engine technology development that combines improved materials, innovative structural design, improved aerothermodynamics, and advanced computational methods. Working together, the U.S. Army, Navy, and Air Force, NASA, the Defense Advanced Research Projects Agency (DARPA), and industry have as an objective, through technology development and demonstration programs, the doubling of gas turbine engine propulsion capability by the year 2000. Involving turbojet/turbofan, expendable, and turboshaft/turboprop engines, a three-phase program targets 30% and 60% increases in propulsion capability in phases one and two, respectively, and a 100% increase in phase three.

James Petty, manager of IHPTET for Wright Laboratory, WPAFB, says that in phase one, to be completed in 1993, high-strength, lightweight organic structural composites have contributed to the achievement of the current 30% increase in propulsion capability and cost effectiveness, including advancements in tooling technology, aerodynamics, and parts reduction, as well as materials application. Injection molded PMR-15 polyimide, reinforced with graphite chopped fiber, for example, is used for an experimental missile engine inlet case. A large, complex graphite-polyimide composite intermediate case for an experimental fighter-type engine also has been fabricated for near-future testing.

Phase two, scheduled for completion by 1997, will focus on higher (1000|degrees~F to 3000|degrees~F) temperature areas of the engine, which could involve ceramic fibers and metal or ceramic matrices, as possible replacements for metal superalloys. In phase three, expected to be completed in 2003, goals will include further development of ceramic composites, as well as further advances in cooling, aerodynamics, electrical actuation to replace hydraulics, nonlubrication-dependent bearing systems, and application of the increased temperature capability of optical sensors rather than wires.

growing pressure

The U.S. construction industries are under increasing pressure to reduce costs, improve quality and long-term reliability, and to develop alternatives for outmoded practices. Some industry experts have likened the situation to that of the U.S. automotive industry in the mid-1970s. "Threats of foreign competition, offering skilled and motivated labor, state-of-the-art materials and construction technology, integrated design/build capabilities, and attractive financing as instruments of foreign government policy, hang over the domestic civil engineering industry like a sword," says Douglas Barno, market development consultant, the SPI Composites Institute. "International competitors are outspending the U.S. in construction R&D by nearly ten to one." Barno emphasizes, however, that a major opportunity exists for structural composites in construction, driven in part by the realization that conventional materials often have not proven durable enough in many construction applications. Corrosion of steel reinforcement, for instance, in concrete bridges, parking garages, buildings, highways, and infra-structure is a serious national problem. R&D to optimize shapes, and resin systems reinforced with glass, aramid, vinyl esters, or thermoplastics, and, possibly, blended fiber hybrids and three-dimensional fiber arrays, is an ongoing requirement.

More than 60 organizations, worldwide, are investigating composites for construction, with notable emphasis on replacing corrodible steel rods in concrete with structural composite reinforcement. Barno suspects that Japan and Europe are well ahead of the U.S. In the U.S., Catholic University, the University of Pittsburgh, the South Dakota School of Mines & Technology, and West Virginia University are among the leaders in developing data on the long-term performance of structural composites for construction. For example, the South Dakota School of Mines & Technology will be building a 60- by 15-ft test pier at the Naval Civil Engineering Laboratory in Port Hueneme, Calif., where prestressed concrete pilings, reinforced with glass-fiber/epoxy and carbon-fiber/epoxy tendons, will be exposed to harsh marine environments.

Government laboratories, including the Federal Highway Administration, McLean, Va.; the U.S. Army Corps of Engineers Construction Engineering Research Laboratories, Champaign, Ill.; and the Naval Civil Engineering Laboratory, Port Hueneme, have new programs designed to quantify the suitability and long-term structural performance of composites. Also, professional organizations such as the American Society of Civil Engineers, the American Society of Testing & Materials, the American Concrete Institute, and the National Association of Corrosion Engineers have their own programs to investigate structural composite technology.

Unfortunately, however, Barno contends, the varied efforts tend to be fragmented and uncoordinated, thus inhibiting the creation of a substantive research database. Barno asserts that structural composite demonstration projects, and development and promotion of design standards, conducted through key trade and professional organizations, stand a much better chance of accelerating penetration of the massive, relatively untapped construction market.

improved thermoplastics

One of two new products recently introduced by Azdel, Inc., a joint venture of GE Plastics and PPG Industries, is a 50% unidirectional random-glass-mat product that offers higher impact strength and lower deflection than the existing 42% undirectional material used in the Honda Accord bumper system. The new polypropylene/glass formulation is used for the front and rear bumper beams on the 1993 Toyota Corolla and GM's Geo Prizm.

The second new entry is a mat-based product that offers improved glass flow into corners, ribs, and boss areas, more like a chopped-glass-fiber formulation. David Nichols, sales manager, Azdel, Inc., says the product, currently targeted for knee bolsters and glove-box doors, exhibits better surface aesthetics and impact strength.

Nichols also mentions the increased development of tailoring performance for specific applications with more cost-effective hybrid molding of Azdel mat-based grades, combined with unidirectional or chopped fiber-based composites. Mazda uses beams incorporating this technology for the MX6 and 626 rear bumpers. Additionally, Buick's 1993 LeSabre features an Azbrite beam, a 40% glass-mat-based/polypropylene product that incorporates proprietary hybrid composites with an insert-molded, chrome-plated steel decorative strip.

While automotive bumper beams seem to have dominated Azdel structural composite applications, Nichols points to the added focus on interior components, such as steering column covers, knee bolsters, and glove-box doors, with some of these uses coming on 1993-1/2 GM and Ford models. The material has also branched off into nonautomotive uses such as the Larson storm door, made of 30% glass-mat-based, high-flow polypropylene skins with a polyurethane foam core; and Ryerson Plastics' bulkhead door for railroad boxcars. The structural composite consists of a three-layer, melt-bonded construction of 40% random-glass-mat panels and a central corrugated layer. The Azdel panels are incorporated into a steel frame, and the unit functions as a movable partition in the rail car. Weight reduction, compared to a 600-lb all-steel unit, allows increased payload.

tough requirements

Material formulation and the structural foam process have combined to meet very demanding strength, dimensional, and multiple subassembly requirements for a complex new Pitney Bowes composite feeder/sealer base that replaces a more costly die-cast aluminum design. Victor Grizzle, industry manager, Computer and Business Equipment Market, and Dan Furlano, technical programs manager, GE Plastics, largely credit an innovative filler system, which balances a 40% glass/mineral combination with a particular sizing, in the company's Noryl FM 4025 PPO/PS blend, for the ability to penetrate the traditional metal die-cast arena. The thermoplastic meets the structural requirements, and, aided by its amorphous nature, repeatedly holds critical dimensions to plus or minus 0.002 inch.

Grizzle and Furlano also say that an inherent assist is given by the low and uniform pressure distribution of the structural foam process, compared to the high pressures of solid injection molding, to maintain the dimensional precision, especially in the long dimension of the 14-lb, 20-inch-wide by 8-inch-long by 8-inch-high part.

The structural composite base minimizes ribs and stiffeners and requires no secondary operations, such as deflashing, drilling, or tapping before assembly. An overall system saving of 35%, compared to the previous metal design, is achieved by reduced piece-part cost, increased part functionality, and the decreased time required to mount multiple subassemblies, because of the base's "z-axis, from-the-top" assembly configuration, which eliminates need for costly handling of the base on the assembly line.

additives for BMC

Recent advances of bulk molding compound (BMC) have been sparked by innovations in additives, says Larry E. Nunnery, Jr., president, Bulk Molding Compounds, Inc. Included are coated filler systems that upgrade scratch and staining resistance, and gloss retention, permitting new applications in housewares; a low-profile additive package that improves colorability, with much deeper and darker blues, reds, and blacks, and systems for BMC that enhance glass integrity after processing.

Several BMC manufacturers have taken advantage of the increased impact capabilities, in conjunction with vinyl ester resins, to develop under-hood automotive applications. Nunnery says BMC's cost, dimensional stability, and heat resistance have made inroads in automotive lighting over the past five years. Valve covers, timing chain covers, cam covers, and intake components have been made possible in near-future models by the tougher, more dimensionally stable BMC systems.

Other growth areas are chemical-resistant BMC heating and air-conditioning systems, as replacements for painted sheet metal. Thermoplastic candidates to meet the 500|degrees~F-plus temperatures are comparatively costly, while painted metal cannot withstand the chemical attack. Nunnery mentions a recently developed Heatcraft heating system, in which chemical- and heat-resistant BMC 400 increased drain pan life from an estimated 5 to 8 years to 15 to 20 years.

New high-heat additive packages also have improved BMC color retention, resulting in better cost competition with stamped and painted aluminum for electrical kitchen products. Refined filler systems provide molded-in decorative capabilities, such as a granite, marble, or stone finish. Some of the BMC systems to be introduced in 1993 will demonstrate harder surfaces, more stain resistance, and tougher properties than have previously been possible.

The SMC Alliance, Composites Consortium, and BMC, Inc., have been evaluating the potential of regrinding the materials for reuse. The MTS Box, a reusable shipping container introduced by BMC, Inc., and made of recycled bulk molding compound, is now used by about 40% of the company's customers.

more alternatives

The role of alternate approaches to producing structural composite parts continues to expand. Under-hood fan shrouds for Class 8 heavy-duty trucks, for example, have traditionally been made of stamped metal. As a result of concerns about weight and corrosion, this application has significantly shifted to hand lay-up or resin transfer molding of fiberglass-reinforced polyester, depending on production volumes and ability to justify tooling costs. Steven Ham, technical marketing manager, Cashiers Plastics, says that glass-reinforced polypropylene, injection molded via the structural foam process, and utilizing a chemical blowing agent (in this case, azodicarbonamide), has replaced thermosets in many applications.

Freightliner Corp. has tooled up for fan shroud production with 20% glass-reinforced chemically coupled polypropylene. Ham says the smaller clamp tonnage requirement for structural foam molding, which typically requires longer cycle times, is offset by the larger machine sizes that would be needed for solid injection. He calculates that with solid injection molding, for a 3-mm wall-thickness, 30% glass-reinforced, 38-inch by 30-inch fan shroud, the exposed surface area of 1140 |in.sup.2~, figured at 3 ton/|in.sup.2~, requires 3420 tons of clamping force. By contrast, only 285 tons of clamping force, figured at 0.254 tons/|in.sup.2~, is needed to produce a 5-mm wall-thickness, 20% glass-reinforced part with equivalent performance. Ham maintains that the lower part cost of structural foam justifies the cost of the mold in 2000 to 5000 parts, depending on the complexity of the design.

team emphasis

The industry trend of team efforts and emphasis on partnerships to foster competitiveness is strongly manifest at Owens-Corning. Activities include part and/or process development with resin suppliers, molders, and end users; work with industry consortia and Composites Institute task forces; and involvement in a variety of university programs, such as development of bridge applications for structural composites. Richard Kaverman, manager, Marketing Support Services, says meeting customer needs and fostering global growth in the reinforcements business, with emphasis on enhanced integration of the resources from the company's diverse R&D centers, are keys to market success.

Specific product/process developments and activities include improved performable glass mats for resin transfer molding and focus on transfer of the SMART (Secured Modular Automotive Rail Transport) car hauler pultrusion composite technology to applications such as truck trailer bodies and containers. Development of a robotized process at Owens-Corning's laboratory in Battice, Belgium, advances the ability to position reinforcement in a precise pattern, providing consistency and repeatability that are often lacking or difficult to achieve in traditional spray-up preforms. The company is working with an industry group to foster the commercial availability of the production process. One possible application is the more efficient production of automotive bumper beams.

Uses also are growing for Owens-Corning's 101C chopped strand for thermoset bulk molding compounds, offering significantly increased impact strength and improved dimensional stability for uses such as valve covers and other underhood components, and for dimensionally critical headlight housings. To be introduced is the first of a series of new chopped strand products for thermoplastics. Number 144 for polypropylene will be followed by two new products for other specific thermoplastic resin families.

development programs

Industry/customer-driven development programs for structural polymer matrix composites at PPG Industries fall within the areas of glass rovings, mats, and preforms, primarily for automotive applications; rovings and chopped fiber strands for thermoplastic and phenolic resins; and defining ultra-high-strength composite performance boundaries.

For automotive, the goal is to extend bumper beam, spring, manifold, valve cover, and oil pan applications. Development of preform technology for frame members is being conducted in conjunction with the Automotive Composites Consortium, and PPG is also a participant in a consortium in Europe to develop an automated method for precise preform manufacture.

Short-term reinforcement development for thermoplastics focuses on improving fiberglass processability and composite property levels. The company says maximum reinforcement strengths have been nearly achieved in some resins; in others, reinforcement efficiency is less than theoretical. For longer-term research, PPG has been selected as the glass fiber participant in the Ford/GE five-year program to demonstrate the ability to make structural composite parts from GE's cyclic thermoplastic chemistry.

Reinforced phenolics are also growing in importance for structural applications. PPG has developed a roving for thick-section phenolic pultrusions of rod and mill shapes in which uniformity of dispersion and mechanical strengths have been maximized and void content minimized.

Consistent with the need for design and performance data for successful replacement of steel and concrete, PPG has studied long-term static fatigue (stress rupture) and creep characteristics of unidirectionally oriented E-glass and thermoset resin composites.

new automotive thermosets

The automotive industry's interest in structural composites is largely driven by continuing weight reduction goals, sparked by the Corporate Average Fuel Economy (CAFE) regulations, and efforts to reduce costs through parts reduction and processing advantages. Douglas Brown, automotive market manager, Reichhold Chemicals, Inc., foresees new high-modulus, low-specific-gravity unsaturated polyester (UPE) and vinyl ester (VE) composites growing in underhood and under-car structural components such as brackets, radiator supports, and fender liners. High-strength UPE composite systems with specific gravities near 1.10 (equivalent to that of thermoplastic polyesters) enhance the potential, Brown says, for the materials in grille-opening reinforcements, and cross-car structural beams in the cowl area and under the instrument panel. New zero-shrink formulating technology, and the ability to sustain E-coat bake-oven temperatures, are providing thermoset polyester composites with the ability to compete with engineering thermoplastics in a wide range of structural applications, where tight under-hood dimensional tolerances are critical. Brown adds that tough structural UPE composites are passing Federal Motor Vehicle Safety Standards for energy management, thus positioning the thermosets for applications such as instrument-panel knee bolsters and glove-box doors.

Brown also says that Reichhold's Atlac improved higher-flow, high-strength ITP (Interstitial Thickening Process) vinyl ester technology facilitates thin-wall sections and improves the composites for lightweight bumper beams; and new UPE technology is providing high-speed cure capability to reduce cycle times for RTM composites to better compete with structural reaction injection molding.

Brown adds that new tough, resilient UPE technology provides automakers with Class A composites for injection or compression molded semistructural vertical body panels that pass high E-coat bake-oven temperatures, and thus reduce costs by facilitating on-line assembly.
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Author:Wigotsky, Victor
Publication:Plastics Engineering
Date:Dec 1, 1992
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