AGATE Methodology Proves Its Worth
The Advanced General Aviation Technology Experiments (AGATE) Consortium was founded in 1994 by NASA, the U.S. Federal Aviation Admin. (FAA) and about 70 other members from industry, academia and other government agencies. Its mission was to revitalize the general aviation industry in the U.S., delivering more aircraft in half the development time. By improving small aircraft, the consortium hoped to shift more air travel to public-use landing facilities, decreasing dependence on the limited hub-spoke airport system. The result would be a significant increase in capacity and shorter travel times.
AGATE was to accomplish this mission through the development of affordable new technologies, standards and certification methods for airframe, cockpit, flight training systems and airspace infrastructure for single pilot, light airplanes. Program work was divided into ten technical areas or work packages. The mission of the Integrated Design & Manufacturing (ID&M) work package was to reduce airframe and propeller cost and weight. The Materials Group within ID&M was tasked with developing a design manual for the use of composites in general aviation (GA) aircraft. Its specific tasks included standardizing test methodologies, developing a database of materials properties, developing methodologies for manufacturing process control and for suppliers to incorporate new materials and processes, and shortening the materials development and certification cycle from five to three years.
In April 2001, the FAA released DOT/FAA/AR-00/47, "Material Qualification and Equivalency for Polymer Matrix Composite Material Systems," known as the AGATE methodology for short. This document presents the detailed qualification plan used to generate statistically based design allowables for composites at the lamina (ply) level. The A- and B-basis allowables give end users a statistical confidence that the material properties will be at least as good as the minimum allowables (see HPC May/June 2000, p. 27, for a more complete discussion). More specifically, the methodology covers carbon or glass prepregs that are cured and processed at 115.5°C/240°F or higher, using both autoclave and vacuum-only cure cycles. The methods also can be applied to a broader range of composite materials.
Although the AGATE program ended in November 2001, the composite qualification methodology remains an active standard today. FiberCote Industries Inc. (Waterbury, Conn.) and Toray Composites (America) Inc. (Tacoma, Wash.) were the first two companies to use the AGATE methodology to develop materials databases, and they have continued testing to add additional materials to the database. In July 2002, Advanced Composites Group Inc. (ACG, Tulsa, Okla.) added a high-temperature prepreg material. Newport Adhesives and Composites Inc. (Irvine, Calif.,) has added an intermediate-temperature prepreg.
FiberCote has applied the AGATE method to a family of materials based on its E-765 resin system. The prepregs include a T300 3K plain-weave carbon, a T300 6K 5-harness satin carbon, a T700 unidirectional carbon and a 7781 fiberglass. A T700 12K twill carbon and 12K plain-weave carbon are currently in the screening phase. The 6K satin prepreg is qualified under a 3.1 bar/45 psi autoclave cure; the other materials are qualified under vacuum bag cures. E-765 cures at 121°C/250°F and provides an 82.2°C/180°F wet service temperature.
Toray has applied AGATE to three materials based on its 2510 resin system. These prepregs include a 7781 fiberglass, a T700G 12K unidirectional carbon tape and a T700S 12K plain-weave carbon fabric. The 2510 system is designed for both autoclave and non-autoclave use. It cures at 132°C/270°F, yet provides a hot-wet glass transition temperature of 128°C/262°F (146°C/294°F dry). Although the carbon fabric is made from 12K tow, it has an areal density of 193 g/m² (5.67 oz/yd²) — the same density as a 3K fabric. The individual fiber bundles are more like miniature unidirectional tapes than tows, making the fabric exceptionally flat and easy to handle, even on highly curved parts. The fabric also demonstrates excellent damage resistance, though Toray has not determined exactly what gives the fabric this property.
ACG is targeting more traditional aerospace materials with its HTM45, a 177°C/350°F cure, toughened epoxy system. Two forms have been tested using the AGATE methodology: a 0.368 mm/14.5 mil, 8-harness satin carbon, and a 0.20 mm/7.7 mil, plain-weave carbon. With this material, ACG is targeting military aircraft manufacturers. ACG also is applying AGATE to its MTM45 resin system, which is designed to cure between 79.4°C/175°F and 121°C/250°F. Future reinforcements to be tested include additional carbon weaves, quartz and glass fabrics, and unidirectional carbon.
Newport has applied AGATE to three material forms based on its NB 321 prepreg resin system: a 7781 glass fabric; a Grafil Inc. (Sacramento, Calif., U.S.A.) 34-700 unidirectional carbon tape; and a 3K 70P plain-weave carbon fabric. The NB 321 system cures between 135°C/275°F and 163°C/325°F. Newport also has three supplemental AGATE method material databases for use with the AGATE materials: aluminum and copper mesh lightning strike materials impregnated with NB 321; and an NB 101-compatible film adhesive. All material forms have been accepted by the FAA and are now flying on production-certified aircraft.
Material manufacturers seem to be unanimous in their support of the new AGATE methodology. Most aerospace materials today are currently qualified to performance-based customer specifications. To sell the material to another company, the vendor must requalify to a new specification. The result is usually a significant investment in testing of a single material that, in the end, produces duplicate data. "The advantage of the AGATE methodology," says Chris Ridgard of ACG, "is the testing is done once and offered to multiple end users. The responsibility of producing the data is more on the material supplier, with the supplier assuming responsibility for selecting the product forms, resin content and other factors that define the specification." This approach is cost-effective for both material suppliers and end users. Ultimately, the goal is to create material standards driven by industry-wide specifications instead of company specifications, analogous to the way metals are currently bought and sold. Once a material is qualified by the AGATE method, it is accepted by the FAA, the U.S. Department of Defense, the U.S. Department of Transportation and other organizations, provided their individual requirements are met.
The original AGATE program covered only lamina properties. The next step, according to Dr. John Tomblin of Wichita State University (Wichita, Kan.), is to extend the methodology to the laminate level. By using the lamina statistics, it should be possible to reduce the amount of testing at the laminate level and use the building-block approach proposed by MIL-HDBK-17. Each initial test series will be performed on laminate samples from three different batches of materials and establish populations characteristics of each failure mode. "The idea is to form a bridge between lamina, one-batch laminate and three-batch laminate data," says Leslie Cooke, director of AGATE prepreg business development at Toray. If bridging works, then three-batch testing may not be required in the future. Tomblin thinks this approach will work because historically it has been proven with MIL-HDBK-17 materials. Toray submitted one-batch laminate data to the FAA in February 2003. FiberCote has received approval of one-batch laminate data for its T300 carbon fabrics and T700 unidirectional carbon tape.
The AGATE methodology, however, is not just about generating numbers. MIL-HDBK-17 gives guidelines for the statistical characterization of composite material properties, but the available data has not found widespread use in applications. The problem, explains Curtis Davies of the FAA Advanced Materials and Structures Branch, is that much of the data in MIL-HDBK-17 is of uncertain pedigree. Although the statistical guidelines were followed, details of the test method may not be clear; testing may have been performed on laboratory instead of production batches or the procurement and processing requirements may not be available.
In contrast, the metal data listed in MIL-HDBK-5 has a reliable pedigree and the materials are widely used. Metal alloys are produced in a factory setting, where the processes can be tightly controlled. Manufacturers who use metals machine the materials to produce their products: they do not create the metals themselves. "It's important to recognize composites are process intensive," says Davies. "You need to have more strict controls on procurement and processing." The final composite properties are not determined until they are cured, so both the material manufacturer and part fabricator are involved in the processing. Two different end users can start with the same prepreg from a single manufacturer, and yet end up with significantly different finished properties.
The AGATE methodology recognizes this process dependence. Material vendors establish the initial database through a qualification program. Airframe manufacturers then perform a small subset of the original testing to show that their procedures result in materials statistically equivalent to the original data set. Although companies have been able to demonstrate equivalency, Davies points out that some have had difficulty because of the small number of test coupons used in certain property tests. Determining the appropriate population of test coupons in cases like those will be part of the ongoing development.
The process dependence is even greater for composite parts made by liquid molding processes such as RTM. With prepregs, a single material supplier is responsible for combining the reinforcement and resin, and then generating allowables. With RTM, the end user takes fibers and resins, usually from different manufacturers, and then combines them when the part is built. In October 2001, Tomblin co-authored two reports that established a qualification methodology and presented B-basis allowables for RTM parts made with Cytec Engineered Materials Inc. (Tempe, Ariz.) PR520 resin and A&P Technology Inc. (Cincinnati, Ohio) braids made with Hexcel (Dublin, Calif.) AS4 carbon fiber. Whereas prepreg allowables were generated solely by the material vendors, the RTM allowables document refers specifically to the part manufacturer, Raytheon Aircraft Co. (Wichita, Kan.).
John Tauriello of FiberCote points out that the AGATE Method also outlines a procedure to qualify an AGATE material as a second source. Recently, FiberCote's E-765 system was qualified as a second source at a globally recognized business jet manufacturer for use on several primary structures. The program established the E-765 system as a statistically acceptable alternate to the originally qualified material. Regulatory approval is pending, but Tauriello believes this is the first time an AGATE method database has been used to streamline the qualification of a material to an existing customer specification.
The first two certified, production airplanes to make extensive use of AGATE technology, including composite materials, were the Lancair Columbia 300 (Lancair International Inc., Bend, Ore.) and the Cirrus SR20 (Cirrus Design Corp., Duluth, Minn.). Since then a number of other companies have chosen to use AGATE materials in both certified aircraft and non-certified programs. For example, FiberCote materials are used on the C-130 (Lockheed Martin), the Dash 8 (Bombardier), a propeller backplate manufactured by Texas Composite Inc. (Boerne, Texas), and a SATCOM radome for A320 and DC-9 aircraft flown by JetBlue Airways; non-certified applications include the NemesisNXT kit plane (Nemesis Air Racing, Mojave, Calif.) and the Atlas V solid rocket motor fairing manufactured by HITCO Carbon Composites Inc. (Gardena, Calif.). A&P RTM/braids have been used for the wing flaps of the Raytheon Premier I, propeller blades by Ratier-Figeac, stator vanes on the GE Honeywell jet engine by Advanced Technical Products Inc. (DeLand, Fla.), and for vertical tail structures on the Joint Strike Fighter (Lockheed Martin).
The Adam Aircraft A500 (Adam Aircraft Industries, Colo.; see HPC July 2002) is the first all-composite aircraft to embrace AGATE. Adam Aircraft chose Toray's 12K carbon fabric, unidirectional carbon tape, and 7781 glass fabric for the aircraft structure. Pierre Harter of Adam Aircraft reports that equivalency qualification of the carbon materials has been completed, and testing of the glass prepreg is almost complete. "Testing has been very straightforward, with no problems at all," says Harter. Test reports were slated for publication by the middle of March 2003. Harter estimates that using AGATE materials saved the company at least one year and $160,000 (USD) in testing costs. Acceptance of Toray's laminate data will save even more by eliminating the need for laminate testing by Adam.
The Liberty XL2 (Liberty Aerospace Inc., Melbourne, Fla.), a new two-seat touring aircraft, also uses the Toray carbon fabric, carbon tape and glass fabric materials. Composites make up about 11 percent of the total structural weight. Jason Russell, chief design engineer, explained that Liberty looked carefully at where it made the most sense to use composites. The fuselage and engine cowls are a thin carbon sandwich laminate; the wing tips, tail plane tips and wing root fairings are glass laminates. A composite wing must carry loads at elevated temperatures. This operating condition would have required Liberty to build an environmental chamber for full-scale testing, adding significant cost, or to use composite static load factors, adding weight. The factors are much smaller for metals, so Liberty chose thin gauge aluminum for the wing structure. The primary load-carrying structure is a 4130 steel truss. Liberty was one of the first companies to perform equivalency testing, so the process took longer than expected. Still, it was very straightforward, and Russell thinks the testing should now take, at most, four months.
The first aircraft incorporating Toray materials to actually go into production and be sold to customers is the SparrowHawk glider from Windward Performance LLC (Bend, Ore.). The SparrowHawk design makes extensive use of Toray's 12K carbon fabric. As a non-certified aircraft, Windward did not need to perform equivalency testing, but Greg Cole, owner and designer, did not even consider a non-AGATE material. Simply having a database of good, reliable material properties enabled Cole to reduce knockdown factors used to account for material uncertainty, thereby saving weight. The total weight of the SparrowHawk is only 70.3 kg/155 lb, yet it is designed to a higher load limit and the same safety standards as a typical 272 kg/600 lb glider. The high conformability and workability of the Toray material also was critical to achieving the extremely low drag of the fully molded design. The SparrowHawk, flown by Gary Osoba, currently holds three world records: the 300 km speed record; the triangle distance record; and the 500 km speed record. The former two records beat the old records by 36 percent and 20 percent respectively, despite sub-optimal flying conditions.
The AGATE databases are proving attractive to users outside of the aviation industry. Delta Velocity Corp. (Leesburg, Va.), for example, recently selected FiberCote's E-765 unidirectional carbon tape for development of a launch vehicle fairing structure under a U.S. Air Force Small Business Innovation Research (SBIR) Phase I contract. A Phase I contract provides $100,000 (USD) in funding and must be completed in six months. "We chose the FiberCote material because it was inexpensive, in stock and had a good properties database," explains Joe Padavano, president of Delta Velocity. "We had to start building parts right away and couldn't afford the time or cost of a material testing program." The same material was chosen for the manufacture of a full-scale flight fairing when Delta Velocity was awarded a Phase II contract to continue development.
The future of shared databases
The AGATE program lives on through continued application of the qualification methodology and the efforts of the MIL-HDBK-17 organization. The general MIL-HDBK-17 Coordination Group Meeting at the end of February 2003, included discussions on incorporating AGATE data into MIL-HDBK-17, extending AGATE to laminate testing and developing procurement standards. In the AMS-P17 Subcommittee meeting, to be held concurrently with the MIL-HDBK-17 Coordination Group Meeting, work also is being directed at standardizing composite specifications. The idea is to specify general classes of composites, such as 121°C/250°F-cure carbon plain-weave prepregs, so it will be easier to show equivalence between different material suppliers.
Material vendors continue to add new materials, and the number of manufacturers using the AGATE methodology is gradually increasing. John McKnight, aerospace business manager at Newport, thinks AGATE growth has been slower than expected, due largely to the overall health of the economy, which also has resulted in slower activity in the general aviation industry. Still, Newport and other suppliers are seeing increased interest from other industries where B-basis allowables are important in product development. In fact, the AGATE methodology proves its worth in tough economic times. As Toray's Cooke says, "The whole thrust of AGATE is affordability, not just of the raw material, but in qualification of the material and the final part."
Wichita State's Tomblin believes the original AGATE program was successful because it brought together the largest consortium of government agencies, companies and universities that had ever worked together. The smaller companies have embraced the AGATE process to the fullest extent, and their successes have shown the rest of the industry that the AGATE methodology makes good business sense. Continued development by these companies and coordination through the MIL-HDBK-17 organization will help to keep the AGATE momentum going.
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