With the growing use of carbonfibre composites in aircraft construction, new certification standards are being developed for an evolving technology
Carbonfibre composites, long used in the primary structures of high-performance combat jets and light general-aviation aircraft, are now the future of commercial transport aircraft design, especially with the advent of the Airbus A380 and Boeing 787. Designers favour composites because of their weight saving and damage tolerance, but their increased use is posing a challenge for airworthiness authorities and repair organisations more used to understanding metallic structures.
While the A380 – 25% composite by weight – has an all-composite centre wing-box, empennage and tailcone, composites account for about 50% of the 787's structural weight, and are the dominant material in the wing, empennage and fuselage. This represents the most extensive use to date of non-metallic materials in airliner manufacture, particularly in major load-bearing structures, and fuel savings are a primary incentive.
For airworthiness authorities, the shift away from conventional metal structures means having to develop new certification standards in tandem with evolving composite technologies. To help in this task, the US Federal Aviation Administration has formed a partnership involving industry, government and academia under an organisational structure known as the Joint Advanced Materials and Structures (JAMS) centre of excellence. The organisation, managed by the agency's technical centre in Atlantic City, New Jersey, was formally announced in December 2003.
JAMS was structured as a "joint award", creating two additional centres of excellence – Advanced Materials in Transport Aircraft Structures (AMTAS) located at the University of Washington in Seattle; and the Center of Excellence for Composites and Advanced Materials (CECAM) at Wichita State University, Kansas. Both work with other academic institutions and industry partners on research projects.
Curtis Davies, the FAA's JAMS programme/research manager, says about 95% of the research projects being managed and funded through JAMS concern composite materials or adhesive-bonded structures technology. The two lead universities are responsible for project supervision and administration, while research funding is sent directly to each institution. The University of Washington's AMTAS partners are Oregon State University, Washington State University and Ed-monds Community College. Wichita State University's CECAM is partnering with Northwestern University, Perdue University, the University of California-Los Angeles, the University of Delaware and Tuskegee University in Alabama. The FAA has a technical oversight role for all of the research.
"The FAA's two major areas of focus concerning advanced materials involve composite damage tolerance and bonded structures technology, because composites are a prime contending technology in aircraft design," says Davies. "With the all-composite fuselage of the new Boeing 787 pushing the envelope, we have to be up to speed on what the safety issues and certification process with the new technologies might be. The challenge is to keep up with the evolution in the composite world, and these centres of excellence have been established so that the FAA's knowledge of composites evolves at the same time."
Research funding
Davies says the FAA promised to commit $300,000-500,000 to the University of Washington, as well as to Wichita State University, from July 2004 to July of this year to fund the 20 advanced materials research projects that the lead schools are overseeing with their partners. At least 15 projects concern carbonfibre composites, or the bonding of those structures. From July this year to July 2007, the FAA has committed a further $300,000 a year for each of the two lead universities to continue research. The funding is in the form of a matching grant, with the extra money coming from non-government sources.
The focus of the projects ranges from damage tolerance to basic structural substantiation, which deals with strength, fatigue and load-bearing, says Davies. "We are also looking at durability, bonded joints and structures, maintenance and repair practices, advanced material forms and fabrication processes, cabin safety and crashworthiness, and the need for shared databases, material standardisation and materials life management."
AMTAS director Dr Mark Tuttle, professor and chairman of the University of Washington's mechanical engineering department, says the centre is concentrating on the polymer-based composites now making inroads into large aircraft structures. The most prominent is carbonfibre epoxy (called graphite epoxy in the USA), he says. "Until recently, material and manufacturing costs were prohibitive. But there have been some reductions in costs, as well as improvements in the manufacturing technology. This is why we will see increased use [of epoxy] in large, load-bearing structures, that use a continuous-fibre primary composite structure."
Major research involving such areas as composite damage tolerance, adhesive bonding and repairs is being carried out at CECAM, says the centre's director, Dr John Tomblin, also executive director of the Wichita-based National Institute for Aviation Research (NIAR). Another major research project involves crashworthiness, which seeks to define the high-rate mechanical properties of composites for use in analytical models to predict factors such as impact absorption. "These tools would give a designer up-front knowledge of the material's crashworthiness, helping him to build a more crashworthy composite structure," he says.
NIAR is the only research body actively studying ageing composites, says Tomblin, through a project that began in March 2004 employing ultrasonic inspections and mechanical testing. "We have been using a horizontal stabiliser from a Boeing 737 which was manufactured in the late 1970s, flew in commercial service for 18 years, and has more than 50,000 flight hours and 45,000 cycles. We are also using composite material from a Beechcraft Starship. Our research has indicated that composites age much better than aluminium structures because they are not susceptible to corrosion and are inherently fatigue-resistant."
Damage tolerance
Tomblin says the research provides a "damage-tolerance approach to composite design", which means a composite structure will be able to meet its design requirements despite the presence of damage that is not readily visible. "Composites need to be designed with this approach in mind, which is the basis of a programme that has been ongoing for five years."
Bob LaMantea, vice-president and chief financial officer for Integrated Technologies (INTEC), says a huge database, generated from about 20 years' industry experience, has helped to assure the safety of composites in large airframe structures. INTEC, based in Everett, Washington, is a leading composites tester and manufacturer, and an AMTAS partner. "At the same time, we now have a good basis for testing and certification, which has enabled the industry to make the progress it has in using composites in more complex structures," he says.
According to LaMantea, with a better understanding of composites and what they can do, airframe manufacturers can build more integrated structures, as opposed to individual parts. "Improved tooling and analysis have made this happen," he says. "Composite manufacturers have shown they can make a better product, and have been responding to the [aircraft] OEMs' [original equipment manufacturer] demands. It has really been a market-driven evolution."
But significant issues remain, he adds. "For instance, there is no book available today which will teach you how to assess long-term degradation of composites. The industry is addressing this, because it will be a key issue for the future." Another major problem is the lack of standardisation for composite products. The composites manufacturing industry largely comprises companies that provide a highly customised, "boutique" type of product, he says.
"They are making materials to the specifications of individual customers, so we have a lot of proprietary items evolving. Given the huge amount of money involved in developing these items, nobody is going to share the design data."
Lack of standards
Some efforts are being made to minimise the effect of this lack of standardisation. One, known as the Military Standard 17 Group, is a consortium of industry and government representatives involved in composites testing. "Their goal is to develop a handbook for structural applications of composite materials," says LaMantea. The AGATE (Advanced General Aviation Transport Experiments) public/private partnership, set up in 1995, also had developing standards for composites fabrication and repair among its aims.
"In the final analysis, it's a matter of training and having the right people available," says LaMantea. "At the same time, as composites are used to a greater extent on transport aircraft, problems will arise for which there are no solutions in place now. It's a little hard to solve many of the problems until they actually occur."
To have the right people available, a workforce experienced in composites and composite repairs must be developed, says CECAM's Tomblin, and this is one of the industry's big challenges. "The military has had a lot of composite expertise for many years, and the general aviation industry has had it for the last decade. For transport-size aircraft, there is experience, but it is mostly with small structures, such as engine nacelles. That is the kind of expertise that will have to be built upon."
LaMantea says there is no question flight-line repairs involving composites will differ a lot from those involving aluminium, especially when working within the time restraints of an aircraft-on-ground situation, where a quick turnaround is mandatory. "The expertise to do this takes an entirely different skill set and training level from those for metal, and a different type of repair altogether. For example, with metal you can just rivet a patch into place. Composites involve a hot-bonding technique."
Asked if the industry will need to invest in expensive equipment for composite repairs, LaMantea says it will not be necessary for local-level, minor work. "It will be more of an issue with training, and maintaining an inventory of repair material such as composite bonding kits. Large amounts of damage will probably result in replacements, rather than repairs."
Dr Frank Simmons, a structures staff scientist and technical fellow with Gulfstream Aerospace in Savannah, Georgia, agrees that the biggest issue with composites maintainability over metallic structures is that of training – and awareness of how to recognise potential damage. "Damage to metallic structure is easily identified because it normally appears as dents, buckles or cracks. However, composite structure damage will appear as broken fibres instead of dents, and interlaminar delaminations instead of cracks."
Roland Thevenin, senior expert in composite structures for Airbus in Toulouse, says some parts of the repair industry are already well prepared to work on large composite structures. He cites the Airbus maintenance, repair and overhaul network of 13 independent support facilities, formally established in March this year. "They have experience on the composite structures used on other Airbus products, and are positioned to support the new A380, as well as the proposed A350, which will be 35% composite by weight," he says. "Although we are using composites on our new aircraft more extensively, the design principle of the composites has not changed. The same composite repair procedures will apply to the A380, and no additional tooling will be needed, especially since many of those repairs will be done on-wing."
By comparison with metal airframes, no special inspection procedures are needed, says Thevenin. "The stresses composite structures are subjected to will be a very low percentage of their maximum stress-bearing capability. With composites, you don't have the kind of crack initiations you have with metal. Any damage not visible to the naked eye will not propagate or lead to additional damage, as you would have with metal structures."
Repair facilities
Southwest Airlines, which has composite repair facilities at its Dallas Love Field, Phoenix Sky Harbor International and Houston Hobby hangars, has been repairing composite structures on its Boeing 737-300 and -500 fleet since 1996 and, more recently, on its new 737-700s. The airline is studying the establishment of an additional composite shop at Chicago Midway airport, says Ed Montalvo, a Dallas-based maintenance supervisor. All the repairs follow the Boeing structural repair manual, he says, with any extensive composite structure overhauls contracted out to specialists. All Southwest mechanics who work on composites have taken special training at Dallas, involving lectures and hands-on instruction.
For Montalvo, the biggest problem with composite repairs concerns the 737-700, specifically the thrust reversers and rudder. "We are working with Boeing to make the structural repair manual for that aircraft more user-friendly, since many of the repairs described for the 737-300 and -500 were taken from the 737-700 manual," he says. "If those repairs were put back into the manual, we could do them without having to consult Boeing, and know if it is something we could do ourselves."
Goodrich, which has one of the world's largest composite repair capabilities, warns the MRO industry to prepare for increased use of composite structures, which will require a high level of process control and complex non-destructive inspection equipment and processes. "Composites have been in place on commercial transport aircraft for over 20 years," it says. "Those who have gained experience over that time are well positioned to meet the needs in the composite environment."
PAUL SEIDENMAN/SAN FRANCISCO
Source: Flight International
