The major objective of this study was to identify engineering issues related to the introduction of new materials and their expected effect on the life-cycle durability of future civil transport aircraft. The committee investigated the likely new materials and structural concepts for next-generation commercial aircraft and the key factors influencing application decisions. Based on these predictions, the committee identified and analyzed the design, characterization, monitoring, and maintenance issues that appear to be most critical for the introduction of advanced materials and structural concepts.
The committee's findings are organized into three sections:
General conclusions, identifying the influencing factors and new materials, processes, and structural concepts likely to see application on next-generation commercial transport aircraft.
A description of the roles of the Federal Aviation Administration (FAA) and of other government, industry, and academic organizations in technology development for next-generation aircraft.
Identification of research opportunities and the committee's recommendations in three primary areas: (1) materials, manufacturing, and structural concepts; (2) methods for assessment of structural performance; and (3) inspection, maintenance and repair.
There has been significant recent progress in the introduction of new materials and structural designs in commercial transport aircraft. For example, toughened, polymeric composite primary structure was introduced on the Boeing 777 empennage, and structural aluminum castings were introduced on the Airbus A340. Aircraft designers continue to apply new materials and structural concepts to provide benefits in performance, durability, compliance with environmental regulations, and most recently, acquisition and maintenance costs. The use of new materials and structures will continue to expand on next-generation aircraft.
As noted, the current turbulent, economic climate affecting the airline, manufacturer, and materials industries has significantly changed the application criteria for advanced materials. As a result, materials performance is no longer the only primary driver for materials selection. Aircraft manufacturers are responding to airline concerns about reducing overall costs, including the costs of acquisition and maintenance. The result is incremental, evolutionary material changes rather than revolutionary ones. The principal barriers to increased use of new high-performance materials are:
Costs (acquisition, manufacturing, certification, life cycle) relative to benefits, vis-à-vis "old" designs based on "old" materials.
Incomplete understanding of basic failure mechanisms and their interactions in advanced materials—particularly composite materials—and their structures.
Industrial conservatism engendered by perceptions of technological risk. The industry lacks the experience that would allow the understanding of the durability of advanced materials and structures.
The state of the materials supplier base. The specialty materials industries find it difficult to make the longterm financial commitment needed to undertake a major development program.
Principal "new" airframe materials expected to realize increased use in the next generation of advanced civil aircraft include polymer-matrix composite primary structure (laminates, tailored forms, woven and sewn three-dimensional configurations, automated tape and tow placement) and advanced metals and alloys (tough aluminum, high-yield aluminum, aluminum-lithium, high-strength titanium, and high-strength steel). Continued incremental improvements in metal alloys represents a low technological risk in that the design tools and characterization methods, analytical tools, and design issues differ little from current procedures. On the other hand, the expanded application of composites in primary structure and the application of innovative composite and metals processes (e.g., net-shape processing) requires substantial improvements in characterization and analysis methods and thus represents a higher technological risk and is unlikely to proceed as rapidly.
In contrast, given the emphasis on incremental technology advances and total costs, the committee does not foresee significant application of metal-matrix composites in the
airframes of next-generation transports. New advanced materials applications in major subsystems such as carbon/carbon composites in brakes and ceramic nozzles in auxiliary power units have been more aggressive. These components have not been included in this study and may be worthy of future consideration by the FAA.
Increasingly, airframe manufacturers are using an integrated product development approach that considers such factors as producibility, cost, nondestructive evaluation (NDE) methods and criteria, and repair and maintenance issues and involves airline designers, manufacturers, and suppliers from the outset of development programs.
Commercial aircraft are built and operated on a global basis with international teaming of manufacturers, suppliers, and fabricators. Accordingly, the development and harmonization of international standards for materials and processes, testing and evaluation, NDE, and repair and maintenance procedures are critical to developing and commercializing new materials and structures technology. In spite of the international teaming involved in developing a new aircraft, the manufacture and operation of commercial transports remains an extremely competitive business. The committee believes that competitive pressures will continue to influence the criteria for the application of new materials and processing technology.
There are a number of organizations—airlines, aircraft manufacturers, suppliers, the FAA, university researchers, the National Aeronautics and Space Administration (NASA), and the U.S. Department of Defense—that are involved in the development and application of new materials and structures technology for aircraft. The airlines are responsible for establishing performance needs that will keep them competitive and for establishing and implementing inspection and maintenance procedures required to operate the aircraft in a safe and cost-effective way. The aircraft industry and their suppliers are ultimately responsible for the development, evaluation, application, and validation of new technologies and operating procedures for production-scale utilization. Industry focus is on short-term developments and technologies that will allow them to remain competitive.
While the industry performs a significant amount of research and development, it does not generally perform basic, precompetitive research. NASA has been described as "the only organization in the United States with both the capability and the mandate to perform the basic research as well as the ground and flight testing necessary to validate new concepts to the extent that they can begin to be incorporated into commercial aircraft" (NRC, 1992). NASA has a history of success in this type of activity, including composite long-term testing and flight-service evaluations and the current Advanced Composite Technology program discussed in chapters 2 and 4.
University research organizations work in concert with industry and government efforts to provide basic research and development tools, evaluation and analysis, and workforce education and training.
The continuing research and development activity in materials and structures conducted by the FAA is fueled by the responsibilities and mandates of the organization. First, the FAA must continue to keep abreast of technology needed to support their Aircraft Certification Service and Flight Standards Service in the certification of new aircraft and the effort to monitor the safety of the aircraft fleet. Second, the FAA has a mandate to undertake research to develop technologies to assess the effect of aircraft design, maintenance, testing, wear, and fatigue and to develop improved technology and practices for maintenance (including NDE) (P.L. 100-591) and to develop technology to assess the risk of and prevent failures or malfunctions that would lead to catastrophic failure (P.L. 101-508). In addition, the FAA is the logical organization to work with industry in the harmonization of standards and operations and maintenance procedures with foreign industries and agencies.
As described in the previous section, technological advances are brought about through the concerted efforts of airline, industry, academic, and government organizations. In forming their recommendations, the committee identified the technologies that are likely to be involved in the development of next-generation aircraft and outlined the work required to bring about those developments. The recommendations are directed toward all of the organizations involved in new materials applications, but also specifically toward what the FAA role should be in these developments.
In general, the committee recommends that the FAA remain involved in all stages of the technology development process, with emphasis on work related to aircraft safety, operations, maintenance, and nondestructive evaluation.
Materials, Manufacturing, and Structural Concepts
The future improvements in aircraft structural components will continue to be based on factors related to materials selection, analytical methods, structural concepts, and processing innovations. With the increased emphasis on affordability, it is probable that fewer new materials will be developed. On the other hand, robust and cost-effective processing methods as well as compliance with environmental
regulations will become paramount issues to provide lower costs. Forming, joining, and finishing processes that contribute to reduced labor hours and fewer detailed part counts will have a major impact on reduced overall acquisition costs.
Specifically, the committee envisions continued improvements in the performance and durability of metal alloys and polymeric composites. Materials and structures more conducive to low-cost processes such as casting, high-speed machining, and superplastic forming/diffusion bonding of metals and fiber placement, resin transfer molding, and nonautoclave processing of composites will be emphasized in the future.
The committee recommends that the FAA work with industry, government, and academic organizations in the development of new materials, processing, and structure technology by the following guidelines:
Keep abreast of innovative materials processing technologies that provide methods for low-cost fabrication of aircraft structure. Emphasis should be placed on the understanding of new product forms, processing methods, and thermal treatments and their possible effects on materials performance.
Support the development of emerging process modeling techniques for definition of processing parameters and requirements.
Establish and maintain databases of material and structural properties resulting from the candidate processing methods. The databases should include test methods, physical and mechanical properties, failure modes, and influences of probable defects and manufacturing processes on property behavior.
Work with the materials, manufacturing, and airline industries to develop industrywide standards to improve consistency in the final products, especially with the increasing globalization of materials availability.
Participate in industry-and NASA-sponsored flight hardware demonstration programs for the introduction of new materials, manufacturing processes, and structural concepts in high-risk applications. FAA emphasis should be on validation of inspection and repair techniques and in the development of technology needed to certify and monitor these structures.
Methods for Assessment of Structural Performance
Current structural design and analytical procedures used by the aircraft industry are largely semiempirical, even though significant improvements have occurred in structural analysis methodology over the last two decades. Accurate, finite element analysis methods are used routinely for predicting the stress, strain, and displacement fields in complex structural geometries. However, the reliable prediction of structural failure modes, ultimate strength, residual strength, and fatigue life has remained elusive to the structural engineer. The current standard practice relies heavily on extensive testing at the coupon, subelement, element, subcomponent, component, and full-scale levels. Design details are frequently optimized through test programs. Scale-up effects are handled through a building-block approach that relies on testing to verify the anticipated structural performance at each scale level. While the committee anticipates that this building-block approach to structural design will continue indefinitely, a more rigorous, analytical prediction methodology will greatly improve the process of introducing new materials into airframe primary structure.
The committee recommends that the FAA work with other industry, government, academic organizations in the development of improved analytical methods by the following guidelines:
Support development and facilitate implementation of advanced analytic and computational methodology to predict residual strength as a function of time.
Support programs to improve the understanding of basic failure mechanisms in advanced materials and their structures. Include the interactions of the various failure modes manifested at the various length scales— from material to structural levels.
Inspection, Maintenance, and Repair
The successful application of new materials and structural concepts relies on an effective maintenance program that is cost-effective, while ensuring passenger safety. The aging aircraft experience has provided the airline industry with significant lessons learned for inspection and repair technologies. These lessons provide a framework for improving inspection and repair processes for next-generation materials. Major issues that continue to limit the effectiveness of an aircraft maintenance program are poor structural inspection standards, inadequate defect indication interpretation, unreliable inspection techniques, high cost of new NDE methods, and limited linkage with design analyses and NDE results. The leadership of the FAA and the continued participation of airlines and manufacturers in developing and implementing improved maintenance and inspection methods is crucial.
The committee recommends that the FAA take a leadership position in the development of improved inspection and maintenance methods by the following guidelines:
Support the development of improved standards for NDE methodologies and their specific materials and structural applications, especially through participation
in industry-and NASA-sponsored component development and flight hardware demonstration programs for the introduction of new materials, manufacturing processes, and structural concepts.
Support the development of cost-effective, quantitative NDE methodologies for in-service inspection of airframe materials and structures. Emphasize improved defect detection reliability, cost-effectiveness, and ease of implementation in field environments. Particular attention should be given to rapid, wide-area inspection with limited or one-sided access.
Develop improved analytic methods to determine NDE reliability and inspectability of materials and structures to support damage tolerance and durability analyses.
Support the development of real-time repair and maintenance processes for materials and structures that use the results from quantitative NDE methods and computational analyses.