Aircraft safety is the overriding concern of the flying public and is the primary design criteria for the aircraft industry. While the aircraft industry enjoys an excellent safety record, the continued increase in the number of passengers and the size of the commercial transport fleet requires significant reductions in accident rates to achieve the Federal Aviation Administration (FAA) goal of 50 percent reduction in fatalities by the turn of the century (FAA, 1991).
In response to the Aviation Safety Research Act of 1988 (P.L. 100-591), the FAA has initiated a comprehensive, long-range research program concerned with all aspects of civil aircraft safety. The overall objective of the FAA program is to anticipate the technological advances that will likely affect future aircraft and to evaluate their implications for aircraft safety.
New aircraft (i.e., aircraft that, in the context of this report, will enter service in the next 15–20 years) will incorporate new materials, fabrication processes, and structural concepts to realize the cost and performance benefits of lighter-weight, more-efficient structures. One very important concern regarding aircraft safety is the degradation of structural materials as aircraft operate beyond their original design life, bringing to the forefront issues of materials stability, corrosion resistance, fatigue behavior, and maintenance procedures. Evaluation of new materials and structures for commercial aircraft application requires an assessment of performance and operating costs over the entire life cycle of the aircraft, from fabrication to maintenance. Also, a thorough understanding of the material properties and anticipated performance in airline service is required to adequately develop inspection and maintenance procedures that assure safety over the service life of the aircraft.
The major objective of this study was to identify issues related to the introduction of new materials and the effect that advanced materials will have on the durability and technical risk of future civil aircraft throughout their service life. The committee investigated the new materials and structural concepts that are likely to be incorporated into next-generation commercial aircraft and the factors influencing application decisions. Based on these predictions, the committee attempted to identify the design, characterization, monitoring, and maintenance issues that are critical for the introduction of advanced materials and structural concepts into future aircraft. This study focuses primarily on airframe structures of large subsonic commercial transport aircraft with the understanding that many of the issues identified will also apply to smaller general aviation planes (both fixed and rotary wing). The emphasis of this study is, for the most part, restricted to primary and secondary airframe structures.
There has been significant progress in the introduction of new materials and structural designs on commercial aircraft. 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 committee believes that the use of new materials and structures will continue to expand on next-generation aircraft.
The current, turbulent economic climate affecting the airline, manufacturer, and materials industries has significantly changed the application criteria for advanced materials. As a result, aircraft manufacturers are responding to airline concerns about reducing overall costs including the costs of acquisition and maintenance. The result is incremental, evolutionary—rather than revolutionary—changes in materials. The principal barriers to increased use of new high-performance materials are acquisition, manufacturing, certification, and life-cycle costs; incomplete understanding of failure mechanisms and their interactions; technological risk; and the state of the materials supplier base.
Principal ''new" airframe materials expected to see increased use in the next generation of advanced civil aircraft include polymer-matrix composites (laminates, tailored forms, woven and sewn three-dimensional configurations, automated tape and tow placement) and metallic alloys (tough aluminum, high-yield-strength aluminum, aluminum-lithium, high-strength titanium, high-strength steel, and cast products). In contrast, given the emphasis on incremental technological advances and total costs, the committee does not foresee significant application of metal-matrix composites in the airframes of next-generation transports.
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 is 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 selection criteria for the application of new materials and processing technology.
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
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 likely 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. 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.
Methodologies 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 methods over the last two decades. Accurate, finite element analysis methods are routinely used to predict 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 have 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 and structures. 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 make use of the results from quantitative NDE methods and computational analyses.
The findings of the committee have been grouped into five parts. The first provides an overview of the emerging trends and the requirements and drivers for new materials applications. This overview is followed with separate discussions on materials, processes, and structural concepts; analytical methods; and aircraft operations. Specific conclusions and recommendations that emerged as a consensus view during the deliberations of the committee are cited in part 5.