AGING OF MATERIALS AND STRUCTURES
To predict the response of a materials system to long-term exposure in a aircraft structures service environment, a fundamental understanding of the physical phenomena associated with damage and failure must be developed. This can only be established by experimental materials characterization and development of the associated mathematical and computational models that describe the physical phenomena. In many cases, testing standards and codes are available to provide guidance on specific types of component design and test methodology. However, for some advanced materials (e.g., composites) different approaches to materials testing may be required, and there may be a lack of standardized methods. Although the requirement for the same basic material properties may be similar, users often develop their own test methods. The different testing methodologies can lead to ambiguities in the test data, resulting in uncertainties in materials selection, design, and production.
The evaluation of long-term aging responses of materials and structures using accelerated testing and analytical methods is very difficult, especially for the complex conditions encountered in aircraft service. Even the best techniques will probably not yield completely satisfactory predictions of materials performance. However, because new aircraft will be designed, and materials and structures evaluated over their entire life cycle, it is important to develop testing and analysis methods that provide the best possible understanding of materials and structures performance to support materials selection and structural design decisions. It is better to have incomplete data available in time to influence key program decisions than to wait for the completely rigorous data to be developed.
Concerns over these issues have led the National Aeronautics and Space Administration (NASA) to request the National Research Council 's National Materials Advisory Board (NMAB) to identify issues related to the aging of advanced materials and suggest accelerated evaluation approaches and analytical methods to characterize the durability of future aircraft materials and structures throughout their service lives. An NMAB study committee was established to (1) provide an overview of long-term exposure effects on future high-performance aircraft structures and materials; (2) recommend improvements to analytical methods and approaches to accelerate laboratory testing and analytical techniques to characterize and predict material responses to likely aircraft operating environments; and (3) identify research needed to develop and verify the required testing, predictive analytical capabilities, and evaluation criteria. The committee emphasized methods to evaluate materials performance and long-term aging responses and methods to develop property relationships for component design. Methods to evaluate specific design details such as part geometry, joint design (bonded, welded, or fastened), or method of manufacture are not considered within the scope of this report.
Aging of current commercial and military aircraft has become a major concern as many older aircraft reach their original design life. Significant work is being accomplished by the aircraft industry, NASA, and the Federal Aviation Administration in this area, with emphasis on corrosion, widespread fatigue damage, and nondestructive evaluation methods to monitor the aging fleet. This report, however, considers long-term aging in the context of materials selection, materials evaluation, and component design for new systems. The emphasis is on new systems that will be subjected to sustained elevated-temperature environments. The goal is to provide the framework to develop sufficient and accurate scientific information such that aging issues can be predicted and considered in the design process. The ability to understand and predict aging processes can have immediate implications for dealing with existing fleets.
The committee chose to examine issues involved in the evaluation of long-term aging of materials and structures being considered for use on a future High-Speed Civil Transport (HSCT) as a case study. The HSCT provides a valuable case study because the anticipated service conditions are harsh, with severe combinations of thermal, environmental, and stress conditions. Despite the complexity involved, a substantial amount of work has been undertaken by airframe manufacturers and NASA to define service conditions in terms of flight profiles and to establish structural design criteria. A wide range of candidate materials are being considered, including high-temperature alloys, composites, and hybrid structures. Most important, consideration of aging issues for the HSCT represents an important need. The HSCT is considered critical to the future competitiveness of the U.S. aircraft industry, and much work is being undertaken to develop and understand materials for this important application.
CASE STUDY: HIGH-SPEED CIVIL TRANSPORT
The HSCT is a potential long-range, supersonic commercial aircraft being considered by the aircraft industry for introduction early in the next century. The development goals of the HSCT—5,000-nautical-mile range, approximately 300-passenger payload capacity, greater than Mach 2 speed at cruise, and ticket costs competitive with subsonic aircraft—represents a substantial improvement in capability over the existing technology (e.g., the Concorde) (Brunner, 1994; Grande, 1994).
The development of an HSCT poses challenges to the aircraft design community unlike any faced before in commercial aviation. New and future federal and international regulations will require that the aircraft meet strict noise and emission requirements in order to protect earth's fragile environment. In addition, service conditions during extended supersonic cruise are significantly more severe compared with those experienced by subsonic aircraft, supersonic fighters, or even the Concorde. Advanced materials and structural concepts for both the engine and the airframe are critical to the performance and economic viability of an HSCT.
Lightweight structural materials with long-term resistance to thermomechanical fatigue, creep, and environmental degradation in temperature ranges between 100 and 200 °C (212 and 392 °F) for the airframe (and much higher for the engine and for limited areas of the airframe) are needed to attain the speed, payload, and range necessary for an economically successful aircraft.
Candidate materials for the many different structural applications include polymer-matrix composites (both thermoplastics and thermosets), high-temperature and low-density aluminum alloys and aluminum-matrix composites, and improved titanium alloys for airframe applications and nickel-based superalloys and ceramic-matrix composites for engine applications. Also of great importance to the viability of future HSCT are support materials, including coatings, seals and sealants, adhesives, and fasteners and joining methods. These structural components will be complex, including bonded honeycomb constructions and dissimilar materials combinations produced using advanced joining methods and manufacturing processes. The final design must meet certification requirements for twice the proposed life-time of the aircraft (i.e., 120,000 hours for the airframe and 20,000 1 hours for the engine). This requires a thorough understanding of the long-term material properties and anticipated failure mechanisms as well as component reliability and durability evaluation methods.
Due to the severe exposures and long service life projected for an HSCT, the time required for complete, real-time evaluation of thermomechanical aging in advanced materials and structures will not be available. Unfortunately, it is difficult to characterize time-dependent phenomena related to materials aging in accelerated tests and to predict the equivalent real-time behavior with a high degree of confidence, especially when multiple aging processes and degradation mechanisms are involved. Existing methods for accelerated service simulation testing and damage accumulation rules may be inadequate for analyzing complex structures operating in new environments.
There has been considerable work associated with the NASA High-Speed Research program and at the aircraft and engine manufacturers to characterize the performance of HSCT-candidate materials under expected service conditions (Brunner, 1994; Williams and Johnson, 1994). These efforts have provided important progress toward understanding the behavior of advanced materials under severe conditions and in identifying degradation mechanisms discussed in this report.
To provide a better understanding of the problem, the committee addresses the following concerns:
What are the issues related to the performance of advanced materials over extended periods of time?
What are the current methods for evaluating long-term aging responses of materials and structures, and how appropriate are they, considering the in-service conditions under which the HSCT must operate?
What testing and analytical methods should be used to predict the performance of high-speed aircraft structures throughout their service life?
What research should be initiated to develop and validate the necessary methodology for accelerated aging of materials and structures?
There has recently been an effort to extend design life for subsonic commercial engines to 30,000 hours. If adopted, this criterion would likely apply to the HSCT.