Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure (1996)

The characterization of long-term responses of materials and structures to complex or cyclic environmental conditions presents a difficult challenge to those involved in materials selection and structural design. The problem is particularly challenging when the required service life is inordinately long compared with the time available for development and evaluation of materials and components. In most cases, service failures due to inaccurate characterization of aging responses result in potentially costly repair or premature component replacement. However, in safety-critical applications such as nuclear reactors and commercial aircraft the cost of inaccurate characterizations could be significantly greater.

Simulated service testing to characterize materials aging is fraught with deficiencies that stem from the inability to reproduce complex service conditions in the laboratory. Important aspects of the environmental conditions encountered in service cannot be accurately simulated. These environments are influenced by changing loads, temperature, humidity, radiation, and other effects that are interactive and cannot be consistently recreated and controlled. Moreover, critical degradation mechanisms and failure modes are not well known and understood prior to testing. While most service simulation testing has provided insights concerning degradation mechanisms and potential failure modes, it has yielded little engineering-quality data useful in the systematic characterization of materials aging. Thus, alternative approaches are required to develop such data.

The National Aeronautics and Space Administration (NASA) requested that the National Materials Advisory Board (NMAB) of the National Research Council identify issues related to the aging of materials and suggest accelerated evaluation approaches and analytical methods to characterize the durability of future aircraft structures throughout their service lives. The NMAB formed a committee 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 has examined the problem of aging characterization using the expected conditions and candidate structural materials for the High-Speed Civil Transport (HSCT) as a case study. The HSCT is a potential long-range supersonic commercial transport being considered by the aircraft industry for introduction early in the next century. The HSCT provides an important and instructive example because (1) the HSCT will be operated in a complex and severe service environment compared with current subsonic aircraft; (2) a broad range of materials with potentially different degradation mechanisms are being considered for structural applications; and (3) the development of the HSCT could be critical to the future commercial aircraft market for the U.S. aircraft industry well into the next century.

The committee believes that an approach based on a more fundamental understanding of materials response, degradation methods, and models and simulations using validated accelerated test methods will lead to increased confidence in aging predictions. In this light, the committee has recommended such an approach to aging characterization and has identified the critical research and data needs.

The committee recommends that, regardless of the material or application, the fundamental approach to the characterization of aging behavior should be the same. The approach that the committee recommends involves five steps. These steps are the basis for organizing the committee 's conclusions and recommendations. The steps required to characterize the aging responses of materials and the evaluation of structures include:

• defining the service environment,

• identifying probable degradation or failure mechanisms,

• characterizing the materials aging responses using accelerated methods and analytical models,

• using test and model results to analyze and understand aging in structural components, and

• validating predictions.

As a general finding, the committee determined that several organizations have important roles in the characterization of materials aging in high-speed aircraft applications. The

aircraft manufacturers should define the service conditions and data needs, apply analytical techniques to predict the aging responses at the component level, and validate predictions against controlled real-time tests. The material suppliers and academic researchers should perform the systematic characterization of materials properties and aid in the identification of potential degradation mechanisms under the full range of service and test conditions. The committee recommends that:

• NASA (1) integrate the efforts to provide fundamental characterization of materials properties, degradation mechanisms, and aging responses; and (2) develop the modeling capability to relate aging responses to component performance.

The service environment must be defined as precisely as possible to characterize the aging of a material or structural component. This can prove very difficult for application of materials under environmental conditions that differ significantly from previous experience. This difficulty arises because actual data are scarce, and conditions are defined by the component manufacturer largely through estimates or analysis. Among the variables that must be considered for the high-speed aircraft applications are operating temperatures, loads, ambient environmental conditions, moisture and fluid exposures, radiation, maintenance, and ground handling.

• The committee recommends that airframe and engine manufacturers, with assistance from NASA, continue to improve their understanding of the service environment expected for future high-speed aircraft. Emphasis should be placed on the analysis of aerothermal and chemical interactions and heat transfer to better define component temperature and thermal gradients and on characterization of ambient exposure conditions throughout the typical flight regimes.

The properties of the aluminum alloys, titanium alloys, nickel-based superalloys, and ceramic-matrix and polymermatrix composites that are candidate materials for high-speed aircraft structures and engines degrade with time when exposed to elevated temperatures. Designers of high-speed commercial aircraft will use physical and mathematical models to characterize long-term behavior of candidate materials. This process requires the designers to continually build on their understanding of the degradation and damage accumulation mechanisms and to improve models used in the analysis of materials properties.

The evaluation of basic materials properties and definition of critical degradation mechanisms for the candidate structural materials is, at present, incomplete. The scarcity of available property data constrains the analysis of aging responses.

• The committee recommends that NASA emphasize fundamental work to characterize materials properties and, most important, to determine critical degradation and damage accumulation mechanisms over a broad range of environmental conditions. This work should allow determination of the mechanisms that are likely to have the greatest influence on component durability.

Once the fundamental materials properties have been defined and the probable degradation and damage mechanisms established, the response of the material to service conditions can be estimated with greater confidence. The characterization of aging responses in structural materials entails establishing the fundamental relationships between service and environmental exposure and structure and property metrics. To be useful in analytical models, the relationship between exposure and material structure or property must include methodologies to integrate degradation rates; dependence on environmental factors such as temperature, pressure, loads, or concentrations of chemical agents; and the effect on significant structural design or performance metrics.

Methods to evaluate fundamental materials responses are fairly well established for specific damage mechanisms acting alone. However, the potential for synergistic effects among mechanisms is not completely understood. Unfortunately, it would be virtually impossible to explore each possible combination. Therefore, a carefully designed evaluation approach using statistical design of experimental techniques is required to determine the interactions with the greatest potential effects on in-service properties.

A critical aspect in the testing and characterization of aging response is the relation to a performance metric (most often related to mechanical properties). Degradation mechanisms can operate over a broad hierarchy of size scales, ranging from intermolecular to macroscopic, and time scales ranging from seconds to years. It is important to test aging response at the scale where the degradation takes place. For example, oxidation of polymeric composites has the most profound effect at edges and at fiber-matrix interfaces. Hence, the most sensitive measure of oxidation effects would be the degradation of the strength of the bond between the fiber and matrix resin.

The committee recommends that:

• Methods be developed to characterize aging responses in structural materials for previously identified

degradation mechanisms by establishing the fundamental relationships among service and environmental exposure conditions, damage accumulation, and structure and property metrics. These relationships must include property degradation or damage accumulation rates; dependence on critical environmental factors such as temperature, pressure, loads, strain rate, concentrations of chemical agents, and synergistic effects; and effects on significant performance metrics.

• Statistical experimental design approaches be used to establish critical dependencies between degradation mechanisms.

The long service-life requirements of high-speed commercial transports and the limited time available for development, evaluation, and validation of material candidates makes acceleration of aging characterization methods necessary. Methods include accelerated testing and accelerated aging. Accelerated testing to reduce the time required for testing material responses is required for mechanisms that involve progressive accumulation of damage or deformation that could lead directly to failure. Examples in which accelerated testing is required include creep and high-temperature fatigue. For accelerated test methods it is important to establish the equivalence between test progression and service exposure time or flight cycles. To reduce the time required to expose materials to aging conditions, accelerated exposuresare used to generate end-of-life microstructure or damage states for subsequent characterization tests. An example in which an accelerated exposure is required is coarsening of metal alloy microstructure, which would be expected to affect strength and toughness after exposure.

For accelerated aging, the calibration of test progression with service exposures is not as critical as the confidence that the microstructural features, produced using accelerated exposure methods, represent realistic end-of-life conditions. In cases that involve multiple interacting damage mechanisms (e.g., thermomechanical fatigue), both accelerated aging and accelerated testing could be required.

Accelerated exposures and testing can be accomplished through a number of schemes, depending on the aging mechanisms and the environmental variables. In many cases, it may be possible to accelerate exposures or tests by increasing temperature and loads, predamaging test articles, shortening hold times on cyclic exposures, or increasing the concentration of a degradative chemical or compound. When using accelerated exposures, it is necessary to anticipate and avoid excursions into or near regimes where other degradation mechanisms are expected to become active.

Accelerated methods become much more complex when multiple mechanisms and synergistic effects are involved because the relationship between accelerated and service conditions is not usually the same for different mechanisms. For this reason it is very difficult to directly test multiple accelerated conditions simultaneously. The committee concludes that the most viable approach to the characterization of multiple aging mechanisms is to incrementally subject samples to accelerated conditions designed to advance single-failure mechanisms. Samples would be cycled through a series of conditions designed to advance different discrete mechanisms, in turn, until end-of-life conditions are reached.

The committee recommends development of:

• Accelerated exposure and test methods, with calibration to service conditions or end-of-life microstructural conditions, for critical degradation mechanisms.

• Testing and exposure approaches that allow incremental application of conditions to evaluate multiple, synergistic degradation mechanisms.

Modeling techniques are valuable in the process of understanding the fundamental aging characteristics of materials and the properties of the structural components fabricated from them. The effects of scale, geometry, surface quality, coatings, and individual service conditions must be considered in the formulation of models. Analyses can also be done of mechanisms and rates of degradation at increasing size scales to provide technical guidance for component design and testing protocols.

The potential degradation of mechanical properties as determined from materials aging-response characterizations and the potential for damage accumulation over the service life must be considered when developing design property test programs and protocols for structural components. Durability predictions can be verified by comparing model predictions with controlled real-time and accelerated laboratory exposure and test methods. Validation of service condition assumptions, characterization test results, and model predictions can be accomplished based on visual and nondestructive monitoring of fleet-leading aircraft and mechanical and microstructural evaluation of components returned for service.

The committee recommends that:

• An integrated modeling capability be developed to relate characterizations of materials aging responses to component performance. An ability to examine the effects of scale, geometry, finishes, and individual service conditions must be part of the model specifications.

• Property degradation and damage accumulation be included as part of component durability evaluations and be considered in the design property test programs and protocols.

• Model predictions be verified using controlled real-time tests and accelerated exposure and test methods.

The findings of the committee have been organized into five chapters. Chapter 1 introduces the study task and report objectives. Chapter 2 describes the service or operating environment expected for airframes and engines expected to be used in future high-speed aircraft. Chapter 3 identifies and describes the candidate structural materials for such aircraft applications. Chapter 4 describes critical degradation mechanisms for the various candidate materials. Chapter 5 describes accelerated methods for the characterization of aging of materials and structures and describes the approach to materials aging recommended by the committee.