National Academies Press: OpenBook

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

Chapter: 5 Accelerated Methods for Characterization of Aging Response

« Previous: 4 Degradation Mechanisms
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

5

Accelerated Methods for Characterization of Aging Response

When the fundamental materials properties have been defined and the important damage mechanisms have been established, the response of materials to service conditions can be determined. 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 degradation rates; dependence on environmental factors such as temperature, pressure, loads, or concentrations; and the effect on a significant performance metric.

Much work has been done to evaluate fundamental materials aging responses for the isolated damage mechanisms identified in chapter 4. Much of this work is described in this chapter for specific materials. However, the potential of synergistic effects between mechanisms is not completely understood. This chapter describes characterization methods to evaluate the aging responses in HSCT- (High-Speed Civil Transport) candidate materials. Current methods are explored, and data needs and gaps are identified for metals, polymeric composites, and ceramic-matrix composites.

The long service-life requirements of HSCTs and the limited time available for development, evaluation, and validation of material candidates makes accelerated aging characterization necessary. Methods include accelerated testing and accelerated aging. Accelerated testing is required for mechanisms that involve progressive accumulation of damage or deformation that could lead directly to failure. For accelerated test methods it is important to develop equivalence between test progression and service exposure time or flight cycles. Accelerated exposures are used to produce end-of-life microstructure or damage states for subsequent characterization tests. 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 exposures represents end-of-life conditions. In some cases, both accelerated aging and accelerated aging methods are required.

METALLIC ALLOYS

The accelerated aging and test methodologies and approaches for monolithic metallic systems (aluminum alloys, superalloys, and titanium alloys) have considerable similarities. The load, stress, and stress-intensity parameter levels; the thermal profiles and test temperatures; and the test environment vary among the variety of alloy systems and applications. However, the testing and data analysis procedures have considerable similarities which are attributable to commonality in the durability concerns for HSCT structure and engine components. The primary durability concerns include:

  • brittle and ductile fracture,

  • mechanical- and thermal-fatigue crack initiation and growth,

  • variable amplitude loading including dwell periods,

  • creep and creep-fatigue crack initiation and growth,

  • environment-assisted crack growth, and

  • long-term microstructural stability under exposure to stress and temperature.

The linear elastic and elastic-plastic fracture mechanics approaches for predicting brittle and ductile fracture, respectively, are well established and are not discussed in this report. Similarly, the approaches for predicting classical fatigue crack growth behavior under rapid cyclic loading are also very well established.

The focus of this section is on methods for characterizing time-dependent degradation mechanisms which include microstructural degradation, creep, and environmental and creepfatigue degradation of materials. Table 5-1 and Table 5-2 show the degradation and deformation mechanisms discussed in chapter 4 and the approaches one might take in characterizing materials response. In each case, the most important variables, mechanistic models, empirical models, and ways to accelerate the test are presented. The characterization models and techniques are generally effective in describing the progression of particular degradation processes. More work is required to determine the synergistic effects between degradation processes and to relate test exposures to the service environment.

The Concorde Experience

Much can be learned from the Concorde experience, which may be relevant to new high-speed transports. There are numerous references to the Concorde's structural development (Harpur, 1968; Strang and McKinlay, 1978), to the

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

TABLE 5-1 Critical Degradation Mechanisms for Advanced Metallic Alloys

Damage Mechanism

Properties Most Affected

Most Important Variables

Mechanistic Models

Empirical Models

Ways to Accelerate Test

Precipitate coarsening

Strength Toughness

Applied stress

Exposure temperature

Exposure time

Coarsening models a

Strengthening model b

Arrhenius analyses

Avrami analyses

Increase temperature

and/or stress

Grain boundary precipitation and precipitate-free zone formation

Toughness

Corrosion

Applied stress

Exposure temperature

Exposure time

 

Arrhenius analyses

Avrami analyses

Increase temperature and/or stress

Grain growth and secondary recrystallization

Toughness

Corrosion

Creep

Applied stress

Exposure temperature

Exposure time

Grain growth models c

Arrhenius analyses

Avrami analyses

Increase temperature and/or stress

Changes in dislocation structure

Creep

Fatigue

Applied stress

Exposure temperature

Exposure time

Dislocation models d

Arrhenius analyses

Avrami analyses

Increase temperature and/or stress

Void formation and cracking

Toughness

Creep crack growth

Fatigue

Applied stress

Exposure temperature

Exposure time

Void formation models e

Arrhenius analyses

Avrami analyses

Increase temperature and/or stress

Sources:

a Lifshitz and Slyozov (1961), Wagner (1961), Speight (1968), Ardell (1972), Shiflet et al. (1979), Davies et al. (1980).

b Brown and Ham (1971), McElroy and Szkopiak (1972), Gerold (1979), Martin (1980).

c Cahn (1983).

d Bendersky et al. (1985).

e Nix and Gibeling (1983).

selection of materials for that aircraft (Doyle, 1969a, b; Murphy, 1972), and to long-term data on those materials (Spuhler et al., 1963; Martinod et al., 1969; Webb, 1977; Butt and Wilson, 1980; Butt, 1985).

The aluminum alloy selected for the primary structure of the Concorde (CM001) has a nominal composition of Al-2.5Cu-1.5Mg-0.22Si-1.1Fe-1.1Ni (Harpur, 1968; Strang and McKinlay, 1978). This material is a specially processed clad sheet of the alloy known in the United Kingdom as RR58 and in France as AU2GN. It was selected over other candidates such as 2024-T81, 7075-T6, and L73 (a British equivalent of alloy 2014) because of its static strength, fatigue strength, and especially because of its creep strength. To evaluate the structural alloy, static strength data were acquired at room temperature and elevated temperatures for materials exposed for 20,000 hours at estimated cruise temperatures. Fatigue was evaluated by testing at 120°C (248 °F) with a constant mean stress of about 25 percent of the ultimate strength. Creep strength was defined as the stress for which creep strains would be limited to less than 0.1 percent for a given time. Creep data were acquired at 120°C (248°F) under a steady stress of 177 MPa (25.6 ksi). 1

There are a number of earlier studies that summarize the results from long-time tensile and creep tests on Concordecandidate alloys (Spuhler et al., 1963; Martinod et al., 1969; Webb, 1977; Findley and Lai, 1978; Butt and Wilson, 1980; Butt, 1985). A review of such prior work may not provide the engineering data needed for HSCT design, but should provide insight, test procedures, and models that could be useful for HSCT materials.

Creep Deformation and Failure

Creep failure has been avoided in high-temperature structures in power plants by choosing maximum service temperatures and maximum allowable stresses to restrict creep deformation to less than 1.0 percent strain in 100,000 hours. In the 20,000 hours of service experience with the Concorde, creep failures have been successfully avoided (Peel, 1994) by use of simple design rules and well-accepted approaches for predicting creep behavior. Figure 5-1 shows the relationship between applied stress and creep test duration to achieve a 0.1 percent creep strain for various high-temperature aluminum alloys demonstrating the superiority of alloy 2618 over other alloys. Extrapolation of the creep data was accomplished by the well-established Larsen–Miller (Larsen and Miller, 1952)

1  

This concept was very different than the concept used by engine designers, where creep-rupture times were of greater importance.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

TABLE 5-2 Critical Deformation Mechanisms for Advanced Metallic Alloys

Deformation Mechanism

Most Important Variables

Mechanistic Models

Empirical Models

Ways to Accelerate Test

Creep

Operating stress

Operating temperature

Operation time

Creep deformation models a

Larson–Miller (and modifications such as Manson–Haferd, Orr–Sherby–Dorn, Manson–Brown) d ; Θ projection e ; minimum commitment method d

Increase temperature and/or stres

Thermal fatigue

Temperature range

Number of cycles

Frequency

Linear cumulative damage analysis b

see f

Increase temperature range

Mechanical fatigue

Operating stress range

Operating temperature

Number of cycles

Frequency

Linear cumulative damage analysis c

see f

Increase temperature and/or stress range and/or frequency

Sources:

aAshby (1972), Lagneborg (1972).

bImig (1976), Kiddle et al. (1976).

cImig and Garrett (1973), Imig (1976).

dManson and Ensign (1979).

eEvans et al. (1990), Wilshire and Evans (1994).

fImig and Garrett (1973), Imig (1976), Kiddie et al. (1976).

and Dorn–Sherby (Dorn and Starr, 1954) parameters. Deformation and failure mechanism maps (Frost, 1985) have been developed since the design of the Concorde. The purpose of the deformation mechanism maps is to ensure that the proper deformation mechanisms are accounted for in predicting material responses under conditions other than those tested. An example of the transient deformation mechanism maps for 316 stainless steel is shown in Figure 5-2. Such maps can be developed for the candidate aluminum and titanium alloys that will serve as guidelines for selecting the allowable stress and temperature ranges to ensure edequate creep resistance.

Creep-Fatigue Failure

Creep-fatigue failures caused by mechanical loading as well as thermomechanical fatigue are a concern in the design of HSCT structures and engines. The materials are likely to experience an increased risk for such failures if adequate consideration to these failure modes is not given during the design. The jet engine and the electric power generation industries have experienced such concerns in the past in the design of their components and therefore have experience in designing against creep-fatigue failures. This problem is new, however, to the aircraft structure design. Creep-fatigue failures have not been encountered in the Concorde service (Peel, 1994). However, due to the more severe service conditions expected for the HSCT, thermomechanical fatigue with

FIGURE 5-1 Relative creep strength of candidate alloys for the Concorde. Source: Peel (1994). © British Crown Copyright 1994, Defence Evaluation and Research Agency. Reproduced with the permission of the Controller, Her Majesty's Stationery Office.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-2 Deformation mechanism maps for 316 stainless steel after exposure for (a) 3 years and (b) 30 years. Note: Strains were calculated from constitutive equations. Source: Frost (1985).

possible contributions from creep could be a significant concern.

Creep and Creep-Fatigue Crack Growth

Since damage-tolerant analysis is standard for all civil aircraft structures and is also built into certification procedures (Kan, 1994), it is important to explore the available fracture mechanics approaches and test procedures for predicting creep and creep-fatigue crack growth in structures. Time-dependent fracture mechanics concepts have been well developed for creep-ductile metals (Saxena, 1991), including the development of a recent American Society for Testing and Materials (ASTM) standard for creep crack growth testing (ASTM, 1994). Figure 5-3 shows creep crack growth rate for Cr-Mo steels used in power plant components. Similar studies have also been performed on titanium alloys (Dogan et al., 1992) and aluminum alloys (Lang et al., 1991; Hamilton and Saxena, 1994). Current data and techniques are insufficient to develop well-accepted creep crack growth approaches to creep-brittle materials such as the high-temperature aluminum alloys and titanium alloys.

Creep-fatigue crack growth rates for trapezoidal waveforms have been characterized by the average value of the Ct parameter as shown in Figure 5-4 for a variety of hold times ranging from 10 seconds to 24 hours and also including creep crack growth rate data. Although such nice consolidation of creep-fatigue data for several hold times into a single trend cannot be expected for all materials, the approach itself of correlating creep-fatigue crack growth rates to a global crack tip parameter is widely used.

Standard ASTM test procedures are available for conducting creep crack growth testing. The current standard does not

FIGURE 5-3 Creep crack growth behavior of Cr-Mo and Cr-Mo-V base materials. Note: The plot includes all ex-service and new material data. Reprinted from Saxena et al. (1988), with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington OX5 1GB, UK.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-4 Comparison between creep crack growth and creep-fatigue crack growth data in terms of Ct Source Yoon et al. (1993) Reprinted with permission of Kluwer Academic Publishers

apply to creep-brittle materials which include many of the HSCT-candidate materials. However, the subcommittee E.08.06 of ASTM is currently actively working on extending this method to the creep-brittle materials. No standard test procedures are available for conducting creep-fatigue crack growth testing, however, it is a subject of active research.

The creep and creep-fatigue crack growth tests require from a few weeks to one year to complete. In tests that last one year, it is possible to obtain crack growth rate information at growth rates as low as 10−5 in./h (1.3 × 10−3 mm/h). These data are in the range useful for design and also for setting realistic inspection intervals. For example, if the flaw tolerance of an aircraft structure that is inspected every 10,000 hours is 0.25 inch (6.35 mm) and the crack size that can escape detection during inspection is 0.05 inch (1.27 mm), the maximum allowable average crack growth rate can be 0.45/10,000 = 4.5 × 10−5 in./h (1.14 × 10−3 mm/h). The measured crack growth rate data is below the above rate. Hence, there is no need for data extrapolation in this case.

Acceleration and Analytical Methods
Microstructural Changes

The mechanistic models referenced in Table 5-1 for deformation mechanisms and microstructural changes can be applied effectively to predict aging responses for single mechanisms. The synergistic interactions that are encountered when multiple, possibly competing, mechanisms are present add significant complexity to aging analysis. Current mechanistic models do not provide accurate predictions under these conditions.

It may be beneficial to use a more empirical approach to aging characterization when degradation mechanisms cannot be evaluated separately. For example, although techniques exist to describe the rate of overaging using a model for precipitate coarsening, the actual overaging process may involve a number of simultaneous processes (e.g., coarsening of one type of precipitate, the dissolution of another, the formation of grain-boundary precipitates, and the development of precipitate-free zones), and, typically, the models for these processes are not particularly well developed. Attempts at summing all of the relevant processes would likely result in very questionable results, making an empirical treatment more appealing.

An Alcoa study used a simple Arrhenius analysis (as described in the following text box and Figure 5-5 ) to determine the activation energy for overaging (Alcoa, 1995). The result of this analysis could be considered to be an “effective ” activation energy for overaging, which would include all of the ongoing processes. To determine the activation energy for overaging, plots of room-temperature tensile yield strength after exposure versus exposure time were used. An example of these data is shown in Figure 5-6. The data from this figure were used to produce the plots of ln 1/t versus 1/T in Figure 5-7. Note that this analysis was carried out using four different

Rate Expressions—Arrhenius Analysis

A simple relationship that underlies many of the empirical and semiempirical degradation rate relationships described in this chapter is the Arrhenius rate equation. The Arrhenius relationship expresses the rate constant (k) as:

k = A exp(−Q/RT).

Assuming that the activation energy (Q) and pre-exponential factor A are constant and independent of temperature, the relationship above can be expressed as:

ln (t/tref) = Q/R[(1/T)−(1/Tref)],

where t and T are time and temperature, respectively. The activation energy can be determined from the slope of ln t versus 1/T at an equivalent property time (Salin et al., 1992), as shown in Figure 5-5. Arrhenius analysis can be applied to a range of rate relationships. For instance, this type of analysis has been applied in the characterization of a number of aging characteristics including secondary (“steady-state ”) creep (Dorn and Starr 1954; Angers, 1991), overaging of aluminum alloys (Angers, 1991), and weight loss due to thermal degradation or oxidation (Salin et al., 1992).

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-5 Schematic showing use of the Arrhenius expression for describing a reaction or property degradation rate. Source: Alcoa (1995).

values for the overaging stress; the parallel lines indicate that the value for the activation energy is independent of stress, at least over the range examined.

Accelerating Creep Tests

There are several empirical and analytical methods for accelerating a test, most focusing on accelerating creep tests (Manson and Ensign, 1979). The Larson–Miller parameter, for example, is an empirical method generally used to predict creep-rupture times or steady-state creep rates at temperatures higher than the service temperature. The Goldhoff–Sherby, Orr–Sherby–Dorn, and Manson–Halford analyses are modifications to the Larson–Miller method and were developed to provide better fits to experimental data. Unfortunately, preliminary analyses on 0.1 percent creep and creep-rupture data from 2XXX-series aluminum alloys suggest that none of these empirical methods are particularly applicable for use with these materials (Alcoa, 1995).

A number of studies addressed the acceleration of creep tests at the time that the Concorde was being developed or used data generated during that time (Harfert, 1970; Imig and

FIGURE 5-6 Room-temperature tensile yield strength as a function of exposure time at various temperatures for 2618-T651 extrusions. Source: Alcoa (1995).

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-7 Plot of ln (1/t) versus 1/T for 2618-T651 extrusions overaged to 50, 40, 30, and 20 ksi. Source: Alcoa (1995).

Garrett, 1973; Imig, 1976; Kiddie et al., 1976; Findley and Lai, 1978; Cho and Findley, 1984; Wilshire and Evans, 1994; Evans et al., 1990). Findley and Lai (1978) and Cho and Findley (1984), for example, developed a viscous-viscoelastic model with a replacement of a temperature-compensated time for actual time, using tension and torsion data for alloy 2618-T61 in the range of temperatures of 200°C (392°F) to 230°C (446°F). Harfert (1970) studied two titanium alloys and alloy 2024-T3 and found that differences in creep temperatures and stresses caused different types of damage, even when the strain was held constant.

One study that achieved promising results and appears directly relevant to the current problem was carried out by Wilshire and Evans (1994). They used the Θ projection concept to predict long-term creep-rupture properties for 2124-T851 from the results of constant stress creep curves. In this method, strain versus time data are fit to an equation involving four constants: Θ1, Θ2, Θ3, and Θ4. These constants have fundamental significance, as Θ1 and Θ2 describe the decaying primary strain component, and Θ3 and Θ4 describe the accelerating tertiary creep component. The key to the concept is that the log Θ values vary linearly with stress. Hence, if one determines the four values for Θ using shorttime tests at high stresses, the long-time values for Θ, and hence, the long-time curves, can be predicted by extrapolation. By analyzing data from tests lasting 1,000 hours, rupture behavior in excess of 20,000 hours was successfully predicted. Additional work is needed to verify that the Θ projection concept could be used to predict the times to produce small creep strains.

Very little work has been published on extrapolating results from high-temperature to lower-temperature tests, specifically in aluminum. Claeys and Jones (1984) attempted to accelerate a creep test of 6061-T6 by accelerating an elevated-temperature exposure. They used overaging data and creep-rupture data to determine the activation energies for the overaging and creep processes. Using this activation energy, they could then relate a long-time, low-temperature exposure to a shorter time at a higher temperature. Samples were then exposed at the higher temperature and then creep tested into the steady-state regime at the lower temperature. A pseudocreep curve was produced by placing line segments, having the slope represented by the steady-state creep rates, onto a creep strain versus time plot at the appropriate position with respect to the time axis (i.e., at times adjusted using activation energy). While this method provided curves that satisfactorily reproduced real long-time test data, it has several drawbacks. The accelerated exposures did not include an applied stress, suggesting that the match between pseudocreep curves and real creep curves was fortuitous. Also, the method does not provide any information about when sampies subjected to the service conditions might enter the tertiary creep regime or fail. Nonetheless, this novel approach warrants further consideration and may be able to be modified to predict behavior in the primary creep regime.

A large body of tensile and creep data from ingot metallurgy 2XXX aluminum alloys (including 2014, 2024, 2090, 2124, 2219, 2519, and 2618) was analyzed at Alcoa using the Arrhenius analysis (Alcoa, 1995). Specifically, creep stresses for different levels of creep strain were of interest. For some alloys, there were data for several tempers and product forms, while for others data were limited. In many instances data were available for creep tests up to 10,000 hours. Plots of the stresses to reach 0.1 percent creep, 0.5 percent creep, and creep rupture were used to determine the activation energy for creep deformation. Examples of these data sets for 2618-T651 extrusions are presented in Figure 5-8. Figure 5-9 shows plots of inverse time versus inverse

FIGURE 5-8 Stress for 0.1 percent creep strain as a function of time at various temperatures for 2618-T651 extrusions. Source: Alcoa (1995).

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-9 Plot of ln (1/t) versus 1/T for 2618-T651 extrusions overaged to 30 ksi, crept to 0.1 percent, 0.5 percent, or rupture. Source: Alcoa (1995).

temperature used to determine activation energies for 0.1 percent and 0.5 percent creep at 207 MPa (30 ksi). The roughly parallel lines indicate that the activation energy for creep does not vary over a wide range of strain and is similar to the activation energy for overaging.

Other work on heavy-metal impurity embrittlement of aluminum alloys has successfully used both single-specimen and multiple-specimen fracture mechanics tests to determine crack growth rates over a range of test temperatures in order to determine an activation energy for crack growth (Lewandowski et al., 1992). This has been successfully used to estimate crack growth rates at different test temperatures. In addition, crack linkage in the near-threshold regime is particularly important for the continued growth of the crack. There may be considerable specimen-to-specimen variations in the crack initiation and growth behavior due to differences in the microstructure, level of impurities present, and the distribution of the heavy-metal impurities. The effects of systematic changes in these parameters on the subsequent threshold and crack growth behavior have been successfully modeled using Monte Carlo simulations (Singh et al., 1992a, b). These simulations were patterned after those successfully used in predicting the behavior of stress corrosion cracks of pipeline steels where it is known that failure is caused by the coalescence of a multitude of individual microcracks (Singh and Parkins, 1990; Singh, 1991). Such modeling enables multiple simulations to run in order to provide information on the probabilistic aspects of crack initiation and growth as well as the effects of extrapolating data from single-specimen tests to multiple specimens. The use of such models in combination with single-specimen tests has been successful in predicting lifetimes of structures in service that contain cracks of various sizes. Other experimental and theoretical treatments of creep crack growth include studies by Fu (1980), Sadanda and Shahinian (1981), Bensussan et al. (1984), and Nikbin et al. (1986).

Accelerating Fatigue Tests

Imig and Garrett (1973) and Imig (1976) investigated the possibilities for reducing fatigue test time for supersonictransport materials and structures in tests of Ti-8Al- 1Mo- 1V and Ti-6Al-4V with simulated Mach 3.0 flight-by-flight loading. The effects of design mean stress, the stress range for ground-air-ground cycles, the simulated thermal stress, the number of stress cycles per flight, and salt corrosion were included in their studies. They conducted accelerated tests (2 seconds per flight) and real-time tests (96 minutes per flight), used a linear cumulative damage analysis, and concluded that they were successful in that the fatigue lives were generally within a factor of two of the lives from real-time tests.

Kiddle et al. (1976) conducted a study of thermal fatigue in box beams of CM001 under mechanical and thermal stresses. They showed that they could accelerate thermal-fatigue tests very predictably by a factor of two by increasing the thermal stresses through increases in the range of temperatures applied to the aircraft.

POLYMER-MATRIX COMPOSITES

Current Methods
Moderate- Temperature Applications

The methodology for substantiating the service life of polymeric-matrix composites for lower-temperature aerospace applications has been developed over the past 25 years (Whitehead et al., 1985; Vosteen and Hadcock, 1994). This work formed the basis for the requirements of MIL-HNBK17 (DOD, 1994). MIL-HNBK-17 provides guidance on the selection, design, and analysis of composite structures based on static ultimate strength considerations and the effects of three primary degradation mechanisms—impact damage, mechanical fatigue, and humidity (or fluid) exposure.

The substantiating approach for each of these mechanisms has been incorporated into the design verification process with evaluations on scales from coupon-level to full-scale structural components. This approach has been successfully developed to help offset the design analysis limitations in terms of predicting interlaminar stresses, damage initiation, and delamination growth. The final step of this approach is typically a full-scale component fatigue test on an impact-damaged structure. The acceleration of the primary damage mechanisms is achieved as described in the following sections.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

Impact Damage. The structure is impacted prior to the fatigue test, substantiating both barely visible impact damage requirement for the life of the structure and the inspection interval required for visible impact damage. This is considered a conservative approach since the impact damage is done prior to fatigue cycling.

Mechanical Fatigue. The acceleration of fatigue testing in fiber-dominated constructions has been achieved by assessment of typical flight cycles. This approach, detailed by Jeans et al. (1980), determines how fatigue cycles are combined and what cycles can be truncated because low stress levels are well below fatigue thresholds. The balance of the cycles are typically conducted with higher loading to account for material data scatter, since typically only one article is tested. Fatigue is not generally a significant damage mechanism in fiber-dominated composite structures that meet impact-damage tolerance requirements. Components that experience significant interlaminar or out-of-plane loading can be susceptible to fatigue damage.

Humidity and Fluid Exposure. The methodology for accelerating humidity and fluid exposure tests are described in MIL-HNBK-17 (DOD, 1994). The accepted approach is to increase the temperature exposure to increase the diffusion rate. Design properties based on coupon tests are typically generated in a fully saturated humidity condition (85 percent relative humidity).

Real-time tests, using both flight service components and ground exposures, have verified the approach taken in first-generation composite applications. For example, NASA has conducted flight service evaluations of 350 components with over 5.3 million total flight hours and a large 10-year ground exposure program (Dexter and Baker, 1994).

High-Temperature Applications

Higher-temperature carbon/polyimide composites have been successfully certified for military airframe and engine applications (e.g., the carbon/PMR- 15 structures on the Navy F18 C/D and Air Force B-2 airplanes). The substantiation test methodology has been very similar to the process described above for lower-temperature applications except that a different criterion is used to determine the upper-use temperature for a material system. Carbon/epoxy systems have typically followed the approach where the material operating limit is considered to 28°C (50°F) below the glass transition temperature under moisture-saturated conditions (Whitehead et al., 1986). This generally keeps the material-use temperature low enough that irreversible resin morphology changes, thermal degradation and oxidation effects, and vapor-induced blistering are not a concern.

The moisture-saturated glass transition temperature is just one of the indicators of a high-temperature polymers upperuse temperature. The other indicators are resistance to transverse matrix cracking and to thermal degradation and oxidation. For the short-life, high-temperature military applications described above, these damage mechanisms are assessed by simulating the application environment in a real-time experiment. In contrast, little data are available on thermal and environmental degradation of high-temperature thermoplastic composites.

Accelerated Methods

For applications such as the HSCT where complex, long-duration exposures are expected, it would be economically and technically difficult to use a purely experimental test approach. Accelerated test methods and durability modeling will be required to predict end-of-life properties for these applications. The challenge will be to carefully augment the current composite verification approach with accelerated testing without overlooking critical damage mechanisms.

There are additional degradation mechanisms for high-temperature applications compared with the degradation mechanisms already discussed for lower-temperature systems. These degradation mechanisms include thermal and oxidative degradation, transverse matrix cracking, and hygrothermal (moisture plus temperature) effects. These degradation mechanisms are discussed in chapter 4. Table 5-3 shows critical degradation mechanisms for high-temperature applications of polymeric composites, the most important influencing variables, modeling approaches, and accelerated methods.

Thermal Degradation and Oxidation

Fundamental modeling of thermal degradation processes for high-performance polymers and composites is currently not available. A promising technique that may be applicable is computer simulation of molecular dynamics. These techniques have been applied to simulate the combustion of polyolefins (Nyden and Brown, 1993), however it is unclear whether these techniques could be accurately applied to high-performance polymers or composites.

The characterization of thermal degradation of a polymeric composite, under both oxidizing and inert conditions, is based on weight-loss measurements as a function of time and temperature. Characterization of weight-loss kinetics can be accomplished using thermogravimetric analysis (TGA) techniques. TGA characterization methods have been developed to determine the initial degradation of composites used in the manufacture of carbon/carbon composites (Nam et al., 1989) and for the degradation of HSCT-candidate materials (Grayson and Fry, 1994). These methods have been applied to both thermoplastic-and thermoset-matrix composites, and

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

TABLE 5-3 Critical Degradation Mechanisms for Polymeric Composites

Degradation Mechanism

Most Important Variables

Modeling Approach

Ways to Accelerate Tests

Missing Information

Matrix cracking

Processing parameters

Temperature cycle range

Static and fatigue loading

Moisture/thermal cycle

Number of cycles

Ply thickness

Residual stresses

Damage accumulation models a

Increase temperature

cycle range

Increase applied stress Increase

Effects of moisture Rate effects

Thermal degradation and oxidation

Temperature

Oxygen concentration

Exposure time

Mass-transfer models

Reaction kinetics models b

Increase temperature

Increase oxygen

concentration

Chemical degradation

models

Effect of

matrix cracking

Hygrothermal degradation

Exposure

temperature

Moisture concentration

Heating rate

Moisture diffusion models c

Increase exposure

temperature

Increase moisture content

Cycle moisture sorption

Effect of moisture

on residual stresses

Desorption effects

Moisture gradient effects

Effect of Matrix cracking

Effect of imposed stress

Phase separation/microstructural changes

Exposure time

Exposure temperature

Moisture content

none

Increase temperature

Increase moisture content

Microstructural

characterization methods

Solubility models

Sources:

aReifsnider and Highsmith (1981), Allen and Lee (1990), Nairn and Hu (1992).

bNam and Seferis (1992), Salin et al. (1992).

cShen and Springer (1976), Paplham et al. (1994).

a generalized methodology to describe degradation kinetics of polymeric composites accounting for the anisotropy of composite degradation and for changes in the dominant degradation mechanism has been developed (Nam and Seferis, 1992; Salin et al., 1992).

In any evaluation of thermal degradation or oxidation of polymers, it is important to define the dominant degradation reaction and the changes in dominant reactions with temperature. This can be accomplished by analyzing the chemical make-up of the evolved gases using methods such as gas chromatography/mass spectroscopy or by analyzing the degraded polymer using methods such as diffuse reflectance Fourier-transform infrared spectroscopy.

While the initial degradation kinetics of the effects of edges and ply orientation can be evaluated using the methods above, the correlation of weight loss with mechanical performance is unsatisfactory. Current methods and previous evaluations have relied on exposure of test coupons or larger panels to isothermal or cyclic conditions (Kerr and Haskins, 1987; Pride et al., 1968). Weight loss is measured and specimens are machined and tested. Unfortunately, these tests do not necessarily account for the directionality of degradation reactions or on the preferred degradation at edges and at the fiber-matrix interface, nor do they take the presence of matrix cracking, component geometry, and coatings into account. Hence, measured property degradation is not sensitive to the level of degradation, and test results are not easily reproduced. The synergistic effects of time, pressure, and atmosphere on composite degradation can be established following the approach employed by Pride et al. (1968). Typical results are shown in Figure 5-10 and are for composites made with 181-style glass cloth and polyimide resin. The results shown in Figure 5-10 have been compressed into the single curve shown in Figure 5-11 by plotting the strength as a function of the Larson–Miller parameter, TR (C + log t), in which TR is the exposure temperature in degrees Rankine (Larson and Miller, 1952). Other properties, including elevated temperature properties, can be presented in a similar manner.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-10 Variation of room-temperature flexural strength with exposure time at elevated temperatures. Source: Pride et al. (1968). Copyright (1968) SPI Composites Institute. Reprinted with permission.

Matrix Cracking

Transverse matrix cracking or in-plane microcracking can be thermally, hygrothermally, or mechanically induced. Current micromechanical and fracture mechanics models to predict matrix cracking susceptibility focus on mechanically (Reifsnider and Highsmith, 1981) and thermally (Bowles, 1984) induced matrix cracking. Matrix cracking can be characterized in terms of a saturation crack density, called the characteristic damage state that depends on ply properties, ply thickness, and stacking sequence (Reifsnider and Highsmith, 1981). The characteristic damage state has been shown to be independent of load history, initial stress, and environmental effects (except as they affect ply properties; Reifsnider and Giacco, 1990).

While the characteristic damage state provides a definition of an end-of-life condition, the development of matrix cracking and the threshold damage conditions are more difficult to model. Typically, these conditions have been identified by cyclic thermal or hygrothermal exposure testing (Brunner, 1994; Sensmeier, 1994), with test acceleration gained by rapid cycling, increased temperature range, or increased ply thickness (multiple adjacent plies of one orientation). The result would be a damage progression plot as shown in Figure 5-12. The results of this empirical approach are difficult to apply to conditions different from those tested. These tests have proven to be useful in screening or ranking materials, but have not enabled predictive capabilities.

Models that could be used to predict the onset and development of matrix cracking based on constituent properties must account for the dependence on processing conditions, residual stresses, thermal cycle range, and the effects of moisture content and distribution on the stresses that lead to cracking. A mechanistic model approach to predict the development of matrix cracking caused by both mechanical and thermal stresses has been developed (Nairn and Hu, 1992). Mechanistic modeling allows results to be applied over a broad range of conditions. The applicability of this type of modeling must be verified.

There are a number of analytical methods to determine stiffness reductions in a composite laminate because of matrix cracking (Allen and Lee, 1990). These methods include a shear-lag model (Highsmith et al., 1981; Highsmith and Reifsnider, 1982; Lim and Hong, 1989), self-consistent scheme (Laws et al., 1983; Dvorak, 1985), strain energy method (Aboudi, 1987), complementary strain energy method (Hashin, 1985, 1987), and the internal state variable approach (Allen and Lee, 1990). Additional work needs to be accomplished to allow the predicted ply properties developed in the theoretical models to be used to predict critical design properties of laminate or components, including thermal properties such as coefficients of thermal expansion, thermal conductivity, and moisture absorption. The effect of microcracking on failure criteria and failure modes needs to be established.

Hygrothermal Effects

As discussed in chapter 4 , the combined actions of moisture diffusion and thermal exposure (hygrothermal effects) can lead to several possible damage modes, including

FIGURE 5-11 Correlation of room-temperature strength after various exposure times and temperatures at 760 torr. Source: Pride et al. (1968).Copyright (1968) SPI Composites Institute. Reprinted with permission.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-12 Damage progression of matrix cracking in a laminate versus number of cycles. Note: Data are for mechanically induced cracking (fatigue loading). Source: NRC (1991).

intrinsic property loss, matrix cracking, microvoid generation, delamination, and surface blistering. Extensive work has been accomplished to understand the diffusion of moisture in composites and the resulting composite properties (Fried, 1967; McKague et al., 1975, 1978; Browning, 1978; Loos and Springer, 1979; Adamson, 1980; Wolff, 1993). These studies were, for the most part, focused on epoxy-matrix composites. Moisture effects on high-temperature polyimides have also been studied (Cornelia, 1994; Paplham et al., 1994).

The diffusion models described above, based on absorption measurements on test laminates, allow diffusion rates to be described under a wide range of conditions and laminate construction. Moisture distributions and the effects of hygrothermal cycling on moisture gradients can be characterized. Work needs to be undertaken to allow prediction of residual stress and mechanical property effects from moisture concentration gradients and diffusion histories and on synergistic effects of matrix cracking and moisture absorption.

CERAMIC SYSTEMS

As described in chapter 3 and chapter 4 , the state of development of high-performance ceramic-matrix composites has not reached a point where much is known about basic materials properties, degradation methods, or the characterization and modeling of aging phenomena. This section describes testing and analytical methods that could be important in evaluating the long-term performance of ceramic composites.

Oxidation and Volatilization

The degradation mechanisms described in chapter 4 illustrate the possibility of rapid loss of a silicon-compound-based material, even when a passive SiO2 layer forms, by the removal of hydrated silica species. The details of the effects of total pressure and gas velocity on the kinetics of the oxidation followed by volatilization must be determined. High-pressure gas burner rigs can be used for these studies but they are expensive and difficult to maintain. High-pressure TGA studies can be performed over a range of pressures and velocities to develop a model for this complex process, and the burner rigs can be used to verify the predictions of the model.

In the active regime where SiO(g) forms directly by oxidation of the substrate, the behavior at one atmosphere and in partial vacuum is well understood. Oxidation measurements at high pressure must be carried out to extend the model into the range of realistic operating conditions under fuel-rich conditions.

Thermal cycling has adverse effects on silicate scales, particularly when they begin to crystallize. Thus, the oxidation experiments enumerated above must also be carried out under conditions of thermal cycling in order to determine these effects on the oxide scales.

Mechanical Response
Behavior of the Constituents

The creep behavior of the fiber and matrix in ceramic-matrix composites can be measured in a straightforward manner. The SiC-based fibers typically show primary creep over their whole life, and phenomenological models can be accurately fitted to the data to allow interpolation and extrapolation (Jia et al., 1993; Lewinsohn et al., 1994). Similar detailed analyses must be performed on the new SiC-based fibers, such as Hi-Nicalon® and fibers produced by Dow Corning, and Carborundum. The matrix behavior can be measured by depositing it on a highly compliant carbon fiber and performing similar measurements to those outlined above for fibers.

The rupture behavior of these fibers at high temperature appears to be controlled by slow crack growth, but the detailed studies to correlate the experimental data with the model predictions are incomplete. DiCarlo et al. (1994) have used a phenomenological model to fit a very limited data set to attempt extrapolations into the long-lived regime. Much more extensive data generation must be undertaken to provide sufficient data to verify model predictions.

Behavior of the Composite

The complications of unstable interfaces and the lack of a large supply of the specific engineering material has hindered

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

the determination of the time-dependent response of the composites at high temperature. Holmes et al. (1993) have shown the creep response of SiC-fiber/Si3N4-matrix composites, illustrating the characteristics of these materials when both components creep. They have also demonstrated the creep recovery characteristics of these materials. In general, SiC/SiC composites can be expected to behave similarly at higher temperatures than composites used for the silicon-nitride matrix materials. Matrix cracks will probably form in the SiC/SiC composites in use because of thermal transients and steady-state thermal gradients. Thus, the behavior of the material at high temperature in the elastic-elastic, elastic-plastic, and plastic-plastic region must be studied.

Microcomposites (single fibers or monodirectional fiber tows in the matrix) are being used to study interfaces at room temperature. Evaluation of microcomposites at high temperatures to determine interfacial properties has not been completed. Also, the effect of HSCT service conditions must be incorporated to develop a realistic appraisal of the materials in service.

Expected HSCT service conditions necessitate study of thermomechanical fatigue in simulated service environments of these composites. In conjunction with the above-listed studies of the degradation processes of the individual constituents, the thermomechanical fatigue studies should lead to an understanding of which process dominates in which regime so that material modifications can be made to defeat the degradation process.

ANALYSIS OF STRUCTURES

Current structural design and analysis procedures in the aeronautics industry make use of materials properties that are largely semiempirical, even though significant improvements have occurred in other aspects of structural analysis methodology over the last two decades (NRC, in press). The standard practice relies heavily on extensive testing at the various levels, including:

  • coupon-scale tests to establish basic static and time-dependent design property limits, or allowables, under pertinent environmental conditions;

  • element tests to relate allowables to design elements; and

  • tests on structures from subcomponent through full-scale components culminating in static and fatigue tests on the complete aircraft to verify scaling models and assumptions.

Design details are frequently improved 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.

Test methods for developing design allowables for metallic and composite structures are fairly well established and are continually reviewed by the Federal Aviation Administration through committees such as MIL-HDBK-5 for metals and MIL-HDBK-17 for composites. While test methods will continue to be refined and updated to establish more-reliable design property values and allowables, the conditions under which the values were established, including product form, processing method, and thermal treatments, is very important.

Modeling techniques are critical in relating the fundamental aging characteristics to complex structural components. The effects of scale, geometry, surface quality, coatings, and diverse individual service conditions must be considered together with their possible synergistic interactions. Analyses of mechanisms and rates of degradation must be evaluated at increasing size scales to provide technical guidance for component design and testing protocols.

Metallic Materials

Metallic materials tend to fail due to the formation and growth of a dominant microcrack (or cracks) that eventually reach a critical length and then more rapidly propagates to failure. While fracture mechanics is now a mature part of the engineering standard practice, rigorous prediction methodology only exists for brittle materials that exhibit limited plasticity. Fatigue crack growth behavior and fracture processes exhibited by ductile materials are reasonably well understood.

It is currently possible to use a deterministic approach to predict the structural durability of metallic components for high-speed aircraft. Such an approach would include stress analysis, crack initiation analysis, and crack growth analysis.

  • Stress analysis determines stress/strain/time history in fracture-critical locations. To perform such analysis, accurate constitutive models are needed that can account for time-dependent deformation and damage response, as well as the influence of microstructural degradation under monotonic, sustained, and cyclic loading.

  • Crack initiation analysis ensures that design-life requirements are completely met. Models for predicting crack initiation must account for all relevant damage and degradation mechanisms.

  • Crack growth analysis provides damage-tolerance calculations and determines inspection intervals and criteria. These models must be able to account for all relevant time-dependent and fatigue-cycle-dependent modes of crack growth and include effects of environment and variable amplitude loading.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Composite Materials

In contrast to metals, composite materials exhibit complex failure modes that may involve the interaction of several different damage mechanisms. The complexity of the failure modes becomes a major design consideration when addressing durability and damage tolerance requirements. Most design allowables depend on the as-cured properties of the constituents and the fiber architecture.

The current design practices rely heavily on empirical approaches to conservatively estimate the effects of damage on durability and damage tolerance. For example, simple tests such as open-hole compression and compression-after-impact tests are often used to establish “ knock-down” factors for reducing the working strain levels to avoid damage growth. In the absence of mechanics methods to predict damage initiation, growth, and failure modes, all test data must be obtained on the specific laminate stacking sequence or fiber preform architecture to be used in the design. Test data at the element and subcomponent levels are relied on to confirm the suitability of the structural design parameters to couple coupon-level material behavior and test data to the actual structural behavior.

Two methodologies used to predict remaining strength (damage tolerance) and life of composite structures are currently being integrated into design codes. The first is the use of “damage mechanics” to predict the changes in stiffness that occur during service life (Talreja, 1985; Simo and Ju, 1987; Lee et al., 1989; Shapery, 1990). This approach is becoming common for the purposes of following the development of damage and for interpreting the changes in stiffness of the structure as well as at the microstructural level in composites. The second methodology is the use of micromechanics and kinetic theory to predict remaining strength. Micromechanical representations of the fundamental composite strengths are constructed in terms of the constituents, their geometry, and their arrangement (Reifsnider and Stinchcomb, 1986; Gao and Reifsnider, 1991; Reifsnider, 1991a, b, 1992; Reifsnider and Gao, 1991; Xu and Reifsnider, 1992). The constitutive parameters in these strength models are studied as a function of the service inputs and environments using kinetic (or rate) theory allowing fatigue, creep, creep rupture, aging, oxidation, and other time-dependent and cycle-dependent effects to be introduced.

CHAPTER SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

Characterization Methods

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; and the effect on a significant performance metric.

Methods to evaluate fundamental materials responses are fairly well established for isolated damage mechanisms. However, the potential of synergistic effects between 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 experiment 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). The degradation mechanisms described in operate over a broad hierarchy of size and time scales. It is important to test aging response at the size scale where the degradation takes place. For example, as described in chapter 4 , 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 expected to 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.

Accelerated Methods

The long service-life requirements of HSCTs and the limited time available for development, evaluation, and validation of material candidates makes acceleration of aging characterization methods necessary. 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

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

accelerate exposures or tests by increasing exposure temperature, increasing 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, and

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

Analysis of Structures

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 using controlled, real-time tests and accelerated methods. While beyond the scope of the current study, it may be of interest to compare laboratory simulations with service experience for established materials such as alloys for power generation applications (see Figure 5-3 and Figure 5-4 ) and first-generation aircraft materials (especially 7XXX-series aluminum alloys and carbon/epoxy composites).

Monitoring of service components provides information useful to validate aging methods and predictions. Validation of service condition assumptions, characterization test results, and model predictions can be based on monitoring of fleet-leading aircraft. Service monitoring can be accomplished using visual inspections and nondestructive evaluations as part of the airlines' maintenance program or by performing destructive mechanical and microstructural evaluations. Nondestructive evaluations would be used to detect unanticipated damage. Unfortunately, available techniques—currently effectively limited to optical, radiographic, and ultrasonic methods—have limitations due to poor structural inspection standards, inadequate defect indication interpretation, poor reliability, high cost, and inadequate linkage between design analyses and nondestructive evaluation results (NRC, in press). Rapid, wide-field nondestructive evaluation methods that can account for limited or one-sided access are needed to effectively monitor in-service components. Destructive evaluations would determine the type and extent of changes in properties and microstructure, and the type size, and distribution of deformation and fracture damage.

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 realtime tests and accelerated exposure and test methods.

Approach to Materials Aging Characterization

The committee believes that an approach based on a fundamental understanding of materials response, degradation methods, and models and simulations based on validated accelerated test methods will lead to increased confidence in aging predictions. Regardless of the material system or application, the committee suggests that the fundamental approach to the characterization of aging behavior should be the same. The approach that the committee recommends is depicted schematically in Figure 5-13.

In the case of high-speed aircraft applications, several organizations have roles in the characterization of aging phenomena. The aircraft manufacturers should define the data needs and service conditions, apply analytical techniques to predict the aging response at the component level, and validate model 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 materials and degradation mechanisms that should be emphasized by NASA for the HSCT depends on the design speed chosen. For the airframe of the current baseline aircraft (Mach 2.4), matrix cracking, thermal degradation

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×

FIGURE 5-13 Recommended general approach to characterization of aging of materials and structures.

and oxidation, and hygrothermal degradation and microstructural stability of high-temperature thermoset- or thermoplastic-matrix composites and microstructural effects, hydrogen embrittlement, and moderate-temperature effects (strain aging, slip localization) of titanium alloys should be emphasized. For the airframe of a Mach 2.2 aircraft, elevated-temperature fatigue, creep, environmental effects, and microstructural stability of high-temperature ingot aluminum alloys could be considered. For engine materials, low-cycle fatigue, creep, oxidation, and microstructural stability of nickel-based superalloys and thermochemical degradation (including corrosion and oxidation reactions) effects on slow crack growth and creep deformation in ceramic-matrix composites should be emphasized.

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.

Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 29
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 30
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 31
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 32
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 33
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 34
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 35
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 36
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 37
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 38
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 39
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 40
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 41
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 42
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 43
Suggested Citation:"5 Accelerated Methods for Characterization of Aging Response." National Research Council. 1996. Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure. Washington, DC: The National Academies Press. doi: 10.17226/9251.
×
Page 44
Next: References »
Accelerated Aging of Materials and Structures: The Effects of Long-Term Elevated-Temperature Exposure Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF
  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!