Accelerated Aging of Materials and Structures The Effects of Long-Term Elevated-Temperature Exposure (1996) / Chapter Skim
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5 ACCELERATED METHODS FOR CHARACTERIZATION OF AGING RESPONSE
5 ACCELERATED METHODS FOR CHARACTERIZATION OF AGING RESPONSE
Pages 29-44
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From page 29... ...
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.
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From page 30... ...
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.
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From page 31... ...
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 adequate creep resistance.
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From page 32... ...
Current data and techniques are insufficient to develop well-accepted creep crack growth approaches to creep-bnttle matenals such as the high-temperature aluminum alloys and titanium alloys. Creep-fatigue crack growth rates for ~apezoidal wavefo~ms have been characterized by the average value of the C, parameter as shown in Figure 5-4 for a vanety of hold times ranging from 10 seconds to 24 hours and also including creep crack growth rate data.
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From page 33... ...
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.
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From page 34... ...
. values for the averaging stress; the parallel lines indicate that the value for the activation energy is independent of stress, at least over the range examined.
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From page 35... ...
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 sarnples subjected to the service conditions might enter the tertiary creep regime or fail.
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From page 36... ...
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)
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From page 37... ...
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.
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From page 38... ...
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.
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From page 39... ...
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~.
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From page 40... ...
ACCELERATED AGING OF MATERIALS AND STRUCTURES 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.
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From page 41... ...
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.
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From page 42... ...
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.
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From page 43... ...
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.
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From page 44... ...
The committee recommends that: · NASA ( 1 ) integrate the efforts to provide fundamental characterization of materials properties, degradation mechanisms, and aging responses, and (2)
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