Accelerated Aging of Materials and Structures The Effects of Long-Term Elevated-Temperature Exposure (1996) / Chapter Skim
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4 DEGRADATION MECHANISMS
4 DEGRADATION MECHANISMS
Pages 17-28
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From page 17... ...
Changes Residua/ Stresses The level of residual stresses in aluminum alloys and aluminum-matrix composites are strongly dependent on processing conditions, heat treatment, and thermal excursions. In monolithic aluminum alloys, changes in the processing conditions (e.g., hot and warm working, cold working, rolling, etc.)
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From page 18... ...
In addition to the residual stresses produced via processing, heat treatment, and straining in the monolithic materials, the introduction of reinforcements with different coefficients of thermal expansion than the matrix alloy induces additional thermal residual stresses as well as the possibility of increased mechanical residual stresses because the reinforcements do not deform in a plastically deforming matrix (Bourke et al., 1993; Liu et al., 1993; Withers and Clyne, 1993~. As with monolithic alloys, the levels of residual stress may be affected by subsequent thermal exposures and prestraining (Liu et al., 1993~.
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From page 19... ...
While there are no theoretical treatments that describe primary creep rates in terms of microstructural features, the effects of alloying, grain size, percent stretch, and extent of aging have been investigated for 2618 and other precipitation-strengthened aluminum alloys. Creep resistance appears to improve with increasing grain size in precipitation-strengthened alloys.
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From page 20... ...
Summary: Aluminum Degradation Mechanisms Potential damage mechanisms associated with high-temperature applications of aluminum alloys include microstructural changes, fatigue, creep, and environmental effects. · Elevated-temperature exposure under applied stress can introduce a number of micros~uctural changes including coarsening of the matrix precipitates (important in stren~th-critical applications)
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From page 21... ...
Concerns include the possibility for dynamic strain aging from solute such as oxygen or carbon to reduce tensile ductility and toughness by affecting microvoid growth and coalescence, the possibility for thermally activated slip localization in locally soft regions of the microstructure, time-temperature effects on hydrogen embrittlement, and deformation and fracture behavior at cryogenic temperatures. Environmental Effects The dissolved hydrogen content of HSCT-candidate titanium alloys could increase during processing, component fabrication, or elevated-temperature service in aggressive environments, such as airplane hydraulic fluid, and subsequently degrade tensile ductility and fracture toughness.
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From page 22... ...
FIGURE 4-3 Effect of grain size on a variety of mechanical properties for nickel-based superalloys. ACCELERATED AGING OF MATERIALS AND STRUCTURES superalloys used in gas turbine engines are metastable under use conditions.
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From page 23... ...
This combined degradation phenomenon can be seen generally after 5,000 hours of exposure and represents a valid concern for longterm HSCT applications. Damage Accumulation: Matrix Cracking Perhaps the most critical damage mechanism operating in high-temperature polymeric composites is the formation of transverse ply cracks (shown schematically in Figure 4-4)
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From page 24... ...
. The fundamental cause of thermally induced transverse matrix cracking in a laminate is the residual stresses resulting from differences in thermal expansion of the lamina in longitudinal (parallel to the fiber orientation)
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From page 25... ...
Concentration gradients (Figure 4-7 - from high moisture levels at the surface to near dryness at the midplies along with subsequent Resorption in the near-surface plies and redistribution of moisture during flight can lead to significant residual stresses or outer-ply delamination or blistering during rapid heating. The effects of moisture are dependent on many factors, including thickness, material type, prevailing humidity, flight temperature profile, time and conditions between flight times, and presence of matrix cracking.
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From page 26... ...
Perhaps the most critical damage mechanism operating in high-temperature polymeric composites is the formation of transverse ply cracks and in-plane microcracks within the matrix of multiaxial composites. Matrix cracking can result from initial laminate processing, mechanical static and fatigue loading, residual stresses resulting from hygrothermal exposures, thermal cycling, and combined effects of mechanical and environmental cycles.
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From page 27... ...
Non-oxide fibers are also subject to the same thermochemical degradative processes if they are exposed to the environment. In addition, the polymer-derived fibers undergo microstructural changes at elevated temperature which lead to altered mechanical properties (Bodes et al., 1993)
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From page 28... ...
The selective attack of certain microstructural features of the composite can lead to the initiation of flaws that affect slow crack growth and creep deformation processes. RECOMMENDATION The development and application of aging characterization tests and predictive models require a thorough understanding of the important basic materials properties and degradation and damage accumulation mechanisms to provide confidence in their ability to accurately assess service life.
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