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The temperature cycle range above is typical for

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Unformatted text preview: indicator of the progression of fatigue damage. engineering stress in a tensile test. the that relates the magnitude of a thermal gradient to the The normalized modulus value E E0 as a function of cycle ratio is shown schematically unidirectional fiber–reinforced composite A comresulting heat flow rate. in Figure 14.5–8, where E is the elastic modulus and E0 is the modulus of the undamaged posite material in which all fibers are aligned parallel to composite. thermal diffusivity The constant of proportionality in each Figure 14.5–9 shows the relationships between the ratio of the applied as the non-steady-state heat flow equation. It is defined maximum stress other. | | pg604 [V] G2 7-27060 / IRWIN / Schaffer 604 Part III iq 13.01.98 plm QC2 rps MP Properties MICROSTRUCTURE AFTER 1000 CYCLES (a) MICROSTRUCTURE AFTER 2000 CYCLES (b) MICROSTRUCTURE AFTER 3000 CYCLES (c) FIGURE 14.5–10 Damage accumulation in the matrix of an alumina fiber-reinforced Al-Li matrix composite as a result of thermal cycling: (a) after 1000 thermal cycles, (b) after 2000 thermal cycles, and (c) after 3000 thermal cycles. The damaged regions appear as noncircular dark patches within the lighter-colored matrix. (Source: Reprinted with permission from A. M. Gokhale.) maximum value at that location. Figure 14.5–10 shows the progression of thermal fatigue damage in a metal-matrix composite containing alumina (Al2 O3) fibers in a matrix of an aluminum-lithium alloy. The specimens were thermally cycled between 300 C and room temperature with no mechanical loading. Cracks and voids appear in the matrix between fibers as a result of thermal fatigue damage. These defects can considerably degrade the load-carrying capability of composites. The temperature cycle range above is typical for supersonic aircraft structures. To minimize thermal fatigue damage, the difference between the thermal expansion coefficients of the matrix and fiber materials must be minimized. This is not always possible; hence, compliant (flexible) fiber coatings are used to ease the thermal stresses at the interfaces. This step, although effective in reducing the occurrence of thermal fatigue failures, adds considerably to the composite cost. 14.6 OTHER APPLICATIONS OF COMPOSITES | v v Magnetic resonance imaging (MRI), commonly used in the medical field for noninvasive imaging of internal organs, uses superconducting solenoids to produce high-intensity magnetic fields. The solenoids are made from a composite of niobium-titanium filament in a matrix of high-purity copper. The superconducting temperature for this alloy is about 23 K. Thus, the superconducting cable is immersed in liquid helium at all times. Any change in the applied field leads to heating of the filament, which in turn may cause the filament to lose its superconducting characteristic. This raises its resistance, leading to additional heat generation. To contain runaway behavior, several small-diameter superconducting filaments are embedded in a copper matrix. When the resistance of the filament changes, the...
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