Unformatted text preview: indicator of the progression of fatigue damage. engineering stress in a tensile test.
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
thermal diffusivity The constant of proportionality in
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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