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288 Fracture Mechanics: Fundamentals and Applications
FIGURE 6.35 Ductile-phase bridging in Al 2 O 3 /Al. Photograph provided by A.G. Evans. Taken from Evans,
A.G., “The New High Toughness Ceramics.” ASTM STP 1020, American Society for Testing and Materials,
Philadelphia, PA, 1989, pp. 267–291.
This toughening mechanism is temperature dependent, since the flow properties of the metal
particles vary with temperature. Ductile phase ceramics are obviously inappropriate for applications
above the melting temperature of the metal particles.
6.2.4 FIBER AND WHISKER TOUGHENING
One of the most interesting features of ceramic composites is that the combination of a brittle
ceramic matrix with brittle ceramic fibers or whiskers can result in a material with relatively high
toughness (Table 6.1). The secret to the high toughness of ceramic composites lies in the bond
between the matrix and the fibers or whiskers. Having a brittle interface leads to higher toughness
than a strong interface. Thus ceramic composites defy intuition: a brittle matrix bonded to a brittle
fiber by a brittle interface results in a tough material.
A weak interface between the matrix and the reinforcing material aids the bridging mechanism.
When a matrix crack encounters a fiber/matrix interface, this interface experiences Mode II loading;
debonding occurs if the fracture energy of the interface is low (Figure 6.36(a)). If the extent of
debonding is sufficient, the matrix crack bypasses the fiber, leaving it intact. Mathematical models
[43] of fiber/matrix debonding predict crack bridging when the interfacial fracture energy is an