Page 308 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)
P. 308

1656_C006.fm  Page 288  Monday, May 23, 2005  5:50 PM





                       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
   303   304   305   306   307   308   309   310   311   312   313