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286 Fracture Mechanics: Fundamentals and Applications

FIGURE 6.31 The microcrack toughening mechanism. The formation of microcracks in or near second-phase
particles results in release of strain energy (modulus work) and residual microcrack opening (dilatational
work). 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 mechanism is relatively ineffective, as Table 6.1 indicates. Moreover, the degree of micro-
crack toughening is temperature dependent. Thermal mismatch and the resulting residual stresses
tend to be lower at elevated temperatures, which implies less dilatational strain. Also, lower residual
stresses may not prevent the microcracks from becoming unstable and propagating through the
particle–matrix interface.
6.2.2 TRANSFORMATION TOUGHENING

Some ceramic materials experience a stress-induced martensitic transformation that results in shear
deformation and a volume change (i.e., a dilatational strain). Ceramics that contain second-phase
particles that transform often have improved toughness. Zirconium dioxide (ZrO ) is the most
2
widely studied material that exhibits a stress-induced martensitic transformation [42].
Figure 6.32 illustrates the typical stress-strain behavior for a martensitic transformation [42].
At a critical stress, the material transforms, resulting in both dilatational and shear strains. Figure 6.33(a)
shows a crack-tip process zone, where second-phase particles have transformed.
The toughening mechanism for such a material can be explained in terms of the work argument:
energy dissipation in the process zone results in higher toughness. An alternative explanation is
that of crack-tip shielding, where the transformation lowers the local crack driving force [42].
Figure 6.33(b) shows the stress distribution ahead of the crack with a transformed process zone.

FIGURE 6.32 Schematic stress-strain response of a
material that exhibits a martensitic transformation at
a critical stress.