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Fracture Mechanisms in Nonmetals 287
FIGURE 6.33 The martensitic toughening mechanism. Transformation of particles near the crack-tip results
in a nonlinear process-zone and crack-tip shielding.
Outside of this zone, the stress field is defined by the global stress intensity, but the stress field in
the process zone is lower, due to dilatational effects. The crack-tip work and shielding explanations
are consistent with one another; more work is required for crack extension when the local stresses
are reduced. Crack-tip shielding due to the martensitic transformation is analogous to the stress
redistribution that accompanies plastic zone formation in metals (Chapter 2).
The transformation stress and the dilatational strain are temperature dependent. These quantities
influence the size of the process zone h, and the strain-energy density within this zone. Consequently,
the effectiveness of the transformation-toughening mechanism also depends on temperature. Below
M , the martensite start temperature, the transformation occurs spontaneously, and the transforma-
s
tion stress is essentially zero. Thermally transformed martensite does not cause crack-tip shielding,
however [42]. Above M , the transformation stress increases with temperature. When this stress
s
becomes sufficiently large, the transformation-toughening mechanism is no longer effective.
6.2.3 DUCTILE PHASE TOUGHENING
Ceramics alloyed with ductile particles exhibit both bridging and process-zone toughening, as
Figure 6.34 illustrates. Plastic deformation of the particles in the process zone contributes toughness,
as does the ductile rupture of the particles that intersect the crack plane. Figure 6.35 is an SEM
fractograph of bridging zones in Al O reinforced with aluminum [40]. Residual stresses in the
2 3
particles can also add to the material’s toughness. The magnitude of the bridging and process-zone
toughening depends on the volume fraction and flow properties of the particles. The process-zone
toughening also depends on the particle size, with small particles giving the highest toughness [40].
FIGURE 6.34 Ductile phase toughening. Ductile
second-phase particles increase the ceramic toughness
by plastic deformation in the process zone, as well as
by a bridging mechanism.