Crack tip shielding

Cracking catalysts Crack-tip shielding Crack-wake bndging Cr3Al Crambe Crambe oil Cranb erry... 

The toughness induced in ceramic matrices reinforced with the various types of reinforcements, that is, particles, platelets, whiskers, or fibers, derives from two phenomena crack deflection and crack-tip shielding. These phenomena usually operate in synergism in composite systems to give the resultant toughness and noncatastrophic mode of failure. 

Crack Deflection Contribution to Toughening. Crack deflection is a phenomenon that leads both to toughening and to the formation of bridges that shield the crack tip from the appHed stress. Little is known of the bridge formation process, but its effect, that is, crack-tip shielding, is considered in the following section. 

In structural ceramic composites, the principal effect considered was one of crack-face closure tractions, or cohesive forces, brought about, for instance, by bridging fibers. A rigorous evaluation of the crack tip fields where the crack faces are not traction free has not yet been attempted. However, an approximate approach for the small-scale creep case is to assume that the crack tip fields are not functionally altered by crack-face tractions, with the effect of the traction being only to introduce a zone of crack tip shielding. This allows for the development of preliminary models for creep crack growth which is inclusive of the role of crack bridging. These preliminary models predict that,... 

Dislocation Screening at a Crack Tip From the standpoint of the linear theory of elasticity, the problem of crack tip shielding may be seen as a question of solving the boundary value problem of a crack in the presence of a dislocation in its vicinity. As indicated schematically in fig. 11.15, the perspective we will adopt here is that of the geometrically sterilized two-dimensional problem in which both the crack front and the dislocation line are infinite in extent and perfectly straight. As we will see, even this case places mathematical demands of some sophistication. The basic question we... 

Ritchie, R.O. (1988) Mechanism of fatigue crack propagation in metals, ceramics and composites role of crack-tip shielding. Mater. Sci. Eng., 103, 15-28. 

Figure 8.56 In crack tip shielding, various processes can be activated by the high stresses near the crack tip and this process zone can be left as a wake behind the crack tip. If this process reduces the stresses at the crack tip, the zone shields the crack tip.

Crack tip shielding by Second-phase brittle fibers with partial... 

When the neat epoxy resin is modified by the incorporation of submicron rubber particles or glass spheres, the fracture energy can be increased by factors of 4-5 at all levels of temperature, as is shown in Fig. 13.41. Moreover, the increase of toughness with temperature is less abrupt. As we demonstrate, these observations represent an almost classical behavior of transformation toughening by crack-tip shielding of a brittle solid (Evans et al. 1986). 

The average ratio of the above quantity measured by Kinloch is 5.7, indicating that there are additional effects of crack-tip shielding attributable directly to the cavitation that we have not accounted for. 

In the Mg-PSZ, the transformation from the metastable tetragonal phase to the monoclinic phase absorbs part of the energy required for crack propagation with consequent increase in fracture toughness, manifested as crack-tip shielding. 

In the above relations, Tq is intrinsic toughness, is the extrinsic toughness mechanism (crack-tip shielding), Rq is the fracture resistance energy and R is the crack resistance energy contribution. The critical condition for crack extension is then given by ... 

Since crack-tip shielding events are irreversible by nature, it is expected that the toughness of ceramics will increase with the crack extension (T- or R-curve behaviors). This implies that the toughening terms, T or R, are functions of crack length, c. Therefore, Eqs. (8.69) and (8.70) may take the form ... 

Fig. 8.60 Schematic representation of the classes and mechanisms of crack-tip shielding [46]. With kind permission of Professor Ritchie...

Fig. 8.61 Schematic illustrations of crack-tip shielding processes in ceramics (redrawn after Lawn) [21], With kind permission of Dr. Jorge Lino...

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