Crack-tip growth

Crack-tip growth mechanisms have been proposed that do not involve dislocation movement explicitly, but rather, in response to the stress field at the crack tip, interstitial atoms diffuse to the region of the stress field to reduce the stress substitutional atoms also will diffuse to the tip if the local stress is thereby reduced. Crack-tip growth would be increased if this local change in alloy composition enhances dissolution during slip displacement or alters the passive film such that it is more easily ruptured by dislocations emerging to the surface. That is, there is continuously produced at the crack tip a film that is more easily ruptured than the more stable passive film on the sides of the crack (Ref 158). 

Figure 3.29. Crack tip growth deformation likened to a sinusoidal cyclic motion, (a) Dark portion resembles first 90° of deformation (b) hypothetical crack tip. (Seward, 1970.)...

The mechanism of subcritical crack growth is the reaction of the corrosive medium with highly stressed bonds at the crack tip. In siUca, in the absence of stressed bonds, the rate of the reaction between the bonds and corrosive media such as water is very low. The introduction of strain energy into crack tip bonds increases the activity of the bond. For siUca glass in water, attack and bond breakage occurs by the following reaction (47) ... 

The utility of K or any elastic plastic fracture mechanics (EPFM) parameter to describe the mechanical driving force for crack growth is based on the ability of that parameter to characterize the stress-strain conditions at the crack tip in a maimer which accounts for a variety of crack lengths, component geometries and loading conditions. Equal values of K should correspond to equal crack tip stress-strain conditions and, consequently, to equivalent crack growth behavior. In such a case we have mechanical similitude. Mechanical similitude implies equivalent crack tip inelastic zones and equivalent elastic stress fields. Fracture mechanics is... 

Fig. 7. Schematic representation of hydration causing crack propagation in a wedge test specimen. The increase in volume upon hydration induces stresses at the crack tip that promote crack growth 19,391.

Another approach to the question of resistance to crack growth is to consider the extent to which yielding occurs prior to fracture. In a ductile material it has been found that yielding occurs at the crack tip and this has the effect of blunting the crack. The extent of the plastic zone (see Fig. 2.70) is given by... 

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