Crack flanks

Fig. 12.8 (a) A mode I crack with a cohesive zone of length c over which the opening tractions at the two tips of the crack are limited to <7c. (b) The shape of a mode I crack with a cohesive zone of length c under a traction of compared with the shape of a mode I crack with a singular field (v is the half crack-flank displacement shown in (a) (from Williams (1984) courtesy of Wiley). 

When the central ligament of thickness 2b becomes just fully plastic, a symmetrical Prandtl slip-line field is established. This moves material by plastic shear from the crack flanks into the central ligament (McClintock 1969). 

Figure 14.10 (a) Point load closure force, P, on crack flanks (b) Closure pressure, p, on crack flanks. 

Hence, the extent of phase transformation toughening and overall toughness is a function of the square root of the width of the transformed zone along the crack flanks. 

Parkins has devised a slow and rapid potentiodynamic scanning method to determine both the relative susceptibility to slip-film rupture-repassivation SCC susceptibility, as well as possible ranges of potential where SCC might occur [60], The method applies to metals and alloys whose oxide films can initially be cathodicaUy reduced. The validity of the method relies on the notion that SCC only occurs when crack walls are readily passivated while the crack tip dissolves at a high rate due to continual destabilization of the passive film. In this method, the rapid scan anodic polarization curve provides a measure of the bare dissolution kinetics over a range of anodic potentials. A slow anodic potentiodynamic scan provides a measure of the passive crack flank dissolution kinetics over the same range of potential. Alloy-electrol5 combinations that produce potential... 

In a second example, CF is promoted by increased impurity-sulfur in ferritic steels subjected to low-frequency loading in pressurized pure water at 288°C (Fig. 15) [22,23], MnS inclusions, which intersect crack flank surfaces, dissolve to enrich the occluded crack solution in sulfide. These anions promote crack advance by increasing the anodic charge that is passed per film rupture event, or perhaps by the HEE mechanism. This effect of steel sulfur content is severe for a stagnant environment within the autoclave, and is eliminated by turbulent solution flow which reduces sulfide buildup within the crack [66],... 

The extent of the transformation at the crack flanks is an important parameter, which depends on temperature, the amount and type of stabiliser, and the grain size of tetragonal zirconia. The maximum capability of a material to experience transformation toughening can be expressed as a shielding stress intensity factor, Ks, which is a function of the transformation zone shape, as evaluated by the parameter a, and the zone size, ho. It may be expressed as... 

In an electrochemical sense, SCC is similar to crevice corrosion and differential aeration corrosion, in that anodic and cathodic areas are spatially separated. With SCC, the anodic processes occur at the crack tip while cathodic processes occur on the crack flanks and near the crack mouth. Hinton et al. (1985) have proposed that the inhibition... 

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