Environmental Engineering Reference
In-Depth Information
Mechanism
In mechanical fatigue, the crack initiation is attributed to the formation of extru-
sions and intrusions at the metal surface because of localized slip within the grain
caused by the stress (Fig. 3.59). The slip is localized because the stress is not
large enough to produce massive slip. Once the crack initiates the tip of the
crack undergoes localized plastic deformation in the same manner and the crack
propagates. Stresses below the endurance limit cannot overcome the work harden-
ing caused by plastic deformation and further slip is impeded.
Since corrosion fatigue involves the lowering of endurance limit or the change
of the fatigue behavior of ferrous metals to that of nonferrous metals exhibiting
no endurance limit, it has been envisaged that the corrosive environment helps
in the process of localized plastic deformation at the crack tip. This may be
achieved either by the removal of the barriers to plastic deformation, such as
dislocation piled up at the metal surface at slip steps, or by favoring plastic defor-
mation by the reduction of surface energy. The process of formation of extrusions
and intrusions thus continues at lower stress levels.
Although the exact mechanism has not been quantitatively presented, it never-
theless explains the combined role of stress and corrosion. The more aggressive
the solution, the more of this effect is achieved. Also, the shortening of fatigue
life with the application of cyclic stresses of lower frequency is explained by the
Figure 3.59
Extrusions and intrusions in copper after 6
10
5
cycles in air silver-
coated after test.
