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Fatigue
Crack Initiation
 While
on the subject of dislocations, it is appropriate to briefly discuss
fatigue. Fatigue is one of the primary reasons for the failure
of structural components. The life of a fatigue crack has two
parts, initiation and propagation. Dislocations play a major role
in the fatigue crack initiation phase. It has been observed in
laboratory testing that after a large number of loading cycles
dislocations pile up and form structures called persistent slip
bands (PSB). An example of a PSB is shown in the micrograph image
to the right.
 PSBs
are areas that rise above (extrusion) or fall below (intrusion)
the surface of the component due to movement of material along
slip planes. This leaves tiny steps in the surface that serve
as stress risers where fatigue cracks can initiate. A crack at
the edge of a PSB is shown in the image below taken with a scanning
electron microscope (SEM).
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