Fatigue Crack Initiation

Slip bands within a grain are very small, but can still be imaged with powerful microscopes. 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.

An extrusion will protrude from the surface of a grain while in intrusion sinks into the plain of a grain.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).

Though slip bands are small regions, they often initiate cracks where they meet the uniform grain surface.