Three stages of fatigue failure:
1. crack initiation in the areas of stress concentration
2. incremental crack propagation
3. final rapid crack propagation after crack reaches critical size
The total number of cycles to failure is the sum of cycles at the first and the second stages:
Crack initiation at the sites of stress concentration (microcracks, scratches, indents, interior corners, dislocation slip steps, etc.). Quality of surface is important.
Crack propagation
Stage I: initial slow propagation along crystal planes with high resolved shear stress. Involves just a few grains, and has flat fracture surface
Stage II: faster propagation perpendicular to the applied stress. Crack grows by repetitive blunting and sharpening process at crack tip. Rough fracture surface.
Crack eventually reaches critical dimension and propagates very rapidly.