Crack Initiation and Propagation

Stages is fatigue failure:

I. crack initiation at high stress points (stress raisers) II. propagation (incremental in each cycle)

III. final failure by fracture

Stage I - propagation

•  slow

•  along crystallographic planes of high shear stress

•  flat and featureless fatigue surface

Stage II - propagation

Crack propagates by repetitive plastic blunting and sharpening of the crack tip.