Crack Initiation and Propagation
Cracks usually initiate at some point of stress concentration
- Common areas include scratches, fillets, threads, and
dents
Propagation occurs in two stages:
- Stage I: propagates very slowly along crystallographic
planes of high shear stress and may constitute either a large or small
fraction of the fatigue life of a specimen
- Stage II: the crack growth rate increases and
changes direction, moving perpendicular to the applied stress