Local criteria for ductile fracture

Strain distributions in specimens suitable for studying the initiation of fracture are reviewed, and distributions are developed for the steady-state propagation of cracks in plane strain lension of fully plastic materials. The functional forms of local fracture criteria are discussed for different metallurgical mechanisms. It is concluded that:

1. pure Mode I (normal) fracture is unlikely to exist except in cleavage.
2. there is both theoretical and experimental evidence for the development of both: sharp and flat-bottomed cracks.
3. simultaneous diffuse and concentrated (Dugdale-Muskhelishvili) flow fields can occur in torsion of longitudinally grooved bars if the stress-strain curve has a maximum which causes band formation, so that a displacement criterion becomes appropriate for final fracture.