STUDY ON UNSTABLE BRITTLE CRACK ARREST TOUGHNESS OF NEWLY DEVELOPED STEEL WITH SURFACE LAYERS WITH ULTRA FINE GRAIN MICROSTRUCTURE; IMPROVEMENT OF ARREST TOUGHNESS BY ENHANCED SHEAR- LIP FORMATION

Unstable running crack behaviour is strongly affected by side- ligaments (shearlips), which is formed in the surface layers of steel plate after an unstable crack has propagated in the mid-thickness of the plate. Since extensive plastic deformation takes place during the formation of shear-lips, enhanced shearlips formation can improve the crack arrestability. Therefore, a new type of steel plate has been developed by controlling the microstructure, especially in the surface layer of the plate. The surface layer of the steel plate has an ultra fine grain ferrite microstructure which possesses excellent fracture toughness even at a cryogenic temperature. Fracture mechanics investigations confirm that shear-lips can be formed in the surface layers with this microstructure whenever a brittle crack propagates in the developed steel plate at an actual service temperature for ships. Furthermore, the brittle fracture propagation models also confirm that crack arrestability is improved by enhanced shear-lip formation.