THE RELATIONSHIP BETWEEN FRACTURE TOUGHNESS AND NOTCH TOUGHNESS, TENSILE DUCTILITIES IN LATH MARTENSITE STEEL

The relationship between fracture toughness and notch toughness, tensile ductilities in lath martensite steel has been investigated. The experimental results show that fracture toughness is controlled by microplastisity in the small local region (1 to 2 times the crack tip opening displacements c at critical) ahead of the crack tip, whereas blunt notch toughness, tensile ductility is controlled by austenite grain size (dγ) or martensite packet diamiter (dp). The characteristic distance corresponds to the extent of effective deformed region (2c) resulted from blunting of crack tip. The relationship between fracture toughness and notch toughness 、tensile ductility depends on the relative size of effective influence region (2c) and austenite grain. When 2c (1 - 2dγ), the fracture toughness coincides with notch toughness, tensile ductility.