Unstable Brittle Crack-Arrest Toughness of Newly Developed Steel Plate with Surface Layers having Ultra Fine Grain Microstructure (2)

Abstract

A new type steel plate with ultra fine grain surface layers (SUF plate) has been developed to improve crack arrestability. In order to prevent the catastrophic damage of hull, which might cause ocean pollution and casualties, caused by serious accidents such as collisions and/or grounding of VLCCs (Very Large Crude oil Carriers), application of SUF plates with ultra-high crack arrestability has been investigated using both computer simulation for a collision of VLCCs and large scale fracture testings for crack arrestability in emergency cases.The computer simulation results suggest that a collision generates significant plastic strain in surroundings of a struck part. For example, the plastic strain reached a 5-10% level before an inner-hull of the VLCC was ruptured at sheer strake plates near the struck part.Therefore, the effect of plastic strain (10% level) on crack arrestability in steel plates (the SUF plate and a conventional TMCP plate) was examined by standard ESSO test, ultra-wide plate duplex ESSO test, and sheer strake model fracture test. The test results suggest the followings : 1) Plastic strain deteriorates crack arrestability.2) Sufficient crack arrestability at 0 deg. C cannot be expected in the conventional TMCP steel plate with 10% plastic strain induced.3) The SUF plate with 10% plastic strain induced possesses high crack arrestability against emergency cases for ships.Therefore it is expected that application of SUF plates can prevent serious accidents in which a large amount of oil flows out through hull structural plates damaged by brittle fracture.