Development of Diagnostics Evaluation Method for Shell-to-Annular Joint of the Oil Tank Under Ultra Low Cycle Fatigue

Since the past decade, diagnostics evaluation method for the oil tank under ultra low cycle fatigue was continuously discussed through several committees in Japan High Pressure Institute (JHPI). It has been supposed that the shell-to-annular joints of the oil tank would suffer from large cyclic plastic strains of several percentages by the effect of bending moments caused by large earthquake. Few studies have been reported about fatigue crack growth estimation for such large plastic strain region. In order to assess the structural integrity of the oil tank, extensive efforts are required to establish the estimation technique of the fracture behaviors and the criterion of maintenance under cyclic plastic strains to prevent or minimize fatal accidents. The JHPI committee aims to establish the Fitness-for-Service procedure for national large oil tank. This paper reports the summary of the activity of the committees toward the establishment of the method to estimate crack growth life under repeated large plastic loading. Crack growth behaviors under large cyclic plastic strains were examined experimentally under both axial loadings and bending loadings of a plate with a surface flaw. A series of three-dimensional FEM analyses was also carried out to obtain J-integral value for shell-to-annular joint with various crack sizes. A diagnostics evaluation method for shell-to-annular joint of the oil tank under supposed large earthquakes were developed by using these results with taking into account of the accompanied various extra phenomena such as new micro-cracks initiation, irregular crack growth due to crack coalescence, crack initiation and growth at the back surface of the specimen, etc.