Along with rapid increase in the size of container ship for a few decades, it is required to use thicker plates in way of the hatch coaming than before. The use of higher tensile steel with yield strength 460 MPa is also inevitable to avoid using excessively thick plates. According to the recommendation of IIW and rule requirements of classification societies, it is defined that fatigue strength of welded joints is independent of the tensile strength and the effect of plate thickness is considered by mean of a correction exponent. Recently, to investigate the effect of plate thickness and tensile strength on fatigue strength for the weld joints, series of fatigue test were conducted on the specimens of butt weld joint and longitudinal fillet weld joint with the different plate thickness and tensile strength steel. From the test results of each series of specimens which were taking into account recent developments concerning material and welding techniques, the effect of plate thickness and material was investigated. In addition, considering the effect derived from this fatigue test, fatigue strength was calculated on butt weld joints and longitudinal fillet weld joints in way of hatch coaming top plate for a large container ship.
The trend toward increasing the size of container ships has been accelerated rapidly since 2005 as shown in Fig. 1. The increase in the size of container ships has led to an increase of steel plate thickness. Fig. 2 shows the relations between the increase of ship size and plate thickness in way of hatch coaming. Furthermore, it is inevitable to use new material class with a yield stress 460 MPa, hereafter referred to as YP47, because the heavy-thick plates make some kinds of problems such as difficulties of welding work and weight limitation to control plate.
