Fatigue Strength Assessment of Crack Flaws in Thick High Tensile Steel Plates on Large Ships

The sizes of large container ship increased during the past years have accompanied by the use of thicker plate and higher steel grade. The fatigue performance of thick high strength steel plate is a problem which has to be investigated. As an example, the fatigue crack growth of the butt welds at the deck plate of a 8530 TEU container ship has been investigated. Crack growth life predication method in combination with failure assessment diagram (FAD) technology has been used to calculate the fatigue life and the critical crack length. It has been discussed the fatigue sensitivity of the steel grades, the plate thicknesses and the positions of initial defect in thickness direction. The FAD methodology used in this paper is a common approach which has been introduced in various standards, which is practical and useful in fatigue strength assessment of ship structures. Crack tip opening displacement (CTOD) tests of high tensile steel YP40 and YP 47 have been carried out to obtain the fracture toughness, which play an important role in FAD calculation. And the minimum value of the test results is used as the calculated value. It is concluded that the fatigue performance of high strength steels do not increase proportionally with the steel grades and plate thicknesses. The calculation results also have shown that there is significant decrease of fatigue performance along with the increase of plate thickness, and in the same initial conditions, the harmfulness of the near surface defects on structure detail is more serious compared to the embedded defects.