The deck plates of single-deck floating roofs for large oil storage tanks are joined by continuous full-fillet welds on the upper side of lap plates. The deck plate area is subjected to cyclic deformation due to strong wind load. Fatigue cracks may occur at the lap welded joints. The objective of this study is to investigate the effect of the imperfect lap joints on the fatigue life. The fatigue life is predicted by the crack growth analysis. The stress intensity factors are analyzed using the influential function method established by Shiratori, and the crack growth is based on the Paris Law in this paper. In the influential function method, the stresses on the virtual cracked surface of the uncracked lap joints had been analyzed and were reported in the previous paper. Under the tensile load, the fatigue life of the joint decreases with increasing the gap size of the lap plates. On the other hand, the fatigue life is not related to the gap size under the bending load.

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High strength Cr-Mo steels are useful for high performance machinery and equipment. Factors that affect hydrogen susceptibility of fatigue lives have been explored and analyzed quantitatively by using SCM435 steel of various purities. Logarithm of fatigue life ratio log (R_Nf) , which means the fatigue life of hydrogen charged specimen normalized by that of non-charged one, is proportional to H (450K) that means the amount of hydrogen evolved up to 450K in thermal desorption spectroscopy (TDS) measurement. The proportional coefficient is negative and changes in accordance with the square of phosphorous content of the specimen. The intercept of log (R_Nf) is zero for soaking in FIP solution, but it becomes log (0. 2) to log (0. 4) for cathodic hydrogen charging. The log (R_Nf) is lowered by increase in tensile strength. Supposing that the degradation be independent with hydrogen and phosphorous contents, this term could be proportional to tensile strength. Within the experimental range, the fatigue life ratio R_Nf can be expressed by the following equation,
log R_Nf(est) =log (r_pit) -1500•H(450K) •P²-0. 0045•TS+4. 8
Here r_pit is the correction factor for corrosion pit, P is phosphorous content, TS is tensile strength in MPa

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Correlation between lower shelf and transition temperature is examined on fracture toughness data for brittle crack arrest. The lower shelf shows a tendency to be higher for steels of lower transition temperature. A simple model is proposed to explain this tendency.

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