| Fatigue failure and brittle fracture are the most common failure modes in the ocean engineering structures. With the tendency of application of ocean structures into deep-sea, the thickness and strength of steel plate used in the ocean structures is increasing. The fatigue performance and the toughness of the steel play more important role for the safety of the ocean structures. If the fatigue performance is good, the fatigue crack initiation will not happen or the speed of the crack propagation is very low. Further more, if the toughness of steel is good enough, the crack propagation will stop even if there have been fatigue cracks in the structures. It will not cause the brittle fracture which is very dangerous for safety of the structures. Therefore, it is a very significant work to evaluate the fatigue performance and toughness of the marine steel.The main contents of this dissertation can be divided into three parts:the first part is study about assessment of the toughness of the marine steel(chapterⅡ); the second part is study about assessment of the fatigue performance of the marine steel(chapterⅢandⅣ);the third part is study on relationship between CTOD and fatigue life of marine steel(chapterⅤ). The steel for ocean structures has the advantages of high strength, heavy thickness and so on, this dissertation selects EQ70 steel as study object which is representative.Firstly, taking the characteristics of the marine steel into account, the dissertation analyzed the current methods of determination of the toughness of material and it is concluded that the CTOD test is the best method to evaluate the toughness of marine steel, then evaluated the toughness of EQ70 steel by this method. The test results showed that EQ70 steel has good performance of low temperature toughness.Secondly, compared the current methods of fatigue testing, the dissertation designed a fatigue testing which is applicable to marine steel and got S-N curve by the method. The testing results showed that EQ70 steel has good fatigue performance. Then further analysis of the experimental data is made to obtain the P-S-N curve which is based on different probability. At the same time, the dissertation has analyzed the distribution of the fatigue data and it proved that the logarithm of fatigue life follows normal distribution.Since to get S-N curve in different reliability is very complicated, P-S-N surface method which can reflect the S-N curve in nearly all reliability is developed. It is proven that the P-S-N surface method has good accuracy and it will help the designers obtain the S-N curve in prescriptive reliability.Finally, the linear relationship between the logarithm of fatigue life and CTOD is got and also the explanation from microscopic aspect is given. The dissertation presents two methods and one model equation. The two methods are called respectively Probability Distribution Method and P-δCurve Method, while the former can give the probability of CTOD of the material meeting the criterion and can compare the toughness of two kinds of steel plate. The model equation has established a foundation of the relationship between toughness and fatigue performance of the marine steel.
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