Research On The Direct Approach Of Assessing Strength Of Marine Structure With Crack Based On Failure Assessment Diagram

Crack is one kind of unavoidable defects in the huge welding structures, such as ships and ocean structures. Therefore it is the key issue of ensuring the structure’s safety to assessment the strength of the structures with crack. Failure assessment diagram (FAD) is a powerful tool to assess the safety of structures with flaws. As the assessment results based on FAD are straightforward and reliable, the concept of FAD is accepted widely in industry. To this day, there are several regulations for assessing integrity of structure with flaws based on FAD. However, the application of FAD in the area of naval architecture and ocean engineering has just begun.In this dissertation, the direct approach of assessing strength of ships and ocean structures based on FAD has been established, which can supply the theoretic proof and technical support in the designing and inspection of marine structures. The main work of this dissertation is as follows,(1) The new formulae of virtual crack closure technique, which can be used in four-node shell element, is proposed base on the theory of energy release rate. The new dummy-node fracture element has been implemented with the commercial FEA software ABAQUS(?) through the user defined element (UEL). In addition, the node definition, building of stiffness matrix and local coordinate system of the new element has been described. Two examples of plate with central crack and cylinder with circumferential through crack are examined by comparison to the analytical formulae. The results illustrates that the new element is accurate, easy to operate, and non-sensitive to the size of mesh.(2) The new formulae for calculating the reference stress is proposed based on the load and geometry information of ligament of cracked structure. Furthermore, a new technique, named virtual section closure technique, has been established based on the new formulae and dispersing concept. The reference stress element has been implemented with the commercial FEA software ABAQUS(?) through the user defined element (UEL). Two examples of plate with central crack and cylinder with circumferential through crack are also examined by comparison to the formulae in the related regulations. The results illustrates that the new proposed element is accurate, non-sensitive to the size of mesh.(3) A new super element is developed and implemented into commercial FEA software ABAQUS(?) with user defined element for determining stress intensity and reference stress simultaneously with one step linear analysis. Thus, the new super element is named failure assessment element. The proposed element and related formulae in regulation are used to assess the strength of plate with central crack and cylinder with circumferential crack. The comparison of results illustrates that the failure assessment element is accurate, non-sensitive to mesh size, and more effective.(4) The failure assessment element and other numerical methods are used to assess the strength of Nishihara model with through crack, cylinder with longitudinal stiffeners with through crack and circular hollow section T-joints with through crack, which can validate the feasibility of the new element in typical ship and ocean structure. The comparison of results illustrates that the results of failure assessment element are accurate and reliable. Furthermore, the new element is more effective than other methods. Therefore, the failure assessement elelemt can be a powerful tool to assessment the integrity of marine structures.