Research On Fatigue Crack Propagation Behavior In High-strength Steel For Marine Structures Under Variable Amplitude Loading

Fatigue failure is one of the most severe problems faced by marine and ocean engineering structures.Thus,an accurate method to assess the fatigue life of marine structures is indispensable to keep offshore workers and properties safe.The fracture mechanics based fatigue life assessment method makes it possible to consider the influence of retardation and interaction caused by overloads and different load sequences.The ability to consider the complexity of actual loadings makes it a more reliable prediction method compared to the S-N curve method based on Miner cumulative damage criterion.Nevertheless,there is a paucity of research focused on the fatigue performance of marine steels under variable amplitude loading.The difference in crack propagation behaviour in various materials is uncertain.It is of great significance to investigate the crack growth behaviour in materials for ship and ocean structures.The main contents of this dissertation are as follows:A series of fatigue crack propagation experiments were designed for compact tension specimens made of AH36 High-strength steel for marine structures.The influence of overload ratio,interval of overloads,and load sequence on FCGR are investigated.Besides,storm model loads and superimposed loads were studied experimentally.The research indicates FCG behaviour under variable amplitude loadings deviated apparently.The difference is mainly caused by the sequence of variable amplitude and the retardation/acceleration caused by tensile/compressive overloads.Digital image correlation technique is applied to obtain the strain fields and residual strain along crack tips.The mechanism of FCG behaviour under variable amplitude loadings is investigated by analyzing the transformation of strain fields in crack tips.In addition,a crack tip plastic zone size calculation method based on XFEM is proposed to improve work efficiency.And the deficiency of analytic formula in nonstandard structures analysis is eked.Three kind of measurement methods for crack closure force is compared with each other.Then,the FCG behaviour is predicted by using crack closure model.The plastic zone size obtained by DIC technique is then applied to improve the modified Wheeler model.The FCG behaviour under variable amplitude loadings is then predicted by the improved modified Wheeler model.