Study On Low-Cycle Fatigue Crack Propagation Of Cracked Hull Plates And Stiffened Plates

The large-scale development of ships has become a trend to adapt to the development of society nowadays.The stress level will be greater as the size of the vessel become larger under the severe sea conditions.The problem of low-cycle fatigue crack propagation has been paid more and more attention due to the frequent occurrence of marine accidents caused by fatigue fracture.A key problem in the study of low-cycle fatigue crack propagation is the analysis of the influence factors on the stress field and crack opening displacement field of crack tip.Serial analysis on the crack propagation rate is also crucial for the study of low-cycle fatigue lives of structures.Different research methods should be adopted according to different structural forms to solve the basic structure fatigue problems of large ships.The present paper focuses on the low-cycle fatigue crack propagation problem of cracked hull plates and stiffened plates and this thesis will start from the following aspects.?1?Theoretical study of stress and strain field as well as crack opening displacement field at the crack tip of cracked hull plates and stiffened plates is carried out considering the passivation mechanism of crack tip area under cyclic loading based on elastic-plastic fracture mechanics theory.?2?The influence of accumulated incremental plasticity under cyclic loading on the stress field and the crack opening displacement field at the crack tip region of plate and stiffened plate under different loading conditions is considered based on the Chaboche kinematic hardening model using finite element analysis software ABAQUS,choosing AH32 high-strength marine steel as material.At the same time,closure effect during fatigue crack propagation is analyzed with the model of CT specimen.?3?Different stiffener stiffness ratios and overload peaks are selected to study the effect of different stiffener stiffness ratio on stiffened plates respectively.Various overload peaks were applied to the CT specimens and stiffened plates whose material is AH32 to analyze hysteresis effect of overload peak on fatigue crack propagation.?4?COD-a curve and P_OP-a curve are introduced.COD-a curve can reflect the relationship between the crack opening displacement field and the crack in the process of fatigue crack propagation more directly and POP-a curve can reflect the change of the open load in the crack tip area during the fatigue crack propagation more directly.The trend of the two curves under different loading conditions was analyzed.The two curves are used to combine the results of finite element and experimental results.In this way,the reliability of finite element analysis is verified.?5?MTS fatigue testing machine in laboratory and CT specimens made of AH32high-strength marine steel material are used for static tensile test,constant amplitude low-cycle fatigue crack propagation test and low-cycle fatigue crack propagation test under overload peak of plate and stiffened plate.The low-cycle fatigue crack propagation behavior of the structure is studied experimentally.The reliability of finite element analysis is verified by the test.