Research On The Bearing Capacity Of Ship Structure With Crack Damage Under Low Cycle Fatigue Load

The purpose of ship structure design is to ensure that the ship can bear all kinds of possible loads in its service life.The traditional ultimate strength assessment methods are all based on the ultimate strength under the one-time monotone load,that is,the total damage of the hull is the result of the combination of the most unfavorable one-time load that the dangerous section of the hull can bear.In fact,under the action of alternating extreme load,the ultimate bearing capacity of hull beam will decrease with the increase of cycle times,and the ultimate strength evaluation method based on one-time monotone load may lead to dangerous results.In addition,in the process of ship service,ship structure will inevitably suffer a variety of damage and defects,crack as a common form of defects,often cause local stress concentration of the structure,affect the collapse behavior of the structure,and then reduce the bearing capacity of the ship structure.Therefore,evaluating the bearing capacity of ship structures with crack damage under low cycle fatigue load can more accurately explain the overall failure phenomenon of ship girders under extreme sea conditions.Hull plate and stiffened plate are the basic components of hull structure.The study on the bearing capacity of cracked hull plate and stiffened plate under low cycle fatigue load can provide a basis for further study on the bearing capacity of the overall structure of ship with crack damage under low cycle fatigue load.The main research work is as follows:(1)The research methods of ultimate strength of ship structure are summarized systematically,and the research progress of ultimate bearing capacity of ship structure with crack damage under low cycle fatigue load and cumulative plastic failure of ship structure is summarized,and the importance of carrying out research on ultimate bearing capacity of ship structure with crack damage under low cycle fatigue load is clarified.(2)The characteristics of the hull structure,the classification of cracks and the necessary assumptions,as well as the application mode of low-cycle fatigue load are described in detail.The simplified formulas for ultimate strength of plate and stiffened plate with crack under one-time collapse are systematically introduced.The reliability of the numerical simulation method in the dissertation is verified,which lays a theoretical and numerical foundation for the subsequent research on the ultimate bearing capacity of ship structures with crack damage under low cycle fatigue load.(3)The research on the bearing capacity of ship hull plates with crack damage under the condition of considering cumulative plastic deformation and crack propagation is carried out.The life-and-death element method in nonlinear finite element method is used to simulate the crack propagation in order to investigate the influence of the crack propagation on the ultimate bearing capacity of the hull plate.The effects of crack distribution,plate thickness and low cycle fatigue load on the hull plate bearing capacity,crack growth rate and crack length were studied parametrically.Furthermore,the residual ultimate strength of the cracked plate where crack extends from the initial small crack to a certain length is compared with that of the cracked plate where crack is prefabricated directly.(4)The research on the bearing capacity of stiffened plate with crack damage is carried out under the condition that cumulative plastic deformation and crack growth is considered The life and death element method is used to simulate crack propagation and the effect of crack propagation on the bearing capacity of ship stiffened plate is discussed.The effects of crack distribution,thickness of strip plate,size of stiffener and low cycle fatigue load on the bearing capacity and crack propagation behavior of stiffened plate were studied.The residual ultimate strength of the stiffened plate where crack extends from the initial small crack to a certain length is compared with that of the stiffened plate where crack is prefabricated directly.