Study On Residual Ultimate Strength Of Cracked Ship Structures Under Cyclic Loading

With the continuous enlargement of ships,the number of marine accidents increases,which has attracted scholars’ attention to the ultimate strength of ship structures.The traditional methods for evaluating the ultimate strength of ship structures are based on the maximum bending moment that a ship can bear under one-off monotonic load.In fact,in the process of transporting goods,ships not only have to bear the loads caused by different static forces,but also bear the cyclic loads caused by loading and unloading goods.With the increase of the number of cyclic loading,the cumulative process of plastic deformation will continue to increase,and the bearing capacity of the dangerous section of the hull will continue to decrease.At the same time,with the increase of the number of cyclic loading,cracks will appear in the stress concentration area of the hull structure（welding seams and ribs）.When the cracks reach a certain length,the effective bearing area of the structure will be reduced,the collapse mode of the structure will be affected,and the residual ultimate strength of the hull structure will be further reduced.Therefore,it is very important to evaluate the residual ultimate strength of cracked ship structure under cyclic loading to ensure the structural safety of the ship in the whole service life.The bearing capacity of cracked hull plates,stiffened plates and box beams under cyclic loading is studied.The main research work of this thesis is as follows:（1）The research methods of ultimate strength of ship structure are summarized systematically and the research progress and research methods of ultimate strength of ship structure under cyclic load and residual ultimate strength of ship structure with crack damage are summarized.The significance of the study on the residual ultimate strength of cracked ship structures under cyclic loading is clarified.（2）The ultimate strength of cracked plates and stiffened plates subjected to uniaxial compressive and tensile loads is investigated by the nonlinear finite element method.A parametric study is conducted considering the effects of the length and location of cracks as well as the plate thickness.（3）A series of nonlinear finite element calculations are carried out for the residual ultimate strength of cracked hull plates under cyclic loading.The effects of crack length,crack location,plate thickness and lateral load on the residual ultimate strength of the plates are studied parametrically,and some relevant factors are analyzed.（4）The numerical simulation of elastic-plastic large deformation of stiffened plates with crack damage under cyclic loading was carried out.The nonlinear finite element method was used to simulate the effect of cracks.The effects of crack length,crack location,stiffener type and lateral load on the residual ultimate strength of stiffened plates were discussed.（5）Based on the concept of gradual-collapse method,a program for calculating the residual ultimate strength of box girder under cyclic load is compiled.At the same time,a series of calculations were carried out using the nonlinear finite element method,and the effects of factors such as crack length,crack location and lateral load on the residual ultimate strength of the box beam were discussed.