Research On Ultimate Strength Of Ship Hull Structure Considering Accumulated Plastic Collapse Under Cyclic Loading

The ultimate strength of hull structures is the most important guarantee for the structural safety of ships.Therefore,the ultimate strength assessment of hull structures has long drawn great attention from the marine engineering community.Although a great deal of research has been done both at home and abroad,in recent years,the accidents of hull structural fractures and failures have continued to occur,and further studies are still needed.After more than fifty years of exploration,the ultimate strength assessment of hull structures has yielded fruitful results in theoretical analysis,numerical calculation,experimental research and the development of simplified algorithms.Naval engineers’ understanding of the ultimate strength of hull structures are also gradually clear.In the meantime,in view of the importance and complexity of the ultimate strength of the ship structures,the technical committee of ultimate strength at the 19 th International Ships and Offshore Structures Conference still emphasizes: “With these developments ultimate strength assessment is now becoming a more important issue to ensure the safety of ship structures.From this point of view the role of this committee continues to be very important.”What needs to be pointed out is that most of the existing studies on the ultimate strength of ship structures are based on the concept of static ultimate strength assessment.That is,the hull structure is considered to collapse under monotonically increasing external loads.Research on the ultimate strength characteristics of ship structures under cyclic loading is scarce.In fact,with the ups and downs of the waves,the deformation of the ship structure is cyclical.In the course of many extreme sea conditions and the consequent extreme external loads,it is entirely possible to generate cumulative plastic deformation in several local structures,even if the hull girder does not collapse as a whole.Under the extreme cyclic loading,the hull structure will fail under the coupled effect of low cycle fatigue and buckling ductile failure(ultimate strength failure).If the low-cycle fatigue damage closely related to the cyclical loads to the hull structure will be discussed separately,the failure of the ship structures under cyclic loading will be dominated by the buckling failure of the compression member with the accumulated plasticity.This thesis focuses on the failure of ultimate strength of hull structures under severe cyclic loading,and in response to the worsening of the sea conditions.The above definition of the scope of this paper is beneficial,which shows that the ship structure will continue to produce new deformation under the cyclic effect of bad sea conditions.Although the structural shakedown of the hull under cyclic loading is also very significant,Like the fatigue problem,it is not the focus of this article.In this paper,the ultimate bearing capacity of hull plate,hull stiffened plate and hull girder subjected to cyclic loading is studied in detail by theoretical analysis,numerical calculation,experimental study and Smith method.Through the research of this paper,the method and method for evaluating the ultimate strength of ship hull structure under cyclic loading is proposed and implemented for the first time in the world.The research content is covered in full in theoretical analysis,model experiment,numerical simulation,and simplified algorithm.Fully break through,improve and open up the research status and direction of the ultimate strength assessment of hull structures,and lead and demonstrate the role of realizing the ultimate strength of hull structures in the real marine environment in the future.The main research work is as follows:(1)The development of the research on the ultimate strength of hull structures and its research status are summarized,and the research methods and research results are also summarized.On the basis of summarizing the existing research on the ultimate strength of ship structure,this paper puts forward and expounds the significance of carrying out research on the ultimate strength of ship structure under cyclic loading,and puts emphasis on the research results directly related to it.(2)From the existing research on the ultimate bearing capacity of plate members under cyclic loading(Fukumoto,Huang),the theoretical construction ideas of the average stress-average strain hysteresis curves of hull plates under in-plane cyclic compression are proposed.And,the theoretical hysteresis curves of the hull plates under the in-plane cyclic compression were compared with the experimental curves.(3)The average stress-average strain curves under the monotonic compression load of stiffened plate elements on the hull girder cross-section was deduced by using the beam-column theory.The average stress-average strain curves obtained from beam-column method was modified by using the nonlinear finite element method.The average stress-average strain relationship of the stiffened plate under in-plane cyclic loading is developed by extending the average stress-average strain relationship under the monotonic increasing compression of the stiffened plate.And,laying the foundation for the calculation procedure for the collapse of the hull girder(Smith method).(4)The ultimate bearing behavior(including ultimate strength,ultimate strain,stiffness,plasticity accumulation,etc.)of hull plates,hull stiffened plates and hull girders subjected to cyclic loading is studied numerically using nonlinear finite element method.The modeling range,element selection,grid size,initial imperfections,boundary conditions,cyclic load loading method and calculation result processing of the finite element model are discussed.(5)Seven square columns models(different thickness to width ratio,aspect ratio,steel yield strength,etc.)were designed and manufactured,and the ultimate bearing capacity tests under axial cyclic compression load were carried out.The failure modes and the residual ultimate strength and stiffness of the hull plates under cyclic loading were studied by experiment and nonlinear finite element method.(6)Five models of box girder(different density of stiffeners,yield strength of steel)were designed and manufactured,then tested under repeated sagging and alternating sagging-hogging cyclic bending.The ultimate bearing capacity of hull girders under cyclic bending was studied through experiments.At the same time,nonlinear finite element simulation is carried out for some model tests.(7)The ultimate strength of hull girder under cyclic bending was calculated by Smith’s method.By using the developed calculation program,the ultimate strength of the box girders with different cross-section was evaluated under repeated sagging and alternating sagging-hogging cyclic bending.In view of the different magnitude of cyclic bending moments,the ultimate strength of two real ships is rapidly evaluated.(8)Summarizes the research work of this thesis,and prospects the follow-up research work.