Research On Bearing Capacity Of Ship Structure With Cracking Defects Under Cyclic Loads

The purpose of ship structural design is to ensure that the ship can bear various possible loads during their lifetime in service.Structural strength is an important aspect of the ship hull structural design process.The traditional ultimate strength assessment method is based on the ultimate strength under one-off monotonic loading,that is,the ship hull collapse is the result of one-off extreme external loads that the hull dangerous section can bear.In fact,the ultimate bearing capacity of the ship hull girder will continue to decrease with the incremental number of cycles under repeated extreme loads.The evaluation methods based on the ultimate strength under one-off extreme loads may result in a dangerous result.Additionally ship structures are inevitable to suffer various kinds of damages and defects during their lifetime in service.As a common defect of ship structures,cracking defect can cause local stress concentration,which is considered to have a significant effect on the buckling and ultimate strength behaviors of ship structural members and reduce the ultimate bearing capacity of ship structures.Therefore,research on bearing capacity of ship structures with crack damage under cyclic loads can more accurately explain the overall collapse of the ship hull girder under repeated extreme loads.Hull plates and hull stiffened plates are the basic components of the ship hull structures.Research on bearing capacity of ship hull plates and stiffened plates with crack damage under cyclic loads can provide the basis for further study on the bearing capacity of the ship's overall structures.The following works and contributions have been made in this thesis:(1)The investigation methods of ultimate strength of ship structures have been summarized in details.The research on bearing capacity of the intact ship structures under cyclic loads has been reviewed thoroughly.The importance of carrying out research on the bearing capacity of ship structures with cracking defects under cyclic loads has been elaborated.(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)The bearing capacity behavior of cracked plates and cracked stiffened plates subjected to cyclic loads is studied experimentally.The effect of crack propagation on the bearing capacity behavior of ship structures is investigated by comparing the ultimate bearing capacity of plates and stiffened plates with cracking defects of preset crack length and the same crack length from crack propagation.(4)Numerical studies on the bearing capacity behavior of cracked plates and cracked stiffened plates subjected to cyclic compressive and tensile loads are conducted.Some parameters including the crack propagation effects,the crack location and the cyclic loads levels are varied during the numerical simulations in order to examine their influences on the bearing capacity behavior of cracked plates and cracked stiffened plates under cyclic loads.(5)A series of numerical investigations are conducted on the ultimate bearing capacity of plates and stiffened plates with crack damage subjected to cyclic compressive loads.The length and location of cracks as well as the plate thickness are varied to check their effects on the collapse behavior of cracked plates and stiffened plates under cyclic compressive loads.