Study On The Influences Of Dimensional Effect And Ambient Temperature On The Ultimate Strength Of Ship Structures

When hull girder is subjected to the combination of self-weight and external cargo loads,still water and wave-induced bending loads,the sagging or hogging mode of deformations for hull girder would progressively increase,and then may result in nonlinear buckling and even collapse of ship hull structures.Therefore,the researches on the ultimate load carrying capacity and collapse mechanisms of both hull girder and stiffened panels are crucial to the safety design of ship structures.Current researches mainly focus on the ultimate strength of ship structures under room temperature.However,the temperature variation in local ship structures would lead to the degradation of material properties and produce thermal stress,and consequently affects the buckling behaviours and ultimate strength of hull girder.Because these influences are still ambiguous,it is necessary to perform the investigation of temperature effect on the ultimate strength behaviours of ship structures.For ship structures,compression tests on ultimate strength are important to study the failure mechanism of nonlinear buckling and verify the theoretical results.Nevertheless,the disadvantages of large size of actual ship structures,expensive fabrication and the objective limitations of experimental setup make it impossible to perform the tests of full scale models;instead,small scale models are often utilized.However,since ship hull structures belong to the thin-walled structures,the ratio of the length to breadth for a plate is usually larger than 10,hence the complete geometrical similarity is always inappropriate for designing small scale models with the same scale ratio in plate length and breadth.The existing partial similarity theories may lead to the change of buckling mode,and thus induce the great difference of ultimate strength between small and full scale models.Hence,it’s essential to investigate the dimensional effects on the failure mode and ultimate strength of ship structures.Based on that,the similarity methods of ultimate strength are developed,which could consider the buckling behaviours of ship structures.First of all,three partial similarity methods were proposed in accordance to the buckling behaviours of stiffened panels,and afterwards some corresponding scaled stiffened panel models were designed based on the dimensions of actual ship structures.Meanwhile,the influence laws on the structural failure mode and ultimate load carrying capacity for different similarity methods were analysed by finite element method(FEM),thus the design criteria of small scale stiffened panels considering buckling modes were derived.In addition,the compression tests of scaled stiffened panels under different ambient temperatures were conducted to verify the effectiveness of proposed similarity methods,and then the influence mechanisms of temperature effects on the collapse behaviours of stiffened panels were investigated by experimental and numerical methods.On this basis,in consideration of the transient effect of hull girder ultimate moment under dynamic loading,the dimensional effects of ultimate bending moment mechanism between full scale hull girders and small scale box girders with different solution settings were discussed through the dynamic explicit method,and then the influence laws of temperature loads on the collapse mode and ultimate moment of hull girder structures were investigated.The study results could provide experimental data and theoretical references for scaled model design as well as the safety specifications of ship structures.This dissertation includes the main innovations as follows: the similarity methods on the ultimate strength of stiffened panels considering their buckling behaviours were developed,and the corresponding design criteria were given to provide the theoretical basis for designing compression tests of small scale models for ship structures.The compression tests of scaled stiffened panels considering ambient temperature effects were designed,and the test data were used to verify the numerical results for obtaining appropriate finite element models.On this basis,the effects of thermal loads on the distributions of thermal stress and thermal strain in stiffened panels were discussed,and then the influence laws of temperature effects on the ultimate strength of stiffened panels and hull girder were studied.The transient effects of hull girder ultimate strength in the dynamic explicit method were investigated,and the influences of different solution settings on the mechanism of ultimate moment for different scale hull girders were discussed.Finally the proper method of evaluating the hull girder ultimate strength considering transient effects was summarized.