Study On Ultimate Strength Assessment And Residual Strength Of Ship Hulls

With the improvement of the demand on the strength and safety of ship hull structure,the traditional allowable stress has been unable to meet the design requirements.Accordingly,it has become much more important in structure design to accurately assess the ultimate bearing capacity of hull structure,which provides reference for structure safety margin in the course of ship design and actual navigation.As the longitudinal ultimate strength of ship hull is much related to strong nonlinear problems,it is necessary to make a comprehensive comparison through a variety of effective research methods while assessing the actual ultimate bearing capacity of hull structure.In this paper,aiming at the ultimate strength assessment and residual strength of ship hulls,the research history and status of ultimate strength is comprehensively elaborated.Based on this,study on longitudinal ultimate strength is conducted by three simplified methods,including Smith’s method,criterion method and single step method,of which the effectiveness is verified on typical box beam structures.Then the calculation and comparison of the ultimate bending moment of the target ship’s cross section is carried on.Subsequently,the ultimate strength analysis of hull cabin based on nonlinear finite element method is conducted,and the ultimate bearing capacity under sagging and hogging condition is considered,as well as the failure mode after reaching the limit state.The influence of various factors like element characteristics,mesh precision,numerical solution and lateral load is taken into account in the finite element analysis,of which the result is compared with the simplified methods.It shows that the finite element analysis and the Smith’s method both have better accuracy.The results of criterion method and single step method,however,seem too conservative for ultimate strength assessment.On the basis of finishing the comprehensive assessment of actual ship,the similar model design is conducted,during which the section characteristic equivalent requirement is deduced through mechanical theory of thin-walled structure,and the factor of local buckling is considered.The numerical results has better validated the similarity of the model and it may do help in providing a better way of thinking for the related ultimate strength experiment design.Crack damage is one of the most important damage types in ship structure,which may lead to the severe decrease of the hull structure’s capacity.Taking the cracked box girder for example,this paper investigates the stress distribution of the structure subjected to torsional load.Based on the numerical results,simplified calculating formulas with better accuracy to assess the effect of crack damage and estimate the residual ultimate strength are proposed,which may provide a certain reference for the study on residual ultimate strength with crack damage.