Research On Ultimate Strength Assessment Of Hull Girder Based On Modified Incremental-iterative Method

The incremental-iterative method is one of the methods for evaluating the ultimate strength of hull girders in harmonised common structural rules(HCSR).It is widely used to evaluate the ultimate strength of ordinary steel hull girders,but the ultimate strength of 921 A steel hull girders has not been verified.The stress-strain curves describing different buckling failure modes of stiffened plates in HCSR specification are indispensable for evaluating ultimate strength of hull girders by incrementaliterative method.Scholars usually study the stress-strain curves of beam-column buckling,torsional buckling and local web buckling of stiffened plates in HCSR code.The stress-strain curves of hard corner elements are usually replaced by stress-strain curves of ideal elastic-plastic materials,so the research on the stress-strain curves of hard corner elements is not comprehensive enough.The stress-strain curve of stiffened plate in HCSR specification has not been verified for stiffened plate of high strength steel with minimum yield stress over 390 MPa.Therefore,it is of great significance to study the application of stress-strain curves of different buckling failure modes of stiffened slabs in HCSR code to stiffened slabs of high-strength steel with minimum yield stress over 390 MPa,and to study the incremental-iterative method to evaluate the ultimate strength of 921 A steel hull girders with minimum yield stress over 390 MPa.The main research work and conclusions of this paper are as follows.(1)In 921 a steel stiffened plate ultimate strength test,the standard tensile test of 921 a steel,designed to simulate beam column buckling and torsional buckling,web local buckling failure mode of 921 a steel stiffened plate sample,this paper strain measuring point layout,test apparatus and test process,carries on the different buckling failure mode of 921 a steel reinforced plate ultimate strength test,The load displacement curves,permanent plastic deformation and deformation data of stiffened plates in different buckling failure modes were analyzed.The results show that the stress-strain curves of the stiffened plates with different buckling failure modes are similar: the curves rise rapidly in the elastic stage,the curves are relatively gentle in the elasticplastic stage,and the curves decline rapidly in the completely plastic stage.Different buckling failure modes produce different permanent plastic deformation.Different buckling failure modes have different failure causes.(2)The ultimate strength of 921 A steel stiffened plate with different buckling failure modes was studied by numerical simulation method and HCSR specification,and whether the stress-strain curves under different buckling failure modes in HCSR specification are suitable for evaluating the ultimate strength of 921 A steel stiffened plate was analyzed.The validity of the modified stress-strain curve of hard Angle element is verified by modifying the stress-strain curve of hard Angle element described in HCSR specification for elastic-plastic failure.The results show that the ultimate strength of single-stiffened plate obtained by numerical simulation has high precision,which verifies the rationality and accuracy of numerical simulation method.The results of the stiffened plate under the constraint of adjacent stiffened plates are more reasonable and close to the ultimate strength of the real stiffened plate.The results of the numerical method for the stiffened plate with three stiffened plates are more accurate than those of the single stiffened plate.The stress-strain curves of the stiffened plate with three stiffened plates are more consistent with the actual situation.The stress-strain curves of beam-to-column buckling,torsional buckling and local web buckling in HCSR specification need not be modified,and the ultimate strength of921 A steel stiffened plate can be directly evaluated.For hard corner elements,the stress-strain curve of hard corner elements in HCSR specification should be modified,otherwise the ultimate strength of hard corner elements should be overestimated.The modified stress-strain curve of hard Angle element in HCSR specification is verified to be effective and can be used to evaluate the ultimate strength of 921 A steel hard Angle element by an example.(3)This paper studies the ultimate strength evaluation of 921 A steel hull girder by incremental-iterative method,introduces the basic situation of 921 A steel box girder and hull girder selected as the research object,expounds the process of evaluating the ultimate strength of 921 A steel hull girder by incremental-iterative method and nonlinear finite element method,and expounds the difference between original incremental-iterative method and modified incremental-iterative method.The ultimate strength of 921 A steel box girder and hull girder was calculated by original incremental-iterative method,modified incremental-iterative method and numerical simulation.The results of original incremental iteration and modified incremental iteration were compared with the results of numerical simulation,and the accuracy of ultimate strength of 921 A steel hull girder was analyzed by modified incrementaliterative method.The results show that the ultimate strength of 921 A steel hull girder is overestimated by the original incremental-iterative method,especially for the 921 A steel hull girder with a large number of hard corner elements.The modified incremental-iterative method is more accurate in the ultimate strength evaluation of921 A steel hull girder,and can be used to evaluate the ultimate strength of 921 A steel hull girder.