Ultimate Strength Of Stiffened Plates Considering Cumulative Plastic Failure Under Cyclic Loadings

The ultimate strength of hull structures is one of the most basic and important strength indexs to ensure the safety of hull structure.At present,most studies on ultimate strength of hull structures are based on the collapse of hull structures under monotonic external loading.As a matter of fact,the deformation of hull structure is cyclic in undulating waves,especially in harsh sea conditions,hull structures often produce cumulative plastic deformation due to extreme cyclic loads,so the ultimate strength of hull structure can be overestimated to some extent by using the single ultimate strength criterion.Stiffened plates are the basic unit of hull structure.It is very meaningful to study the cumulative plastic failure process of stiffened plates under cyclic loading.At present,the researchs on ultimate strength of hull structure under cyclic loading have not been thoroughly carried out,and the researchs on ultimate strength of stiffened plate under cyclic loading are not perfect.Based on the above research background,this paper took the stiffened plate as the research object and studied the ultimate strength of the stiffened plate under cyclic loading by using the experimental method,nonlinear finite element method and empirical formula method.The main research work and conclusions of this paper are as follows:(1)The standard tensile test of the stiffened plate material at room temperature was carried out to obtain its elastic modulus,yield stress,tensile strength,elongation,section shrinkage and other material parameters,and drew its real stress-strain relationship curve,providing parameters input for the subsequent finite element simulation.(2)The ultimate strength test of stiffened plate under cyclic loading was carried out.Three stiffened plate samples were designed to simulate the beam column buckling mode,the tripping buckling mode and the web local buckling of the stiffened plate respectively.The test results show that the ultimate strength of the stiffened plate varies with the number of cycles in the whole cycle,which can be divided into three stages: the stage of rapid attenuation of ultimate strength,the stage of slow attenuation of ultimate strength and the stage of stability of ultimate strength.In the first stage,the attenuation ratio of the ultimate strength of each model is more than 60%,and in the third stage,the ultimate strength of each model is almost unchanged with the increase of the number of cycles.The rapid attenuation of the ultimate strength of each model is often accompanied by the rapid accumulation of plastic deformation.The decay process of cyclic plastic work is similar to that of ultimate strength.(3)Ultimate strength test of stiffened plate under cyclic loading was simulated by using the finite element software ANSYS.The validity and reliability of the finite element numerical simulation method are verified by comparing the numerical simulation results with the experimental results.The mechanism of the ultimate strength decline of the stiffened plate caused by plastic accumulation under cyclic loading was explored.The results show that the reduction of ultimate strength of stiffened plates due to plastic accumulation under cyclic loading can be attributed to the following two reasons:(a)After each complete cycle loading,the stiffened plate will be plastic accumulated outside the surface deflection,the deflection become the next cycle load of stiffened plates with initial outside surface deflection(b)Due to the continuous accumulation of plastic deformation and local buckling,the high stress area of the stiffened plate is concentrated in the middle of the stiffened plate in the cyclic limit state,which leads to the continuous reduction of the effective bearing area of the stiffened plate,and then weakens the overall bearing capacity of the stiffened plate.(4)The similarities and differences of ultimate strength and buckling modes of stiffened plates under cyclic load and monotone load were studied by using finite element software ANSYS.The variation of ultimate strength of stiffened plates under different cyclic loads was studied.The results show that the ultimate strength of the stiffened plate under cyclic load is usually less than that of the stiffened plate under monotonic load.There is no obvious difference between the buckling modes of the stiffened plates under monotone load and cyclic load in most cases.For the 7 types of cyclic loads considered,cyclic load type 3 is a very dangerous cyclic load type;During the study,a parameter which can characterize the degree of plastic accumulation was proposed.Based on Z-K formula,an empirical formula for ultimate strength of stiffened plates considering the degree of plastic accumulation was established.This formula can be used to predict the ultimate strength of stiffened plates under cyclic loading.