Researchon The Ultimate Bearing Behavior Of Ship Structure Under Cyclic Loading

It is very important to estimate the load carrying capacity of a ship’s hull as awhole from the view of safety, and now the evaluation methods are based on the staticultimate strength criterion. When the ship hull structure are subjected to large cyclicloading, a certain degree of yield, buckling failure and plastic deformation may occurto some of the longitudinal components of the hull girder, leading to the decrease ofthe load carrying capacity of the hull girder, which may result in damage on the hullgirder under the bending moment which has not reached a one-time longitudinalultimate bending moment. The ultimate strength of hull structure evaluation will beoverestimated based on the static ultimate strength criterion and it does not confirmthe actual situation. Therefore, there is a need to introduce the increasing plasticfailure criterion based on ultimate bearing capacity of hull under the cyclic loadingand consider the hull structure response under cyclic loading.Stiffened plate is the main load-bearing member of ship hull, the one-timeloading ultimate strength of stiffened plate has been widely studied, but theperformance analysis of stiffened plate under cyclic loading is rare. According to alarge number of finite element calculation, this paper mainly modifies the formula ofthe stiffened plate element stress-strain relationship from Rahman method and getsthe stress-strain relationship of stiffened plate element under cyclic loading. Thiswork has great significance on the research of stiffened plate deformation performance.At last, this paper discusses the hull girder bending moment–curvature changerelationship on the basis of the deformation performance of stiffened plate elementunder cyclic loading, and the main research works are as follows:1) Two kinds of methods to calculate the stress-strain relationship of stiffenedplate element are introduced: Rahman method and nonlinear FEM. The calculationresults of the two methods are compared to verify its accuracy and reliability, and theadvantages and disadvantages of two methods are discussed.2) The explicit theoretical formulas of Rahman method are modified based onthe results of nonlinear FEM, and the error is analyzed to get a more accurate andsimple calculation of stiffened plate element stress-strain curves relationship. 3) The simple calculation formula based on the modified Rahman method is usedto analyze the deformation properties and get the stress-strain relationship of stiffenedplate under cyclic loading. Two typical cyclic loading process is introduced:increasing one-way cycle mode and increasing two-way cycle mode. The stress-strainrelationship of stiffened plate under the two typical cyclic loading process iscalculated by some examples and compared with the the finite element results.4) Based on the traditional progressive-collapse-method, this paper improves thecalculation program by introducing the concept of incrementally accumulative plasticcollapse and adopting the stress-strain relationship under cyclic loading in order to getcyclic bending simplified method calculating program. The new modified program isused to discuss the bearing behavior of hull girder under the two typical cyclicloading process by taking two box beams and a ship as examples, compared with theFEM results to confirm the accuracy of the modified program.