Study On Fatigue Strength Of Typical Joints For Ship Structures Considering The Effect Of High Frequency Vibration

During the navigation,the large ships will not only be affected by the bending load caused by the waves,but also due to the influence of wave-induced vibration and slamming vibration,so that there is a certain amplitude of high-frequency vibration in the course of the low-frequency wave load actually suffered.The existence of this high frequency vibration component can easily cause the composite fatigue damage of the hull structure,which seriously affects the safety and reliability of the ship.Therefore,based on the analysis of the nonlinear wave load response characteristics of the hull structure,the fatigue test of the notch specimens and the typical welding joints,considering the influence of high frequency vibration,is designed and carried out.On the one hand,the effect of high-and-low frequency composite damage in LHC(the load history of high frequency component)is explored by scientific experiments.The method of fatigue cumulative damage analysis and life prediction under LHC are also explored by scientific experiments.On the other hand,taking the actual ship structure as the research object,the influence of composite damage on the crack propagation life of typical welded joints of hull is analyzed,and the fatigue damage evolution law of hull joints under LHC is further revealed.The main research contents and conclusions of this paper are as follows:(1)Taking the river-sea going container ship with wide and flat hull,shallow draft and large deck opening as an example,based on the frequency domain spectrum analysis method and time domain measurement data of wave loading model test,the vertical wave load response characteristics,structural hot spot position distribution and typical joint fatigue cumulative damage are analyzed,and the HLL(high-and-low frequency load)considering the influence of hull-grider vibration is proposed.(2)Based on the notch specimens fatigue test under different HLL conditions,the effects of frequency ratio,amplitude ratio and average stress on the fatigue life and fatigue damage of the specimens are investigated.The main influencing factors leading to the life loss of the incision specimens were analyzed.Based on the statistical results of the fatigue cumulative damage of each frequency component under the rainflow counting method,the fatigue life prediction method considering composite damage and the threshold value for characterizing the life loss of the notch specimens are proposed.At the same time,through the statistical fatigue test data,the threshold value for characterizing the life loss of the notch specimens is also proposed.By comparing the fatigue life prediction accuracy of four analysis methods(the rain-flow counting method,the equivalent stress method,the T-K model method and the method of this paper),the accuracy of the prediction method in this paper is verified.(3)By carrying out the crack propagation test of the hull welding joints under typical HLL conditions,the crack propagation characteristics of the longitudinal bone through the transverse bulkhead joints under constant amplitude load and superimposed load are analyzed.Based on the joint simulation analysis method of ABAQUS and FRANC3 D,the evolution law of effective stress intensity factor in the process of crack propagation is obtained.On this basis,the nonlinear variation characteristics of composite damage in different crack propagation stages are discussed,and the effect of composite damage on the fatigue life of the model under each critical crack length is summarized.Then the effect of composite damage on crack morphology(a/c)during crack propagation is analyzed.Based on the research results of this paper,it is possible to provide a basic technical reserve and test data support for establishing a typical fatigue strength evaluation method for hulls considering nonlinear wave-induced vibration and slamming vibrations.