Affect Study Of Structural Fatigue Strength Under High-low Frequency Combined Stress

In recent years,as the competition in the shipping Market has intensified,the increase in the main scale of the container ship hull is a mainstream trend,many problems have been brought about.The whipping and springing phenomenon are more likely to occur with the decrease of natural frequency and the unique structure.The springing and whipping response contribute a lot to the fatigue damage of the hull structure.In this paper,the load response of a 20000-TEU container ship is analyzed by numerical calculation method,and response trend arc obtained.According to the linear relashion of load response and stress response,methods are adapted to cauculate the fatigue strength of the hull,furthermore,contrastive analysis are made.Based on this,the fatigue test under high and low frequency combined stress is designed.The effects of stress response on the fatigue strength of the hull structure are obtained under different frequency ratios.A direct calculation corrected coefficient is obtained according to the test data,and applied to the 20000-TEU container ship fo r fatigue damage analysis.The specific work of this paper is as follows(1)The transform matrix method and finite element method are used to calculate the modes of hull structure,and the causes of the difference are analyzed.Then the linear and nonlinear load characteristics of the target ship in the wave are calculated.The frequency domain spectrum analysis of the load time history shows that the influence of the high frequency load on the amplitude frequency response function is very severe(2)calculation methods for fatigue damagw of hull structure under the action of springing and whipping are given.The springing damage calculation of the typical stucture of the ship is carried out,and the load amplified coefficient of dynamic analysis is obtained.The load is obtained by the bimodal spectral density function.A simplified calculation of the mid ship section longitudinal is used to analyze the contribution of the high frequency stress response to fatigue strength.The whipping fatigue strength is calculated by the rain flow counting method,and the contribution of the high frequency load to the fatigue damage is analyzed(3)The fatigue trial under the combination of high and low frequency combined stress is designed.By combining the high frequency load and the low frequency load at different frequency ratios,the high frequency stress in the springing and whipping response are obtained for the super large container ship,the fatigue damage contribution of hull structure is analyzed.The influence of high and low frequency combined stress on the fatigue strength of hull structure under different frequency ratios is analyzed and a correction coefficient for calculating the springing and whipping response by rain flow counting method is obtained.The trial results are applied to the calculation of the actual ship fatigue strength.