A New Freguency Domain Fatigue Damage Computational Method Considering Rain-flow Counting

With the development of large-scale vessels and ocean engineering structures in the deep sea and the widely use of high strength steel,the fatigue problems of ocean structures raise dramatically concerns.The spectral analysis which takes the randomness of wave loads into consideration has a mature theoretical basis,making it an applicable and efficient tool to analyze fatigue problems of ocean and engineering structures.With the advantages of considering the non-linear effect of slamming and whipping,time-domain analysis method provides a capability to simulate the hotspot stress under real sea conditions,which makes the fatigue damage calculation more accurate.However,the calculation procedure requires extremely long computational time and it is almost impossible to analyze all the conditions in the wave scatter diagram.In order to avoid the running time,a new frequency domain assessment combining with rain-flow counting is presented in this paper,which can provide a more accurate fatigue result than spectral method.The main contents of the thesis are as follows:(1)Hydrodynamic analysis and Finite Element calculation in frequency domain were finished to obtain the stress response amplitude operator of hotspots.The hotspot stress power spectral density was obtained according to the wave spectrum and wave scatter diagram.Then,the fatigue accumulation damages of hotspots are presented by spectral analyses.By comparing the accumulation damages,the effects of different sea states are investigated.And the parameters like Wirsching's correction factor are considered in the contrastive study.(2)Hotspot stress-time histories of 3 different sea states are calculated by using time-domain analysis method.Then,fatigue accumulation damages are computed based on the rain-flow counting method and S-N curve.A segmented model test was carried out.The motion responses and vertical wave bending moments of the sections under regular and irregular waves were measured.Comparison of vertical bending moment results between segmented model test and numerical simulation are made to verify the reliability of the loads obtained by simulations.(3)Power spectral density of the hotspots from spectral analysis and time-domain method are compared,as well as the fatigue damages.It shows that narrow band approximation in spectral method and the rain-flow counting in time-domain method is the main cause for the difference between the cumulative fatigue damage values calculated by the two methods.Therefore,a frequency-domain calculation method based on rain-flow counting method is proposed.It transforms the power spectral densities of the hotspot stress obtained from frequency analysis into stress-time history,then the fatigue accumulation damage can be calculated by applying the rain-flow counting and S-N curve method.The fatigue damages obtained by the proposed method,spectra analysis method and time-domain analysis are compared respectively to verify the reasonability of the proposed rain-flow counting based frequency-domain method,and it is found to be reliable in damage assessment of ship structures.