Fatigue Assessment Of Ship And Marine Structures Under Random Multi-loads

Ship and marine structures are commonly subjected to various dynamic loads.The combination of two or more processes usually reduces the fatigue life,since it increases hot-spot stress range greatly.Fatigue assessment methods under a single load have been very mature,but fatigue assessement method under multi-loads are very few.This paper introduces some fatigue issues of ship and marine structures under random multi-loads,based on this,a series of fatigue assessment methods are developed to deal with these fatigue problems.(1)Fatigue assessment methods in bimodal narrowband stress spectra are studied.Parameterized rectangular bimodal spectra are constructed.A new numerical technique is proposed,which can transform a stress spectrum into a stress time series.Some bimodal spectra methods in the literature are adopted to predict fatigue damage,and the results of damage predictions are validated with the proposed technique.Finally,some accurate and simple bimodal methods are suggested to evaluate fatigue damage.(2)Fatigue assessment methods based on combination of fatigue damage are researched.Based on superimposition of two ideally narrowband random stress processes,two new formula are developed.Based on parameterized rectangular bimodal spectra,The accuracy of two new formula is validated and they are proven to give an accurate fatigue damage prediction.Some methods of combination of fatigue damage in the literature and rules are aslo used to calculate fatigue damage and compared with two new forumula.Compared with methods in the literature,new methods are more simple and accurate.Finally,the effect low frequency loadings induced fatigue damage or high frequency loads induced fatigue damage on fatigue damage is investigated.The results show that high frequency loads induced fatigue damage cannot be neglected.Low frequency loads induced fatigue damage can be neglected in some limited conditions.(3)Fatigue assessment methods in non-narrowband bimodal stress spectra are studied.A few accurate bimodal spectral methods and newly proposed methods are generalized to assess fatigue damage in non-narrowband bimodal loads and bimodal loads with overlapped frequency.By adjusting the parameters of rectangular spectra,non-narrowband bimodal spectra and bimodal spectra with overlapped frequency are obtained.Considering different bandwidth combination,the accuracy of new methods is validated,compared with rainflow counting results.The effect of bandwidth on fatigue damage is analyzed,some suitable methods are finally given to assess fatigue damage in in non-narrowband bimodal loads.(4)Fatigue assessment methods in hybrid time/frequency domain response are investigated.Spectral parameters of combined stress processes are derived,based on that,a hybrid fatigue damage assessment methods are built,and the results are proven to be reliable.Next,the effect of spectral shape on fatigue damage is conducted,the results show that spectral shape has little influence on fatigue damage.According to this conclusion,a concept of 'pseddo spectra' is introduced,subsequently,a new hybrid fatigue assessment method are proposed.New method is validated to be accurate and suitable for assess fatigue damage in hybrid time/frequency domain response.(5)Fatigue assessment methods in trimodal and multimodal loads are studied.A new technique which is called as spectral decomposition is developed,based on this,a new trimodal method is proposed.Parameterized trimodal triangular spectra are built to check the precision,and the precision of the proposed method is validated with numerical simulations.The new method is proven to be reliable and practical to evaluate fatigue assessment methods in trimodal and multimodal loads.