Study On Time Domain Analysis Method Of Multiaxial Fatigue Of Marine Structure

Marine structures have been under the combined action of wind,wave and current.Fatigue failure has become one of the most significant failure modes of marine structures.For the fatigue problem of welded structures,classification societies of various countries mainly use the uniaxial fatigue analysis theory.However,most of the welding areas of marine structures are in multiaxial stress state,and the uniaxial fatigue assessment theory only considers a single stress component,which cannot reflect the real stress situation of the structure.The multiaxial fatigue assessment theory considers the multiaxial stress state,which is more accurate to describe the real stress state.Compared with the uniaxial theory,it is more suitable for the fatigue assessment of marine structures.At present,the frequency domain method is mainly used to deal with the random loads,but even in the case of uniaxial loading,the frequency domain method also leads to scattered results.The time domain method can truly reflect the response of the structure under random load,and it is the benchmark of checking the frequency domain method.Therefore,the establishment of time-domain multiaxial assessment method for marine engineering structure is the premise of applying multiaxial fatigue assessment theory to engineering practice,which is of great significance to promote the development of fatigue assessment theory and improve the safety performance of marine engineering structure.The application of multiaxial fatigue evaluation theory in engineering is mainly faced with three problems.In stress analysis,the mainstream methods in engineering have some limitations.In terms of fatigue criterion,most fatigue criteria can be applied to proportional load and non-proportional load,but the accuracy of random load is not well,and the counting method under uniaxial condition cannot be simply extended to multiaxial case.The fatigue analysis method under random load needs further study.Although the time domain fatigue analysis method has good accuracy,its calculation cost is high.In the design stage,frequency domain method is generally used,and the time domain method is used as the benchmark for checking the accuracy.Therefore,it is necessary to compare the frequency domain method and time domain method in the multiaxial cases,which can provide reference for engineering application.The specific work of this paper is to use a new structural stress method which is more convenient,accurate.Combined with MWCM method,which still has good results under random load,a multiaxial fatigue assessment method is established.In view of the problems existing in traditional counting methods,a modified multiaxial fatigue counting method is proposed.The rain flow counting method is extended from uniaxial cases to multiaxial cases,which improves the accuracy of counting,and is suitable for multiaxial fatigue analysis.In view of the problem that the stress distribution is difficult to obtain in time domain,a new multiaxial stress monitoring method is proposed.By arranging sensors far away from hotspot,multiaxial stress condition in hotspot area is obtained.It can avoid the stress concentration in hotspot area affecting the measurement accuracy,reduce the difficulty of sensor layout,and can obtain multiple stress components at the same time.It is beneficial to the popularization of multiaxial fatigue in practical engineering.By taking a FPSO as an example,the fatigue damage of the structure is calculated according to the time domain method proposed in this paper and the frequency domain method is compared.The characteristics and rules of the uniaxial and multiaxial method in time and frequency domain are also analyzed.