Research On Fatigue Life Estimation Method Of Welded Structure Under Complex Loading

In order to meet the lightweight development needs of modern machinery equipment,welded structure is widely used in practical engineering.During the service period,the welded structure is often subjected to multi-level variable amplitude loading,and fatigue failure is easy to occur.The mechanism of fatigue and fracture is also very complex,the traditional fatigue life assessment method is difficult to accurately estimate the fatigue life,which causes great difficulties to the design and maintenance of the structure.Study on fatigue life assessment of welded structures under complex loading,combined with variable amplitude loading history,small load and material memory performance,can establish a new fatigue life evaluation system,with which we can improve precision of welded structure fatigue life evaluation,reduce or avoid the serious accident.Therefore,the study of this thesis has important theoretical meaning and engineering value.Aiming to solve the problem that the traditional fatigue life assessment method of welded structure does not take into account the loading history,small load and material memory performance and other factors,this paper carried out the following works:1.In order to solve the problem that the linear damage accumulation model did not consider the loading history,based on the S-Nds curve model,the material memory performance function was constructed by using the cyclic exponential decay function of load.Combined with the influence of the damage state on the memory performance and the change law of the memory performance of different materials,an improved model was established.This model can not only consider the effects of material property degradation and damage state,but also consider the effects of loading sequence.2.In order to solve the problem that variable amplitude loading interaction factor is difficult to determine in the fatigue driving force energy model,two new loading interaction factors were presented by using real-time fatigue damage degree,cyclic ratio function and stress function,and introduced into the fatigue driving force energy model,and two improved models were proposed.The improved model can well consider the influence of real-time loading interference,loading sequence effect and real-time damage.3.In order to solve the problem that the fatigue driving force energy model did not consider the material property degradation mechanism,based on the fatigue driving force energy damage parameter model,an improved nonlinear fatigue damage accumulation model was established by using load cycle ratio and memory performance function to construct the material property degradation attenuation coefficient.The model can well consider the effects of material property degradation,loading sequence and the interference between loads.4.In order to solve the problem of strengthening under small load,based on the Corten-Dolan model,the residual strength attenuation coefficient was constructed by using the ratio of the residual strength to the initial strength of the material.A new method to define D was proposed,and a new improved model was established.The improved model can not only consider the influence of small loads on damage evolution,but also better consider the influence of loading sequence,loading interaction and material residual property degradation.Through several groups of numerical examples,it is proved that the models established in this paper can effectively evaluate the fatigue life of members.Compared with the traditional models,the proposed models can take into account the loading history factors of variable amplitude load better,and the prediction accuracy is higher,which provides a valuable reference for the research on the fatigue life assessment methods of welded structures.