Study On The Low Cycle Fatigue Crack Propegation Behavior And Residual Strength Of Ship Structure

The large scale development of ships has led to an increasingly prominent problem of low cycle fatigue,so crack propagation law is very important for current research.The hull structure is mostly composed of the welded structure of plate and rib.In order to simplify the analysis,the basic structure of the hull is taken as the research object carrying out the crack propagation research.At the same time,the relevant experimental research is also carried out to further explain the crack propagation law.With the increase of the cyclic load,the crack will appear to extend,and the residual strength of the structure becomes the research object to evaluate the structural strength.By studying the effect of prefabricated crack and fatigue crack on the residual strength,the fatigue crack growth and residual strength can be combined to be analyzed.This is of great significance for the research of residual strength changes during crack propagation.The main research work of this thesis is as follows:(1)The thesis describes the research status of fatigue crack growth and residual ultimate strength of the structure on the system,and analyzes the relevant theories to lay the foundation for the subsequent work.(2)Based on the low cycle fatigue experiment and finite element simulation of cracked plates,the variable law of stress field,displacement field,crack closure and accumulative plastic strain are investigated.And the related factors such as stress ratio,cycle number and crack length are emphatically discussed,which provide a feasible way to investigate the law of low cycle fatigue crack propagation.(3)The thesis takes the welded plate with crack damage as the research object,and investigates the effect of residual stress on crack propagation by the crack closure method.The stress ratio,maximum load,overload ratio and number of overloads are emphatically discussed to explore the law of fatigue crack propagation with welding residual stress.(4)The thesis has carried out the fatigue crack propagation experiment of the plate and stiffened plate,and discussed the effect of stress ratio,maximum load,the distance of double ribs on the crack propagation life.At the same time,the maximum opening displacement of the plate and stiffened plate is obtained by the finite element method to characterize the crack propagation behavior,and the sensitivity of related factors is also discussed.(5)To study the effect of crack propagation on the residual strength of the structure,this thesis mainly analyzes the compressive and tensile residual strength of pre-crack and fatigue crack specimens,respectively.The crack length,stress field,out-of-plane deformation and load level are emphatically discussed,which provides some guiding function on evaluating the structural residual strength during fatigue crack propagation.