Research On Low Cycle Fatigue Crack Initiation Life For Notched Plate

Along with increasing in ship dimensions and more use of high strength steel in recent years,the stress and deformation of ship structures are higher and larger,which result in prominent problem of low cycle fatigue damage to large scale ships.At present,most of the research achievements and applications of fatigue strength are basically for high cycle fatigue of ship hull structures.However,the local hull structure may be in plastic state during the voyage due to impact of adverse sea conditions,structural discontinuities and defects in the initial structure which will introduce greater local stress concentration.Thus it is necessary to research the low cycle fatigue strength of large scale ships.In this paper,the low cycle fatigue crack initiation life of Q345 notched plateunder stress cyclic loading is researched by theoretical analysis,numerical simulation and experimental observation method.Based on existing research,this paper mainly includes the following aspects.(1)According to the point of energy conservation,fatigue crack initiation critical dimension has been explored.Assuming that fatigue crack initiation is caused by the maximum tensile stress.The dissipated energy computed by damage mechanics equals to fracture mechanics when the fatigue crack initiated to a critical size based on continuum damage theory.Thus expression of fatigue crack initiation critical size is obtained for plastic strain as basic parameter.The fatigue crack initiation critical size can be well applied to the assessment of fatigue life.Meanwhile,the influence of mean stress and stress amplitude to plastic strain was analyzed.(2)The low cycle fatigue crack initiation life model and accumulative plastic strain model of notched plate were proposed based on damage mechanics.Assume fatigue crack initiating as material reaches critical damage,local stress near notches were recalculated by introducing fatigue notch coefficient.The model is convenient to estimate the fatigue crack initiation life under stress cyclic loading and also has a certain precision(3)The law of stress and strain distribution near the notch under different mean stress and stress amplitude was calculated by finite element method.And relative stress gradient and strain gradient methods were used to obtain effective strain amplitude,substituted them into Morrow fatigue life estimation formula,the fatigue crack initiation life were predicted.Compared with experimental values,fatigue crack initiation life obtained by strain gradient method is more effective.(4)The low cycle fatigue experiment of Q345 notched plate under asymmetrical stress cyclic loading was conducted.Thematerial parameters of Chaboche model was deterred based on fatigue test data.Through the experiment,law of fatigue life and strain distribution under asymmetrical stress cyclic loading was studied.The experimental results were compared to determine the rationality of theoretical model and finite element method.(5)Notch radius and plate thickness are important factors affecting notch local stress and strain distribution,finite element simulation method is adopted to analyze the difference of notch local stress and strain state in various radius and plate thickness.It is vital for further research of low cycle fatigue strength on notch plate with different geometry under asymmetrical stress cyclic loading.