Research On The Low Cycle Fatigue Crack Growth Behavior Of Ship Plate Considering Of The Accumulative Plastic Damage

The traditional study on fatigue strength for ship structures usually focuses on high cycle fatigue and ignores low cycle fatigue.However,the trend of large scale development of ships is inevitable in recent years,the stress and deformation of ship structures are so high and large,which causes the low cycle fatigue damage of ship structure to be paid more and more attention.The research results show that the failure of ship structure is usually caused by low cycle fatigue failure and accumulative plastic damage.Therefore,considering the effect of accumulative plastic damage,the paper will study the behavior of low cycle fatigue crack growth of ship plate from theoretical analysis,numerical simulation and experimental study.The main work of the paper is summarized as follows:(1)Combined with Newton-Raphson iteration method,the theoretical solution of accumulative plastic strain for ship plate under asymmetric stress cyclic loading is deduced.Then based on the cyclic stress-strain field of the fatigue crack tip,by introducing the correction term of accumulative plastic strain rate to reflect the influence of accumulative plastic damage on fatigue life under asymmetric stress cyclic loading,a new low cycle fatigue crack growth rate prediction model of ship plate considering the effect of accumulative plastic damage is proposed.Meanwhile,an experimental study on the coupling interaction of accumulative plastic damage and low cycle fatigue crack propagation of AH32 steel was carried out.In the low cycle fatigue crack propagation experiment under asymmetric stress cyclic loading,the gradual accumulation of plastic deformation at ship plate cause a reduction in stiffness,which results in additional damage to low cycle fatigue crack propagation life.The low cycle fatigue crack propagation life of ship plate is affected by accumulative plastic damage and low cycle fatigue failure subjected to asymmetric stress cyclic loading.The experimental results show that the accumulative plastic damage and low cycle fatigue crack propagation life of AH32 steel depend greatly on mean stress,stress amplitude and stress ratio.Two kinds of failure modes,accumulative plastic damage with obvious necking due to large accumulative plastic strain and low cycle fatigue failure due to crack propagation,take place respectively,depending on the stress level prescribed in the experiments.The prediction result of the low cycle fatigue crack propagation behavior is consistent with the experimental result,suggesting that the low cycle fatigue crack growth rate prediction model allowing for the influence of accumulative plasticity is feasible.(2)Based on the theory of elastic-plastic fracture mechanics,an analytical model is presented to determine the CTOD for ship plate under cyclic loading,then the relationship between crack propagation rate and CTOD is established on the basis of fatigue fracture mechanism.Meanwhile,an experimental study on the low cycle fatigue crack propagation behavior was conducted using compact tension specimens of AH32 steel.The prediction result of the low cycle fatigue crack propagation behavior is consistent with the experimental result under different stress ratio and maximum load,suggesting that the low cycle fatigue crack growth rate prediction model represented by CTOD can characterize the low cycle fatigue crack propagation behavior of cracked plate under cyclic loading.(3)Based on the cyclic stress-strain field of the fatigue crack tip,and combined with the Willenborg's residual stress theory,a new low cycle fatigue crack growth rate prediction model considering the effect of accumulative plastic damage under overload is proposed.At the same time,a series of fatigue crack growth experiments were conducted using compact tension specimens of AH32 steel,the influence of overload ratio,stress ratio and maximum load were firstly observed and studied with the AH32 steel.Apparently,after the application of single tensile overload,an initial slight increase is followed by a sharply decrease in da/ d N,which intensively depends on the overload ratio.Then,the subsequent crack growth rate gradually increases to the steady state of constant amplitude loading.The retardation effect is described by overload length and minimum crack growth rate,the crack retardation increases with increased overload ratio and maximum load,and decreases with increased stress ratio.The predicted results were compared with the experimental results,showing that the prediction model can quantitatively predict the crack propagation behavior after the application of single tensile overload.(4)The finite element model with dynamic crack is established to analyze the stress and displacement field near the crack tip after overload,and the intrinsic mechanism of overload effect in fatigue crack growth is discussed.The residual compressive stress caused by overload effectively slows down the crack growth rate,and the crack propagation goes through the retardation phase.The reverse plastic zone formed after the unloading of tensile overload does not bring the crack growth rate to the lowest point immediately,and the crack propagation goes through the delayed retardation phase.Therefore,the residual stress field related to loading history changes the crack propagation behavior after overload.Through the finite element analysis of the residual stress field near crack tip of CT specimen after applying two tensile overloads,it is found that the overload ratio and interval cycles between overloads have significant influence on the distribution of residual compressive stress field.When the interval cycles between overloads is small,the peak of residual compressive stress does not appear at the position of overload,but occurs at a later stage of first tensile overload,and the reverse plastic zone is much higher than that of single tensile overload.