Research On Low-cycle Fatigue Strength Of Ship Hull Plate Based On Accumulated Plastic Damage

The fatigue strength of ship’s structure has very important significance on the safety and survivability.At present all the research achievements and applications of fatigue strength are basically for high cycle fatigue of ship hull structures.Along with the increasing in ship dimensions and more use of high strength steel in recent years,the stress and deformation of ship structures are so high and large,which result in very prominent problem of low cycle fatigue damage to large scale ships.This has become the key issues demanding prompt solution in the development of large-scale ships.Analysis shows that most overall fractures of ship hulls tend to be the coupling interaction result of low cycle fatigue and accumulative plasticity.Therefore,the low cycle fatigue analysis and estimation based on the accumulative plastic damage can more realistically predict the fatigue strength of ship hull structures.Based on the accumulative plastic damage,this paper will carry out theoretical,numerical and experimental research on the low cycle fatigue of ship notched plate.Firstly,a theory model of fatigue crack initiation life for ship notched plate based on accumulative plastic damage was proposed.Combined with Newton-Raphson iteration method,the accumulative plastic deformation at notch root of ship plate subjected to high level asymmetrical low cyclic loading was derived,and the theoretical solution of the tangential stiffness of ship notched plate in accumulative plastic destruction was also obtained.In addition,in the process of low cycle fatigue crack initiating to a critical length,the stress concentration factor at notch root was corrected allowing for the influence of short crack theory.Meanwhile,experiments for ship plates with characteristic notch radius and low cycle fatigue crack length were carried out in order to studying the law of accumulative plastic deformation and low cycle fatigue crack initiation life.In the analysis of low cycle fatigue crack initiation life,the effect of accumulative plastic damage at notch root and short crack were taken into account by introducing stress concentration factor which was corrected for short crack influence.The results showed that the theoretical analysis method in this paper was closer to the experimental value.And also,the influence of mean stress and stress amplitude for accumulative plastic damage were studied to obtain valuable theoretical analysis results.Secondly,low cycle fatigue crack propagation model of ship notched crack plate was proposed through theoretical analysis,and the experiment was conducted to validate theoretical study.Simultaneously,the stress and strain field at crack tip for ship notched plate under constant amplitude low cyclic loading was analyzed,to obtain the mechanism and influencing factors of accumulative plasticity destruction at fatigue crack tip.By introducing single tension or compression overload in low-cycle fatigue constant amplitude loading form,the applicability of low cycle fatigue crack growth rate forecast model under variable amplitude loading was researched.Through theoretical analysis,numerical simulation and experimental study,it was find out that,the model can rightly estimate crack growth rate and low cycle fatigue crack propagation life of ship plate under different loading conditions(including constant amplitude and variable amplitude loading).By comparing with experimental results,the crack propagation prediction model and experimental results under various conditions were in good agreement;the model can quantitatively predict the low cycle fatigue crack propagation rate in each stage caused by overload.Studies have shown that there is a clear plastic accumulative damage phenomena at crack tip under low cycle fatigue loading,and it has significant impact on the low cycle fatigue life.Therefore,the CTOD(crack tip opening displacement)for central through cracked plate and 3D surface cracked plate is studied based on the accumulative plastic deformation theory,which can provide reasonable assessment and techniques of fracture toughness for ship hull plate.Based on the accumulative plastic damage theory,the research for hull low cycle fatigue strength carry out with theory,experiment and numerical simulation method,and the model can be more accurate in evaluation the low cycle fatigue strength at extreme sea conditions.It is useful exploration for low cycle fatigue strength assessment.