Low-cycle Fatigue Crack Propagation Behavior Of Hull Plates Considering Accumulative Plastic Strain

With the rapid development of modern machinery and ships,the forces on the hull structure are becoming more and more complex and diverse,and the problem of lowcycle fatigue is becoming more and more prominent.When the ship sails in the waves for a long time,under the action of cyclic loading,the local structure is easy to exceed its yield.The ultimate load will produce plastic strain within a short time.As the sailing time increases,the cumulative plastic strain will promote the initiation and propagation of cracks,which will eventually lead to the destruction of the hull structure.Therefore,the initiation and propagation of cracks and the destruction and damage of hull structures caused by low-cycle fatigue problems are currently a problem that cannot be ignored in the development and change of ships.At present,most studies considering structural damage caused by crack propagation remain within the range of linear elasticity.The actual low-cycle fatigue problem is caused by elastoplastic deformation.Since the study considers that the time history of low-cycle fatigue problems is mostly one-time failure or high-cycle fatigue,this article uses finite element method to model by parametric.Discuss the problem of multi-crack propagation under the influence of cumulative plasticity,combined with the cumulative plastic strain and residual stress in the crack propagation,from the perspective of the crack closure effect,the following work has been done:(1)This paper summarizes and summarizes the research and development history of crack propagation caused by accumulated plasticity,provides a theoretical basis and basis for the subsequent solution of low-cycle fatigue problems,and systematically expounds the meaning of crack-related parameters.(2)Using the finite element software ABAQUS,two models of multi-crack plates were established,the double cracks at the edge of the hole and the collinear multicracks.The crack propagation process under cyclic loading was dynamically simulated,through the evolution of the residual stress along the crack surface and the cumulative plastic strain This trend verifies the influence mechanism of accumulated plasticity in crack growth,and provides corresponding support for the subsequent further research on the damage of ship structure caused by crack initiation and propagation.(3)Using CTOD theory and discussing the growth rate of multiple cracks in the process of multi-crack growth,experiments have found the growth law of the center crack and the side cracks through different stress ratios and plate thicknesses.That is,long cracks will inhibit short cracks,and side cracks are easier to grow than central cracks.(4)The closed parameters and crack tip stress field changes of multiple cracks and single cracks under different overload ratios are analyzed.The study shows that the overload hysteresis of multi-sided cracks is more obvious than that of single cracks,and greater tensile stress will be generated near the crack tip at the moment of overload.Adjacent cracks will cause more serious stress concentration at the tip of the propagating crack.(5)The cumulative plastic strain rate is used to calculate the low-cycle fatigue life of the multi-crack hull plate.Combined with different stress ratios,it is found that the propagation life of multiple cracks is longer than that of a single crack.This indicates that multiple cracks are more sensitive to changes in stress ratio.The larger the ratio,the greater the growth of the multi-crack propagation life.