| 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 get into plastic state rapidly during the voyage due to impact of adverse sea conditions which will introduce greater local stress concentration.Thus it is necessary to research the low cycle fatigue strength under overload of large scale ships.The crack propagation experiments and an approach of fatigue crack growth modeling is used to study the low cycle fatigue behavior under overload of ship-used high tensile steel AH32.A series of fatigue crack growth experiments were conducted using compact tension specimens of AH32 steel.Based on existing research,this paper mainly includes the following aspects.(1)Based on the theory of fracture mechanics and elastoplastic mechanics,considering the blunted radius of crack,plastic stress-strain relationship of the crack tip is elaborated.The prediction model of fatigue crack propagation rate under classical constant amplitude load and variable amplitude load is introduced in detail.(2)The dynamic propagation model of low cycle fatigue crack is established using finite element software ABAQUS combined with the node releasing technique considering reasonable minimum size of finite element and the scheme of node releasing.Stress and displacement field near the crack tip is analyzed to study the fatigue behavior under variable amplitude loading.(3)A numerical modeling approach has been developed to quantitatively simulate the fatigue crack propagation for AH32 steel.In the proposed approach,the fatigue crack advances when the plastic energy density at crack tip accumulates to a critical level.The crack growth rate is simulated under different forms of overload using the established propagation model.The predicted results achieve high conformity with results of the crack growth experiments.(4)The influence of crack closure effect on the overload retardation effect is discussed according to simulating the crack opening load under various load case which agrees well with the experiments.The crack opening load related parameter S is put forward to illustrating the effect of different forms of overload on retardation effect by means of drawing crack opening load-crack length curve.(5)A series of fatigue crack growth experiments were conducted using compact tension specimens of AH32 steel.Monotonous static load tensile tests,constant amplitude cyclic loading tests and different kinds of overload tests are investigated respectively.The material parameters of fatigue property and Chaboche model was deterred based on fatigue test data.The influence of overload ratio,stress ratio and number of constant cyclic load between overloads on crack growth rate,crack opening load and fatigue life were analyzed and discussed with the experimental results. |
