Analysis Of Biaxial Non-proportional Low-cycle Crack Propagation Behavior Of Hull Cracked Plate

Generally,the ship is faced with multiple extreme wave loads in harsh environment with the process of service.That is,the ship components are subjected to periodic alternating loads with high stress and high frequency for a long time.As a result,the carrying capacity of the hull structure gradually decreases,which eventually leads to the overall fracture failure of the hull.So the fatigue fracture failure of ship structures is of great significance to the safety and viability of ships.In recent years,there has been a systematic theoretical analysis and experimental study on the low-cycle fatigue crack propagation of metal structures under uniaxial loading.However,the study on the low-cycle fatigue crack propagation behavior under multi-axial non-proportional loading is uncertain,as well as no substantial research results have been obtained so far,which is far from solving the problems existing in the engineering application of ships.Therefore,considering the influence of accumulative plastic damage in this paper,the cruciform of the hull cracked plate is taken as the research object,which analysis the behavior of low-cycle fatigue crack propagation under the biaxial non-proportional loading from theoretical analysis,numerical simulation and experimental research.The main work of this paper is summarized as follows:（1）The theoretical solution of biaxial low-cycle fatigue crack tip of hull cracked plate based on biaxial fatigue crack propagation theory was analyzed,which combined with Newton-Raphson iteration method.And on the basis of theoretical analysis and experiment,reasonable Chacoche model parameters are obtained by numerical simulation,as well as the mechanical mechanism of accumulative plastic failure of low-cycle fatigue crack tip was studied.（2）Based on the theory of elastic-plastic fracture mechanics and the accumulative plastic strain at the crack tip,the CTOD calculation model was established withΔε_pas the control parameter.The effect of different phase difference,biaxial stress ratio and stress ratio on the relationship between accumulative plastic strain incrementΔε_pandΔCTOD was investigated by numerical simulation.The crack tip stress-strain curves under different conditions and the function curves between accumulative plastic strain incrementΔε_pandΔCTOD were obtained.The calculation model was verified by experimental results.（3）Based on the low-cycle fatigue crack propagation finite element simulation of cruciform cracked plate,to study the crack tip stress–strain field,plastic zone radius and residual stress and so on.The effect of the phase difference,biaxial stress ratio and stress ratio are emphatically discussed to explore the law of low-cycle fatigue crack propagation with biaxial non-proportional loading.（4）The thesis has carried out the accumulative plastic experiment of the Q235 steel cruciform specimen,and discussed the effects of phase difference,biaxial stress ratio and stress ratio on accumulative plastic deformation.As the same time,the coupling relationship between accumulative plastic failure and low-cycle fatigue is verified.（5）The biaxial non-proportional low-cycle fatigue crack propagation experiments were carried out on Q235 steel cruciform specimen in this paper.The effects of different phase difference,biaxial stress ratio and stress ratio on the crack propagation rate were investigated.As well as the theoretical model of crack growth rate was compared with the experimental result to verify the rationality of the theoretical model.