| The exploration of deep-sea resources and the development of the marine economy have been elevated to the national strategic height.Deep-water submersibles are one of the indispensable and important equipments in the marine development process.During the process of floating and sinking,their pressure-resistant structures are influenced by the cyclical seawater pressure,which will probably cause structural fatigue damage.However,fatigue problems under pressure and cyclic loads are significantly different from fatigue under conventional pull-pull and tension-compression cyclic loads.Therefore,as for the design and safety assessment of deep-sea structures,it is significant to carry out the research on crack propagation characteristics under cyclic compression loading,to reveal its expansion driving force and expansion law,and to propose a fatigue crack propagation life prediction method under pressure and pressure of deep-sea structure.In this paper,based on the Natural Science Foundation of Jiangsu Province,the method of finite element and experiment is used.Firstly,the stress field and plastic residual stress of the crack tip under compressive load are calculated,and the calculation method of stress intensity factor is proposed.Secondly,the compression fatigue crack propagation experiment is carried out.Based on this,a fatigue crack propagation life prediction method under pressure and pressure is established.Taking a pressure-reducing spherical shell scale model as an example,the specific process and steps for predicting the fatigue life of compression are expounded.The main research contents of the paper are as follows:(1)The domestic and foreign research status and main analysis methods in the field of fatigue research are summed up.The advantages and disadvantages of various methods and the scope of application are discussed.Finally,the fracture mechanics method based on the improved McEvily expansion rate model is used as the theoretical basis of this paper.Moreover,these are the foundation of follow-up elasto-plastic finite element analysis,experimental research and establishment of fatigue crack growth life prediction methods.(2)The stress field and plastic residual stress of crack tip under compressive load are analyzed and the calculation method of stress intensity factor is put forward.With the usage of multi-load step combined with nodal release analysis technique,the plastic residual stress near the crack tip is calculated and the effects of different cell sizes at the crack tip region are studied.The optimal crack tip cell size is determined as one-fortieth the size ofthe crack tip plastic zone.Compared with the experimental results in the literature,the validity and correctness of the finite element analysis method are verified.(3)The quasi-static tensile test of AH36 steel and the fatigue crack propagation rate test of AH36 steel are conducted,and the mechanical performance parameters and fatigue crack propagation data are measured.The model parameters in the improved McEvily model of AH36 steel are obtained by least squares fitting.(4)A compression fatigue crack propagation test for AH36 marine high-strength steel is designed and carried out.Fatigue crack propagation under three compressive loads with constant amplitude,single peak overload and single peak load is further studied.Based on the improved McEvily model,the compressive fatigue test is theoretically calculated.compared with the experimental results to verify the validity and correctness of the proposed method are varified.(5)The pressure shell of the scale ratio submersible is designed and used as the research object to calculate the fatigue crack growth life using the fatigue evaluation method proposed in this paper.The analysis process and steps of this method are described in detail. |
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