Study On Three-dimensional Fatigue Crack Propagation Behavior Of Submarine Pressure Structures

Conical-cylindrical shell is an important part of submarine withstand pressure structure.Due to the particularity of cone-column joint position welding,the conical-cylindrical shell is subjected to alternating sea water load and welding residual stress in the process of submarine structure continuously diving and floating,resulting in stress concentration in the convex cone position of submarine cone-column structure,which is easy to generate fatigue cracks and expand continuously.It even caused structural failure of the submarine structure.Therefore,it is of great practical significance to study the propagation behavior of submarine conical-cylindrical shell surface crack for the fatigue life prediction and reliability research of submarine cone-column structure.The main contents and conclusions of the study are as follows:(1)In this work,the finite element model of the conical-cylindrical shell of the 300 m submarine with cracks is established by taking the pressure structure of the submarine as the research object.The corresponding boundary conditions and working loads were applied in ABAQUS according to the submarine working conditions under water and the alternating sea water loads suffered by the submarine in the process of diving and floating.Combined with the professional fracture analysis software FRANC3 D,surface cracks were introduced into the convex cone welding toe of the conical-cylindrical shell,and the welding residual stresses were set in a linear distribution along the thickness direction of the cone-column.The stress intensity factor of crack front was solved based on the M integral method,and the distribution law of stress intensity factor of crack front was obtained when initial cracks of different sizes were introduced into the toe of submarine conical-cylindrical shell.(2)Based on the three-dimensional fracture toughness of Type I cracks,a threedimensional fracture criterion for surface cracks in the 921 A steel submarine conicalcylindrical shell has been derived,which serves as the basis for three-dimensional fracture evaluation of the crack body structure.Based on the three-dimensional fracture criterion,the three-dimensional residual strength coefficient of the crack body was defined,and the threedimensional stress intensity factor and residual strength coefficient of the crack front of a semi-elliptical surface crack were solved through MATLAB programming.Through a combination of numerical and experimental research,the accuracy of the three-dimensional residual strength coefficient of the cracked body based on the three-dimensional fracture criterion in evaluating the fracture of the cracked body structure was verified.(3)Based on the research results of residual strength of structures with cracks,it can be directly applied to the structural fracture assessment and residual strength analysis calculation of conical cylindrical shells with surface cracks.By calculating the threedimensional stress intensity factor of the crack front of a semi-elliptical surface crack and the distribution of residual strength coefficient of the conical-cylindrical shell,the propagation trend of the submarine conical cylindrical shell surface crack and the loadbearing capacity of the structure were predicted.Furthermore,the influence of the shape parameters of the semi-elliptical surface crack on the three-dimensional fracture parameters and related evaluation standards was analyzed.(4)Multiple sets of cracks at the convex cone welding toe of submarine cone-column structure were simulated through FRANC3 D,and the new crack front was fitted.The influence of typical crack characteristics on crack propagation on semi-elliptical surface cracks was analyzed.By comprehensively considering the various factors affecting the surface cracks of submarine structures and comparing the propagation results of cracks under the conditions of three-dimensional fatigue fracture theory,based on the threedimensional fatigue crack closure theory and strip yield model,a propagation model of nonpenetrating three-dimensional cracks on the surface of conical-cylindrical shells is established,and the influence of semi-elliptical surface cracks of different sizes on the fatigue life of the structure is analyzed.