Study On Low Cycle Fatigue Reliability Of Structures With Large Submergence Depth

In recent years,with the rapid progress of global exploration technology,countries have devoted great efforts to the development of underwater structures.Manned submarines and submarines,as typical representatives of underwater structures,play an important role in the fields of science and technology and national defense respectively.Manned submersibles have a large depth of submergence,reaching the level of kilometers or even tens of thousands of meters.Pressure hulls are usually spherical.As military weapons serving national defense,submarines have a large volume of pressure hulls,usually cylindrical,with a depth of about 300 meters.Generally,the structure with the ultimate submergence depth greater than 500 m is called the structure with large submergence depth,and the conventional submarine can rarely submerge to a depth of more than 500 m.At present,there are relatively few studies on deep diving cylindrical pressure hulls.In the process of frequent upstream and downward submergence,pressure hulls are periodically subjected to hydrostatic pressure of different magnitudes,resulting in low cycle fatigue failure of structures.The introduction of reliability theory can more accurately predict the life of pressure hulls.In this paper,the submarine with a submarine depth of 600 m is taken as the research object.In view of the difficulty in collecting historical submarine data,the probability model of submarine depth distribution is established by fitting the submarine data of "Alvin" manned submarine,and its fatigue load spectrum is obtained.By comparing the finite element analysis results with the theoretical calculation results,it is verified that the numerical simulation of the life of the pressure shell based on the fatigue load spectrum method is more accurate than that based on the limit load method.On this basis,the fatigue reliability of the pressure shell is studied,and the reliability research method considering the fatigue and stability co-failure mode is proposed.Specific research contents are as follows:(1)Using the "Alvin" manned submersible's submergence data as the original data,the original data are fitted and analyzed by mathematical statistics,Weibull distribution and Gumbel distribution functions are compared and analyzed,and a suitable probability model of submergence depth distribution is established to determine the fatigue load spectrum.(2)Taking the conical-cylindrical composite shell as the typical part of submarine pressure hull structure as the research object,two different correction methods(S-W-T method and Morrow method)are used to analyze the crack initiation in the hot spot area of fatigue of pressure hull,and fracture mechanics method is used to analyze the crack propagation at the convex corner of shell outer surface.It is found that the fatigue crack propagation life of this part is longer than that of fatigue crack initiation.Life expectancy is much smaller.(3)The crack initiation stage and the crack propagation stage along the depth direction on the outer surface of pressure shell are studied by combining theoretical calculation with finite element method.It is found that the numerical simulation results by the fatigue load spectrum method are in good agreement with the theoretical calculation results,and the error is less than the error of the numerical simulation results by the limit load method and the theoretical calculation results,which verifies the accuracy of the finite element analysis by the fatigue load spectrum method.The numerical simulation under this method obviously obtains a fatigue life closer to the theoretical calculation.At the same time,it is verified that the load spectrum information fitted in this paper is reliable.(4)The program for calculating fatigue reliability index of submarine pressure hull is compiled by Fortran language,and the influence of random parameters on reliability is discussed.Based on the reliability study of single fatigue failure mode,considering the correlation between fatigue failure mode and stability failure mode,and on the basis of traditional reliability model of series and parallel systems,the reliability model of pressure hull under the combined action of two kinds of loads is established,and the reliability analysis method of pressure hull under fatigue and stability co-failure mode is put forward.