Fatigue Life Evaluation Of Deep-sea Pressure Structure Based On Equivalent Structural Stress

Deep submersible is an important equipment in deep sea development,and its key component is pressure shell,which bears great pressure in the sea water,and plays a role in protecting personnel and equipment in the cabin.After diving for many times,the spherical shell will appear fatigue damage,especially the weld area of pressure hull,which has been a threat to the fatigue life of structure.Equivalent structural stress method is a method to evaluate the fatigue life of welded joints.The corresponding S-N curve can be replaced by the master S-N curve,which is convenient to use and the results are more accurate.There are many welds on the pressure shell,which determines the life of the whole structure.Therefore,the fatigue life of the pressure shell is evaluated by using the equivalent structural stress method.Firstly,the theory of equivalent structural stress method is introduced.The hot spot stress and structural stress results of fillet weld of cross joint are compared under different grid sizes.The results show that the grid of structural stress is insensitive,which is the important reason for this paper to choose structural stress method to calculate fatigue life;Then the welding characteristics of titanium alloy TC4 are analyzed.Most of the pressure resistant spherical shells in service are made of titanium alloy.The size and properties of the materials after welding are changed.The metallographic test,hardness test and tensile test are carried out for the welded titanium alloy plate.The material properties of TC4 welding seam are obtained.Based on the experiment,the welding simulation is combined,The fitting curve of the weld size corresponding to the thickness of the welding plate of TC4 thick plate is obtained,and the weld size on the ball shell is obtained by fitting curve;The results of the method are compared with the results of finite element before calculation.The feasibility of the finite element method is verified.Then the stress analysis of the spherical shell under different working conditions is carried out;Based on the finite element results,combined with the submarine data of a 7000 meter manned submersible,the fatigue of the spherical shell is systematically analyzed by using the equivalent structural stress method,including the transverse comparison of different parts of the weld under the same load,and the longitudinal analysis of fatigue in high stress area under different working conditions.Finally,the calculation results of fatigue life are given.In this paper,the fatigue life of different welding joints on pressure shell is calculated with equivalent structural stress as parameter,and the guidance for maintenance and replacement of pressure shell during service is given;In the process of research,the dimensionless fitting curve between weld width and plate thickness is proposed,which provides a reference for the width of weld joint of large thickness titanium alloy TC4;At the same time,the application of structural stress method in fatigue analysis of underwater pressure equipment is enriched,which provides a reference for the fatigue life evaluation of deep-sea pressure structure.