Total Fatigue Life Analysis For Pressure Spherical Shell Of Human Occupied Vehicle

With the development and progress of manned submersible technology,the exploration of deep sea resources has become a new field of human scientific research.Titanium alloy manning shell is an important structure of deep-sea submersible,which is related to the safety of scientific research personnel and its importance is self-evident.For the titanium alloy structure,the fatigue failure caused by the alternating stress is inevitable.Therefore,the fatigue life prediction of the pressure spherical shell is particularly important for the service safety guarantee of the deep submersible.In this paper,the titanium alloy manned shell of the American deep-sea submersible "Alvin" is taken as the research object,and the two-stage method based on the fatigue cumulative damage theory and fracture mechanics theory is used to predict the total fatigue life of the spherical shell,and the fatigue reliability theory is introduced to improve the accuracy of the fatigue life prediction.The historical diving data of "ALVIN" were collected,and the data were analyzed and the submersible load spectrum was fitted with the appropriate distribution function.Through error comparison,Gumbel distribution is selected as the probabilistic model of diving depth.In the crack initiation life prediction stage,the finite element simulation analysis was carried out by ANSYS,and the strength check of the pressure spherical shell was carried out to verify the accuracy of the model,and the fatigue crack initiation location was located by the stress nephogram.Based on the local stress-strain method,the fatigue crack initiation life of pressure spherical shell was evaluated,and the influence of welding residual stress on the fatigue crack initiation life was analyzed according to the further study of various parameters.In the prediction stage of crack growth life,the finite element software FRANC3 D was firstly used to split the model,and the crack stress intensity factor(SIF)was calculated based on displacement extrapolation and its distribution was discussed.The classical Paris crack propagation model was used to evaluate the crack propagation life,and the results of multiple calculations all met the design requirements.The influencing factors such as the initial crack depth,the half-width ratio of the initial crack depth and the threshold value of the stress intensity factor amplitude are discussed.The fatigue reliability analysis program of pressure spherical shell was programmed by MATLAB.The fatigue reliability analysis of manned spherical shell was carried out by response surface method(RSM),and the results of RSM were verified by Monte Carlo method.The results show that the fatigue failure probability of pressure spherical shell is very low and the calculation results of response surface method are accurate.Finally,the parameter sensitivity analysis shows that the material parameters and the design number of dives have a great influence on the failure probability.