| In this thesis,the corrosion fatigue crack expansion of the submerged 4000 m class egg-shaped pressure shell is the object of study,and the relevant research is carried out by combining experimental research,theoretical analysis and numerical calculation.The corrosion law of martensitic nickel steel in seawater was studied.The corrosion fatigue crack expansion rate model of egg-shaped pressure shells with welding residual stresses is established,and the structural corrosion fatigue life is predicted.It will lay a theoretical foundation for the design of damage tolerance of egg-shaped pressure shells,which is of great significance to the establishment of fatigue damage prediction and safe operation evaluation system of special-shaped pressure shells and the reliability of deep-sea submersible use.The main study contents and findings are as follows:(1)Pre-corrosion fatigue tests were carried out on standard C(T)specimens of martensitic nickel steel 18Ni(250)material.The corrosion rate and surface corrosion morphology processes of 18Ni(250)material in seawater were obtained by immersing C(T)specimens of martensitic nickel steel material in artificial seawater environment for 18 months.Through the fatigue crack expansion test,the corrosion fatigue crack growth rate of18Ni(250)material was studied,and the corrosion fatigue crack growth and fatigue crack propagation difference Δa-N curves of 18Ni(250)material were obtained.And the Δa-N relationship is fitted into a polynomial function,which provides a basis for studying the corrosion fatigue crack growth characteristics based on the fatigue crack propagation characteristics.(2)According to the egg profile curve function and strength theory,the egg-shaped pressure shell is designed to meet the requirements of 4km underwater operation.The numerical model of the welded blank was established,and the modeless expansion process was simulated by the finite element method.Based on the numerical model,the improvement of the segmented eggshell tending to egg shape by internal expansion deformation and the stress distribution in the welded parts of the eggshell were studied.The effect of dieless bulging on the bending stress of egg shell welding part was analyzed.Through finite element simulation and residual stress test,the distribution of welding residual stress along the thickness direction was studied.(3)Through theoretical calculation and finite element simulation verification,the specimen that can equivalently produce bending stress and welding residual stress under hydraulic pressure load is designed,and the supporting fixture for fatigue test is designed.The Newman-Raju formula is reasonably corrected to add the stress intensity factor caused by welding residual stress,and get the unified form of surface crack intensity factor of compression-bending stress with welding residual stress.The fatigue crack growth rate of the equivalent specimen was studied by fatigue crack growth test.The crack expansion rates at different crack shape ratios were fitted to obtain a unified fatigue crack growth rate benchmark model for the equivalent specimens.(4)The fatigue crack growth of the equivalent specimen was analyzed by FRANC3 D software,and the reliability of the unified fatigue crack growth rate model under the welding residual stress of the equivalent specimen 18Ni(250)was verified.The crack expansion behavior of the equivalent specimen is analyzed to obtain the crack expansion paths at the surface and depth directions of the specimen.Based on the baseline model of unified fatigue crack growth rate of equivalent specimens,the relationship equation of corrosion fatigue crack growth and fatigue crack growth crack length difference ?a and cyclic load N is added,and the unified model of corrosion fatigue crack growth rate is obtained.The corrosion fatigue life of pressure eggshells was predicted using the unified model of corrosion fatigue crack growth rate,and its corrosion fatigue life was obtained. |
PDF Full Text Request