| In order to further exploit and utilize deep-sea resources,the research and development of deep-sea manned submersible has received unprecedented attention.For deep-sea manned submersible,the key of its research is the pressure shell material of manned submersible.Titanium alloys offer better corrosion resistance and specific strength than steels used in conventional Marine structures.Since the middle of the last century,titanium alloy has been widely used in the field of ships,from Jiaolong to the present 10,000-meter submersible "Striver".As the new alloy is processed into the deep sea manned submersible pressure shell,it will inevitably be welded and other processing technology.In order to ensure the safety and reliability of the pressure shell of manned submersible in the deep sea area,it is necessary to study the characteristics of titanium alloy and its welded joints used in the pressure shell.During the service period,the deep-sea manned submersible pressure hull will not only be subjected to the periodic loading of floating and diving,but also will be subjected to the variable amplitude loading due to the different depth of diving each time,so that the pressure hull will often face fatigue damage and load order problems.Therefore,it is very necessary to study the fatigue damage and load sequence of pressure shell titanium alloy.In this thesis,the main content is based on the theory of fatigue crack propagation,for new titanium alloy of pressure shell material,the preparation of new titanium alloy welded joint,and the parent metal and welded joint to carry out the basic mechanics test,fatigue test under load,often comparative analysis after welding material performance and the change of the fatigue crack growth behavior,and in view of new titanium alloy welded joint,Carry out load sequence fatigue tests with introduced overload and low load.Based on the test data,the crack growth prediction formula for the new titanium alloy welded joint under complex load is presented.Based on the above purposes,the research work of this thesis is as follows:(1)According to the typical welding structure evaluation methods and fatigue crack propagation theories at home and abroad,the data collection and review are carried out,and the fatigue life prediction method of titanium alloy welded joints of submersible pressure shell is emphatically researched and breakthrough is made,which lays the foundation for the research of this thesis.(2)Preparation of new titanium alloy welded joints.Experimental study was carried out on the mechanical properties of the new type of titanium alloy base material and welded joint at room temperature,and the changes of the yield strength,tensile strength,fracture toughness and other mechanical properties before and after welding of new titanium alloy were obtained,which laid the experimental foundation for the following research.(3)Study on fatigue crack propagation behavior of new titanium alloy base metal and welded joints under constant amplitude loading.Combined with the fatigue crack growth model,the fatigue crack growth tests of new titanium alloy base metal and welded joint under constant amplitude loading were designed.The fatigue test results were compared and analyzed,and the differences of fatigue crack growth between new titanium alloy base metal and welded joint under constant amplitude loading were obtained.(4)The influence of loading sequence effect on fatigue crack propagation behavior of new titanium alloy welded joints was studied.Combined with the fatigue crack growth model,the fatigue crack growth rate of new titanium alloy welded joint under single peak overload and low load was studied.Based on the analysis results,the influence of loading sequence effect on the fatigue crack propagation of new titanium alloy welded joint was obtained.(5)The fatigue life prediction method of new titanium alloy welded joints under complex loads is studied.Based on the fatigue crack growth prediction formula of new titanium alloy base metal,the parameters of the prediction model were modified,and the fatigue crack growth prediction formula considering the effect of load order was put forward for welded joints under complex loads. |
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