| Due to its excellent mechanical properties,titanium alloy presents a favorable application demand in deep-sea equipment manufacturing and is widely used in deep-sea equipment and structures.Titanium alloy structures in deep-sea service are not only subject to extreme fatigue loads,but also face severe corrosion environments,where corrosion and fatigue act together and load simultaneously,posing a huge challenge for the safe service of deep-sea structures.Therefore,the evolution of damage to deep-sea structures caused by fatigue loading with deepsea corrosion should be quantified to achieve structural integrity design of deep-sea equipment.However,the simultaneous acceleration tests on fatigue and corrosion are still challenging due to the lack of equivalent acceleration ratio between fatigue and corrosion.In this thesis,a deepsea TC4 ELI titanium alloy is selected as the research material,and the corrosion-fatigue crack growth behavior of the material is systematically tested based on a self-designed and manufactured corrosion-fatigue equivalent acceleration equipment.The special phenomenon of deceleration followed by acceleration was observed in the corrosion-fatigue crack growth of TC4 ELI titanium alloy,and the subsequent analysis and characterization attributed this special phenomenon to the competition between corrosion-induced crack tip blunting and material performance decreasing.The main researches in thesis are as follows:1,first,the material was subjected to quasi-static tensile testing and pre-corrosion tensile testing.The relationship between the effect of corrosion on the static strength of the material was quantified by comparing the test data obtained at different levels of corrosion,and the results showed that corrosion had a significant effect on the elongation of the material.Based on the test results and macroscopic analysis,the mechanism of corrosion influence on the static strength of the material was subsequently clarified,and the corrosion-strain influence model was summarized.2.Secondly,according to the theory of simultaneous loading of corrosion and fatigue,a corrosion-fatigue crack growth equivalent acceleration experimental device was designed and fabricated to perform fatigue crack growth test and corrosion-fatigue crack growth test on the material,and it was found that in the framework of three-dimensional fatigue crack growth,the material affected by corrosion would show a special phenomenon of growth rate decreasing first and then increasing.The mechanism of " deceleration" and " acceleration" of corrosion in fatigue crack growth is also qualitatively described according to the relevant extreme tests,and the relevant phenomenological model is compiled.3.Subsequently,the material was characterized and analyzed from macroscopic to microscopic scale by various measures such as microscopy,white light interference,SEM,etc.The two mechanisms of corrosion in fatigue crack growth were discussed in detail from various aspects,and the accuracy and practicality of the conclusions obtained were verified.4.Finally,the relevant tests were simulated and verified by using finite element ABAQUS software and fatigue crack growth Zencrack software.The same simulation results as the test phenomenon were obtained,which verified the accuracy and universality of the intrinsic constitution equation and crack growth model obtained from the test and provided data support for the subsequent study.In this work,the fatigue crack growth behavior of deep-sea TC4 ELI titanium alloy under corrosion acceleration is systematically studied,and the influence mechanism of corrosion in fatigue crack growth is revealed and verified by combining different measures such as systematic test,characterization analysis and numerical simulation,which provides data support and reference value for the safety operation of large deep-sea equipment in China. |
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