| With more and more attention paid to marine resources in the world,more and more countries are aiming at deep-sea development.To some extent,the research and development level of deep-sea manned submersible represents the level of deep-sea development of a country.In recent years,China has made great progress in many important research fields of deep-sea development,including Jiaolong 10,000-meter manned deep submersible,Qianlong unmanned submersible and Hailong unmanned cable-controlled submersible.The fatigue problem of the pressure structure of manned deep submersible is the key technology in the design of manned deep submersible because of the special service environment.In the working process,the manned deep submersible will experience frequent submergenceoperation-floating process,and in this process,the pressure hull of the deep submersible will experience the process of load increase-load dwell-load reduction.Due to the complexity of ocean environment and the uncertainty of working process of deep submersible,in order to ensure the safety of manned deep submersible pressure-resistant structure,it is necessary to fully consider the influence of random loads,such as overload,lower load,in addition to conventional triangular wave load,sinusoidal wave load and trapezoidal wave load in the process of crack propagation analysis and life prediction.Based on the theory of fatigue crack growth,the dwell-fatigue crack growth rate test of TI-6AL-4V manned deep submersible pressure hull material under different dwell time was designed.The effect of load sequence on crack growth was fully considered in the test design.The crack growth behavior of TI-6AL-4V was studied in view of the special load form of overload dwell fatigue.Based on the experimental results,the existing prediction model is modified and a prediction model for the overload dwell fatigue crack growth rate of pressure hull structure material of manned deep submersible is proposed.At the same time,the model is used to predict the overload dwell fatigue crack growth rates of different titanium alloys,and the predicted results are compared with the experimental results to verify the reliability of the model.The main contents of this paper are summarized as follows:(1)The research progress of fatigue crack propagation at home and abroad is reviewed.The research contents of different experts and scholars in the fields of fatigue crack growth rate prediction model and dwell-fatigue crack growth rate prediction model considering the effect of load sequence are analyzed and summarized in this paper.Laid the foundation for the follow-up research.(2)Fatigue crack growth rate test and dwell-fatigue crack growth rate test are designed and carried out.The minimum dwell saturation time of titanium alloy material TI-6AL-4V is proposed,and the control group test is carried out.The main environmental factors besides the test variables are excluded from the test results,and the section of the specimen is analyzed to prove the accuracy of the test results and the reliability of the tests.(3)The fatigue crack propagation behavior of titanium alloy TI-6AL-4V at room temperature is studied.Based on the load sequence effect,the crack growth rate and crack propagation mechanism considering overload effect are studied and designed.The crack growth rate tests of load-fatigue under different overload ratio and different dwell time were carried out.The cross-section of the specimen was analyzed and the cause and influence condition of the overload line were studied.(4)The dwell-fatigue crack growth rate model is modified,and some parameters of the model are determined by fitting the prediction model based on the test results,the overload dwell-fatigue crack growth rate prediction model considering the effect of load sequence is put forward.The fatigue crack propagation rate tests of different kinds of titanium alloy materials were carried out to verify the accuracy of the model and compare the fatigue resistance of the new titanium alloy and TI-6AL-4V. |
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