| Nowadays,more and more countries position development and utilization of deep-sea resources as important strategic targets,so the competition for marine resources becomes more and more intense.Deep-sea manned submersible is one of the important deep-sea resources exploitative instruments,and plays crucial role on deep-sea geographical environment research,marine resource investigation and deep-sea biological gene research,etc.Because of the complexity and variability of the marine environment,submersible is subjected to periodic fluctuating loads during its service.Therefore,the fatigue failure is one of the main failure models of the marine structure during service.The life of the pressure hull material of deep-sea manned submersible is closely related to the reliability and service life of the submersible.Although the load spectrum used in calculating the fatigue life of submersibles is always fatigue cyclic load,in the actual work process,submersible pressure hull structure is subjected to the dwell load when diving to the work depth except cycle load when dive and come-up.Therefore actually,the fatigue of the pressure hull structure is also dwell-fatigue issue.Because of favorable mechanical properties,titanium alloy is an important material for the manufacture of pressure hulls for submarines and deep submersibles,and a large number of tests have proved that the structure life under dwell-fatigue loading is significantly lower than that under pure fatigue conditions.Therefore,it is important to study the method of predicting the dwell-fatigue crack growth rate of the titanium alloy material used in the pressure hulls of deep-sea manned submersibles,which has important scientific research foreground and practical engineering significance.The main purpose of paper is to propose a prediction method for dwell-fatigue crack growth behavior used in pressure hull structures of deep-sea manned submersibles,and use this method to predict preserve dwell-fatigue life and crack growth rate of the typical titanium alloy material used to submersible pressure hulls.For this purpose,the main contents of paper are as follows:(1)The domestic and overseas prediction method for fatigue and dwell-fatigue crack growth rate are reviewed.Especially the prediction methods for dwell-fatigue crack growth rate are summarized,which is the foundation for the research of this paper.(2)The prediction model for dwell-fatigue crack growth rate is modified,and a parameter η related to the hold time is proposed.The modified prediction model is more suitable for crack propagation under the influence of hold time.(3)The sensitivities of the parameters in the dwell-fatigue crack growth rate prediction model are analyzed.The effects of hold times and the material parameter ? in the dwell-fatigue crack growth rate prediction model are studied.Then prediction results and the corresponding test results were compared.Fatigue and dwell-fatigue crack growth behavior of titanium alloy IMI834,Ti-6242 and Ti-6Al-4V were predicted by the modified predictive model of dwell-fatigue crack growth rate,and compared with the experimental results.Then the applicability of the modified model for predicting the dwell-fatigue crack growth rate is verified.(4)Fatigue and dwell-fatigue crack growth rate of Ti-6Al-4V is studied by test.The basic mechanical properties of titanium alloy Ti-6Al-4V are obtained by tensile and fracture toughness tests.Finally,the dwell-fatigue crack growth rate test is carried out,and the results were compared with the predicted dwell-fatigue crack growth rate.Then,the applicability of the modified model for predicting the dwell-fatigue crack growth rate is validated and the effects of different hold times on the dwell-fatigue crack growth rate are compared.(5)The fatigue and dwell-fatigue crack growth behavior of titanium alloy Ti-6Al-4V were studied by numerical simulation.In order to observe the crack growth process visually,the finite element model is established by Abaqus software,the crack growth behavior was simulated by Zencrack software.Then,the crack growth rate and the leading edge shape,stress distribution and growth path of the crack growth process were obtained.The innovative work of this paper is mainly embodied in: Based on the prediction method of the dwell-fatigue crack growth rate of marine structure,the existing prediction model of dwell-fatigue crack growth rate is modified,and the dwell-fatigue crack growth behaviors under different dwell parameters are studied;the dwell-fatigue crack growth rate of titanium alloy was tested,under different hold times,thus providing an engineering application method for dwell-fatigue crack growth behavior of marine structures... |
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