Experimental Study On Dwell-Fatigue Crack Propagation Behavior Of A New Titanium Alloy

At present,more and more countries position the exploitation and utilization of deep-sea resources as an important strategic goal,so the competition in marine resources is becoming more and more fierce.As an important tool of deep-sea scientific exploration,deep-sea manned submersible plays an important role in deep-sea geographical environment research,marine resource survey and deep-sea biological gene research.The life of the pressureresistant shell material of the deep-sea manned submersible is closely related to the reliability and service duration of the submersible.Due to the complexity and variability of the marine environment,the submersible bears a complex fluctuating load during the service period.So fatigue failure has become one of the main damage forms of deep submersible and other marine structures during service.Because of its outstanding mechanical properties,titanium alloy has become an important material in the manufacture of submarine and deep submersible pressure-resisting shell.It is found that in the actual working process of the submersible,the pressure-resistant shell structure not only bears the cyclic load of diving and rising,but also bears the load-keeping load of diving to the working depth.Therefore,the fatigue of pressure-resistant shell structure is actually a dwell-fatigue problem.A large number of tests have proved that the structural life under dwell-fatigue is obviously shorter than that under fatigue conditions.At the same time,many studies have found that the change of stress ratio in the test has a great impact on the fatigue life and crack growth rate of the material.As the country's desire to exploit deep-sea resources grows stronger and stronger,the depth of the manned submersible is also increasing,and there's also a gradual increase in the number of divers.It is difficult to meet this requirement with the pressure-resistant shell material of manned submersible.Therefore,it is of great scientific research prospect and practical significance to carry out experimental research on fatigue and load-holding-fatigue crack propagation behavior of new titanium alloy materials for pressure-resistant shell of large-depth manned submersible.On the basis of fatigue crack propagation theory,the main purpose of this paper is to study titanium alloy materials used in manned tanks of deep submersible in the whole sea.Basic mechanical tests,fatigue tests and dwell-fatigue crack growth rate tests at room temperature were carried out for the characteristics of crack growth in titanium alloys.The fatigue and dwell-fatigue crack growth rate of a new titanium alloy under different stress ratios and different holding time were tested in combination with the relevant theory of dwellfatigue.The saturation value of holding time was discussed based on quantitative analysis.Combined with the application of micro-scanning electron microscope,the fracture plane will be scanned after the test,and the mechanism of dwell-fatigue crack propagation of the new titanium alloy under different test parameters will be discussed.Based on the abovementioned research purposes,the main research work of this paper is as follows:(1)The research on fatigue and load-holding-fatigue crack propagation behavior at home and abroad is reviewed in this paper.The emphasis is placed on the study of dwellfatigue crack propagation behavior,which lays a foundation for the study of this paper.(2)The basic mechanical properties and fatigue properties of titanium alloy materials were studied.The basic mechanical properties such as tensile and fracture toughness of the titanium alloy material are tested,and relevant material parameters are determined for later test.The fatigue performance tests of titanium alloys under different stress ratios were carried out,and the effects of different stress ratios on fatigue crack propagation behavior were obtained.(3)The dwell-fatigue crack propagation rate of titanium alloy at room temperature was studied experimentally.The dwell-fatigue crack growth rate of titanium alloy under different holding time and different retention stress ratio is studied,and the experimental results are summarized and analyzed and summarized.The effects of different holding time and stress ratio on the behavior of dwell-fatigue crack propagation are obtained.(4)Based on the method of quantitative analysis and the application of microscanning electron microscope,the two methods are incorporated into the analysis of the test results in this paper.After obtaining the fatigue and dwell-fatigue test results of titanium alloy materials,the concept of quantitative analysis method is introduced to quantitatively analyze the fatigue and dwell-fatigue test results,and reveal the influence of holding load on fatigue crack propagation behavior.Scanning electron microscope was used to scan the fracture surface of the sample and the scanning results were analyzed.Based on the above-mentioned analysis results,the mechanism of dwell-fatigue propagation behavior of titanium alloy is discussed,and the mechanism of the influence of load-holding on fatigue propagation behavior is revealed.