Research On Dwell-Fatigue Crack Growth Test And Prediction Method Of New Titanium Alloy

Nowadays,land resources are increasingly scarce,and the development of marine resources,especially deep-sea resources,has become an important strategic goal for all countries in the world.The deep-sea manned submersible,as a platform for the exploration and collection of marine resources,undoubtedly has a very important position in the development of deep-sea resources.As a kind of engineering equipment whose working environment is in the extreme environment of the deep sea,the safety of deep-sea manned submersibles is also the research focus of researchers at home and abroad.Therefore,the focus of studying the safety of deep-sea manned submersibles is to study the service life of submersible pressure shells.Titanium alloy has strong seawater corrosion resistance and specific strength,so the main material of the pressure shell of deep sea manned submersible is titanium alloy.There are three main working states of deep-sea manned submersibles during service,that is,diving,working,and floating.These three working states are similar to the three states of aircraft take-off,cruising,and landing.Therefore,the main problem of the safety and service life of the pressure shell is the dwell-fatigue problem of the titanium alloy.The domestic and foreign researches on the dwell-fatigue crack growth of titanium alloys mainly focus on the dwell-fatigue crack growth under single dwell at room temperature,and some studies have shown that low temperature will affect the dwell-fatigue crack growth of titanium alloys.At the same time,the change of the waveform will also affect the dwell-fatigue crack growth behavior of the titanium alloy.Therefore,it is meaningful to carry out the test and prediction method of the dwell-fatigue crack growth rate under different temperatures and different waveforms for the new titanium alloy material that is suitable for the pressure shell of the deep sea manned submersible.The main research purpose of this paper is to select a suitable titanium alloy dwell-fatigue crack growth prediction model,carry out the fatigue crack growth threshold test of the new titanium alloy material at different temperatures and load ratios,and obtain the fatigue crack growth threshold of the new titanium alloy at different temperatures and load ratios.And reveal the influence law of temperature and load ratio on the threshold value of fatigue crack growth.Carry out the test of dwell-fatigue crack growth rate under different temperatures and different waveforms,and reveal the effect of temperature and waveform on the dwell-fatigue crack growth rate of new titanium alloy materials.Based on the selected model,the dwell-fatigue crack growth rate under different temperatures of the new titanium alloy is predicted.The influence of the waveform is considered,and the model is revised.Based on the revised model,the dwell-fatigue crack growth rate under different waveforms of the new titanium alloy is predicted.The test results are compared with the prediction results to verify the accuracy of the model.For this purpose,the main research content of this article is divided into the following aspects:(1)This paper summarizes the commonly used methods of prediction of dwell-fatigue crack growth at home and abroad.Compared with several applied dwell-fatigue crack growth prediction models,and analyzed the prediction capabilities of each model,a dwell-fatigue crack growth prediction model suitable for new titanium alloy materials was selected;(2)The fatigue crack growth threshold test of new titanium alloys at different temperatures and different load ratios were carried out.The influence of temperature and load ratio on fatigue crack growth threshold of new titanium alloy was analyzed.Fatigue crack growth threshold of the new titanium alloy were obtained.(3)The dwell-fatigue crack growth rate test of the new titanium alloy at different temperatures and different waveforms were carried out.The characteristics of the dwell-fatigue crack growth rate curve of the new titanium alloy at different temperatures and different waveforms were analyzed.The influence of temperature and waveform on dwell-fatigue crack growth behavior of the new titanium alloy was obtained;(4)Based on the selected titanium alloy dwell-fatigue crack growth prediction model,dwell-fatigue crack growth rate at different temperatures of the new titanium alloy was predicted.The test results and prediction results are compared to verify the accuracy of the selected model.Considering the influence of the waveform,the model is modified,the dwell time term is divided into two parts,and the dynamic influence law of the waveform is considered.Based on the revised model,dwell-fatigue crack growth rate under different waveforms of the new titanium alloy is predicted,and the test results are compared to verify the accuracy of the revised model.