Room Temperature Load-holding-fatigue Testing And Crack Prediction Methods For Titanium Alloy Welded Structures

As land resources become increasingly scarce,countries around the world are turning their attention to the oceans and seas,which has led to an intensified competition for marine resources in recent years.But compared with land,the marine environment is more complex and more difficult to develop,so the competition in the marine field is essentially a competition of science and technology.China has clearly recognised the importance of marine resources and has formulated a series of marine development plans.Deep-sea manned submersibles,as the core equipment for the exploration and development of marine resources,have received much attention from the country.Unlike land,there are more uncertainties in the marine environment,and deep-sea manned submersibles are subjected to cyclical fluctuations in load during operation.As the first line of defence for protecting deep-sea manned submersibles,it is important to analyse the form of load on the pressure-resistant shell and to predict the damage to the shell from fatigue.Conventional fatigue studies have been carried out for general cyclic loads,but deep-sea manned submersibles need to operate deep under the sea,so the fatigue of the pressure-resistant shell structure is actually a loadholding-fatigue problem.Fatigue and load-holding-fatigue test results show that the loadholding-fatigue life of the structure is significantly lower than the fatigue life,so it is dangerous to use fatigue cyclic loading to predict the life of the submersible’s pressureresistant shell structure.In addition,the pressure-resistant shell structure is not a single unit,but is made up of multiple pieces of welded material,and research has shown that welding has a certain effect on material properties,so we have to consider the load-holding-fatigue problem at the welded joints on top of the load-holding-fatigue problem of the pressureresistant shell structure.Titanium alloy is the main material used in pressure resistant shells today due to its excellent specific strength and corrosion resistance.Therefore,it is necessary to carry out load-fatigue studies on new titanium alloy base materials and welded joints.In this paper,basic mechanical properties tests,fatigue crack expansion rate tests and scale effect tests are carried out on new titanium alloy base materials and welded joints for submersible vehicles;load-holding and fatigue crack expansion rate tests are carried out on new titanium alloy base materials and welded joints.Based on the test results,the new titanium alloy base material and welded joints for submersible submersible and fatigue crack expansion rate model.The main research content of this paper is as follows:(1)The study of domestic and foreign metal materials and welded joints load-fatigue crack expansion behaviour research,analysis of titanium alloy base material and welded joints load-fatigue crack expansion behaviour and failure mechanism,to determine the load-fatigue of each influencing factor,for the following to carry out load-fatigue crack expansion behaviour test to lay the theoretical foundation;(2)To carry out basic mechanical properties tests of new titanium alloy base material and welded joints at different scales,to obtain the scale effect of the mechanical parameters such as yield stress,tensile limit and elastic modulus of titanium alloy;at the same time,through the test,to obtain the fatigue crack expansion rate of new titanium alloy base material and welded joints at different scales,to analyse the scale effect on the mechanical properties and fatigue crack expansion rate of new titanium alloy base material and welded joints(2)the effect of the scale effect on the mechanical properties of the new titanium alloy base material and welded joints and the fatigue crack expansion rate of the impact law,comparative study of titanium alloy welded joints and base material fatigue crack expansion behaviour;(3)To carry out different load-holding waveforms of new titanium alloy base material and welded joints load-holding-fatigue crack expansion rate test research,to obtain different load-holding waveforms of new titanium alloy base material and welded joints load-holding-fatigue crack expansion rate,analysis of the base material and welded joints load-holding-fatigue crack expansion behavior with the upper peak load-holding time and the lower peak load-holding time change law,to obtain different load-holding waveforms of new titanium alloy load-holding-fatigue crack expansion behavior failure mechanism-fatigue crack expansion behavior failure mechanism;(4)Based on fracture mechanics theory,combined with the new titanium alloy base material and welded joints fatigue and load-holding-fatigue crack expansion rate test,the establishment of a new titanium alloy load-holding-fatigue crack expansion prediction model,the use of the model to predict the load-holding-fatigue crack expansion rate of the new titanium alloy base material and welded joints under different upper peak load-holding time and lower peak load-holding time,and with the corresponding test results for comparison research,so as to verify the accuracy of the prediction model.