Research On Crack Propagation Behavior Of Titanium Alloy Pressure Spherical Hull Specimens Under Trapezoidal Load

The large depth manned submersible is beneficial to the exploitation of deep-sea resources.Manned cabin is the key part of submersible.The strength and stability of its structure are very important to ensure the safety and reliability of the submersible.Due to the harsh and complex environment,the pressure spherical shell is subjected to high pressure cyclic alternating load during service,and it is easy to produce plastic strain and fatigue crack in the stress concentration area(such as weld area),and fatigue failure of the pressure structure will be caused by long-term cyclic accumulation damage.Fatigue fracture failure mode is the main failure mode of submersible in service.Accurate prediction of fatigue crack growth rate is conducive to effective evaluation of its fatigue life.In this paper,the pressure spherical shell of the deep-sea manned cabin is taken as the research object,and its stress distribution characteristics and fatigue characteristics are taken as the starting point.The research status of pressure spherical shell of manned cabin at home and abroad is briefly summarized,and the technical problems that have not been solved in this field are summarized.The research contents and main achievements of this paper include the following aspects:(1)The material properties and structural forms of pressure spherical shell are compared.The service environment and load spectrum characteristics and its simplified method are analyzed and the specific structural form and the basic material properties of the spherical pressure shell made of Titanium alloy used for the current study are determined.(2)According to the requirements of China Classification Society(2018)"Classification Code for Submersible Systems and Submersible ",a pressure spherical shell model(including an opening structure)was designed for the 10,000-meter deep-sea manned cabin,and the finite element calculation of stress distribution characteristics of the model was carried out.The results show that the stress concentration exists in the reinforced area of the opening structure.(3)In view of the high cost of manned cabin spherical hull test,the equivalent deformation specimen is considered to replace the whole pressure spherical shell for the test.Based on the design criterion to ensure that the size and distribution of stress and strain endured by the simulated specimen should be the same or similar to the actual reinforced area of the spherical shell wall as far as possible,the equivalent deformation specimen was designed as the welding joint.After parameter optimization and consideration of the processing difficulty,the welding Angle was determined to be 175°and the thickness to be 24 mm.(4)In order to obtain the residual stress distribution of the welded joint and lay the foundation for the subsequent fatigue characteristics research,the welding test and numerical simulation analysis method were carried out.TIG welding method,X groove,multi-layer and multi-pass welding were used for welding test,and the residual stress was measured by blind hole method.Based on the SYSWELD welding finite element analysis software,the welding process finite element analysis of the welded joint model was carried out,and the validity of the numerical analysis method was verified by comparing the finite element results with the test data.(5)Compression creep constitutive model,fatigue crack growth rate model and creep fatigue crack growth rate model suitable for pressure spherical shell of deep-sea manned cabin in high pressure environment are sorted out.The latest and most complete model was selected for calculation,and the fatigue crack growth rate curve,creep fatigue crack growth rate curve and life curve were compared.The results were consistent with the research conclusion in the reference: the fatigue life of dwell-fatigue was reduced by about 16 times compared with the fatigue life.(6)Based on the condition that the residual stress distribution of the equivalent deformation specimen is clear,a fatigue test with R=0.1 is carried out.During the test,the crack length is recorded once every 2000 load cycles,and a total of 113,402 cycles are completed before the interruption.The trend of crack propagation and the upper and lower fracture were observed by stereoscopic microscope and metallographic microscope.It was found that the interaction of multiple cracks led to the final fracture of the sample,and the fracture section morphology was consistent with the macroscopic morphology of the low-cycle fatigue fracture.(7)Based on ABAQUS and FRANC3 D,the welding area and edge of multiple crack propagation simulation,calculates the fatigue crack growth rate curve and structure life curve,and combined with the test data contrast to verify the validity of the numerical simulation method,considering the equivalent deformation of the specimen is expected to replace the whole pressure spherical shell experiment and numerical methods,for deep-sea manned cabin pressure spherical shell fatigue life evaluation method to provide the reference.In conclusion,this paper studied the pressure spherical shell of manned cabin by use of finite element analysis considering the interaction effects of various factors such as material properties,structural form,fatigue load spectrum characteristics and environment.The welding residual stress,initial defects and fatigue properties of the spherical shell of the deep-sea manned cabin made of Titanium alloy were analyzed in detail,to establish a relatively complete on titanium alloy system of pressure spherical shell system performance research to provide the reference value.