Research On Influence Of Residual Stresses On Bearing Limit Of Marine Pressure Structure

Submersible vehicle is an important tool to explore marine resources. Theenvironment of deep ocean requires high structural performance of the vehicle. Theocean areas that human can explore is limited by structure property. Theimprovement of external bearing limit is of great significance to the development ofmarine resources. In order to maximize the limit of submersibles, engineers choosethe highest strength material, titanium alloy and the best structure—sphericalstructure. At present, it is impossible to enrich bearing limit in those aspects. Thethick wall cylinder is the object studied in this thesis and a way is put forward toenhance the pressure-resistant properties of the structure with a kind of residualstress distribution. The numerical simulation of residual stress formation is carriedout and the influence of this kind of stress distribution on structure’s bearing limit isdeeply investigated.Analyzing the characters of the cylinder structure, the expansion plans of thickwall cylinder and springback methods, this thesis decides to use internal pressure forexpansion and the springback solution of explict method combined with static. As aresult, the process of two-dimensional numerical simulation is determined. Hoopresidual stress produced keeps tensile on inside surface and this stress distribution ishelpful. Different numerical models are established and analyzed. Facts studiedconsist of physical dimensions, material properties and the expansion rate. Theinfluence of those factors on the formation of residual stress distribution isinvestigated respectively. The regularities of factors’ influence are attained and itgets to be known how to control the formation of residual stress. From the point ofstrain and stress, the difference of rebound velocities between inside and outsidewall is the main formation cause of the beneficial residual stress distribution.On basis of the3D cylinder structure, finite element models are established.The bearing capacity of external pressure is investigated respectively under theconditions, including ideal state, initial geometric imperfection, initial imperfectionof residual stress and the superposition of the two imperfections. The influence ofinitial defects on the bearing limit is gained by comparisons of the analysis results.The greater initial geometric imperfection, the lower the bearing capacity of thickwall cylinder. The degree of the initial stress imperfection depends on the stressdistribution and its values. Structure models of three kinds of uniform residual stressstate are analyzed. The higher the value of hoop residual stress, the greater thebearing limit. The development process of stress is proved. The changes of stuctrueroundness are analyzed during the expansion, which reveals that expansion process is helpful to reduce the initial geometrical imperfection of the structure.Based on improvement of the bearing limit, a new kind of structure is chosen totest structures’ bearing capacity of external pressure. According to the availablemanufacturing conditions and structures of environmental simulation devices athome and abroad, a kind of new structure form is created. The volume of thisstructure is small and it can be easily manufactured and operated. Indexes of thestructure are determined. By finite element methods, the structure strength ischecked. It is indicated that the pressure bearing performance of this kind structureis superior to spherical structure. In addition, sealing system is projected andcompleted. Fatigue strength is checked, which proves that the service life of thestructure completely meets the requirement.