| Accelerating the development of marine engineering equipment is the only way to build a strong marine nation in China.The shell is a closed shell structure to withstand the external pressure of seawater,as a key component and buoyancy unit,widely used in various underwater marine engineering equipment.The egg-shaped shell is a kind of thin-walled structure that meets the multi-focus and return curve of the positive Gaussian curve and has the advantage of the streamlined profile,which can achieve the deflection and resistance reduction without additional light shell,but the egg-shaped shell structure belongs to the multi-curvature positive Gaussian rotary shell,and the processing and manufacturing problem has not been solved.Therefore,in response to this problem,this thesis proposes a new shell-forming technology-free hydroforming technology,free hydroforming is a kind of internal pressure forming technology after the shell is filled with liquid medium without using molds and presses,which has the advantages of low cost,small difficulty,short cycle time and high-quality manufacturing,and is a kind of high-efficiency shell forming technology,so in this thesis,we will carry out a study on the free hydroforming characteristics of egg-shaped shells and the external pressure buckling characteristics after expansion to verify the expansion quality through a combination of theoretical,numerical and experimental methods.The main research elements of this thesis are as follows.(1)Based on the principle of closed-shell expansion,a segmented egg-shaped prefabricated blank structure is proposed;then,the general design and geometric analysis of the contour of the segmented egg-shaped blank are carried out according to the N-R equation of the ideal egg-shaped shell contour curve;finally,mechanical analysis of the free hydroforming of the egg-shaped shell is carried out in combination with the research results of the egg-shaped shell at a water depth of 4km to lay a theoretical foundation for the free hydroforming of the egg-shaped shell.(2)A numerical method was used to study the free hydroforming of egg-shaped shells.Firstly,the mathematical model of the egg-shaped prefabricated blank corresponding to the egg-shaped shell at a water depth of 4km is determined and the mesh convergence analysis is carried out;then the general static stress mechanical analysis is carried out,the expansion process is simulated and the effects of the number of segments,wall thickness and weld form on the expansion are compared respectively;finally,the changes of geometric parameters(wall thickness and shape)before and after the expansion are studied,the accuracy and quality of the free hydroforming of the egg-shaped shell are analyzed and a(2)The numerical analysis method of free hydroforming of egg-shaped shells is established.(3)To investigate the free hydroforming of egg-shaped shells utilizing scale model tests.Firstly,a scale model of prefabricated egg-shaped blanks of different sizes was machined and geometrical parameters such as contour scanning and thickness testing were measured,followed by an expansion test.Finally,the changes in geometrical parameters(wall thickness and shape)before and after expansion were analyzed and compared with the numerical analysis results to verify the feasibility of free hydroforming technology for egg-shaped shells.(4)Analytical,numerical and experimental methods were used to study the flexural characteristics of the egg-shaped shells without mold expansion.Firstly,the theoretical equations for the linear elastic buckling of expanded eggshells are derived;then,the linear elastic and elastoplastic buckling characteristics are studied numerically based on the mathematical model of mold-free expanded eggshells;finally,hydrostatic pressure tests are carried out on the proportional model of expanded eggshells,and the results are consistent with the numerical analysis,and the law between the expanded load and the buckling of expanded eggshells is derived. |
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