Integral Hydro-Bulge Forming Method of Spherical Pressure Vessels Using a Triangle Patch Polyhedron

Abstract

This paper proposes an integral hydro-bulge forming (IHBF) method using a triangular patch polyhedron as the closed preform shell. When triangular flat parts are welded along the edges in sequence, triangular patch polyhedra are naturally formed. From the radius of the spherical pressure vessel, a design formula was derived to calculate the side lengths of the triangular flat plate parts. To verify the forming performance of the spherical pressure vessel using the IHBF method, the finite element method was carried out, and a stainless-steel spherical pressure vessel with a thickness of 1.0 mm and a diameter of approximately 500 mm was fabricated using the proposed IHBF method. As a result, the diameter forming error was 5.86%, the shape error expressed as roundness to diameter ratio was 0.48%, and the average plastic strain was 0.02, which was approximately 1/19 times of the forming limit strain of the material. The amount of springback after forming was approximately 0.7 mm, indicating that the amount of water required for IHBF was 5.90% of the volume of the spherical pressure vessel, while the required water pressure was less than 2.4 MPa. The process directly utilizes triangular flat plate parts, eliminating the need for molds to process closed preform shells resulting in a low average plastic strain during forming, thereby improving the quality of the formed spherical pressure vessels.

Mechanical Properties of Spherical Deep-Sea Pressure Structures by Integral Hydrobulge Forming with Triangular Patch Polyhedrons

“…Therefore, an IHBF processing method using a ball-shaped preforming box, as shown in Figure 2, was proposed and studied [26]. Furthermore, to reduce the amount of plastic deformation during IHBF processing and obtain a more uniform plate thickness distribution, a preformed box consisting of triangular patches obtained from a soccer-ball shape was devised [28]. [26,28].…”

“…Furthermore, to reduce the amount of plastic deformation during IHBF processing and obtain a more uniform plate thickness distribution, a preformed box consisting of triangular patches obtained from a soccer-ball shape was devised [28]. [26,28].…”

“…Figure 2 Soccer-ball-type and triangle-type preformed boxes using the IHBF method [26,28] As shown in Figure 2, if the radius of the spherical shell formed by the IHBF method is R, the side lengths, a, of the regular hexagon and pentagon of the soccer-ball-shaped preforming box can be calculated using the following formula [26]:…”

“…Furthermore, each regular hexagon and pentagon was divided into triangles. The side length b6 of an isosceles triangle divided by a regular hexagon and the side length b5 of an isosceles triangle divided by a regular pentagon were calculated using the following formulas [28]:…”

“…When utilizing the IHBF method, water pressure is applied inside the preformed box welded from the flat-plate parts to expand the preformed box into a spherical hull structure [23][24][25]. Using a preforming box as close to a spherical shape as possible and with a perfectly symmetrical water pressure acting inside, the wall material distribution of the expansion-molded spherical hull structure has the advantage of being relatively uniform [26][27][28].…”