A method for designing the ultimate path of three-dimensional strain forming for high-pressure bulging in pipes

Abstract

This invention discloses a method for designing the three-dimensional strain forming limit path of high-pressure bulging in pipes, comprising the following steps: gridding is applied to the surface of all pipe test pieces; the pipe test pieces are placed in a mold; rubber is placed inside the pipe test pieces, and high-pressure bulging is used to induce a concentrated instability state in the pipe test pieces, causing the portion of the pipe test pieces near the insert mold to burst under high pressure; after bulging, the degree of deformation in the thickness direction is obtained by measuring the change of the grid at the fracture point of the pipe test pieces; the first principal strain and the second principal strain are obtained by measuring pipe test pieces with different structures; multiple forming limit curves are plotted according to the different structures of the pipe test pieces, and the curves are connected to form a surface to obtain a three-dimensional forming limit diagram. This invention fills the gap in the three-dimensional strain forming limit diagram of high-pressure forming of rubber, maintains the integrity of the three-dimensional strain limit of high-pressure bulging of rubber in pipes, and is safe, convenient, efficient, requires fewer molds, and has low cost.

Description

Technical Field

[0001] This invention relates to a method for designing high-pressure bulging paths inside pipes, and more particularly to a method for designing the three-dimensional strain forming limit path for high-pressure bulging inside pipes. Background Art

[0002] Internal high-pressure bulging is an advanced forming technology that utilizes the elasticity and plasticity of rubber to expand tubing outwards along its thickness under high pressure. With its widespread application, this technology has reduced experimental costs and technical difficulty, while improving forming efficiency and performance. Rubber high-pressure forming uses rubber to replace some traditional rigid molds for tubing, leveraging its high elasticity and viscoelasticity to achieve reusability, weight reduction, and material savings. It also offers numerous advantages such as high part strength, high material utilization, high processing accuracy, and ease of processing complex parts, and is currently widely used in the automotive, aircraft, and aerospace industries. Traditionally, high-pressure forming is considered a bulging process, with all parts of the tubing in a state of expansion-expansion. However, for complex parts, both expansion-expansion and expansion-compression states exist. Traditional bulging experiments only display the forming limit points on a two-dimensional diagram, failing to analyze the material's forming limits from multiple dimensions, thus lacking a reliable basis for judging the forming limits of more complex parts. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to propose a three-dimensional strain forming limit path design method for high-pressure bulging inside pipes, so as to realize the three-dimensional path design for high-pressure bulging inside pipes.

[0004] Technical solution: This invention includes the following steps:

[0005] Step 1: Apply a grid to the surface of all pipe test pieces;

[0006] Step 2: Place the pipe test piece into the mold;

[0007] Step 3: Place the rubber into the pipe test piece and use high pressure to expand the pipe test piece to reach a concentrated unstable state, so that the part of the pipe test piece near the insert mold bursts under high pressure.

[0008] Step 4: After the bulging is completed, the degree of deformation in the thickness direction is obtained by measuring the change in the grid at the fracture point of the pipe test piece;

[0009] Step 5: Measure the first principal strain and the second principal strain of pipe test specimens with different structures;

[0010] Step 6: Draw multiple forming limit curves according to the different structures of the pipe test pieces, connect the curves to form a surface, and obtain a three-dimensional forming limit diagram.

[0011] The mold includes a left mold and a right mold, and a block mold is installed on the lower part of the right mold.

[0012] The pipe test specimens include pipes without pre-fabricated notches and pipes with different pre-fabricated notches.

[0013] The opening of the insert mold is circular or elliptical.

[0014] The pipe test piece without pre-fabricated notches and the insert mold with an elliptical opening shape are grouped together; the pipe test piece with different pre-fabricated notches and the insert mold with a circular opening shape are grouped together.

[0015] When the elliptical-shaped insert mold is paired with a pipe test piece without pre-made notches, the first principal strain and the second principal strain are both measured to be in an expansion-expansion state.

[0016] When the circular-opening insert mold is paired with a pipe test piece containing different pre-made notches, the first principal strain and the second principal strain in the forming limit diagram are measured to be in the expansion-compression state, respectively.

[0017] The rubber surface is coated with lubricating oil to reduce the coefficient of friction with the pipe.

[0018] Beneficial effects: This invention fills the gap in the three-dimensional strain forming limit diagram of rubber high-pressure forming and maintains the integrity of the three-dimensional strain limit of rubber high-pressure bulging in pipes; in this bulging experiment, the left and right molds and the insert mold ensure that different pipe test pieces are fully compressed during the bulging process, ensuring uniform gap and promoting full deformation of the pipes; this experiment has the advantages of safe and convenient operation, high efficiency, fewer molds, and low cost. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0020] Figure 2 This is a schematic diagram of the insert mold with an elliptical opening in this invention;

[0021] Figure 3 This is a schematic diagram of a pipe test piece containing different pre-fabricated notches according to the present invention. Detailed Implementation

[0022] The invention will now be further described with reference to the accompanying drawings.

[0023] like Figure 1 As shown, the present invention includes a left mold 1 and a right mold 2. A block mold 3 is connected to the lower part of the right mold 2. The structure of the block mold 3 is as follows: Figure 2As shown, a cross-shaped structure is adopted, with a circular or elliptical hole in the center. The diameter of the circular hole is 2 / 3 times the diameter of the pipe test piece. The insert mold 3 is bolted to the right mold 2, and the right mold 2 is bolted to the left mold 1. The pipe test piece 4 is placed between the right mold 2 and the left mold 1, with its bottom edge aligned with the bottom edge of the mold. The radius of the pipe test piece 4 is R. Rubber 5 is placed inside the pipe test piece 4. High-pressure forming of the pipe is achieved by applying a load along the thickness direction of the inner wall of the pipe using rubber 5, ensuring that the pipe test piece reaches a rupture state. By measuring pipes with different structures, the first principal strain, the second principal strain, and the thickness strain are obtained, thereby obtaining three-dimensional strain forming limit diagrams for all types of pipes.

[0024] Pipe test specimens are divided into four categories: those without pre-fabricated notches and those with different pre-fabricated notches, such as... Figure 3 The diagram shows pipe test pieces 4 with different pre-fabricated notches. In this embodiment, five different pre-fabricated notches are used, with notch curvatures of 10°, 30°, 50°, 70°, and 90°. Complete pipe test pieces 4 without pre-fabricated notches are grouped together with elliptical-shaped insert molds 3; pipe test pieces 4 with different pre-fabricated notches are grouped together with circular-shaped insert molds 3 for later use. When elliptical-shaped insert molds 3 are used with pipe test pieces 4 without pre-fabricated notches, both the first and second principal strains are measured as expansion-expansion states. When circular-shaped insert molds 3 are used with pipe test pieces 4 with different pre-fabricated notches, due to the different widths of the pre-fabricated notches, the first and second principal strains in the forming limit diagram are measured as expansion-compression states. From this data, a two-dimensional pipe forming limit diagram can be plotted. After the bulging is completed, the degree of deformation in the thickness direction can be obtained by measuring the change of the grid at the fracture point of the pipe test piece 4. Thus, a complete three-dimensional strain forming limit diagram of the pipe can be drawn. By connecting the curves into a surface, the forming limit diagram in the three-dimensional coordinate system can be obtained.

[0025] In a three-dimensional coordinate system, the quadrant bounded by the positive directions of the first principal strain, the positive directions of the second principal strain, and the negative direction of the thickness strain is designated as quadrant I; the quadrant bounded by the positive directions of the first principal strain, the negative directions of the second principal strain, and the negative direction of the thickness strain is designated as quadrant II. When a set of tests is conducted using an elliptical-opening insert mold and a pipe test piece without pre-fabricated notches, the curve for the quadrant I can be plotted, representing the expansion-expansion state; when a set of tests is conducted using a circular-opening insert mold and pipe test pieces with different pre-fabricated notches, the curve for the quadrant II can be plotted, representing the expansion-compression state.

[0026] The method for designing the limit path of high-pressure bulging three-dimensional strain forming in pipes according to the present invention specifically includes the following steps:

[0027] Step 1: Use an electro-corrosion marking machine to mark a grid on the surface of each pipe test piece 4 so that the deformation in the thickness direction can be analyzed after the bulging is completed;

[0028] Step 2: Place the pipe test piece 4 into the mold; stack the left mold 1 and the right mold 2 together and connect them with bolts; connect the right mold 2 and the insert mold 3 with bolts, align the bottom end of the pipe with the bottom edge of the mold, fix the pipe, and ensure that the gap is uniform so that it can deform fully; apply lubricating oil evenly to the rubber for later use to reduce the coefficient of friction between it and the pipe.

[0029] Step 3: Slowly place the rubber into the pipe test piece 4, place the assembled test piece on the ultra-high pressure hydraulic press, align the pressure head with the center of the top of the rubber, and use high pressure to make the pipe test piece 4 reach a concentrated instability state, so that the part of the pipe test piece 4 near the insert mold 3 bursts under high pressure.

[0030] Step 4: When using an elliptical-shaped insert mold 3 and a pipe test piece 4 without pre-made notches as a set, the first principal strain and the second principal strain can be measured to be in an expansion-expansion state; when using a circular-shaped insert mold 3 and a pipe test piece 4 with different pre-made notches as a set, due to the different widths of the pre-made notches, the first principal strain and the second principal strain in the forming limit diagram are measured to be in an expansion-compression state, respectively. From this data, a two-dimensional pipe forming limit diagram can be drawn.

[0031] Step 5: After the bulging is completed, the degree of deformation in the thickness direction can be obtained by measuring the change in the grid at the fracture point of the pipe test piece;

[0032] Step 6: Based on the different structures of the pipe test pieces, multiple forming limit curves can be drawn. By connecting the curves into a surface, a forming limit diagram in a three-dimensional coordinate system can be obtained.

[0033] This invention obtains several sets of data on the first principal strain, the second principal strain, and the thickness strain by measuring combinations of pipes with different pre-formed notches and insert molds, thereby creating a complete three-dimensional strain forming limit diagram of the pipe. It is applicable to high-pressure bulging forming limit testing of pipes with a thickness of 0.5-4mm in the field of metal plastic forming of aluminum alloys, stainless steel, and other materials.

Claims

1. A method for designing the limit path of three-dimensional strain forming for high-pressure bulging inside pipes, characterized in that, Includes the following steps: Step 1: Apply a grid to the surface of all pipe test pieces; Step 2: Place the pipe test piece into the mold; Step 3: Place the rubber into the pipe test piece and use high pressure to expand the pipe test piece to reach a concentrated unstable state, so that the part of the pipe test piece near the insert mold bursts under high pressure. Step 4: After the bulging is completed, the degree of deformation in the thickness direction is obtained by measuring the change in the grid at the fracture point of the pipe test piece; Step 5: Measure the first principal strain and the second principal strain of pipe test specimens with different structures. The pipe test specimens include pipe test specimens without prefabricated notches and pipe test specimens with different prefabricated notches. Step 6: Draw multiple forming limit curves according to the different structures of the pipe test pieces, connect the curves to form a surface, and obtain a three-dimensional forming limit diagram. In the three-dimensional coordinate system of the three-dimensional forming limit diagram, the quadrant bounded by the positive direction of the first principal strain, the positive direction of the second principal strain, and the negative direction of the thickness strain is the first quadrant; the quadrant bounded by the positive direction of the first principal strain, the negative direction of the second principal strain, and the negative direction of the thickness strain is the second quadrant.

2. The method for designing the limit path of three-dimensional strain forming under high pressure in pipes according to claim 1, characterized in that, The mold includes a left mold and a right mold, and a block mold is installed on the lower part of the right mold.

3. The method for designing the limit path of three-dimensional strain forming under high pressure in pipes according to claim 1, characterized in that, The opening of the insert mold is circular or elliptical.

4. The method for designing the limit path of three-dimensional strain forming for high-pressure bulging inside a pipe according to claim 3, characterized in that, The pipe test piece without pre-fabricated notches and the insert mold with an elliptical opening shape are grouped together; the pipe test piece with different pre-fabricated notches and the insert mold with a circular opening shape are grouped together.

5. The method for designing the limit path of three-dimensional strain forming for high-pressure bulging inside a pipe according to claim 4, characterized in that, When the elliptical-shaped insert mold is paired with a pipe test piece without pre-made notches, the measured first principal strain and second principal strain are both in an expansion-expansion state.

6. The method for designing the limit path of three-dimensional strain forming for high-pressure bulging inside a pipe according to claim 4, characterized in that, When the circular-opening insert mold is paired with a pipe test piece containing different pre-made notches, the first principal strain and the second principal strain in the forming limit diagram are measured to be in the expansion-compression state, respectively.

7. The method for designing the limit path of three-dimensional strain forming for high-pressure bulging inside a pipe according to claim 1, characterized in that, The rubber surface is coated with lubricating oil.

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