Influence of the internal pressure on the change of the wall thickness in the case of tubes subjected to bending.

Ceclan, Vasile Adrian ; Achimas, Gheorghe ; Lazarescu, Lucian 等

Abstract: The paper presents a study on the influence of the internal pressure on the change of the wall thickness in the case of tube bending. Cold bending of the metallic tubes is an important production method due to the fact that such parts are widely used in a great variety of industrial fields, such as automobiles, aircrafts, air conditioners, air compressors, exhausting systems, fluid lines, etc.

Key words: Tube bending, internal pressure, wall thickness, finite element analysis

1. INTRODUCTION

One of the most troublesome problems that are facing the tube production is the change of the wall thickness (Jin, 2001).

In our previous works (Achimas, 2005) and (Lazarescu, 2005), we have developed a finite element model for the simulation of the rotary draw bending process. This finite element approach can be used to optimize the product, the tool design and the bending parameters. In the literature there are a few papers which use the finite element simulation for studying the tube bending process. The finite element model, developed in ABAQUS/Explicit, was used to study the influence of the bending radius on the wall thickness change both in case of applying a pressure inside the tube and in case of bending without pressure.

2. PRINCIPLE OF PRESS BENDING

The device used during the manufacturing consists of the rolls 2 and 3 which are fixed on the table of a press, while roll 1 is attached to the ram of the same press (Fig 1).

[FIGURE 1 OMITTED]

1-upper roll; 2, 3-lower rolls; 4-tube (semi-processed); 20-bending angle; h-displacement of the upper roll during the bending; R-bending radius.

In order to produce the bending, the tube 4 is placed on the rolls 2 and 3, while the roll 1 moves vertically against the tube causing its deformation. The bending radius, as well as the bending angle are controlled using the dimensions x and h. The distance between rolls 2 and 3 can be adjusted by moving them horizontally. The three rolls are placed on a surface with a channel that comes into contact with the tube. The channel dimensions are proportional to the diameter of the tube.

The advantage of this procedure is that it can produce different bending radii without changing the rolls, because the bending radius is not dependant on the radius of the roll, but on the position of the three rolls.

On the other hand, the disadvantage of this procedure consists in the fact that the bending radius is determined indirectly, using the parameters h and x, and it is a little more difficult to be adjusted.

3. FINITE ELEMENT MODELING OF THE PRESS BENDING

A finite element model of the press bending was developed in ABAQUS/CAE as shown in figures 2 and 3.

The tube was modeled as a 3D deformable part made from a material having an elastic-plastic behavior (Hill, 1998), while the tools were modeled as 3D discrete rigid bodies. Shell elements S4R were used to mesh the tube.

The contact associated to various pairs of surfaces (bending die-tube, pressure die-tube, wiper die-tube) is defined using *CONTACT_SURFACE_TO SURFACE option, which allows sliding between these surfaces with a Coulomb friction model. The friction coefficient was chosen 0.1.

The tube material was a rolled steel OLT 35 (STAS 8183-87). The mechanical properties of the tube material were determined by tensile tests performed on straight tubular specimens. The stress level was derived from the axial load force and the instantaneous geometry of the tube.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. EXPERIMENTAL RESEARCH

During the experiments, we used test-tubes made of OLT35, having a length of 280 mm and a wall thickness t = 2.7 mm (the test-tubes most used in practice). In order to observe the differences that might occur during the bending of the pipes, we used different bending angles (60[degrees], 80[degrees] and 100[degrees]). The trials were done on a hydraulic press (Fig. 4).

5. RESULTS

5.1 Results of thee experimental research

After the bending operations were finished, the test-tubes were cut in the critical section, and then the thickness of the walls was measured. The measurement of the bent tubes was performed using a 3D scanner CYCLONE 2 as shown in figure 5. Scanning is a method of acquiring data about an unknown 2D contour or 3D surface. The data obtained can be used to create NC programs as well as CAD files. This machine can work by mechanical palpation, analogical scanning, laser or video scanning.

[FIGURE 5 OMITTED]

5.2 Results of the finite element simulation

Figure 6 presents the distribution of the wall thickness of the tube bent at the angle 20 = 25[degrees] with internal pressure, as predicted by the finite element model.

[FIGURE 6 OMITTED]

Figure 7 shows the comparison between experiments and the results of the numerical simulation.

[FIGURE 7 OMITTED]

6. CONCLUSIONS

The aim of this paper was to study the influence of the internal pressure on the wall thickness and the comparison between experimental data and the finite element simulation.

A finite element model was developed for the simulation of the press bending of tubes. Using the finite element model, the bending with or without internal pressure was studied. It was observed that, if a pressure acts inside the tube, the deformation of the wall thickness is smaller than in the case of the bending without pressure.

After obtaining the experimental results, they have been compared with the predictions of the simulation model.

7. REFERENCES

Achimas Gh. Ceclan V.A. Lazarescu L. Groze. F. (2007): Experimental Research Concerning the Influence of the Bending Radius on the Wall Thickness of the Bent Pipes microCAD pp. 7-11 ISBN 978-963-661-753-0

Achimas, Gh., Crisan, L., Grozav, S., Lazarescu, L. (2005):Quality Assurance of the Bent tubes Using finite Element Simulation, 3-rd International Congress on Precisition Machining (ICPM),Austria, Vienna, pp. 43-48 ISBN 3-901-888-31-4

Hill, R. (1998). The Mathematical Theory of Plasticity, Oxford University Press, ISBN 0198503679, Oxford

Jin, Z.; Luo, S.; Fang. X. D. (2001).: KBS-Aided design of tube bending processes, Engineering application of artificial intelligence, Vol. 14, PP. 599-606)

Kuhn, R. (1981): Querschnittsanderung am gebogengen Rohr, Umformtechnik UT 15 6, Seite1-7

Lazarescu, L., Achimas, GH., Ogneanu, D., Groze, F. (2006),: Studies on wall thickness change of bent tubes using Finite Element Simulation International Conference Modern technologies, quality, restructur-ing, P.527-534, ISSN 1011-2855

Lee, H., VAN Tyne, C.J., Field, D. (2005): Finite element bending analysis of oval tubes using rotary draw bender for hydroforming applications, Journal of Materials Processing Technology 168 327-335

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