TPT September 2009

using solid 8-node hexahedral elements. To simulate the action of the boost die, an axial pressure was applied directly to the end of the tube cross section. As shown in Figure 1, a half model was used instead of a whole model to take advantage of symmetry and thus reduce computation time. The maximum allowed boost load at different bending angles was determined using a series of FEA models, and the effect of the boost load schedule on bending results was also investigated.

Boost Load

Tube Detaching from Bend Die

Symmetric Plane

Boost Load

Bend Die

Rotation

 Figure 3 : Determination of the maximum final boost load

such as tube detachment from the bend die or buckling. At different bend angles the maximum allowed boost loads are different. For example, it becomes increasingly easier to detach the tube from the bend die as the bend progresses, so the boost load must be decreased during bending. The maximum allowable boost loads during the bending process were determined by a series of FEA models. An acceptable boost load schedule in terms of the bend die rotational angle that is predicted to minimise OD wall thinning is plotted in Figure 4. The relative boost load is defined as the ratio of the axial boost load pressure to the yield strength of the tube material, so that this schedule could be used for other steel grades that may have different yield strengths. Using the boost load schedule shown in Figure 4, two FEA models were simulated with two bend die geometries. One bend die is referenced as the original bend die, and the other is referred to as the

Pressure Die

Clamp Die

 Figure 1 : Finite element model setup of boost tube bending process To start the tube bending process, the head of the tube is forced against the bend die by the clamp die. Then the pressure die translates to contact the tube OD surface with a fixed amount of load applied. Once the position of the pressure die is locked, the bend die and the clamp die rotate to bend the tube. In this example, bending

processes with and without a boost load applied were simulated. The position of the pressure die is locked during the entire bending process.

start of bend

Bend Die

 Figure 2 : Ovality

calculation using cross section dimensions of a bent tube

end of bend

A typical cross section of a tube is shown in Figure 2. The ovality ratio, the OD wall thinning ratio, and the ID wall thickening ratio are the three important parameters to characterise the geometric quality of the bend. These three parameters are calculated using the following equations:

Relative Boost Load

Bending Angle (Degree)

 Figure 4 : Optimised boost schedule during tube bending

(1)

Ovality Ratio = Vertical OD – 1.0

Horizontal OD

 Figure 5 : Equivalent plastic strain distribution after bending

(2)

OD Wall Thinning Ratio = Original Wall Thickness – Final Wall Thickness

Original Wall Thickness

(3)

ID Wall Thickening Ratio = Final Wall Thickness – Original Wall Thickness

0°

Original Wall Thickness

Where vertical OD and horizontal OD are shown in Figure 2. For a tube bending process that uses a boost load, the magnitude and timing of the boost load affect the wall thinning ratio at the outside of the bend significantly. Higher boost load leads to less OD wall thinning and higher ID wall thickening. However, too much boost load may cause tube buckling, or the tube may detach from contacting the bend die, as shown in Figure 3. The maximum allowable boost load at a specific bend angle is defined as the highest load that can be applied without causing a bend defect,

180°

90°

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S eptember 2009

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