TPT November 2009

2.1.5 Use a piece of sheet material to replace a whole coil of sheet material A whole coil of sheet material is used in actual production. In order to reduce computational complexity, use a piece of sheet material to simulate forming with a whole coil of sheet material. In order to guarantee stable forming between passes, length of sheet material for simulation should not be less than 1.5 times the distance between passes. In order to avoid influence caused by errors at start of work piece and end of sheet, the middle 1 / 3 part of simulation Since round tube is symmetric, only half of it is simulated. The centre distance between adjacent two passes is 100mm, length of sheet material is 150mm, and sheet width is calculated according to various diameters and sheet thickness. Divide the forming body into 20 equal parts horizontally (direction X), and three layers along direction of thickness (direction Y), and 150 parts along the direction of feeding (direction Z), total 9,000 units. sheet material is selected during analysis. 2.1.6 Partition of sheet material units  Figure 4 : The co-ordinate used for modelling The element type seven (an eight-node, isoparametric, arbitrary hexahedral) is chosen. The equivalence simplifies and reduces computational complexity of contacting search during the computational process. 2.2 Definition of geometrical features Since stress, strain and displacement changing of sheet material in X, Y and Z directions during roll forming are considered normally, it can be defined as a 3D problem. Here 3D solid units are adopted. Because there is lack of shearing behaviour description, in order to remedy it, the method of assumed strain is used. By using selected interpolation to improve describing capacity of shearing (bending) behaviour. Since volume of material before and after distortion is constant, choose the item of constant dilatation. 2.3 Definition of material characteristics During this simulation, sheet material is Q235 with Young’s Modulus of 210Gpa, Poisson’s Ratio is 0.3. 2.4 Set boundary conditions An important content during simulation modelling is definition of boundary conditions. Fix longitudinal displacement of the sheet first, apply direction Z fixed displacement at both the front and rear of the sheet and try to avoid applying any constraint at the node of distorting area. Since only half of symmetrical simulation is analysed, apply lateral constraints on the symmetry axis. Direction X displacement constraints. In addition, a single sheet with a certain length is used during simulation, while it is coil continuous forming during actual production, so apply proper constraints vertically to prevent the swinging of work piece and sheet end. Apply certain direction Y displacement constraints to avoid too large displacement in vertical direction. During direction Y constraints applying, pay attention to the positions, which must be nodes without vertical displacement changing before and after forming.

 Figure 3 : An example forming roll flower of 12mm x 1.8mm

The example for 12mm x 1.8mm (outside diameter x thickness) tube flower pattern is shown in Fig.3

2 Pre-processing of finite element modelling (FEM) 2.1 Geometric modelling The 3D finite element modelling used for ERW tube forming in this example includes rolls and strip coil. The roll drawing is generated in COPRA in AutoCAD format with extensions of DWG or DXF. Before being imported into Marc, the roll drawings are simplified, leaving only the outer contour line and axis, and deleting the other information. For the symmetric rolls drawing, in order to reduce the time of computing, only half of it is for simulation. The rolls DWG files are converted into DXF (AutoCAD2000) format, then rolls outline drawings are imported into Marc, and the rolls contour lines are rotated around the shaft axis in Marc to form roll surface. 2.1.1 Idealising rollforming machine In order to simplify the process of roll forming, assume that the roll forming machine is ideal for the purpose intended: during numerical simulation, assume rigidity of the roll forming machine is just enough and do not consider deflection caused by the force applied on the shaft. Also assume positioning of rolls to be correct, ignore errors from installation and assume sheet material for forming is also ideal. Finally, ignore thickness errors of coil. 2.1.2 Rolls are rigid bodies Since rolls material is die tolling steel with little distortion during forming, the rolls should be regarded as rigid bodies. 2.1.3 Relative motion equivalent During actual production, the rolls rotate to drive sheet material passing the roll-forming machine. According to the principle of relative motion, it is equivalent to the rolls slide static sheet material. Static sheet material is convenient for applying boundary conditions and simplifies distortion analysis. 2.1.4 Ignore friction The reasons: during forming, force of friction drives sheet material forward, so there is little effect for forming. If friction is ignored, computational complexity is obviously reduced, and the result is easy convergence. In situations of small diameter thick-wall tube, no obvious simulation analysis precision error is found by comparing results considered friction and ignored friction.

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N ovember 2009

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