Roll forming article on using finite analysis to roll-form tubes.

This article appeared in The Fabricator.com, TPJ - Tube & Pipe Journal, dated October 9, 2003. TPJ - The Tube & Pipe Journal Using finite element analysis to roll-form tubes By Karan Shah, Yingyot Aue-u-lan, and Taylan Altan October 9, 2003
Roll forming is a common method for producing steel tubes. It is a continuous process in which a strip is guided through several sets of rolls that form the strip into the desired shape. After the final shape is achieved, tube edges are welded together to form a closed section. After the welding operation, the tube is sized through another set of rolls to obtain the required diameter.
Roll forming causes yield stress, flow stress, and hardness variations in material properties around a tube’s circumference. These variations may lead to premature bursting or excessive thinning in hydroformed parts. This makes it necessary to determine the effect roll forming has on a tube’s material properties to find the optimal process sequence and material selection. The quality of incoming roll-formed tubes is a major concern in a hydroforming operation, because variations in tube material properties, such as yield stress, flow stress, and hardness, can lead to premature bursting or excessive thinning during hydroforming. One previous study investigated the effect of roll forming by measuring with the hydraulic bulge test variations in the formability and flow stress of tubes at different locations around their circumference1 (see Figure 1). The study’s main objective was to develop a finite element method (FEM) model that could simulate tube roll forming with a 2-D FEM software package. The goal of the FEM was to obtain strain distribution around a tube’s circumference as induced by a roll forming process. In a similar study, it was found that plastic strain distribution around a tube’s circumference is not uniform.2 As a result, during hydroforming metal flow and excessive thinning may be concentrated in certain regions of the tube.	Figure 1 Roll forming causes yield stress, flow stress, and hardness variations in material properties around a tube’s circumference. These variations may lead to premature bursting or excessive thinning in hydroformed parts. This makes it necessary to determine the effect roll forming has on a tube’s material properties to find the optimal process sequence and material selection.

Software Simulation Even though roll forming is a continuous process, each pass can be investigated approximately with 2-D simulation by taking into consideration only transverse deformation and neglecting longitudinal deformation. Because the roll forming process is approximated by consecutive press brake operations, strain distribution around a tube’s circumference can be determined. The roll forming sequence used in the study consisted of 15 rolling passes.
The simulation model and the assumptions made can be summarized as follows: Each roll pass was considered as plane strain bending, in which upper and lower rolls are modeled as a punch and a die, respectively. Starting with a flat sheet, the section was formed by all the roll sets (punch and die) sequentially until the final section was obtained. The lower rolls were kept fixed, while the upper rolls were moved downward until the sheet was gripped between the rolls with a clearance equal to that found in actual roll forming.	Figure 2
Welding and sizing processes could not be included in the simulation. This means that the property changes caused by welding and sizing were not incorporated in the workpiece. Tube material flow stress was determined with the hydraulic bulge test using Krupkowsky’s law, σ = K(ε₀ + ε)ⁿ (see Figure 2).³ The final equation for flow stress was found to be σ = 1,095(0.06 + ε)^0.361 MPa (megapascals).

Estimating Strain Distribution Effective plastic strain around a tube’s circumference at the end of the 15th pass is shown in Figure 3. These strain values were obtained on a sheet’s midplane at the end of the 15th pass. The figure also shows that deformation was not uniform around the tube’s circumference. This can cause material properties such as flow stress and formability to be variable along the periphery. Figure 4 illustrates the deformation pattern of the sheet during the roll forming process.	Figure 3 These strain values were obtained on a sheet’s midplane at the end of the 15th pass and show that deformation was not uniform around the tube’s circumference. This can cause material properties such as flow stress and formability to be variable along the periphery.
Simulation of the roll forming process consisting of 15 rolling passes using 2-D FEM code required approximately 10 CPU hours. On average, simulation of each roll pass consumed 40 minutes of computation time, whereas simulation of a single bend angle on a channel section using 3-D FEM code can take up to 120 CPU hours.4
The simulation concluded that: Strain distribution around the circumference of a roll formed tube is not uniform, and for a given sheet thickness and material, the nonuniformity depends on the design of the roll forming sequence. Because of nonuniform strain, strain hardening, flow stress, and maximum elongation before fracture are nonuniform. Tubes to be used in a hydroforming application should be produced by a roll forming sequence that provides nearly uniform straining around the tube’s circumference. This study is preliminary and should be extended to compare predictions with measurements. It indicates that strain distribution can be estimated approximately, if the roll forming sequence is known, using 2-D FEA. Karan Shah and Yingyot Aue-u-lan are graduate student associates, and Dr. Taylan Altan is a professor and the director of ERC/NSM at The Ohio State University, 339 Baker Systems, 1971 Neil Ave., Columbus, OH 43210, 614-292-5063, fax 614-226-7219, altan.1@osu.edu, www.ercnsm.org.	Figure 4 Effective strain distribution of a sheet at the end of selected passes during a roll forming process. As the sheet moves from the first pass to the 15th pass, the gradual increment in strain and change in shape can be seen. Based on this analysis, a roll forming sequence should be designed that provides nearly uniform straining around the tube’s circumference.
Notes Yingyot Aue-u-lan, “Determination of Biaxial Formability and Flow Stress of Tubes for Low-Carbon Steel Tubes,” ERC Report THF/ERC/NSM-02-R-14, 2002. B.D. Carleer, “FE Process Simulation for Tube Hydroforming, Starting With the Tube Forming Process,” in proceedings from the International Conference on Hydrofoming, sponsored by the University of Stuttgart, Germany, Nov. 6-7, 2001. Yingyot Aue-u-lan,“Determination of Flow Stress and Formability of Tubes for Hydroforming,” ERC Report THF/ERC/NSM-00-R-11, 2000. Carl McClure, “Roll Forming Simulation Using Finite Element Analysis,” Manufacturing Review 8 (1995), p. 114. Go back.