The length of the section is L. When the section is rotated about the center axis (see Figure 3A), the process stretches the top and bottom edges; the increase in length is ∆D. When it is rotated about the bottom axis, the top edge elongates ∆B. For the same rotational angle,
∆B ≈ 2∆C
Springback Versus Permanent Deformation. For low-carbon steel, the elastic elongation ratio, ∆/L, is less than 0.2 percent.3 In other words, any section of low-carbon steel elongated less than 0.2 percent will spring back when it exits the forming rolls; the deformation isn't permanent. This is the goal in longitudinal elongation. If it exceeds the material's elastic limit, it will develop permanent flaws—it will buckle, bow, twist, or wrinkle. Keep the longitudinal elongation ratio less than 0.2 percent to prevent defects.
Keep in mind that rotating a section about the center axis reduces the amount of elongation, helping to prevent forming flaws.
