The extrados is stretched and thins, the intrados is compressed and wrinkles, and insufficient tool support under bending moment lets the section ovalize. Risk is governed mainly by centerline-radius/diameter (CLR/D), diameter/thickness (D/t), ductility, weld-seam orientation and tooling support.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
RISK SCREEN
CLR≈1.5D is a common manufacturability starting point; near 1.0D is a tight bend. When D/t exceeds roughly 25–30, discuss mandrels, a wiper die and multi-ball support early. These ranges are not acceptance criteria for pressure or fatigue service. General frames may begin discussion around 5–8% ovality and 10–15% wall thinning, but pressure, safety or regulated parts require code-specific limits and testing.
MEASURE AND CONTROL
Ovality may be calculated as (D_max−D_min)/D_nom×100%; thinning as (t_0−t_min)/t_0×100%. Measure mid-bend and tangent transition zones, not one section only. Controls include larger CLR, thicker wall, lubrication, mandrel/wiper tooling, clamp pressure, pressure-die assist and controlled ERW seam orientation. Section or ultrasonically measure the first part’s minimum wall and record springback/end growth.
PROJECT CHECKLIST
D, t and CLR; bend angle; tangent grip length; material/seam; ovality/thinning limits; inner/outer surface; downstream assembly gauge.