Frequently Asked Question

The minimum practical inside radius depends on alloy, temper, thickness, ductility, surface condition, grain direction, and forming method. Bending parallel to the rolling direction can increase cracking risk in some materials because deformation is anisotropic.

Use supplier data and a bend test to establish a design rule. Avoid specifying a sharp radius merely to reduce nominal size; a larger radius can improve repeatability and fatigue performance.

Outer-fibre strain

Cracking risk is driven by tensile strain at the outside of the bend, material ductility, surface damage, thickness, inside radius, grain direction, and forming method. A smaller radius-to-thickness ratio increases outer-fibre strain. Rolling makes sheet properties anisotropic, so bend orientation can change both cracking risk and angle recovery.

ε_outer is a screening estimate of outer-fibre strain, T is thickness, and R is inside radius. It is not a forming-limit curve and should not replace supplier data or a bend test.

Worked example

For T = 2 mm and R = 2 mm, the estimate is 2/[2(2+1)] = 0.333, or 33%. Increasing R to 4 mm gives 2/[2(4+1)] = 0.20, or 20%. The comparison shows why a larger radius reduces strain, but the final minimum radius must also consider alloy, temper, surface quality, and grain orientation.

Control: establish the rule with material data and representative coupons, then inspect the outside radius for cracks under suitable lighting and magnification.

Engineering check

For Minimum Bend Radius, Grain Direction, and Cracking, record the material condition, tool or fixture identity, process settings, measurement method, sampling plan, and reaction rule. Confirm that the measurement system is capable of resolving the requirement and that the process is stable before using capability or first-article data to justify release. Reconfirm the limits after a material, tool, or route change.