In sheet-metal bending, material on the outside of the bend stretches while material on the inside compresses. Between them lies a neutral axis whose longitudinal strain is approximately zero.

A flat blank must contain enough material to become the final bend. The length assigned to the curved region is the bend allowance. A common engineering approximation is

$$BA=\theta(R+Kt),$$

where $\theta$ is bend angle in radians, $R$ is inside bend radius, $t$ is sheet thickness and $K$ locates the neutral axis through the thickness.

For a $90^\circ$ bend, $R=3$ mm, $t=2$ mm and $K=0.4$,

$$BA=\frac{\pi}{2}(3+0.4\times2) \approx5.97\ \mathrm{mm}.$$

After the forming load is removed, elastic strain recovers. This springback makes the final angle and radius differ from the fully loaded tool geometry. Higher elastic-to-plastic recovery generally means more springback.

Manufacturing compensates through tool geometry, overbending or process control rather than assuming that the unloaded sheet exactly reproduces the die position.

Small bend radii also create high outer-fibre strain and may cause cracking. Bend design therefore couples sheet thickness, material ductility, bend direction, tooling radius and required final geometry.