Length of neutral line after bending sheet metal

[HvanBaal]

Hello,

I'm looking for the right calculation method for calculating the length of the neutral line of a piece of sheet metal after bending it into a tube.
We use 304 and 316L stainless steel. I know it depents on a lot of properties (type, temp, etc). I'm looking for a methode that is as accurate as possible.

Thanks a lot...

 

[GBor]

Someone correct me if I'm wrong (admittedly I am not a metal bender), but I would think that the neutral line should remain the same length for a pure bending process. Shear deformation occurs about the neutral axis, right?

Garland E. Borowski, PE

 

[limct]

distance from the inner surface to the neutral plane, let say X, is a variable factor depending largely upon the ratio of the stock thickness T to the bend radius R, the optimum value may be considered to be

R<2T X=0.33T
R=2T to 4T X=0.4T
R>4T X=0.5T

Best regards,
ct

 

[eth]

By definition, the length of the neutral line does not change.

The location of the line must of course change - limct's post provides a good approximation of how much.

 

[BillPSU]

I agree with limct the neutral axis varies based on bend radius and thickness. In previous forming operations I used .46 as the general rule for the neutral axis. If you are dealing in sheet metal why don't you make the part on an open setup and check the length. Actual neutral axis is going to need to check the actual material thickness actual punch radius and process used. In production material thickness variations and die wear will change the length of the flat pattern. The process used will also change the length. Are you air bending the part or coining the part? If you are airbending the part the V-die width opening will change the effective radius being formed and the length of material being consumed. If you are coining the bend then how much you coint the material will change the length produced.