There are other formulas that are used to obtain this bend allowance also. Some are fairly accurate some are not. One in particular also helps to show that the material indeed stretches. It is called the Neutral line theory. The neutral line is an imaginary line that passes somewhere close to the middle of the material. It is the area where the material neither stretches nor compresses in the bend. If the material did not stretch at all then the neutral line would be at 50% of the material. I believe that there is more stretching than compressing occurring due to the fact that the neutral line is closer to the inside of the bend. Mild cold roll steel would be about 45 percent of the material thickness if you are air bending and 43 percent for bottom bending. The next three pages show you how to use the neutral line to figure bend allowance. In this manual we will use the empirical formula to obtain the bend allowance. Page:19
NEUTRAL LINE THEORYNote: 43% is usually used for CRS.Neutral line is closer to the inside bend.