During a tube bending process, a section of the tube becomes and outside bend also called an extrados, and a section directly opposite to it becomes an inside bend also called intrados. The original tubes in the outside bend gets elongated during this process and its thickness decreases based on the formula below. The original tubes in the inside section is compressed and its thickness increases.

t_ex = t * 2 * R / (2 * R + d)

t_ex - thickness of extrados
t - original tube thickness
d - tube diameter
R - centerline bending radius