The bending machine processes pipe materials in a similar manner to sheet material bending. In pure bending, a pipe with an outer diameter of D and a wall thickness of t, when subjected to an external torque M, the outer wall of the neutral layer experiences tensile stress σ1, causing the wall to thin. Conversely, the inner wall of the neutral layer is subjected to tensile stress σ1, resulting in the wall thickening. Moreover, the cross-sectional shape, under the combined forces of F1 and F2, transitions from circular to an approximate ellipse. When the deformation is excessive, the outer wall may develop cracks, and the inner wall may show皱纹.