The elongation within the stretching deformation area of the outer side of the neutral layer does not exceed the material's allowable plastic deformation value, resulting in a fracture.

(2) The thin-walled structure section within the compressive deformation zone on the inside of the neutral layer, under the action of tangential compressive stress, will not exceed instability and wrinkle.

(3) When there is a requirement for ellipticity in the pipe fittings, control the distortion of the cross-section.

(4) When there is a requirement for the strength of the fitting to withstand internal pressure, control the forming limit of wall thinning.