The elongation deformation within the stretch deformation area on the outer side of the neutral layer does not exceed the material's allowable plastic deformation value, thus avoiding fracture.

(2) The thin-walled structural part within the compression deformation zone on the inside of the neutral layer, subjected to tangential compressive stress, will not exceed instability and wrinkle.

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

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