(1) The elongation deformation within the stretching deformation zone on the outer side of the neutral layer does not exceed the material's plastic allowable value, thus preventing rupture.

(2) Within the compression deformation zone of the neutral layer, the thin-walled structural parts subjected to tangential compressive stress will not exceed instability and wrinkle.

(3) If there are requirements for ellipticity in the pipe fittings, control the distortion of the cross-section.

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