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

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

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

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