(1) The elongation deformation within the outer stretching deformation zone of the neutral layer does not exceed the material's plastic allowable value, resulting in fracturing.

(2) Within the compression deformation zone of the neutral layer, the thin-walled structural part subjected to tangential compressive stress will 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 piping fittings have a strength requirement to withstand internal pressure, control the forming limit of wall thinning.