The elongation deformation within the stretching deformation area of the neutral layer does not exceed the material's plastic allowable value, thereby causing rupture.

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

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

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