The elongation deformation within the stretching deformation area of the outer side of the neutral layer does not exceed the material's allowable plastic deformation value and causes cracking.

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

(3) If there is a requirement for ellipticity in the piping 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.