The development of fracture mechanics has defined one.; Crack tip opening displacement; (Crack Tip Opening Displacement, CTOD), which can accurately evaluate the toughness of welded joints. In 1991, the British Welding Research Institute put forward the standard BS7448 Part 1, which gives the test methods of critical CTOD, J integral and KIC of metal materials. In 1997, the second part of the bid, BS7448 Part II, Method for Determining KIC, Critical CTOD and J-integral of Weld Metal Materials, was put forward. In view of the characteristics such as non-uniformity of performance and residual stress in each area of welded joints, BS7448 Part 1 was supplemented, which is generally recognized by the international engineering community, Test standard for determining the CTOD fracture toughness value of welded joints.

The non-metallic contraction joint meets the requirements for the economy of pipe system establishment and the reliability of safe operation, brings great convenience to the construction, operation and maintenance of pipe system establishment, and makes the domestic pipe system establishment process close to or catch up with the development of prosperous countries. The functional design of non-metallic contraction joints in each component part is as follows
1. The corners of non-metallic contraction joints form a rectangular cross section by large arc transition, and the stress degree of corner section will be relatively reduced, eliminating the dead angle of deformation and the overall average deformation of the product during online operation, thus greatly reducing the risk of damage caused by regional high stress and extending the service life of non-metallic contraction joints.
2. The non-metallic shrink joint reduces the thrust of the low pipeline system under the condition of not losing stability as a whole and adhering to the fundamental working stiffness, which improves the thermal compensation ability of the non-metallic shrink joint and reduces the cost of flexible parts in pipeline establishment.
3. Due to the compliance change of geometric shape, non-metallic contraction joints can work reliably in a benign force system under the action of external tensile stress, compressive stress and alternating stress, avoiding some stress risk areas. When working in the atmosphere containing H2SO, CO and H2 smoke, stress corrosion cracking (SEE) and corrosion fatigue cracking (CFF) are effectively improved Under the premise of not affecting the service life, the allowable number of thermal cycles of non-metallic shrink joints is improved.
4. When the non-metallic contraction joint is welded on the corrugated pipe, the weld can be far away from the arc tangent point. When the length of one side of the rectangle is less than 2M, the weld can be missed by one. The misalignment of the weld seam is greatly reduced. The error of corrugated pipe section shape and corrugated envelope length at any position of the product is very small. The corrugated forming and stretch bending forming are full of sophistication; It is consistent with the theoretical criterion of good maximum fatigue life of bellows.
5. The arc steel plate at the corner of the non-metallic contraction joint is welded and reinforced. The interface is made of channel steel, which strengthens the structural rigidity of the non-metallic contraction joint. The overall product line is fluent and the appearance is bright.