Stress Characteristic Study On Local Thinning Area In The Boundary Stress Region

In view of the local thinning problem in boundary region of pressure vessels, local stress fields are analyzed utilizing FEA method. Two typical kinds of boundary region are considered: one is the nozzle-cylinder junction (Model I) and another is head-cylinder junction (Model II). The local thinning area is simplified as an elliptic-sphere-segment-like pit in the analysis, and the influences of pit sizes and their location on surface stress distribution and stress concentration factor are discussed. The results indicate that, for Model I, the stress level on pit surface increases with the increase of the major axis length and the depth of pit, decreases with increase of the distance from junction line; for Model II, the stress level on pit surface increases with depth, and decreases with the increase of the distance from junction line and the major axis length of pit.Based on analysis of stress concentration factor on pit surface, it can be concluded that pit slope (C/A), pit relative thickness (C/T) and pit relative position (D/Ln, pit major axis is parallel or perpendicular to cylinder axial, pit is located on inside or outside surface) are primary influential factors. Formulae of stress concentration factor for the two typical kinds of boundary region are presented. The law is also discussed in this paper that how the membrane stress, the membrane stress plus bending stress and the limit load are influenced by the dimension and relative position of pit. According to the result of the limit load analysis, the G0 criteria presented in Article 40 of"Pressure Vessel Periodical Inspection Regulation" is discussed in the boundary stress region, and suggested if G0 is less than 0.026, the pit in boundary stress region is acceptable. This conclusion lays the foundation for the further revision of the applicable scope of G0.