Brittle Fracture Initiation in Line Pipes Estimated from Surface Crack COD

Abstract

The brittle fracture initiation from the surface crack in the large diameter pipe subjected to inner pressure is treated from the practical point of view. The crack opening stretch model is applied to the surface crack and the ligament of the crack is subjected for the crack closure force.The hydrostatic tests are carried out for two kinds of pipes. One is a heavy wall and large diameter pipe, API 5 L X 70 grade, 940 mm in outer diameter and 39. 2 mm in wall thickness. Another is a relatively thin wall and small diameter pipe, API 5 L X 65 grade, 508 mm in outer diameter and 12. 7 mm in wall thickness. The test pipes have various notches, which are ma. chined by electrical discharge. And the crack closure force is experimentally determined from the notch tip COD, which is directly measured by the clip gage installed on the center of the notch tip, by use of the Dugdale Model with regard to the bulging factor for the pipes.Consequently crack tip COD of the surface crack center is calculated by the proposed crack opening stretch model. And corresponding to the defect size, the required fracture toughness to prevent the brittle fracture initiation from the surface crack of the pipes subjected to inner pressure is estimated.