Direct measurement of the resistance to ductile fracture propagation

Abstract

The resistance to ductile fracture propagation remains a weak point in gas transmission pipelines’ safety since the emerging of this risk more than half a century ago. Full-scale burst tests of pressurized gas pipeline sections are performed to verify the ability of pipeline steel to arrest the crack propagation. However, motivated by the commitment to specific predictive models, the main outcome from full-scale burst tests (FSBT) has been in form of a binary “propagation/arrest” metric corresponding to each pipe in the test section. In the absence of a reliable laboratory-scale test demonstrating correlation with the full-scale resistance to ductile fracture propagation, such approach does not appear reasonable since it leaves line pipe manufacturers with very little clue regarding the preferential metallurgical designs. This work aims to better quantify the crack propagation resistance in FSBT providing metallurgists with a target property and facilitating the development of a new laboratory-scale mechanical test that shows reasonable correlation with FSBT. In order to improve FSBT testing methodologies, the pipe wall strain related to the crack propagation should be assessed along the entire test line. This can be implemented by means of modern 3D-measurement technologies that can digitize very large objects with high accuracy at a reasonably low cost. The method is demonstrated in the current work using the results of a recent FSBT carried out on X80 grade pipes. The results indicated that the plastic strain zone in FSBT can spread from the fracture surface over distances equivalent to tens of pipe wall thicknesses, while in the widely adopted drop weight tear test (DWTT) this spread hardly reaches one specimen thickness. Single-edge notched tensile (SENT) configuration provides much larger extent of the plastic zone into the specimen gauge when compared to DWTT whose major deformation mode is bending. Furthermore, the correlations between the strain parameters in different tests are discussed, and conclusions are drawn to promote a new laboratory test method for ductile fracture propagation resistance assessment in high strength steel pipe.