An Interim Approach to Determine Dynamic Ductile Fracture Resistance of Modern High Toughness Pipeline Steels

Abstract

The ductile fracture toughness of steel is used to assess the ability of a pipeline to resist long running ductile fractures in a burst event. In modern low carbon clean steels with high toughness, conventional measures of ductile fracture toughness (standard Charpy and DWTT energy) are under review, and alternatives are being studied. The major factor causing concern is the inability of these tests to isolate the energy associated with crack propagation from the total energy absorbed during the specimen fracture. This is significant in modern high toughness steels because their initiation toughness is extremely high. To resolve crack propagation energy, a novel modification was evaluated for both Charpy and DWTT specimens by employing a back-slot including a snug fitting shim to replace the removed material. In most cases, this modification was effective in curtailing the load-displacement trace when the propagating crack interacted with the slot on the backside of the specimen, without affecting the initial portion of the trace; this allowed crack propagation energies to be resolved. The propagation energy determined by this method is compared with the total energy and conventional test parameters. The crack propagation energy values inferred based on this should be validated, in future burst test.

Crack tip opening angle measurement through a girth weld in an X100 steel pipeline section*

“…The literature contains a number of different specimen geometries for studying ductile crack propagation by means of the CTOA criterion, such as Middle Tension specimens, M(T), 4,6,13 Compact Tension specimens, C(T), 4,6,13 Drop Weight Tear Test, DWTT specimens (with methodologies based on one 20,21 or two specimens 22 ), 3‐Point Bending specimens, 3‐PB, 23,24 and Modified Double Cantilever Beam specimens, MDCB 9,25 …”