Simulation of Ductile Crack Propagation in Pipeline Steels Using Cohesive Zone Modeling

This paper presents recent results of numerical studies on stable crack extension of high toughness steels typical of those in modern gas pipelines using cohesive zone modeling. Two sets of materials are modeled. The first material set models a typical structural steel, with variable toughness described by four traction-separation (TS) laws. The second set models an X70 pipe steel, with three different TS laws. For each TS law, there are three defining parameters: the maximum cohesive strength, the final separation and the work of separation. The specimens analyzed include a crack in an infinite plate (small-scale yielding, SSY) and a standard drop-weight tear test (DWTT). Fracture propagation characteristics and values of crack-tip opening angle (CTOA) are obtained from these two types of specimens. It is shown that cohesive zone models can be successfully used to simulate ductile crack propagation and to numerically measure CTOAs. The ductile crack propagation characteristics and CTOAs obtained from SSY and DWTT specimens are compared for each set of steels. In addition, the CTOA results from numerical cohesive zone modeling of DWTT specimens of X70 steel are compared with those from laboratory tests.