Simulation of ductile crack propagation and determination of CTOAs in pipeline steels using cohesive zone modelling

Abstract

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 modelling (CZM). The main focus of the work is on the determination of crack‐tip opening angles (CTOAs) of these steels from CZM. Two sets of materials are modelled. 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 analysed 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 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 CZM of DWTT specimens of X70 steel are compared with those from laboratory tests.

Simulations of Ductile Fracture in Pipeline Steels Under Quasi-Static and Dynamic Loading Conditions

“…In the work of (Dunbar, 2011) and (Parmar, 2014), DWTT models were developed and compared to experimental results for X70 and X100 under quasi-static loading conditions. In both of these efforts the bilinear TSL was used to describe the fracture process.…”

“…The Load-LLD data was corrected for compliance by the author's predecessors for X70 and X100, see (Dunbar, 2011) and (Parmar, 2014), using the non-dimensional analysis technique from (Anderson, 2011). The compliance correction analysis adjusts the experimental data accounting for elastic deformation in the testing structure, as well as the local indentation of the hammer in the specimen.…”

“…The CZM has since been implemented to model crack propagation in ductile materials in the works of (Needleman, 1987), (Tvergaard & Hutchinson, 1992), (Tvergaard & Hutchinson, 1994), (Seigmund & Brocks, 2000), (Dunbar, 2011), and (Parmar, 2014).…”

“…In the work of (Dunbar, 2011) and (Parmar, 2014), 2-dimensional DWTT models of X70 and X100 were analyzed under quasi-static loading conditions. In those efforts, a specific focus was placed on the comparison of the measured CTOA with that of experimentation.…”

“…In the work of the authors predecessors (Dunbar, 2011) and (Parmar, 2014), the sizing of the mesh used in the cohesive zone was assigned based on the work of (Diehl, 2008), which stated that at least three cohesive elements were required per adjacent bulk element. This lead to very fine meshes.…”