The crack tip opening angle (CTOA) has been extensively studied as a fracture propagation parameter with great interest over the past several years.  It is of particular interest in the gas pipeline industry.  ASTM E3039 describes the standard test method for determining the CTOA from a drop weight tear test (DWTT) specimen.  This standard was developed to provide a consistent method of determining the CTOA of a DWTT specimen from the load-displacement data.  Experimental work has been performed showing that the CTOA determined from ASTM E3039 matches the mid-thickness CTOA of the material (highest constraint).  One key aspect to using the CTOA as a design parameter is the idea that it is transferrable from one loading mode to another i.e from a bending loading mode (DWTT) to a tensile loading mode (pipe).

In this paper, the ability of the CTOA to be transferred between small-scale test specimens and large-scale structures was examined.  This work also compared the results of a cohesive zone model (CZM) to the results of a constant CTOA model.  The line pipe steels examined in this work were an American Petroleum Institute (API) Standard X65 steel and a structural steel with the grade designation of STPG370.  The commercial finite element code ABAQUS 2017x was used to generate the models and solve the analyses.  The surface-based cohesive zone model was used to simulate crack propagation.  The CZM parameters were calibrated based on matching the surface CTOA measured from small-scale specimen (DWTT and modified cantilevered beam) finite element models to the experimental surface CTOA of the material.  The CZM parameters were then applied to pipe models.  A user-subroutine (VDLOAD) was used to model the internal pressure distribution and decay during the fracture process for the pipe models. The transferability of the CTOA between small-scale specimens and the pipe model was demonstrated using the CZM.  The results of the analysis demonstrate that the CZM can be used to model fracture propagation and produce results which are very similar to experimental observation.  Moreover, the numerical results of the CZM compared well with the results of a constant CTOA model. The experimental results of the STPG370 steel were matched within 10%.  These results give confidence to the transferability of the CTOA between small-scale specimens and large-scale structure under different loading modes.


Application of the Cohesive Zone Model to Crack Tip Opening Angle Design Methodology for Ductile Fracture in Pipeline Steels

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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