The Battelle Two-Curve method has been widely used to predict the required toughness to arrest running ductile fractures in line pipes, characterized by the minimum CVN (Charpy V-Notch absorbed energy) upper-shelf energy. However, the Charpy test specimen is too small and hence problematic for characterizing stable crack propagation toughness of high-strength pipe steels. To replace the Charpy-test-based methodology, the crack tip opening angle (CTOA) has been proposed as a toughness parameter to characterize fast ductile fracture propagation and arrest of axial cracks in gas pipelines. Drop-weight tear test (DWTT) specimens are routinely used in pipe mills to characterize ductile-to-brittle fracture transition behaviour, and measurement of CTOA using DWTT-type specimens can be done readily with the simplified single-specimen CTOA method (S-SSM) in ASTM E3039 - 20. Transferability of CTOA from the DWTT specimens determined according to E3039 to other specimens and especially to pipes is critical to application of CTOA.This work forms part of the research effort to investigate the transferability of CTOA from DWTT specimens to pipes. The focus of this work is on the effect of ligament size and thickness of DWTT specimens on CTOA. Because experimental tests for large DWTT specimens are difficult and expensive due to the excessive loads required, FEA has been used as an effective and efficient method to investigate these size effects.The damage mechanics models used in this work, namely the modified Xue-Wierzbicki (XW) model and the Modified Mohr-Coulomb (MMC) model, utilize stress-state dependent fracture criteria and hence depend on stress triaxiality and Lode angle.&nbsp; The XW and MMC models for X80 developed in previous research were used to simulate crack propagation. CTOA values at 1 mm behind the crack tip were extracted using a python script developed to automate the process. The finite element models validated with the standard DWTT tests (initial ligament length = 66 mm) were used to investigate CTOA in larger DWTT specimens with various thicknesses and initial ligament length = 140 mm.Both the XW model and the MMC model predict similar trends regarding the effect of ligament size and thickness on CTOA. For both the standard and large DWTT models with various thicknesses investigated in this research, steady-state crack growth was achieved at a reasonably low crack extension. The CTOA values measured on the surface are larger than those measured in the interior, which is consistent with experimental observation. Increase of thickness has almost no effect on interior CTOA, but leads to higher surface CTOA. The interior CTOA is found to be very similar in all cases with various ligament sizes and thicknesses. These results support transferability of CTOA from standard DWTT tests according to ASTM E3039 to larger specimen sizes and especially to geometries with larger ligaments (e.g. pipes). The effect of thickness on fracture mode transition was also investigated with the XW model in this research. The implicit analysis with the MMC model predicted flat fracture mode in all cases, while the explicit analysis with the XW model was able to predict flat-to-slant fracture mode transition depending on the thickness-to-width ratio of the DWTT models. For the standard DWTT (initial ligament length = 66 mm) and large DWTT model with smaller thickness-to-width ratio (initial ligament length = 140 mm, thickness = 13.7 mm and 20 mm), the failure mode was flat with substantial crack tunnelling. However, when the thickness for the large DWTT model was increased to 32 mm or 40 mm, the fracture mode transitioned to slant fracture after a short crack extension of flat tunnelling. The effect of fracture mode transition on CTOA will be investigated in future research.Keywords: CTOA, DWTT, finite element analysis, damage-mechanics model, specimen size&nbsp;

Finite Element Analysis to Investigate the Effect of Geometries of DWTT Specimens on the CTOA - An Interim Report

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

ASME 2021 Pressure Vessels & Piping Conference (PVP2021)

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