PSI - Issue 28

J. Xue et al. / Procedia Structural Integrity 28 (2020) 1047–1054 J. Xue, B. Williams, S. Xu and W.R. Tyson/ Structural Integrity Procedia 00 (2019) 000–000

1048

2

Nomenclature CTOA

crack-tip opening angle Charpy absorbed energy cohesive zone modelling drop-weight tear test

CVN CZM

DWTT EMC2 FEA MMC S-SSM

Engineering Mechanics Corporation of Columbus

finite element analysis

modified Mohr-Coulomb model simplified single-specimen CTOA method

X-W

Xue-Wierzbicki model

1. Introduction The Battelle Two-Curve method is often 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 in Maxey (1974). However, the Charpy test specimen is too small for this task, and problematic for characterizing stable crack propagation toughness of high-strength pipe steels in Horsley (2003). To replace the Charpy-test-based fracture arrest methodology, the crack tip opening angle (CTOA) has been proposed as a material parameter for control of fast ductile fracture propagation and arrest of axial cracks in pipelines by Newman (1995); Newman et al. (2003) and Xu et al. (2015). Drop-weight tear test (DWTT) specimens are routinely used in pipe mills to characterize ductile-to-brittle fracture transitions, and measurement of CTOA using DWTT-type specimens can be done readily with the simplified single-specimen CTOA method (S-SSM) in ASTM E3039 - 18 (2018) and Xu et al. (2010). Transferability of CTOA from the DWTT specimens determined according to E3039 to other specimens and especially pipes is critical to the applications of CTOA. This work forms part of the research effort to investigate the transferability of CTOA from DWTT specimens to pipelines in which the stress state is normally biaxial. The focus is on the effect of loading modes (bending vs. tension) of DWTT specimens and biaxial stressing of cruciform models under various boundary conditions on CTOA based on finite element analysis (FEA). Experimental tests for DWTT specimens under tension are difficult due to the excessive loads required, which exceed those of typical load frames. FEA is an effective and efficient method to investigate transferability of CTOA. Widely used in the FEA of crack initiation and propagation in pipelines, the cohesive zone modelling (CZM) method has been used to investigate the transferability of CTOA in Parmar et al. (2018) and Xu et al. (2019). The CZM method has proven to be efficient and accurate, but the main limitation is its independency of stress state. The CZM method employs a simple traction-separation law, which normally doesn’t include any effect of stress state. The damage mechanics models used in this work, such as the Modified Mohr-Coulomb (MMC) model by Paredes et al. (2016), utilize a stress-state dependent fracture criterion, which takes into consideration the effect of stress state, such as the stress triaxiality and Lode angle. As far as we know, there is no investigation of transferability of CTOA using the stress-state-dependent damage-mechanics models so far. Four finite element models were investigated in this work, which include DWTT bending model, DWTT tension model, displacement-controlled cruciform model and force-controlled cruciform model. The MMC damage model for X80 was validated with round robin (RR) test results of the Engineering Mechanics Corporation of Columbus (Emc 2 ) Wilkowski et al. (2019) and Xue et al. (2019) was used to simulate crack initiation and propagation. Finite element models were first validated using DWTT test and FEA results from the previous research by Simha et al. (2014). A meshing technique utilizing tie constraint was used to reduce computational cost, and a python script was developed to automate the process of measuring CTOA. The two techniques proved to be efficient and accurate and build upon the previous research by Simha et al. (2014). The validated model was then used to investigate the CTOA in DWTT specimens under bending and tension to investigate the effect of stress state on CTOA.