Experiments and Fracture Modeling of High-Strength Pipelines for High and Low Stress Triaxialities Kirki Kofiani 1 , Aida Nonn 2 , Tomasz Wierzbicki 1 , Christoph Kalwa 3 and Carey Walters 4 1 Massachusetts Institute of Technology, Department of Mechanical Engineering, Cambridge, MA, USA 2 Salzgitter Mannesmann Forschung GmbH, Duisburg, Germany 3 EUROPIPE, Mülheim an der Ruhr, Germany 4 Centre for Mechanical and Maritime Structures TNO Built Environment and Geosciences, Delft, Netherlands ABSTRACT This paper provides results from a comprehensive study on mechanical characterization of high-strength pipeline steel, grade X100 using experimental and numerical methods. The material was characterized for anisotropic plasticity, fracture initiation for various states of stress, (pre-cracked) fracture toughness and uncracked ductility. The experimental program included tests on flat butterfly-shaped, central hole, notched and circular disk specimens for low stress triaxiality levels; as well as tests on round notched bar specimens and SENT fracture mechanics tests, for high values of stress triaxiality. This program covered a wide range of stress conditions and demonstrated its effect on the material resistance. Parallel to the experimental study, detailed numerical investigations were carried out to simulate all different experimental tests. Using an inverse method, a 3-parameter calibration was performed on the Modified Mohr-Coulomb (MMC) fracture model. Subsequently, the predictive capabilities of the MMC were evaluated by the comparison to the fracture toughness tests results, used extensively in the pipeline industry. The capabilities of the MIT fracture model have been demonstrated on an example of high strength offshore steel, X100. The outcome of this study was not only to provide, the overall characterization of the fracture behavior of this material as an example, but also to present the methodology on how to use the MMC model as a practical tool in pipeline design. KEY WORDS: ductile fracture; SENT tests, high-strength pipeline material; X100; Modified Mohr-Coulomb model; triaxiality and Lode angle dependence. INTRODUCTION The growing need for energy resources located in remote regions with complex ground and extreme climate conditions has led to increasing demands on pipeline material to ensure safe and cost-efficient design of gas transmission pipelines. One of the main challenges of research in the pipeline industry is to achieve both high strength and high toughness material properties which will allow pipelines to sustain large deformations, even under multi-axial loading. However, there are still open issues regarding the deformation and fracture performance of pipelines when subjected to different loading scenarios. In recent decades, numerous fracture models have been developed with the aim of characterizing ductile fracture of steel materials. The major characteristic of these models is their capability to describe failure mechanisms by taking into account the influence of local stress and strain variables. One of the models which is widely used for the simulation of the damage evolution in the pipelines is Gurson- Tvergaard-Needleman (GTN) model (Gurson, 1977; Tvergaard, 1982; Tvergaard and Needleman, 1984). The simulation of ductile fracture for X100 material model has been performed recently using GTN model in high stress triaxiality range (Ishikawa, Sueyoshi and Igi, 2010; Nonn and Kalwa, 2010; Tanguy, Luu, Perrin, Pineau and Besson, 2008). Although this micromechanics-based model describes the ductile failure mechanism of void nucleation, growth and coalescence adequately, it requires the calibration of numerous parameters. The recent modifications of GTN, e.g. to account for shear-stress dominated fracture (Jackiewicz, 2011; Nahshon and Hutchinson, 2008; Nielsen and Tvergaard, 2011) , lead to an increase of the additional parameters and hence the challenge for finding a representative parameter set. Furthermore, the work of Dunand and Mohr (2011) shows that this large amount of parameters is not necessary for achieving a good correlation. As an alternative to the GTN model, the modified Mohr-Coulomb (MMC) model has been introduced by (Bai and Wierzbicki, 2010) and successfully applied for the prediction of ductile fracture initiation and propagation in advanced high-strength sheet and low-strength steel materials. This extended model describes the equivalent strain to fracture depending on stress triaxiality and Lode angle, parameter related to the normalized third invariant of stress deviator. To derive the three-dimensional fracture locus, it is necessary to calibrate only three model parameters by at least the same number of the experiments. The suitability of the MMC model to characterize the ductile fracture has been demonstrated in many publications, primary for the ranges of low stress triaxiality (η <0.7) see (e.g. Bai, 2010, Dunand and Mohr, 2011). On the other hand, there is no sufficient knowledge about the accuracy of the model prediction for the high triaxiality levels η >1.0, which is very important for analysis of cracked structures. This paper seeks to close this gap by providing the results from experimental and numerical characterization of the ductile fracture 511 Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering Conference Rhodes, Greece, June 17–22, 2012 Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-94–4 (Set); ISSN 1098-6189 (Set) www.isope.org