Issue 36

R. H. Talemi, Frattura ed Integrità Strutturale, 36 (2016) 151-159; DOI: 10.3221/IGF-ESIS.36.15 151 Numerical simulation of dynamic brittle fracture of pipeline steel subjected to DWTT using XFEM-based cohesive segment technique Reza H. Talemi ArcelorMittal Global R&D Gent-OCAS NV, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium Reza.HojjatiTalemi@ArcelorMittal.com A BSTRACT . In the past several numerical studies have addressed the ductile mode of fracture propagation. However, the brittle mode of pipeline failure has not received as much attention yet. The main objective of this study is to predict brittle fracture behaviour of API X70 pipeline steel by means of a numerical approach. To this end, the eXtended Finite Element Method (XFEM)-based cohesive segment technique is used to model Drop Weight Tear Test (DWTT) of X70 pipeline steel at -100°C. In this model the dynamic stress intensity factor and crack velocity are calculated at the crack tip at each step of crack propagation. K EY WORDS : Dynamic brittle fracture; Pipeline steel; DWTT; XFEM; Cohesive segment. I NTRODUCTION oncerns have been raised that leaks in a CO 2 pipeline could escalate to brittle fracture crack propagation, due to the large temperature drop associated with the expansion of dense phase CO 2 to ambient conditions [1]. In order to avoid a long running brittle fracture minimum requirements for the shear area in a DWTT are specified. There are many studies aimed to develop Finite Element (FE) models to describe the impact phenomena [2-4]. Wu et al. [2] used the Gurson-Tvergaard-Needleman (GTN) model to simulate the fracture behaviour during DWTT. They analysed the equivalent stress, nucleation of voids and void size distribution using their FE Model. They reported that the fracture propagates in a triangular shape at the crack tip, and inverse fracture occurs when the fracture propagated about 3/4 of sample width. They found that in some of their simulations the transition during DWTT is from the brittle to the ductile and then again to the brittle zone. Scheider et al. [3] have simulated ductile dynamic fracture propagation using a numerical approach with application of damage mechanics models and a cohesive zone method. Basically they used the GTN model to simulate the DWTT with pressed notch and pre-fatigued crack. They have derived numerical fracture resistance curves employed for the assessment of ductile fracture resistance. Nonn et al. [4] modelled the ductile fracture behaviour of API X65Q pipeline steels subjected to DWTT using the GTN model. They have applied their model to describe and evaluate dynamic crack propagation in DWTT and pipe. The majority of available studies in the literature, including the above reviewed ones, concentrated on numerical modelling of ductile behaviour of materials. There are limited studies that focus on brittle fracture of the DWTT or Charpy V-Notch (CVN) impact tests at low temperatures. For instance Sainte Catherine et al. [5] have developed the Beremin (1983) cleavage model to simulate CVN and Sub-Size CVN impact tests at low temperature (-90°C). From their study they found that the results showed a good transferability potential. More recently, Hojjati-Talemi et al. [6] have implemented a novel C