13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Transition of ductile and brittle fracture during DWTT by FEM YouYou WU 1 , Hailiang YU 1,2* , Cheng LU 1 , Kiet TIEU 1 , Ajit GODBOLE 1 , Guillaume MICHAL 1 1 School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong, NSW 2500, Australia 2 School of Mechanical Engineering, Shenyang University, Shenyang, 110044, China * Corresponding author: [email protected] or [email protected] Abstract Globally, steel pipelines are widely used to transport energy in the form of liquid petroleum and natural gas. The steel used in the manufacture of these pipelines must have high strength and toughness, and high resistance to fracture. The Drop Weight Tear Test (DWTT) is the most widely used test to assess brittle fracture characteristics in steel. The zones of ductile and brittle fracture during DWTT characterize the quality of pipeline steels. In this paper, the Gurson-Tvergaard-Needleman (GTN) fracture models are coupled in a Finite Element model. The ductile and brittle fracture zones in the samples are analyzed under different conditions. The results show that the change in fracture mode during the DWTT is from the brittle to the ductile, then again to the brittle. The calculated absorbed energies during DWTT compare well with experimental findings. Finally, we present an analysis of the transition from ductile to brittle fracture under different conditions. Keywords Energy pipeline; ductile fracture; brittle fracture; DWTT 1. Introduction Oil and gas provide 60% of the world’s primary fuel and a large proportion is transported in pipelines. There is more than 33,000 km of high-pressure steel pipelines in Australia. The pipelines are designed, built and operated to well-established standards and rules, because the products they carry can pose a significant hazard to the surrounding population and environment. A combination of good design, adequate material properties and sound operating practices are therefore necessary, to ensure that transmission pipelines operate safely and efficiently. Line pipe specifications specify minimum requirements for the shear area in a Drop Weight Tear Test (DWTT) to ensure the arrest of a long running brittle fracture. The DWTT, as specified in API RP 5L3 [1] or ASTM E436 [2], was developed by Battelle Memorial Institute in 1962 during the course of the American Gas Association NG-18 Research Program [3] to overcome some limitations of the Pellini drop-weight test which was developed by the US Naval Research Laboratory. In a DWTT, the test specimen is a rectangular bar with a length of 305 mm, a width of 76 mm and of the full material thickness (up to at least19mm). The specimen has a shallow pressed notch and is subjected to three-point bending, as shown in Fig. 1. The standards specify a 5 mm deep notch made by a sharp indenter with a 45° included angle resulting in a tip radius that is normally between 0.0127 to 0.0254 mm [1]. A series of specimens are broken under impact loading at various temperatures and the proportions of ductile fracture (shear) and brittle fracture (cleavage) on the fracture surfaces are measured. From correlations with full-scale pipe burst tests, a transition temperature corresponding to about 85 percent shear is normally defined in application standards as the fracture propagation transition temperature (FPTT) [4-5]. In this paper, a numerical method has been used to simulate the fracture behavior of pipelines. The most commonly used fracture model in computational fracture mechanics to characterize the toughness of line pipe steels is the modified Gurson model [6]. The Gurson model includes the influence of micro-voids on the plastic flow in a constitutive framework. The Gurson model was later modified extended by Tvergaard [7] and Needleman [8]. The modified Gurson- Tvergaard- Needleman(GTN) model is used to describe the acceleration of void growth. In this model, the
Transition of ductile and brittle fracture during DWTT by FEM
May 23, 2023
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