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Dynamic fracture behavior of a high strength pipeline steel
The occurrence of a crack propagating along a pipeline is a catastrophic event, which involves both economic losses and environmental damage. Therefore, the study of the fracture initiation and propagation properties of a pipeline is an essential part of its integrity assessment. Fracture prediction, however, is a challenging task, since it requires knowledge of the interaction between the dynamic forces driving crack growth, and the resistance forces opposing fracture propagation. Moreover, plenty of material properties should be taken into account. Aiming at a better understanding of the plastic hardening, damage and fracture properties of an API 5L X70 pipeline steel, and how these are affected by the strain rate, in present contribution, a comprehensive set of test results is presented. The program includes static and dynamic tensile tests on smooth and notched samples, and compression tests on cylindrical samples. Test result analysis is supported by finite element (FE) modelling. As such, the study aims at providing data needed for both fundamental material research and constitutive material modelling.
Fracture of X100 Pipeline Steel: Combined Experimental-Numerical Process | NIST
2011
Accurate characterization of material fracture properties is necessary for reliable numerical prediction of rupture in gas pipelines. In the current work, the ductile fracture properties for high strength X100 pipeline steel were evaluated with experimental data from a single type of experiment. The material characterization was based on a combination of experimental results with numerical simulation for a modified double cantilever beam (MDCB) specimen. The material properties were obtained by parametrical adjustments of the numerical results to the experimental measurements at points along the propagation of a crack in the specimen. The values of the equivalent plastic strain and the stress triaxiality (ratio of the pressure stress to the equivalent stress) were computed at the crack tip for different crack lengths. The fracture properties were evaluated parametrically through an adjustment between the experimental and the computational force-displacement curves. Damage at fracture initiation was considered as a function of the stress triaxiality and plastic strain rate at quasi static and dynamic testing rates. The numerical computations were performed with a commercial finite element code.
Prediction of crack initiation and propagation in X70 pipeline steels
The present work deals with the concept of post-initiation softening applied to ductile crack initiation and propagation problems in thick structural components such as pre-cracked pipes and fracture specimens. The methodology includes a set of calibration steps to determine model parameters of crack initiation and propagation under monotonic quasi static loading condition. The phenomenological Modified Mohr-Coulomb (MMC) fracture criterion is used for fracture initiation along with post-initiation softening for crack extension. A comprehensive experimental/numerical program provides access to the load histories of each specimen geometry at the critical point, where fracture is assumed to occur. This first step enables the construction of the limits of ductility of the material in terms of a weighted von Mises equivalent plastic strain, which strongly dependent on stress tri-axiality and Lode angle evolution. Post-initiation softening parameters are then calibrated from tearing analysis built upon resistance curves of single edge notch tension (SENT) specimens. The calibrated parameters model along with detailed FE analyses shows that the present fracture criterion accurately predicts the onset of stable crack growth and propagation along the uncracked ligament of low constraint specimens.
2014
Brittle out-of-plane cracking (by delamination or brittle tilted fracture) affects the impact toughness of pipeline steels. It has been investigated on notched tensile specimens using a local approach to fracture. Smooth and notched bars taken along four directions (including the short transverse direction, ND) have been tested in tension at temperatures between 20°C and-196°C. Delamination partly results from the lower value of the critical cleavage stress along ND, linked to the microtexture anisotropy, but also from the presence of ductile microcracks acting as stress concentrators triggering fracture along the rolling plane. Brittle tilted fracture was associated to a relatively lower value of the critical cleavage stress in that plane, but prior delamination was necessary to trigger it. The relevance of a macroscopic critical stress criterion for delamination is finally discussed.
Quantitative Investigation of Brittle Out-of-plane Fracture in X70 Pipeline Steel
Procedia Materials Science, 2014
Brittle out-of-plane cracking (by delamination or brittle tilted fracture) affects the impact toughness of pipeline steels. It has been investigated on notched tensile specimens using a local approach to fracture. Smooth and notched bars taken along four directions (including the short transverse direction, ND) have been tested in tension at temperatures between 20°C and -196°C. Delamination partly results from the lower value of the critical cleavage stress along ND, linked to the microtexture anisotropy, but also from the presence of ductile microcracks acting as stress concentrators triggering fracture along the rolling plane. Brittle tilted fracture was associated to a relatively lower value of the critical cleavage stress in that plane, but prior delamination was necessary to trigger it. The relevance of a macroscopic critical stress criterion for delamination is finally discussed.
2021
For the first time, the fracture toughness of pipeline with outer diameter of 168.3mm (thickness: 6.9mm; grade: API X46) was determined using the J-integral (according to ASTM standard E1820), single edge bend [SE(B)], and single-specimen method. The pre-crack was created using fatigue and the crack propagation was measured using the unloading compliance method. In each stage of crack propagation, the J-integral parameter was calculated and JQ was obtained using the J-R curve. The results indicated that satisfied the test’s validity criteria, and was equated to JQ. Subsequently, KIC was gained from the relationship between JIC and KIC. For the given pipeline, JIC and KIC were equal to 51 kJ/m2 and 105.4MPa√m, respectively. In addition, assessment of longitudinal cracks with different depths and lengths on the pipes body was conducted using fracture toughness and Failure Assessment Diagrams (FADs) for levels one and two of BS7910 standard. Results showed that a longitudinal crack wit...
A Damage Mechanics based Evaluation of Dynamic Fracture Resistance in Gas Pipelines
Procedia Materials Science, 2014
Investigation of running ductile fracture in gas transmission pipelines and the derivation of reliable crack arrest prediction methods belong to major topics in pipeline research. The yet available crack arrest criterion, known as the Battelle Two-Curve Method (BTCM), leads to reliable predictions up to grade X70 line pipe steels for which it has been validated. This includes specific limits in terms of mechanical properties, pressure and geometry. The application of this criterion to modern pipeline steels, i.e. especially grades X80 and beyond in combination with larger diameters and high pressure, has led to mispredictions of the BTCM. Hence, in order to ensure safe design of pipelines, new methods are required based on in depth knowledge and appropriate characterization of material resistance. This paper presents a procedure for the assessment of dynamic ductile fracture resistance based on combined experimental and numerical investigations. The procedure involves quasi-static and dynamic dropweight tear testing (DWTT) on modified specimens with pre-fatigued crack for grades X65, X80 and X100 materials, and the application of cohesive zone (CZ) and Gurson-Tveergard-Needleman (GTN) models to describe ductile material damage. The damage model parameters are calibrated on basis of DWTT results and subsequently used to simulate dynamic crack propagation in a pipeline. The influence of material properties (strain hardening, toughness), pipe geometry, usage factor and decompression behaviour on ductile fracture propagation behaviour is studied and evaluated. The results will contribute to an enhanced understanding of major parameters controlling ductile fracture propagation and will help to establish a reliable procedure for safe design of new high-capacity pipelines with regard to crack arrest.
Modeling of ductile tearing of pipeline-steel wide plates
The fract,ure resistance of steel plates used to build pipelines is commonly characterized by measuring macroscopic rupture parameters such as the energy dissipation rate, the thickness reduct,ion or the crack opening angle. In this study, the local approach to fracture is used to modcl tests cnrricd out on wide plates and to predict global parameters. The proposed model is based on an extension of the Gurson model including plastic anisotropy and viscosity. The mechanical behavior of the material is characterized using tensile bars tested along different directions. Rupture is characterized using notched bars. The inclusion content is measured using metallography. The model is used to numerically investigate the effect of plate thickness, through thickness hardness gradients and plastic anisotropy on the fracture resistance.
Experiments and fracture modeling of high-strength pipelines for high and low stress triaxialities
2012
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 predict...
Calibration of Pipeline Steel Model for Computational Running Ductile Fracture Assessment
2019
A novel fracture calibration method is presented for material models applied in finite element analysis of pipeline steels exposed to running ductile fracture. The calibration is based on the drop-weight tear test which is commonly applied for qualification of pipeline steels. The method is applied on three L450 steels with low, medium and high impact toughness. The calibrated fracture models are used in a numerical analysis of a full-scale fracture propagation test where the crack-driving force stems from a CO2-rich mixture that initially is in a dense phase. The results from the simulations are compared with experimental results.