Fracture Of Pipeline Steel Research Articles

Micromechanical modelling of ductile fracture in pipeline steel using a bifurcation-enriched porous plasticity model

In this paper, we investigate the possibility of predicting ductile fracture of pipeline steel by using the Gurson–Tvergaard–Needleman (GTN) model where the onset of void coalescence is determined based on in situ bifurcation analyses. To this end, three variants of the GTN model, one of which includes in situ bifurcation, are calibrated for a pipeline steel grade X65 using uniaxial and notch tension tests. Then plane-strain tension tests and Kahn tear tests of the same material are used for assessment of the credibility of the three models. Explicit finite element simulations are carried out for all tests using the three variants of the GTN model, and the results are compared to the experimental data. The capability of the simulation models to capture onset of fracture and crack propagation in the pipeline steel is evaluated. It is found that the use of in situ bifurcation as a criterion for onset of void coalescence in each element makes the GTN model easier to calibrate with less free parameters, all the while obtaining similar or even better predictions as other widely used formulations of the GTN model over a wide range of different stress states.

Finite element simulation of dynamic brittle fracture in pipeline steel: A XFEM-based cohesive zone approach

Leaking in a CO2 pipeline could escalate to sudden crack propagation, due to a large temperature drop. The resulting drop in fracture toughness together with the pressure stresses at the defect plane leads to pipeline brittle fracture. The main objective of this study is to monitor and predict brittle fracture behaviour of API X70 pipeline steel by means of experimental and numerical approaches, respectively. Dynamic fracture properties of CO2 line pipe steels are generally assessed using the Charpy impact test. To this end, Charpy V-notch tests are performed at different temperatures in order to study the resistance of materials subjected to impact loading conditions. The Charpy test provides valuable indications on the impact properties of materials. Using the experimental results the ductile to brittle transition temperature curve is presented. The extended finite element method based cohesive zone approach is introduced to model the brittle fracture at low temperature. After validation of the developed model against experimental observation significant results from the simulation are graphically presented and discussed.

The effect of hydrogen concentration on fracture of pipeline steels in presence of a notch

Abstract The comparative assessment of local fracture initiation at notches in API grade pipeline steels: X52, X70 and X100 have been done for conditions of electrochemical hydrogenating. The factor of hydrogen concentration in metal was taken into account. The relationship between hydrogen concentration in metal and work for initiation of the local fracture emanating from the notch has been derived. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown and discussed.

Fracture Appearance Evaluation of High Performance Pipeline Steel DWTT Specimen with Delamination Cracks

The delamination cracks and its effects on the fracture of pipeline steel are investigated experimentally by using of Drop-Weight Tear Test (DWTT). The delamination cracks are produced by the stress perpendicular to the weak interfaces before main crack beginning or accelerating, no new delamination crack is produced during the stabile propagation of fracture. The quantity, splay degree of delamination crack and the space between two delamination cracks are influenced by the stress state of the crack tip at beginning or accelerating point of main crack and the length of delamination crack is influenced by the stress state of the crack tip during the propagation of fracture. The surface of delamination crack is cleavage fracture appearance with large cleavage facet. There is no delamination crack on the brittle fracture surface below the brittle-to-ductile temperature or on the brittle fracture region of mix-mode fracture surface with ductile and brittle region. The part of fracture surfaces with delamination crack ought to be evaluated as the shear area because the delamination cracks are produced only on the ductile fracture surface or on the ductile part of fracture surface.

A model of fracture of pipeline steel in the process of hydrogenation

We develop a model of fracture of pipeline steel in the process of hydrogenation and present a classification of the origins of hydrogen-induced fracture. Possible modes of the formation of systems of stepped cracks with different origins are determined and investigated and the process of multiple fracture is analyzed.