A modeling framework is established to describe running ductile fracture in vintage API grade X52 offshore pipelines. For the structural model, the plasticity and ductile fracture properties were characterized by various laboratory scale tests. Tensile tests up to strain rates of 1000 1/s were performed to calibrate the strain rate dependent plasticity model. Using notched tensile specimens with a wide range of stress states, a hybrid experimental-numerical procedure was performed to determine the parameters of a ductile fracture (FL) model. The material model was successfully verified against the instrumented Battelle Drop-Weight Tear (BDWT) test results. The decompression of the CO2-rich gas mixture was described by the GERG-2008 equation of state and implemented as an idealized pressure decay model to reduce the computational cost. Finally, the established modeling framework provides a valuable tool for investigating and evaluating ductile fracture propagation and arrest behavior in the vintage offshore pipelines.
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Accurate finite-element simulation of the fracture of metals requires the calibration of plasticity and fracture modelling parameters based on mechanical tests on the material. Depending on the complexity of the model, each different material that is modelled requires a number of non-standard tests followed by a calibration process. This paper derives relationships between mill test certificate data and the plasticity and damage model parameters for S355-S690 steel in order to enable the quick application of generally representative plasticity and damage models to these steels without the need for repeated manual calibration of each material. The relationships are obtained by regression analysis between a database of 2597 mill test certificate results (of tensile and Charpy tests) and a parametric finite element study in which the parameters of a Hollomon-type stress-strain model and the Modified Mohr-Coulomb damage model were varied.
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Research on ductile fracture under high stress triaxiality has been performed considerably, whereas research on ductile fracture under low stress triaxiality has not been performed sufficiently. In this paper, torsion testing of a bar which is prestrained by drawing is performed using a torsion testing machine, and ductile fracture during torsion testing is predicted using an ellipsoidal void model and an analytical model.
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Maraging steels are a class of precipitation hardened steels wherein different micro-mechanisms of deformation such as planar slip, interaction with coherent/incoherent precipitates, and reverted austenite affecct the overall mechanical behavior of the material. High-pressure-torsion (HPT) processing introduces a large density of dislocations that form sub-grain boundaries within the refined nano-scale structure, leading to changes in precipitate morphology compared to hot-rolled maraging steels. The impact of such nanostructuring on the deformation and fracture micro-mechanisms is being reported for the first time using in-situ characterization techniques along with transmission electron microscopy and atom probe tomography analysis, in this study. Digital image correlation has been used to quantify the full field strain maps in regions of severe strain localization as well as to determine the fracture toughness through critical crack tip opening displacements.
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The fracture behavior of the stainless-steel SS-304L is assessed by loading microtubes of 2.38 mm diameter under combined axial force and internal pressure, using a custom apparatus. The force/pressure ratio is controlled in the experiments, to generate different biaxial stress paths that are proportional or nearly proportional. The results from the experiments are used to calibrate the non-quadratic anisotropic yield function Yld2004-3D. Then, finite element (FE) models of the microtubes are created after incorporating the anisotropic material modeling framework, and compared with the experiments to establish their fidelity. The FE models are then used to probe the fracture behavior under the proportional loading. The failure modes of the microtubes are different depending on the stress state being axial- or hoop-stress-dominant. It is found that the structural instabilities that precede necking are different and appear at different levels of strain. The strains at the onset of fracture, as determined by probing the FE model, reveal significant fracture anisotropy, that can be possibly also attributed to the specimen geometry, beyond the material processing.
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In this work, a comprehensive experimental campaign is conducted to investigate the effect of pre-strain on the mechanical properties of X140 steel used in high performance threaded connections. Mechanical tests are used to characterize the plastic and fracture behavior of the material. Smooth tensile (ST), notched tensile (NT), plane strain (PE) and shear tests (STC) were performed. Cyclic tension-compression tests are used to characterize kinematic hardening. Initially qualified as isotropic, this material showed an anisotropic behavior after undergoing a pre-strain expansion as its plastic flow becomes loading direction dependent. This pre-strain effect is well reproduced using a phenomenological modeling combining isotropic and kinematic hardening contributions with a Hosford’s criterion.
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