Advances in running ductile fracture assessment for the Longship full-scale CCS project

As an integral part of the Norwegian full-scale CCS project Longship, the Northern Lights joint venture owned by Equinor, Shell and Total, is responsible for transporting liquified CO2 by ship from the industrial capture sites in the south-eastern part of Norway to an onshore terminal located on the Norwegian west coast. From the terminal, the liquified CO2 will be transported by a 12 ¾’’ OD offshore pipeline to the Johansen reservoir in the North Sea for permanent storage. Fracture control of the pipeline is essential for safe and robust CO2 transportation during the whole operational lifetime. The presently available international guidance for CO2 transport (in the form of Standards and Recommended Practices) rests with ISO 27913 and DNVGL-RP-F104. This involves designing the pipeline such that a rupture, that might occur due to e.g. accidental loads, will arrest rather than evolve into a more extensive running ductile fracture. However, the multi-phase behavior of expanding liquified CO2 induces a larger crack-driving force on a propagating crack compared to lean or rich natural gas. Several studies have shown that prediction of the crack arrest/propagation boundary is challenging under such conditions. In this study a previous concept-phase assessment of the Northern Lights pipeline is revisited, and a new evaluation is conducted based on some of the latest developments in analytical methodologies for design against running ductile fracture. The new evaluation is performed in combination with recent minor amendments to the design basis for the pipeline. The main conclusion from the present study is that the wall thickness of 15.9 mm remains sufficient for arresting a propagating crack.acceptedVersio

Ductile Fracture in Dual-Phase Steel : Theoretical, Experimental and Numerical Study

The thesis is focused on quasi-static ductile fracture in the low range of stress triaxiality of the dual-phase steel Docol 600DL. The study includes mechanical testing, theoretical aspects of ductile fracture and numerical simulations. Ductile fracture in the low range of stress triaxiality is an interesting topic since experimental data from various studies suggests that the ductility of the material is not only governed by the hydrostatic stressstate, but is also influenced by the deviatoric stress-state. The thesis consists of four journal articles bound together by a synopsis, where the introduction gives the motivation for the work and a state-of-the-art of the topics examined in this study. After the introduction, the objectives for the study are given followed by a summary of the work, conclusions and suggestions for further work. Finally the four articles are given. The articles are referred to as parts as they give detailed information of the different parts of the study. In Part I and Part II the experimental side of the study is described. These parts give detailed information regarding the material, test set-ups, specimen geometries and optical measurement procedures. The results from the experiments in form of the minor versus major principal strain as well as the stress triaxiality, the Lode parameter, and the equivalent strain at fracture are presented and used to describe the material’s fracture characteristics. Macroscopically the material displayed typical ductile behaviour with large strains before fracture. This was confirmed by scanning electron microscopy of selected specimens since all showed ductile dimples. Further, the material displayed a significant drop in ductility when exposed to plane-strain loading; an observation that suggests that the material’s fracture properties are Lode dependent. In Part III several uncoupled criteria for ductile fracture which explicitly take into account the effect of the Lode dependence were assessed. One of the criteria, the modified Mohr-Coulomb criterion, was taken from literature, while two of the criteria, the extended Cockcroft-Latham criterion and the extended Rice-Tracey criterion are augmented versions of existing criteria. The data given in Part I and II were used in evaluating the various criteria and it was found that the proposed extended Cockcroft- Latham criterion managed to give accurate predictions on the equivalent strain at fracture, while a somewhat larger spread was observed for the modified Mohr-Coulomb criterion and the extended Rice-Tracey criterion. Part IV focuses on numerical modelling of ductile crack propagation. Here the experimental tests from Part I were used in assessing the extended Cockcroft-Latham and modified Mohr-Coulomb criteria on crack propagation. Additionally the effects of using a high-exponent yield surface and material softening were investigated. It was found that the different fracture criteria as well as a change of yield function had little effect on the crack propagation. By including material softening through damage coupling, slant shear fracture as observed in some of the experiments was captured, but this did not alter the global response in form of the force-displacement curves

Strain localization and ductile fracture in advanced high-strength steel sheets

An experimental-numerical approach is applied to determine the strain localization and ductile fracture of high-strength dual-phase and martensitic steel sheet materials. To this end, four different quasi-static material tests were performed for each material, introducing stress states ranging from simple shear to equi-biaxial tension. The tests were analysed numerically with the nonlinear finite element method to estimate the failure strain as a function of stress state. The effect of spatial discretization on the estimated failure strain was investigated. While the global response is hardly affected by the spatial discretization, the effect on the failure strain is large for tests experiencing necking instability. The result is that the estimated failure strain in the different tests scales differently with spatial discretization. Localization analysis was performed using the imperfection band approach, and applied to estimate onset of failure of the two steel sheet materials under tensile loading. The results indicate that a conservative failure criterion for ductile materials may be established from localization analysis, provided strain localization occurs prior to ductile fracture.acceptedVersio

Simulation of ductile crack propagation in dual-phase steel

The modified Mohr–Coulomb and the extended Cockcroft–Latham fracture criteria are used in explicit finite-element (FE) simulations of ductile crack propagation in a dual-phase steel sheet. The sheet is discretized using tri-linear solid elements and the element erosion technique is used to model the crack propagation. The numerical results are compared to quasi-static experiments conducted with five types of specimens (uniaxial tension, plane-strain tension, in-plane shear, 45° and 90° modified Arcan) made from a 2 mm thick sheet of the dual-phase steel Docol 600DL. The rate-dependent J2 flow theory with isotropic hardening was used in the simulations. The predicted crack paths and the force–displacement curves were quite similar in the simulations with the different fracture criteria. Except for the 45° modified Arcan test, the predicted crack paths were in good agreement with the experimental findings. The effect of using a high-exponent yield function in the prediction of the crack path was also investigated, and it was found that this improved the crack path prediction for the 45° modified Arcan test. In simulations carried out on FE models with a denser spatial discretization, the prediction of localized necking and crack propagation was in better accordance with the experimental observations. In four out of five specimen geometries, a through-thickness shear fracture was observed in the experiments. By introducing strain softening in the material model and applying a dense spatial discretization, the slant fracture mode was captured in the numerical models. This did not give a significant change in the global behaviour as represented by the force–displacement curves

Calibration of Pipeline Steel Model for Computational Running Ductile Fracture Assessment

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.acceptedVersio

Determination of Ductile Fracture Parameters of a Dual-Phase Steel by Optical Measurements

Marciniak–Kuczynski and Nakajima tests of the dual-phase steel Docol 600DL (www.ssab.com/) have been carried out for a range of stress-states spanning from uniaxial tension to equi-biaxial tension. The deformation histories of the specimens have been recorded by digital images, and the displacement and strain fields have been determined by post-processing the images with digital image correlation software. The fracture characteristics of the material are presented by means of the stress triaxiality, the Lode parameter and the equivalent strain. These parameters are evaluated on the surface of the specimens based on the optical field measurements and assumptions regarding the mechanical behaviour of the material. Additionally the minor versus major principal strains up to fracture are presented. It is found that the material displays a significantly lower ductility in plane-strain tension than in uniaxial tension and equi-biaxial tension, and that it, in the tests exposed to local necking, undergoes large strains between the onset of necking and fracture. Fractographs of selected specimens reveal that fracture is due to growth and coalescence of voids that occur in localised areas governed by shear-band instability.acceptedVersio

Ductile Fracture of Steel Sheets under Dynamic Membrane Loading

Failure prediction assessment is conducted based on validated finite element simulations of impact tests performed on 1.8 mm dual-phase and 1.0 mm martensitic steel sheets. The sheets were clamped between two steel rings and subjected to lateral loading by a punch with a hemispherical nose. Three different specimen geometries were applied. These were chosen to provide membrane loading in stress states near uniaxial tension, plane-strain tension and equi-biaxial tension. Thus, the most important stress states that may occur for thin sheets in an impact situation are covered. Finite element simulations of the impact tests are run with the nonlinear code LS-DYNA. The plastic behaviour of the materials is modelled using the Hershey yield function in combination with the associated flow rule and isotropic hardening. The specimens are discretized by shell elements, thus imposing a state of plane stress. Three different approaches for modelling ductile failure are evaluated by comparing the experimental and simulated force-displacement curves from the experiments on the two steel materials

Low-velocity impact on high-strength steel sheets: An experimental and numerical study

Low-velocity impact tests were performed on dual-phase and martensitic steel sheets and compared with corresponding quasi-static tests. The geometry and loading condition of the specimens were similar to formability tests, and the average strain rates before failure were in the range 80–210 s−1 for the low-velocity tests and 0.002-0.005 s−1 for the quasi-static tests. For both loading rates, the sheets failed under pre-dominant membrane loading, and by varying the specimen geometry, the stress states prior to failure ranged from uniaxial tension to equi-biaxial tension. Thus, the most important stress states occurring during an impact event in a thin-walled structure are covered. The experiments were complemented by nonlinear finite element simulations, where higher-order solid elements and a refined mesh were applied to capture the failure of the sheets. The materials were modelled using the Hershey high-exponent yield function combined with the associated flow rule and isotropic hardening. Quasi-static tensile and shear tests and tensile tests at elevated strain rates were performed to calibrate the constitutive relation. The results in terms of force-displacement curves and strain histories at critical positions in the specimens were similar for low-velocity and quasi-static loading, independent of material and specimen geometry. This indicates that the quasi-static test gives a good description of the sheet behaviour under low-velocity impact loading. The numerical simulations were found to be in good agreement with the experimental results, and strengthened the experimental finding that all the sheet-impact tests, except the martensitic steel sheet in a state close to equi-biaxial tension, displayed local necking before final fracture

Low-velocity impact behaviour and failure of stiffened steel plates

The behaviour and failure of stiffened steel plates subjected to transverse loading by an indenter is studied in this paper. Low-velocity dynamic and quasi-static tests of stiffened plates with geometry adopted from a typical external deck area on an offshore platform were conducted. The results show that the quasi-static tests provide a good reference for impact loading situations, although they displayed a larger displacement at fracture. Finite element simulations of the steel panel tests were performed, using the elastic-viscoplastic J2 flow theory and a one-parameter fracture criterion. A relatively fine spatial discretization in the load application area was needed to capture accurately the onset of fracture. In order to locally refine the mesh, a method for automatic mesh refinement based on damage driven h-adaptivity was implemented and evaluated against results obtained with fixed meshes of various element sizes.acceptedVersio