The critical crack-tip-opening angle or displacement (CTOA/CTOD) fracture criterion is one of the oldest fracture criteria applied to metallic materials. Improved computer-aided photographic methods have been developed to measure CTOA during the fracture process; and elastic-plastic, finite-element analyses (ZIP2D) with a constant CTOA and a plane-strain core have been used to simulate fracture of laboratory specimens. The fracture criterion has been able to link the fracture of laboratory specimens to structural applications. This paper analyzes fracture of cracked thin-sheet 2219 aluminum alloy over an extremely wide range in width, crack-length-to-width ratio, and applied loading. The results from the critical CTOA fracture analyses on the thin-sheet material showed that the stress-intensity factor at failure (KIe) was linearly related to the net-section stress (Sn), as predicted by the Two-Parameter Fracture Criterion (TPFC).
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Themes: Ductile Fracture Under Complex Loading
CHARACTERIZATION AND NUMERICAL SIMULATION OF DUCTILE CRACK INITIATION AND PROPAGATION IN CT SPECIMENS OF DIFFERENT SIZES MACHINED FROM A 316L THICK PLATE
Measuring fracture toughness for ductile materials requires the specimen size to be large enough for the tests to be valid. The higher the toughness is, the larger the specimen must be. This paper uses experimental and numerical approaches to study the fracture behavior of as-received and aged 316L(N) steel and the effect of the size and thickness of the specimens on the evaluated toughness.
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APPLICATION OF A NOVEL UNIFIED PARAMETER ON CHARACTERIZING IN-PLANE AND OUT-OF-PLANE CRACK-TIP CONSTRAINTS FOR AL7075 T651 SEN(B) SPECIMENS
Crack-tip constraint can have a significant effect on fracture toughness. A loss of crack-tip constraint can cause an increase in fracture toughness. In this paper, a novel unified constraint parameter λ based on the plastic strain energy was proposed to quantify the crack-tip constraint level. The application of this parameter for assessing the in-plane and out-of-plane constraints of Al7075 T651 alloy SEN(B) specimens was investigated with a series of fracture bending experiments and numerical modelling.
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FRACTURE MODELLING AND ANALYSIS OF MULTIPLE SITE CRACKS IN PLATES UNDER LATERAL PRESSURE
Results of experimental and finite element study on fracture behavior of damaged thin plate specimens subjected to lateral pressure are presented. Plate specimens with a single crack or an array of collinear cracks were tested applying lateral pressure load by using a specially designed experimental setup. The elastic plastic fracture mechanics concept (EPFM) was employed in FE analyses, as large scale yielding occurred in ligaments of fractured specimens. The critical J-integral and crack tip opening displacement (CTOD) values associated with fracture onset were inferred from finite element simulation results. Assessed critical pressure loads for considered plate specimens were compared with experimentally obtained results and a good agreement was ob-served.
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ASSESSMENT OF EXISTING OFFSHORE GAS TRANSIMISSION PIPELINES IN TERMS OF DUCTILE FRACTURE CONTROL USING A MODELING FRAMEWORK
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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ESTIMATING PLASTICITY AND DUCTILE DAMAGE MODEL PARAMETERS FOR S355-S690 STEEL FROM MILL TEST CERTIFICATE DATA
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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PREDICTING DUCTILE FRACTURE DURING TORSION TESTING USING ELLIPSOIDAL VOID MODEL AND ANALYTICAL MODEL
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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EFFECT OF HPT PROCESSING ON FRACTURE BEHAVIOUR OF MARAGING STEELS
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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DUCTILE FRACTURE OF SS-304L MICROTUBE UNDER COMBINED AXIAL FORCE AND INTERNAL PRESSURE
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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MODELING OF THE ELASTO-PLASTIC BEHAVIOR OF HSLA X140 STEEL: EFFECT OF PRE-STRAIN AND TRIAXIALITY
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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