Two case studies are presented to investigate the use of 3-D crack meshes in Warp3D FEA for FFS assessments. A focused type crack mesh is used for ductile tearing analysis to determine if a cylinder model can be used as approximation for a pipe elbow. The limiting flaw curve results comparison shows an elbow model is needed to obtain more accurate results. A cell type crack mesh is used to examine pipeline hydrotest pressures to cause fatigue crack retardation by crack face closure. The test pressure giving the most benefit is determined by comparing crack closure results from several load cases.
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Localized microstructural defects that often lead to unpredictable fracture have limited the wider adoption of additively manufactured (AM) alloys in critical components. In addition to the background porosity responsible for ductile failure in conventional alloys, defects resulting from the AM process can include large pre-existing voids (~30 μm) resulting in a dual-scale porosity failure process in AM alloys. In the present work, we undertook a numerical approach to explore the dual-scale void and crack interaction processes in both two and three dimensions in AM Direct Metal Laser Melted (DMLM) Ti-6Al-4V. A small-scale yielding modified boundary layer model with monotonically increasing applied displacement was used. The Gurson ductile damage model was implemented to model typical background pores, while the larger AM defects were explicitly represented in a finite element mesh. Fracture resistance curves were numerically generated for random instantiations of AM void distributions with increasing levels of AM defects. Individual outliers of fracture resistance, both over and under perfroming, were analyzed in more detail. It was seen that AM defects may activate a larger fracture process zone ahead of the crack tip, promote crack tortuosity, and on occassion lead to increased local material toughness over the conventional alloy.
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Calibration of the cohesive zone models reqires determination of a number of critical parameters in the traction-separateion law. This paper introduces an approach to determine the traction-separation law, namely the Park-Paulino-Roesler model, through the node-release analysis implemented in the finite element research code WARP3D. The validation of the proposed approach utilizes results from the single-edge-notched bend, SE(B), specimens with varying levels of crack-front constraints and welded plate specimens.
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This investigation addresses a numerical investigation of the crack front fields and effects of crack-tip constraint in subsized SE(B) specimens with transverse delamination. Nonlinear numerical analyses of very detailed 3-D finite element models of SE(B) fracture specimens for nanostructured ferritic alloy (NFA) material enable assessing the effects of prescribed delamination cracks on the crack front fields with increased deformation levels as characterized by the J-integral. Overall, the present analyses reveal important features of 3-D crack front fields in fracture specimens with transverse delamination that have a direct bearing on the often observed toughness increase in fracture testing of materials with through-thickness anisotropy in mechanical properties.
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Control of ductile fracture propagation to ensure arrest is an essential element of design of pipelines transporting natural gas and other fluids such as dense-phase CO2. Full-scale pipe rupture simulations have been carried out using Finite Element Analysis (FEA). The simulations captured an accurate pressure loading profile behind the crack tip resultant from the escaping fluid. Results for loading profiles representative of both natural gas and a CO2 mixture are compared, the difference being that the CO2 mixture can exist as a two-phase fluid (liquid and gas) compared to the single-phase natural gas.
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Residual stresses caused by the welding process can drive stress corrosion crack growth, affect fatigue crack growth, lead to reheat cracking issues if the components are operated in the creep regime, and can affect the fracture response of components. The Virtual Fabrication Technology (VFTTM) computational weld modeling code, which can be used to predict the weld residual stresses (WRS) caused by the welding process, was adapted to the WARP3D open-source code recently. This effort describes the weld modeling process using VFT/WARP3D, provides predictions of WRS fields in several welded components, and provides crack growth predictions and fracture response in several example problems.
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Ductile cracks which form in steel components of civil structures due to ultra-low cycle fatigue may display significant growth prior to component failure. A computational framework was developed to simulate the growth of ductile cracks in structural steel utilizing the WARP3D platform. The basic model formulation is presented, followed by selected results from a small-scale experimental testing program. Results of simulations utilizing the proposed framework demonstrate good agreement with the experimental results.
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This paper examines time to minimum creep rate and its uncertainty with respect to a set of fourteen material parameters. The microstructural model of Grade 91 steel includes both dislocation creep and grain boundary opening/sliding within a finite element model, and hence the simulations are relatively expensive and have several sources of nonlinearity. We will propagate uncertainty in the input material parameters of these two mechanisms and determine the aggregate uncertainty in the predicted time to minimum creep rate as well as the sensitivities of the parameters. The cost, stability, accuracy of the polynomial chaos expansion as a means for stochastic dimensional reduction is assessed against the classical Monte Carlo method.
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