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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Themes: Fracture in Large Scale Metallic Infrastructure: Advances, Challenges, and Opportunities
A NODE RELEASE APPROACH TO CALIBRATE COHESIVE PROPERTIES FOR FRACTURE SPECIMENS AND WELDED PLATE CONNECTIONS
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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3-D CONSTRAINT EFFECTS IN SUBSIZE SE(B) SPECIMENS OF NFA-14YWT WITH TRANSVERSE DELAMINATION
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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PIPE RUPTURE SIMULATIONS FOR TWO-PHASE CO2-MIXTURE
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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VFT COMPUTATIONAL WELD MODELING CODE ADAPTED TO WARP3D: PROBLEMS OF CRACK GROWTH AND FRACTURE IN RESIDUAL STRESS FIELDS
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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DEVELOPMENT AND VALIDATION OF A COMPUTATIONAL FRAMEWORK TO SIMULATE DUCTILE CRACK PROPAGATION IN STEEL STRUCTURES DUE TO ULTRA-LOW CYCLE FATIGUE USING WARP3D
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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MIXED MODE FATIGUE CRACK GROWTH BEHAVIOUR UNDER MICROSTRUCTURAL VARIATION IN FLASH-BUTT WELDS
The strength loss due to welding process poses a high risk of catastrophic rolling contact fatigue failure in the heat affected zones of flash-butt rail welds. Accurate characterisation of fatigue crack growth behaviour in such regions can provide a database for developing safer and more efficient maintenance strategies. This extended abstract details an experimental study on fatigue crack growth behaviour in flash-butt welds in a hypereutectoid rail steel with a hardness level of over 400 HV. Groups of mixed mode fatigue crack growth tests were carried out at parent rail region, partially spheroidised region, fully spheroidised region, re-austenitised region and bond line region. Fractographic analysis was performed to aid the application of the marker band method as well as to analyse the morphology of fracture surfaces. Once all experiments are finished, an equivalent stress intensity factor formula will be fitted to quantify the mixed mode crack driving force in different regions, and modifications of crack growth direction prediction criteria will be proposed for crack growth under the influence of microstructural variation. The current work will provide a reliable database for predicting rolling contact fatigue crack growth at different regions in flash-butt rail welds.
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FRACTURE TOUGHNESS CHARACTERIZATION OF 316L STAINLESS STEEL WELDED PLATES AT LIQUID NITROGEN (77 K) AND LIQUID HELIUM (4 K) TEMPERATURES
In the framework of a collaborative project between ASME, NASA, and NIST, quasi-static fracture toughness tests were performed at liquid nitrogen temperature (77 K, or 196 °C) and liquid helium temperature (4 K, or -269 °C) on precracked SEN(B) specimens extracted from the centers of four separate lots of welded 316L stainless steel plates. Although the plates were produced in accordance with the same specifications from the same material (316L), large differences in fracture toughness have been observed, with the toughest weld exhibiting almost twice the critical toughness of the least tough at 77 K (219 kJ/m2 vs. 113 kJ/m2), and about seven times the critical toughness of the least tough at 4 K (146 kJ/m2 as compared to 21 kJ/m2). Charpy absorbed energies previously obtained at 77 K for three of the four welds were found to be strongly linearly correlated with fracture toughness at both test temperatures, with an exception represented by the fourth weld, which provided the highest impact toughness and the second lowest quasi-static fracture toughness. Dynamic toughness measurements at impact loading rates were also performed on precracked SEN(B) specimens, in order to deconvolute the roles of strain rate and notch sensitivity on the fracture properties.
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EFFECT OF BOLT PRELOAD ON FRETTING FATIGUE BEHAVIOUR OF DOUBLE LAP BOLTED JOINTS WITH CLASS B SURFACE FINISH IN HIGH-CYCLE FATIGUE: EXPERIMENTAL AND NUMERICAL INVESTIGATION.
Slip-critical bolted connections are prone to developing fretting damage between the contacting surfaces. In this study, the effect of two different bolt preload values, 90 and 145 kN, on the fretting fatigue behaviour of steel double-lap bolted joints with a Class B (shot-blasted) surface under high-cycle fatigue conditions is investigated experimentally and numerically. The experimental results show that increasing the bolt preload decreased the total fretting fatigue life significantly. Moreover, the proposed numerical method was able to predict crack initiation and crack propagation behaviours successfully.
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