Application of the standard options of the FITNET procedure to the structural integrity assessment of welded specimens containing cracks

In this paper, the structural integrity of welded specimens is assessed. The specimens were welded using a conventional technique, MAG, and an unconventional one, laser technology. Welded specimens were of the central crack under tension (CCT) type. The elements were loaded, the maximum load was recorded, and the second and the third options of the FITNET procedure were utilized to estimate the critical loading. Estimated and recorded loads were compared to verify the conservatism of the estimated results. For comparison, the results obtained using Option 1 of the FITNET procedure (not recommended in the case analysed) are also presented. Results of the mechanical properties, hardness, fracture toughness for the base and the weld material and residual stress distributions are reported for both MAG and laser welding techniques. The results obtained confirm that the FITNET procedures were properly deigned. The higher the level of analysis, the smaller the conservatism of predictions observed. The overconservative conclusions, following from the lower level of analysis, suggest that the structural element is endangered, do not have to exclude it from exploitation. The higher level of analysis can, in some cases, confirm that the structural element containing a crack can still be exploited.     

Assessment of local thin areas in a marine pipeline by using the FITNET FFS corrosion module

This paper analyses the structural integrity of the marine stretch of a pipeline which is placed over a natural bay. The pipeline is part of a 30-year-old installation used for the provision of petrochemical products to a nearby chemical plant. Although there have been no relevant leaks in the past, both the visual inspections performed (revealing numerous local thin areas) and the fact that it is located in a highly sensitive place with high ecological and tourist value recommend the assessment of the pipeline in order to ensure that it is working in safe conditions and that there are no risks for the environment or the people living in the surrounding area. The assessment has been performed using the newly developed FITNET FFS procedure, whose local thin areas assessment methodology is also explained and compared to the analyses proposed by other well known procedures.     

Finite element analysis of metallurgical phase transformations in AA 6056-T4 and their effects upon the residual stress and distortion states of a laser welded T-joint

Aircraft industry makes extensive use of aluminium alloy AA 6056-T4 in the fabrication of fuselage panels using laser beam welding technique. Since high temperatures are involved in the manufacturing process, the precipitation/dissolution occurrences are expected as solid state phase transformations. These transformations are likely to affect the residual distortion and stress states of the component. The present work investigates the effect of metallurgical phase transformations upon the residual stresses and distortions induced by laser beam welding in a T-joint configuration using the finite element method. Two separate models were studied using different finite element codes, where the first one describes a thermo-mechanical analysis using Abaqus; while the second one discusses a thermo-metallo-mechanical analysis using Sysweld. A comparative analysis of experimentally validated finite element models has been performed and the residual stress states with and without the metallurgical phase transformations are predicted. The results show that the inclusion of phase transformations has a negligible effect on predicted distortions, which are in agreement with the experimental data, but an effect on predicted residual stresses, although the experimentally measured residual stresses are not available to support the analyses.     

An overview of the European flaw assessment procedure SINTAP and its validation

A three-year, European project structural integrity assessment procedures for European industry (SINTAP) was completed in March 1999. The project involved analytical, numerical and experimental work on a number of fracture mechanics topics. The main output was a flaw assessment procedure, which should form the basis of a future European standard. This paper describes the SINTAP procedure, illustrating how the output may be presented in terms of either a crack driving force (CDF) or a failure assessment diagram (FAD). Novel features of the SINTAP work are then described and, finally, numerical and experimental validation of various parts of the procedure is summarised.     

Effect of vibratory weld conditioning on the residual stresses and distortion in multipass girth-butt welded pipes

For a fully welded body valve, the last procedure is welding, so it is important to control the residual stress and distortion in order to assure spool rotation, valve watertightness, stress corrosion resistance and non-deformability in active service. In this study, the effects of vibratory weld conditioning (VWC) on the residual stress and distortion were studied in multipass girth-butt welded pipes through comparison between VWC and normal submerged arc welding. The results show that VWC can reduce the residual hoop stresses at the outer surface and the radial distortion significantly; but VWC has only a slight effect on the residual axial stresses at the outer surface and axial distortion. Moreover, the residual stresses decrease and are lower than the yield strength using VWC, which decreases the susceptibility of a weld to fatigue damage, stress corrosion cracking and fracture, and improves the safety of welded structures.     

Microstructure and sulfide stress corrosion cracking of the Inconel 625/X80 weld overlay fabricated by cold metal transfer process

The use of cold metal transfer process (CMT) is an excellent alternative to depositing nickel-based alloys on oil and gas pipelines due to the relatively small dilution. However, sulfide stress corrosion cracking (SSCC) of dissimilar weld joints has proven to be an issue. Therefore, this study investigates the microstructure and SSCC behavior of an Inconel 625/X80 weld overlay fabricated CMT process focusing on the tempering effect. Four-point bending tests show a high SSCC susceptibility of the non-overlap zone due to the formation of coarse structure in the heat affected zone and narrow martensite layer along the fusion boundary. However, microstructural characterization indicates a tempering effect in the overlap zone, producing homogenized microstructure and less residual strain in the heat affected zone. Therefore, the SSCC resistance is greatly improved, especially for the fusion boundary, because reversed austenite in the martensite layer could decrease the accumulation of hydrogen at lath boundaries.     

Stress field and failure probability analysis for the single cell of planar solid oxide fuel cells

An analytical model is developed to predict the residual thermal stresses in a single cell of solid oxide fuel cells (SOFCs), which consists of a thick porous 8 mol% Y₂O₃ stabilized zirconia/nickel oxide (8YSZ/NiO) anode, a dense 8YSZ electrolyte and a porous lanthanum strontium manganite (LSM) cathode. The simulated stresses in the cell at room temperature, which are resulted from the contraction mismatch of its components, indicate that the major principal stress in the anode is tensile while the electrolyte and cathode are under compressive stresses. The stress in one component decreases with the increase of its thickness when the thicknesses of the other two components are fixed, and the decrease of the tensile stress in the anode will cause the increase of the compressive stresses in both the cathode and the electrolyte, and vice versa. The analysis also reveals that the anode is the part that is most susceptible to fracture since the tensile thermal stress is so high that it reaches to the fracture strength of the anode material. The Weibull statistic is employed to estimate the failure probability of the anode. The simulation results indicate that the anode failure probability decreases with the increase of the anode thickness and the decrease of the electrolyte thickness. To keep the anode failure probability less than 1E−06, the anode thickness should be greater than 0.7 mm for a cell with an electrolyte thickness of 10 μm and a cathode thickness of 20 μm.     

Stress changes in electrochromic thin film electrodes:: Laser beam deflection method (LBDM) as a tool for the analysis of intercalation processes

The mechanical effects of the intercalation processes in electrochromic WO₃ thin films are reported here and discussed. In particular the electrochemical insertion of H, Li and Na ions in WO₃ was studied by means of laser beam deflection method (LBDM). Linear changes of WO₃ stress were observed for small amounts of the inserted charge and linearity was always associated with a reversible mechanical behavior of thin films. An explanation in terms of homogeneity of the insertion is given. An analogous trend was also verified in the absorbance vs. charge curves. As a consequence the constancy of the electrochromic efficiency values for the three different ions was found. In this way it was possible to determine a full reversible behavior for M_xWO₃ in the composition range 0x0.2. The onset of new phases formation when x exceeded previous upper limit, was observed during Li and Na intercalation. Such transitions brought about the loss of optical and mechanical reversibility. During prolonged hydrogen insertion a reversible slope inversion occurred in stress curve so that it was necessary to take into account different possible mechanisms of the WO₃ electrochromic reaction.     

Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

In this work, the influence of hydrogen on the microstructure and fracture toughness of API 5L X80 high strength pipeline steel welded by friction stir welding was assessed. Samples were hydrogenated at room temperature for a duration of 10 h in a solution of 0.1 M H₂SO₄ + 10 mg L⁻¹ As₂O₃, with an intensity current of 20 mA cm⁻². Fracture toughness tests were performed at 0 °C in single-edged notched bending samples, using the Critical Crack Tip Opening Displacement (CTOD) parameter. Notches were positioned in different regions within the joint, such as the stir zone, hard zone, and base material. Hydrogen induces internal stress between bainite packets and ferrite plates within bainite packets. Besides, hydrogen acted as a reducer of the strain capacity of the three zones. The base metal had a moderate capacity to resist stable crack growth, displaying a ductile fracture mechanism. While the hard zone showed a brittle behavior with CTOD values below the acceptance limits for pipeline design (0.1–0.2 mm). The fracture toughness of the stir zone is higher than that of the base metal. Nevertheless, the stir zone displayed higher data dispersion due to its high inhomogeneity. Hence, it can also show a brittle behavior with critical CTOD values.     

Fuzzy-inference-based failure mode and effects analysis of the hydrogen production process using Thermococcus onnurineus NA1

Hydrogen energy can be effectively converted from various energy sources, such as renewable energy or hydrocarbon fuel, and therefore, it is a promising energy source. Various hydrogen production processes have been proposed worldwide because conventional energy conversion systems, after minor design modifications, can be used for such conversions. However, hydrogen gas can be more dangerous than other flammable gases if released into the atmosphere, where it can rapidly reach its explosion limit because of its high diffusion velocity. Therefore, a failure mode and effects analysis (FMEA) for hydrogen production processes is required. In this study, FMEA is performed for hydrogen produced from coal syngas using Thermococcus onnurineus NA1, which was discovered in the Indonesian deep sea. A fuzzy inference methodology is introduced for a quantitative analysis of the linguistic ambiguity of risk priority number estimated from a conventional FMEA. To estimate the objective severity ranking, we introduced the potential asset loss combined with the proportionate cost of each piece of equipment under total capital investment and a weight value influenced by environmental and mankind. Moreover, we proposed a fuzzified risk matrix to effectively represent the fuzzy risk priority number (f-RPN) under the risk matrix; variation with and without fuzzification of risk priority number is then expressed to ascertain why this variation has occurred through a vector diagram. Based on the f-RPN vector diagram, we have performed a design revision of the hydrogen production process using Thermococcus onnurineus NA1 to adjust the risk priority number downward from a broadly unacceptable region.     

Determination of the autofrettage pressure and estimation of material failures of a Type III hydrogen pressure vessel by using finite element analysis

The autofrettage process of a Type III hydrogen pressure vessel for fuel cell vehicles with preset winding pattern was simulated by finite element analysis (FEA). For a precise finite element analysis, the ply based modeling technique was used for the composite layers; a contour function was derived for the fibers at the dome part to determine the exact winding angle; and the exact composite thickness was also considered. In order to determine the most appropriate autofrettage pressure, stress analysis of the pressure vessel according to its internal pressure was carried out with consideration of the international regulations about pressure vessel design. The minimum stress ratio, the permanent volumetric expansion and the generated residual stress were investigated, and the failure of the pressure vessel under minimum burst pressure was predicted by application of various failure criteria of anisotropic composites.     

Holistic minimization of the life cycle environmental impact of hydrogen infrastructures using multi-objective optimization and principal component analysis

Assessing the environmental performance of hydrogen infrastructures is essential for determining their practical viability. Previous optimization approaches for hydrogen networks have focused on optimizing a single environmental metric in conjunction with the economic performance. This approach is inadequate as it may leave relevant environmental criteria out of the analysis. We propose herein a novel framework for optimizing hydrogen supply chains (SC) according to several environmental indicators. Our method comprises two steps. In step one, we formulate a multi-objective mixed-integer linear program (MILP) that accounts for the simultaneous minimization of the most relevant life cycle assessment (LCA) impacts. Principal Component Analysis (PCA) is next employed in the post-optimal analysis of the MILP in order to facilitate the interpretation and analysis of its solution space. We demonstrate the capabilities of this approach through its application to the design of the future (potential) hydrogen SC in Spain.