Plastic and ductile fracture behaviour is studied in AISI 304L austenitic stainless steel widely used for pipes and pipelines. The specimens were manufactured from three different heats, including standard and notched cylindrical specimens subjected to tensile loading, notched tubes subjected to torsional and tensile loading, small punch testing, grooved hollow cylinder subjected to compression and compression–shear specimens. This complex experimental campaign maps the ductile fracture of the investigated material for various stress states in the broad range of stress triaxiality and normalized third invariant of deviatoric stress tensor to bring a value to failures of piping made of 304L steel. The elastic–plastic behaviour is modelled using the von Mises yield criterion and the newly proposed one dependent on the normalized third invariant of deviatoric stress tensor and equivalent plastic strain, which better predicted the material responses, especially in stress states close to generalized shear. However, it still predicted greater necking in the notched tube under tension, so a parametric study was conducted for this type of specimen. Numerical simulations served to calibrate the three uncoupled ductile fracture criteria, which were validated using the three newly designed specimens. The validation was based on the force responses, deformations and failures of two grooved hollow cylinders under compression and of the compression–shear specimen. The elastic–plastic–damage models were implemented through the VUMAT user subroutine in the commercial code of Abaqus with crack initiation and propagation realized using the deletion of finite elements.
The present study systematically investigated the high-cycle fatigue (HCF) behavior of a 12%Cr/30Cr2Ni4MoV dissimilar metal welded joint (DMWJ) for steam turbine rotor applications under testing conditions of 330°C. Fatigue parameters and fatigue curves were obtained for the base metals (BM-1 and BM-2), buttering layer (BL), weld metal (WM), and heat-affected zones (HAZ-1 and HAZ-2). Although the HAZs had finer grain sizes, higher proportion of high-angle grain boundaries, and relatively higher dislocation densities than the BMs, they exhibited lower fatigue strengths. Fracture in the HAZ specimens occurred in the weld regions adjacent to the fusion boundary (FB), and HAZ-1 and the BL showed the lowest fatigue strengths. The carbon-denuded layer (CDL) near FB-1 between HAZ-1 and BL was identified as the weakest micro-region in the entire DMWJ. The blocky ferrite and coarse carbides in BL make its fatigue performance weak. In the WM, the staggered distribution of needle-like bainite, finer grains, and a relatively higher dislocation density collectively contributed to the enhanced fatigue strength. Dislocation density increased after HCF testing, with dislocations primarily accumulating around carbides. The dispersed small carbides on BM-1 and BM-2 can effectively pin dislocations, thereby improving fatigue strength. Coarse Cr7C3 carbides caused fatigue damage in the BL and WM. A carbide-related fatigue fracture model was proposed to explain crack propagation in these regions. Achieving superior high-cycle fatigue properties in the 12%Cr/30Cr2Ni4MoV welded rotor requires further optimization of both the chemical composition of the BL and the corresponding welded technique.
Since the 1972 establishment of the ASME Boiler and Pressure Vessel Code's fracture toughness index—specifically the reference nil-ductility transition temperature (RTNDT)—drop weight and Charpy V-notch tests for transversely oriented specimens have been required for evaluating ferritic materials. However, nuclear power plants constructed under pre-1972 codes often lack sufficient data for fracture toughness assessments. To address this, the U.S. NRC developed NUREG-0800 Branch Technical Position (BTP) 5-3. Although initially intended for older plants, BTP 5-3 remains essential for post-1972 plants when material test data are incomplete or missing. This paper evaluates the applicability of BTP 5-3 by analyzing test results for ferritic pressure vessel materials in South Korean nuclear power plants. The study assesses the conservatism of the BTP 5-3 methodology and proposes an enhanced approach that utilizes shear fracture appearance information to mitigate excessive conservatism. Product-form-specific reference temperature ranges are derived, providing explicit decision criteria for verifying RTNDT estimates and a conservative surrogate where unirradiated data are entirely absent. The results provide a reliable framework for maintaining the structural integrity of reactor pressure vessels even when available data is limited.
The structural integrity of corroded steel pipelines is a critical concern in fluid transportation systems because corrosion-induced defects can significantly reduce load-bearing capacity and lead to catastrophic burst failure. Although the finite element method (FEM) has been widely used to predict burst pressure and mechanical response, the progressive deformation behavior prior to final failure and its relationship with acoustic emission (AE) characteristics remain insufficiently understood. To address this limitation, this study proposes an FEM-guided AE framework for stage-wise damage identification in corroded steel pipelines subjected to internal pressure loading. In the proposed methodology, raw AE signals are processed using band-pass filtering, wavelet denoising, STA/LTA-based event detection, and pressure synchronization. Simultaneously, FEM simulations are conducted to evaluate the structural response of corroded pipelines and identify representative deformation stages associated with the Yield point, ultimate tensile strength, and burst failure. AE event activity and signal features, including peak amplitude, cumulative energy, and frequency-domain characteristics derived from Fast Fourier Transform, are analyzed in relation to the FEM-derived deformation stages through pressure-based synchronization. The results demonstrate that the evolution of AE characteristics is associated with the FEM-derived deformation stages. The proposed FEM-guided AE framework provides a physically meaningful interpretation of AE responses by linking signal evolution with mechanically defined damage stages. This approach improves the understanding of deformation and failure mechanisms in corroded steel pipelines and demonstrates the potential of integrating FEM and AE techniques for structural health monitoring and failure assessment.
This study investigates the fatigue crack growth (FCG) behavior of IN718 nickel superalloy across a temperature range from room temperature (RT) to 600°C. The primary objective is to understand the temperature-dependent variations in fatigue damage mechanisms and crack propagation under cyclic loading. At RT, fatigue damage is primarily driven by dislocation slip, as evidenced by the formation of slip lines within the γ matrix, with cracks propagating transgranularly. Carbide inclusions were identified as initiation sites, influencing the crack path and deviations within the γ matrix. Experimental results show a notable increase in crack growth rates with rising temperature. Specifically, the crack growth rate (da/dN) exhibits a nonlinear increase with the stress intensity factor range (ΔK), particularly at elevated temperatures. Further analysis of damage mechanisms reveals that oxidation and dislocation pile-up at grain boundaries significantly contribute to crack initiation and propagation at higher temperatures, as confirmed by SEM and EBSD analysis. A temperature-dependent crack growth model, based on the Arrhenius equation, was developed to predict the crack growth rate, effectively capturing the temperature sensitivity of the material's fatigue behavior. The model's predictions are in good agreement with experimental data, demonstrating its reliability for high-temperature applications. This proposed model serves as a valuable tool for predicting fatigue crack growth in materials used in critical high-temperature environments, such as gas turbines and aerospace components, where material performance is essential.
The presence of backing plates significantly affects the stress distribution and fatigue behavior of butt joints. This study employs the strain energy density (SED) method to systematically investigate the effects of tack weld spatial layout (including offset distance and number), weld geometry, and plate thickness on the fatigue failure characteristics of 6082-T6 aluminum alloy butt joints with backing plates. The predicted fatigue failure locations are validated against observations from fatigue tests. The results demonstrate that reducing the tack weld leg size increases the SED at the tack weld by 37%, whereas increasing the plate thickness raises it by 52%, relative to butt joints with a 12 mm plate thickness and a 6 mm tack weld leg size. The number of tack welds is a key factor governing the fatigue failure location: a single tack weld promotes failure initiation at the butt weld root, whereas two tack welds shift crack initiation to the tack weld toe. By contrast, the relative tack weld positions do not change the failure site, although offset configurations reduce the SED at the critical locations by 4.9% for a single tack weld and 12% for two tack welds.
This paper proposes a new method for extracting two creep constants of a material that exhibits a hyperbolic sine constitutive creep relationship using a ball indentation creep test. Extensive finite element elastic-plastic-creep analyses were performed to assist the analytical development of the extraction method, which does not require further finite element analysis, for obtaining the three principal stresses and strain rates at a reference point during the indentation creep test. The reference point lies on the axisymmetric axis just beneath the ball indenter. Mises stresses and Mises strain rates at the reference point were calculated using the indentation stresses and strain rates during the indentation creep test. Two creep constants in the hyperbolic sine constitutive relationship were extracted from the Mises stress–Mises strain rate relationship. Experimental indentation creep tests were performed on a modified 9Cr-1Mo steel, which exhibits a hyperbolic sine creep constitutive relationship at 650 °C, and the two creep constants were extracted experimentally following the developed method. The Mises stress–Mises strain rate relationships obtained from the indentation creep tests were compared with those from uniaxial creep data. The extracted relationships showed reasonable agreement with those obtained from uniaxial creep experiments, confirming the applicability of the newly developed extraction method.
This study develops a continuous-parameter creep model for reactor pressure vessel (RPV) steel SA533B1 based on the Liu–Murakami continuum damage mechanics (CDM) framework to overcome the limitations of conventional creep models in capturing temperature-dependent creep behavior and tertiary creep over broad temperature ranges. Globally continuous functions are established for the material constants, allowing the model parameters to vary continuously over the severe-accident temperature range of 900–1623 K. This formulation avoids unphysical parameter discontinuities and rigid regime-switching in transient thermo-mechanical simulations. The model was calibrated using INEL and JAEA datasets and validated against OECD lower head failure (OLHF) tests. Compared with conventional creep models, the proposed model more accurately reproduces the transient displacement history and spatial damage localization up to rupture, improving the prediction of RPV lower-head failure under severe accident conditions of light water reactors.
An unexpected corrosion failure occurred in an ASTM A106 Grade B dimethyl disulfide (DMDS) injection spool. The root cause of this failure was investigated using visual and field inspections (videoscopy), metallography, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), and DMDS compositional analysis. The system had operated with neat DMDS at ∼33 °C and ∼9 bar for approximately four years, experiencing periodic stagnation during plant shutdowns. Both field and laboratory observations revealed localized pitting and under-deposit corrosion beneath fragile deposits on the internal surface. SEM analysis revealed under-deposit "roof-like" morphologies and fractured corrosion products, while EDS maps indicated an enrichment of sulfur and oxygen in the affected regions. XRD analysis of the scraped deposits identified magnetite and γ-FeOOH, along with a weak feature consistent with mackinawite; however, non-stoichiometric iron sulfide (Fe1−xS) could not be definitively resolved. Furthermore, a DMDS assay confirmed the commercial purity of the chemical, containing only trace amounts of total iron (∼0.2 ppm). Based on the maximum pit depth of 2.06 mm observed over the four-year service period, a conservative average pit penetration rate of ∼0.515 mm/y (∼20.3 mpy) was estimated. The findings indicate that the in-situ decomposition of DMDS was catalyzed by trace iron and exacerbated by moisture ingress during idle periods. The resulting reactive sulfur species (H2S and mercaptans) produced a mildly sour environment despite the absence of an external H2S source. Therefore, systems handling sulfur-bearing organics such as DMDS may require sour-service controls, including the use of suitable alloys or coatings and strict moisture management.
To investigate the failure mechanism of X80 pipeline steel in hydrogen environments, this study developed a mathematical model describing the evolution of its mechanical properties during hydrogen diffusion. Material constitutive parameters accounting for hydrogen diffusion effects were incorporated, and a numerical model of crack propagation under force-hydrogen coupling was established. Multi-scale testing and analysis were conducted to characterize the crack-propagation behavior and microstructural evolution of X80 pipeline steel during hydrogen diffusion. The results indicate that with increasing pre-hydrogen charging time, the average grain size decreases, the variation in crystal orientation becomes more pronounced, and the geometrically necessary dislocation (GND) density increases gradually. Hydrogen-induced dislocation density increases around grain boundaries and intragranular Fe3C carbides, accompanied by intensified dislocation pile-up. The stress gradient is identified as a key factor regulating hydrogen accumulation, influencing the distribution of hydrogen concentration in micro-regions. Additionally, as hydrogen charging time increases, the extent of the high-stress region at the crack tip increases, the specimen's equivalent stress increases gradually, and the rate of increase in the maximum equivalent stress shows an upward trend. This study provides theoretical support for the quantitative prediction of hydrogen-induced crack propagation behavior and offers reference values for the safety assessment of pipeline steel in hydrogen-containing environments.
The application of TC4/304 dissimilar welded joints was of paramount significance across diverse aerospace and industrial sectors.However, the presence of brittle Ti-Fe intermetallic compounds (IMCs) seriously damages the mechanical properties of TC4 and 304. The study proposes a novel method of adding filling metals, a laser pre-melting strategy, by introducing CeO2 Ceramic particles into the FeCoNiCr high-entropy alloy molten pool. The addition of FeCoNiCrCeO2 leads to the refinement of the microstructure of the matrix and the welded joint and the formation of strengthening phades through the element competition mechanwasm. Compwered with the unfilled welded joint, the introduced CeO2 achieved in-situ reaction, generating CeTi2O6, and the precipitation of Fe4Cr phade could be observed. These phenomena indicate that the generation of the Ti-Fe IMCs layer had been successfully suppressed. As revealed by the tensile test investigation, the joint strength had increased from 65.33 MPa to 210.4 Mpa, and a relatively excellent weld appearance and refined microstructure had been obtained. Evidenced by the fractography, the failure mechanwasm shifted from a purely brittle to a mixed ductile-brittle character. The research had developed a new method for obtaining high-performance welded joints of titanium/steel dwassimilar metals, and had obtained valuable experimental data and theoretical analysis.
A machine learning framework is presented that simultaneously identifies elastoplastic material parameters and non-equi-biaxial residual stress from single spherical indentation tests, addressing the inherent non-uniqueness problem in indentation-based inverse analysis. A three-dimensional finite element model with Voce hardening was developed to generate a training dataset of 1400 cases spanning diverse material properties and residual stress states. A multi-layer perceptron neural network was designed to process force-displacement curves and residual imprint profiles from two orthogonal directions, extracting six parameters: elastic modulus, three Voce equation parameters, and two normalized residual stress components. Comparative analysis revealed that networks trained on force-displacement curves alone accurately predicted elastic modulus but failed to identify residual stresses, while models using only residual profiles determined stresses but not elastic properties. Integrating both data types yielded an order-of-magnitude improvement over single-source approaches. Robustness evaluation showed that for friction coefficients within the typical experimental range (0.15–0.40), flow stress predictions maintained mean absolute percentage errors of 2.2–3.2% and residual stress predictions achieved coefficients of determination of 0.986–0.994. By recovering the principal residual stress directions without prior alignment of the measurement frame and remaining robust to perturbations that reproduce the discrepancy between finite element analysis and experiment, the framework demonstrates practical applicability for simultaneous characterization of material behavior and residual stress from a single test.
The long-term thermal stability of FeCrAl ODS steels for use in lead-cooled fast reactors remains a critical issue. The 15Cr-4Al-2W-0.1Ti-0.6Zr-0.35Y2O3 (wt. %) ODS steel, following a 10,000 h aging treatment at 700 degrees C, was studied by HRTEM and S/TEM to evaluate the thermal stability of the matrix grains and the phase, distribution and matrix/particle interface structures of particles. The matrix grain size slightly increased from 0.97 mu m to 1.0 mu m. For the particles, the average size shifted from 5.1 nm to 5.2 nm; the interparticle spacing increased from 48.1 nm to 51.4 nm, and the number density reduced from 7.26 & times; 1022 m-3 to 6.92 & times; 1022 m-3. The fraction of Y-Zr-O particles shifted from 86.5% to 84.7%, while that of Y-Ti-O changed from 8.3% to 9.5%. The fraction of coherent/semi-coherent oxides shifted from 94.2% to 91.7%. Kinetic analysis of the oxide coarsening behavior reveals that the exceptional resistance to Ostwald ripening is reflected by an extremely low coarsening rate constant (KLSW approximate to 2.8 & times; 10-35 m3/s). This stability arises from the low diffusivity of Y and Zr in the ferritic matrix and, more critically, from a Zr-induced reduction in interfacial energy. By enhancing interfacial coherency, Zr significantly diminishes the thermodynamic driving force for particle growth. As a consequence of the thermally stable multiscale microstructure, the hardness decreased marginally from 351 HV to 334 HV.
In this study, a new vertical linear electromagnetic stirring (V-LEMS) method was proposed to regulate the microstructure of Incoloy 800H alloy, and the effects of this method on the creep behavior of the alloy under 750 °C/90 MPa conditions were systematically investigated. The results show that the application of V-LEMS can effectively remove the large size TiN particles from the alloy and increases the proportion of fine TiN particles by 25.6%. This reduced the number of creep cavity nucleation sites associated with large TiN particles and simultaneously decreased the probability of particle-stimulated nucleation (PSN)-induced discontinuous dynamic recrystallization (DDRX). The fine TiN particles pinned the grain and subgrain boundaries, which significantly hindered the movement of dislocations and the migration of grain boundaries, thereby maintaining a high dislocation density and enhancing grain boundary stability. The more homogeneous distribution of Ti(C,N) particles alleviated the accumulation of local orientation discrepancies and mitigated microstructural softening caused by local recrystallization. In particular, the volume fraction of Σ coincident site lattice (CSL) grain boundaries in the Incoloy 800H alloy increased significantly after V-LEMS was applied. This low-energy grain boundary network delayed the expansion of cavities and the connection of cracks during the creep process, representing the primary mechanism for the reduced creep rate and the prolonged minimum creep rate stage. The Incoloy 800H alloy prepared by applying V-LEMS enhanced the microstructural stability through a synergistic regulation mechanism dominated by ΣCSL grain boundaries and aided by TiN and Ti(C,N) particles, which strengthened its resistance to DDRX and creep damage, consequently extending the creep life of the alloy from 371 h (sample without EMS) to 422 h under 750 °C/90 MPa.
Magnetic Flux Leakage (MFL) technology is widely applied in oil and gas pipelines. Reliable defect identification from large-scale MFL signals remains challenging since background magnetic disturbance and noise induce blurred defect boundaries and reduced feature clarity. Moreover, defect identification involves large-scale data processing for defect localization and fine-grained feature analysis for defect classification. The two tasks have different requirements for computational efficiency and feature extraction. To address this issue, a cascaded deep learning model is proposed to improve the reliability of defect localization and classification. In the localization stage, a one-stage detection strategy is employed to scan long-range MFL data and extract magnetic anomaly regions. Multi-channel MFL data comprising axial, radial and circumferential components are jointly analyzed to improve the precision of defect localization. In the classification stage, a self-supervised pretrained feature extraction model is proposed to analyze localized defect regions. The model exhibits good generalization under small samples and can capture subtle differences in amplitude, gradient, and spatial distribution under noise. Experiments using a self-constructed defect MFL dataset show that the proposed method achieves a detection precision of 96.03% and a classification accuracy of 87.04%, with high computational efficiency. The t-SNE visualization shows the reliability of the proposed method.
Structural integrity of nuclear components under excessive loading by a large earthquake becomes a big issue. A crack growth evaluation procedure beyond small scale yielding condition (SSY) under cyclic loading has not been established. The FDF-III subcommittee of the Atomic Energy Research Committee of Japan Welding Engineering Society (JWES) has been tackling this issue. Crack growth analysis typically uses Delta Kas a parameter, which is not applicable to elastic-plastic state beyond SSY. Delta J is a candidate parameter for crack growth analysis beyond SSY under cyclic loading. Since it takes time to calculate J-integral by finite element analysis (FEA), several simplified J-solutions for pipes and plates have been developed. However, the applicability of those solutions is limited, and their calculation accuracy has not been confirmed enough. The authors investigated appropriate simplified Jsolutions based on the reference stress method for a pipe with a circumferential surface flaw under bending and evaluated the accuracy of the reference stress method by comparing its results with those obtained by FEA. Since some gaps between the simplified and numerical J-integrals were found at the stress levels just below the yield stress, the authors introduced adjustment parameters into the simplified J-solution based on R6. As a result, the calculation accuracy was improved due to this new formula.
The cold metal transfer (CMT) process has proven to be an optimal wire arc additive manufacturing (WAAM) technique for the economical production of large, complex duplex stainless steel (DSS) components. However, it significantly influences the microstructure and ferrite-to-austenite ratio of the alloy. Hence, a detailed investigation of the microstructural evolution and its effects on mechanical properties and corrosion resistance is essential before deploying it in applications. This study investigates the mechanical properties and stress corrosion cracking (SCC) behaviour of a WAAMed cylindrical component fabricated from DSS 2209 alloy via the CMT process. The microstructure analysis of the cylinder revealed a gradual decrease in the ferrite fraction and an increase in grain size along the build direction. The austenite phase has higher nanohardness than ferrite due to its lower stacking-fault energy and strengthening from partitioned nitrogen. In response to the growth in grain size along the build direction, the microhardness decreased. The tensile properties of the component met the all-weld mechanical property requirement of the ER2209 filler. The hoop strength was higher than the uniaxial tensile strength, but its ductility was comparatively low. In a boiling MgCl2 solution (45 wt%), SCC tests were performed at 20%, 40%, 60%, and 80% of the yield strength, and the resulting corrosion elongation curves were analysed. The fractographic study of the SCC-tested samples revealed that lateral cracks were initiated by preferential dissolution of ferrite in the chloride environment. The primary crack propagated by quasi-cleavage fracture and dissolution of the ferrite phase.
