Abstract The Extended Finite Element Method (XFEM) has recently emerged as a highly effective tool for analyzing crack propagation in complex structures, but its use in pipeline fracture studies, particularly with cohesive zone models (CZM), is still developing. Current XFEM fracture criteria are not calibrated for pipeline steels, relying on fixed fracture stress or strain to initiate crack propagation. While the stress-based criterion works for brittle fractures, it fails for ductile ones, either accelerating cracks or preventing them altogether. The strain-based criterion better predicts both fracture types, but its numerical accuracy remains inadequate, highlighting a need for further research. This numerical study explores the use of XFEM to predict crack propagation in standard fracture specimens of single edge notch bending (SENB) made of X52 pipe steels. First, an XFEM-based cohesive zone model was developed to simulate the specimens. The maximum principal strain (MAXPE) and fracture energy (Gc) were selected as key damage parameters to characterize the fracture process, controlling crack initiation and resistance to crack propagation, respectively. These damage parameters were adjusted until the model closely matched experimental results (Load-crack tip opening displacement (CTOD)) for different initial notch sizes in (SENB) specimens. Subsequently, experimental results for (CTOD-R) and the strain distribution around the crack tip, both at crack initiation and during unstable crack propagation, were compared with the numerical model's predictions to validate the chosen XFEM input damage parameters. The research confirms the effectiveness of XFEM in predicting fracture characteristics.

Abstract Non-destructive evaluation techniques are increasingly applied to estimate the buckling load of shell structures without inducing failure. This study employs the force-stiffness (F-s) method to evaluate the buckling load of oblate ellipsoidal shells subjected to external pressure. Experiments on five steel shells with comparable R/t ratios are conducted, recording pressure, strains, and crown deflections up to failure. Using deflection data up to 80% of the buckling load, the F-s technique predicts buckling with ~90% accuracy, while incorporating crown strain data improves accuracy to ~95%. All shells fail within a pressure range of 2.7-3.4 MPa. The method is further validated on 3D-printed aluminum shells, showing promising consistency, though additional testing is required to refine accuracy. Finally, the F-s approach is demonstrated for real-time prediction of a large-scale shell, nearly three times larger than the experimental models, highlighting its scalability and potential for structural applications.

Abstract A finite element (FE) model is developed to predict the burst pressure of internally corroded elbows with high accuracy and reliability. This model considers various parameters, including the length, depth, and width of the corrosion defect, as well as the pipe diameter and elbow bending radius. It is found that increases in the depth and length of the corrosion defect have a significant impact on the reduction of the corroded elbow burst pressure. Meanwhile, the corrosion defect width has a minimal effect. Corroded pipeline elbows with a smaller diameter or bending radius are more prone to burst than those with larger dimensions. A novel burst pressure prediction model for corroded pipeline elbows is proposed, and its predictions are compared with the FE simulations and several existing models. The mean value and coefficient of variation (COV) of the ratio between the FE results and the proposed model's predictions are 0.994 and 4.42%, respectively. These results demonstrate that the proposed model provides more accurate predictions than other models. This study offers a reliable foundation for predicting burst pressure and evaluating the integrity of pipeline elbows with internal corrosion defects.

Abstract In ocean engineering, spherical shells often serve as common pressure vessels. Under external pressure, these shells tend to suffer from buckling damage. To ensure the stability of spherical shells and optimize their structural design, it is necessary to predict their buckling load under external pressure. However, current predictions based on the knockdown factors are relatively conservative, and predictions based on common machine learning methods lack physical orientation. Based on the energy barrier method, a physics-informed 1D-CNN model is proposed to address this problem. These newly developed physics-informed features and loss functions improve the accuracy of the model. Both metallic and non-metallic spherical shells datasets are used to establish the models. Then, we determine the optimal hyperparameters using K-fold cross-validation and the Optuna optimization framework and compare its predictive performance against models like Decision Trees (DT), Random Forests (RF), and Artificial Neural Networks(ANN). The results indicate that our model outperforms the others in predictive accuracy. Additionally, the potential of th model, guided by the energy barrier method, is demonstrated by comparing various loss functions. For the actual spherical shell, the trained model can be used for buckling analysis and the optimized design of spherical shells in engineering practice.

Abstract Pressure-reducing valve is the most vital component of the hydrogen supply system for the on-board fuel cell stack. A clear understanding of the decompression behavior can help to optimize the valve structure. In this work, a two-step pressure-reducing system is proposed, consisting of a conical throttling valve and a disc throttling valve. The decompression process of high-pressure hydrogen gas is numerically and experimentally investigated in the two-step pressure-reducing system. The flow characteristics are represented by analyzing the variations of flow variables (e.g., pressure, temperature, velocity and turbulent quantities) at different conditions. A linear relationship between the inlet pressure and the mass flow rate is achieved. Besides, the decompression performance is evaluated for the first and the second throttling part separately. The experimental results show that the two-step valve can successfully reduce the hydrogen pressure of 35 MPa to 0.14 MPa, which meets the requirement for the fuel cell stack. The gas leakage tests confirmed that the valve has a good gas tightness, eliminating the safety concern during use. This work is expected to offer in-depth insights into the hydrogen decompression behavior and provide references for the design of pressure-reducing valves.

Abstract To verify the structural integrity of power generation facilities, many fracture toughness tests are being performed on high-toughness materials, and efforts are being made to obtain reliable J-R curve. In the case of nuclear piping system to which Leak Before Break (LBB) design is applied, high-toughness materials are used, and fracture toughness testing is also required to be conducted until sufficiently long crack extension is possible. However, when performing a J-R curve testing of a high-toughness material using a compact tension (CT) specimen to a crack extension size as long as possible, it is difficult to obtain reliable test results due to the phenomenon of rotation of the specimen. In our previous paper, the author proposed a new crack size estimation method considering the effect of load line changes due to specimen rotation and proved its validity through testing. In this study, the mechanical mechanism for crack size estimation from rotated specimens has been investigated. In addition, the crack size prediction accuracy of the ASTM E1820 method and the proposed method has been compared and evaluated through finite element (FE) analysis of specimens deformed by rotation. In the FE analysis, the proposed method showed better accuracy than the ASTM E1820 method. Thereby, it was verified that the author?s method for estimating crack size considering rotation of CT specimens has better accuracy in both FE analysis and experimental evaluation compared to the ASTM E1820 method.

Abstract The efficiency, quality, and reliability of shell and tube heat exchangers are significantly influenced by the design and execution of tube-to-tubesheet (TTS) welding. The key factor includes the selection of materials, welding process, and accuracy of dimensional specifications. There are three commonly used TTS joint designs such as the expanded joint (no welding), seal-welded joint (single pass welding with or without expansion), and strength-welded joint (two pass welding followed by expansion) as per the American Society of Mechanical Engineers (ASME) Section VIII Div. 1 UW-20. An extensive literature review revealed a strong reliance on various welding processes but the designs were limited to traditional joint configurations. In the proposed study, an attempt was made to investigate TTS welding using the gas tungsten arc welding (GTAW) process with a three pass or three layer technique following the inspection standard (ASME Section IX QW 193). Experimental investigations were conducted on a 50 mm thickness carbon steel tubesheet (SA 516 Gr. 70 N) and 19.05 mm outside diameter (OD) with 2.11 mm thickness of carbon steel tubes (SA 179) using a GTAW welding machine for ten samples. The quality of the TTS joint was assessed through various inspection techniques in accordance with ASME Section IX QW 193.1-191.3, encompassing visual inspection, liquid penetrant testing, and macroscopic examination to identify the minimum leak path (MLP). The experimental findings demonstrated that the three pass GTAW technique significantly enhances the quality and reliability and also extends the life of TTS welds in heat exchangers due to better MLP.

Abstract Localized metal loss (LML) in pressurized tubes is a primary contributor to tube failures and unplanned shutdowns in industrial boilers. Initially, these tubes may have inconspicuous anomalies introduced during manufacturing, testing, and inspection. Over time, as the vessels are deployed and exposed to harsh environmental, operating, or aging conditions, these anomalies evolve, eventually leading to their failure. In this paper, detailed investigative studies on various metal loss flaws were carried out to acquire insights on their failure behavior and mechanism, especially as they progress from very small size to lengthy size. Three investigations were carried out to identify which geometric parameters are most critical for predicting failure of the tubes and to what extent will they influence their failure. First, a range of real flaws that failed while operating under high internal pressure in a steam boiler were considered. The study compared two different modeling methods for evolving flaws to their failure geometry. The outcome shows that for most flaws, the failure evaluation is insensitive to the method of evolving the flaw to failure. Second, all the flaw geometries were modeled using the same material and operating conditions and their failure behavior studied. The findings show that failure pressure depends solely on the flaw thickness, and not on the other flaw dimensions, except for pinhole flaws. Finally, a range of flaw geometries, spanning the space from localized pinhole flaws to wide lengthy flaws, were investigated. Outcomes of these studies show that failure pressure for small flaws is sensitive to the flaw geometry. However, the failure pressure ceases to depend on the flaw length once the flaw length is greater than twice the tube diameter, and ceases to depend on the flaw width when the flaw length is greater than the tube diameter. These results suggest that gross plastic collapse is not the dominant failure mechanism for small flaws. However, for flaws with a length at least twice the tube diameter, the flaw dimension which predicts failure is the tube remaining thickness in the deepest part of the flaw. These studies establish the applicability limits of gross plastic collapse analysis for pressurized tubes experiencing metal loss and demonstrate opportunities for simplifying flaw analysis for sufficiently large flaws. Ultimately, the insights from this study will contribute toward appropriate preventive maintenance of these critical assets while in operation.

Abstract The emergence of welding defects and uneven distribution of mechanical properties will easily make the welded joint a weak part of the pipe structure. It is prone to failure when further subject to complex loads. Therefore, optimization of the welding process and design of the strength and toughness properties of the welded joints are important means to ensure the safety and smooth operation of the pipeline. To this end, five X80 steel girth welding joints with different welding processes were manufactured in this study. Then, through the hardness test, small punch test and three-point bending test, the distribution laws of the hardness, strength, and fracture toughness of the five different girth welding joints were determined, respectively. On this basis, the main control factors on the mechanical properties in the welding process parameters were identified. Among them, automatic welding method made the weld zone having high strength properties, but poor fracture toughness compared with the semi-automatic method. In the automatic welding process design, attention should be paid to avoid the use of excessive strength of the welding material to ensure its toughness properties. When reducing the welding speed in semi-automatic welding process and making the welding heat input significant increased, the strength properties of the heat-affected zone and weld zone decreased, meanwhile the fracture toughness of the weld zone decreased significantly. In the semi-automatic welding process design, the welding current and speed parameters should be strictly controlled to ensure the mechanical properties of the welded joint.

Abstract With the Earth's temperature steadily rising due to carbon dioxide emissions, transitioning to alternative energy sources like hydrogen is imperative. However, the safe and efficient transport of hydrogen poses significant challenges, particularly regarding structural design and compliance with regulatory codes. This study evaluates the structural performance of carbon steel piping systems for hydrogen transport, comparing the ASME B31.12 and ASME B31.3 design codes. Using the Allowable Stress Design, maximum allowable pressures (MAP) were analyzed across various piping configurations, with emphasis on the penalties imposed by the material performance factor (Mf) in ASME B31.12 to address hydrogen embrittlement. The results underscore the superior performance of certain materials in resisting pressure penalties, especially those with higher efficiency under hydrogen service conditions. Additionally, the analysis revealed a significant discrepancy in allowable pressures between the two codes, particularly in small-diameter pipes, where the effects of Mf are most pronounced. These findings emphasize the tradeoffs between safety, structural performance, and cost-effectiveness in hydrogen piping design, providing a robust foundation for advancing sustainable hydrogen energy systems.