Toward a sustainable society, there is a demand for multi-material structures that utilize aluminum, which has high electrical conductivity, is lightweight, and is easy to recycle. To realize the manufacturing of multi-material structures, the challenges are to develop a low-cost and versatile aluminum dissimilar material joining technology and a structure that facilitates a circular economy. We have clarified a short-time bonding technique of less than one second that uses a eutectic reaction induced by electric current in air to fracture the base material, and have systematized the bonding conditions. In this study, in order to clarify the bonding mechanism of the current-induced high-speed eutectic reaction, bonding experiments and observations were carried out, and the interface behavior and morphology were analyzed during the interface temperature rise process and the isothermal process after the eutectic temperature of 548°C was reached. As a result, in the isothermal process, while the metabolism of the generation and discharge of the eutectic melt was repeated, the thickness of the reaction layer at the interface was constant (thickness 2.0 to 2.9 μm), interface morphology was maintained, and the soundness of bonding behavior was confirmed. The time required for the metabolism to reach the bonding strength required for base material fracture was 0.1 s or more. It was confirmed that the ϒ1 phase was formed by solid-state diffusion at the aluminum/copper interface due to current induction, and it was found that the eutectic melt appeared from both the aluminum side and the copper side during the heating process. We have analyzed the behavior of the interface during current-induced high-speed eutectic bonding and clarified the importance of the metabolism accompanying the generation and discharge of the eutectic melt.

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The automotive industry is working to improve the fuel efficiency and reduce the running resistance of vehicles from the perspective of preventing global warming. As a countermeasure, the use of aluminum alloys is being used to reduce the weight of car bodies. Aluminum alloys are divided into wrought alloy and die casting alloy according to the manufacturing method, and wrought alloy are widely used for body parts, while die casting alloy are used for the integral molding of large parts. Against this background, joining technology is required for wrought and die casting alloys. Therefore, a clinching technique using resistance heat generation was investigated as a new joining technique. This method can join with a lower load than mechanical clinching because plastic flow is accelerated by the softening of the joint due to resistance heat generation. However, radial cracking was observed in a joint using F material aluminum alloy without heat treatment as the die casting alloy from a preliminary study, and the fracture surface observation confirmed that this was hot cracking. It is considered necessary to reduce the strain rate inside the joint to suppress hot cracking. Therefore, it is considered that the control of strain generation state by electrode force control is effective. In this study, the state of radial cracking and the suppression of radial cracking using electrode force control were investigated. As a result, it was found that reducing the applied pressure during applying current by electrode force control reduces the rapid deformation and strain rate of the joint and suppresses radial cracking.

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Most arc-welded joints in automotive components are lap fillet welded joints, and analyzing their joint strength is important for their effective maintenance and management. The infrared thermoelastic method measures stress distribution directly without contacting the structure and evaluates minute temperature changes in structures under cyclic loading conditions. However, thermal diffusion significantly impacts measurements at low-frequency cyclic loading, raising concerns about measurement accuracy. To simulate the thermoelastic effect, we developed a stress field-temperature field thermoelastic finite element (FE) analysis technique that calculates the heat transfer from the generated and absorbed heat due to fluctuations in tensile stress. In addition, we investigated the effect of load frequency on lap fillet arc-welded joints. The calculated stress distribution aligns with the thermoelastic stress distribution measured using the infrared ray method, implying the effectiveness of the proposed technique in reproducing the thermoelastic effect. Using thermoelastic FE analysis results, we then compared the temperature history at each node in the plate thickness direction. In lap fillet arc-welded joints, the front-side temperature decreases due to tensile stress, while the back-side temperature increases due to compressive stress. Consequently, the front-side temperature does not drop completely, causing the heat absorption peak to shift slightly from the load peak, creating a phase difference. This shift explains the minimal temperature change relative to the load stress. Therefore, when applying the thermoelastic method to stress concentration fields, the influence of load frequency must be considered.

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CO₂ gas-shielded arc welding is widely used in Japanese industry, but one of the major drawbacks of this process is weld spatter caused by unstable metal transfer during the welding. ′Pulsed Gas MAG ′ welding process has been proposed as a new approach to control metal transfer and reduce spatter, in which a small quantity of Ar gas pulses were periodically injected into CO₂ shielding gas so that gas composition of arc atmosphere was largely modified in a very short time. In previous studies, the Ar addition conditions (frequency, valve open time and Ar gas flow rate) that enable metal transfer control were experimentally clarified. However, these results were obtained under fixed welding conditions (welding current, wire feeding speed and wire diameter). In practical application, it is necessary to select appropriate welding conditions corresponding to the workpiece thickness, welding position, etc. In this study, in order to expand the range of application of this process, we experimentally investigated the Ar addition conditions that enable metal transfer control at welding current from 235 to 350 A using solid wires with diameters of φ1.2 mm and φ1.4 mm. As a result, the range of Ar addition conditions that enables metal transfer control for each wire diameter was obtained. In addition, the results showed that current density and droplet volume at the wire tip were dominating factors in synchronizing droplet transfer. Minimum droplet volume was assumed to exist for metal droplet detachment synchronizing with Ar addition even under the condition of such a high current density where spray transfer would be induced in conventional MAG welding. The minimum volume was estimated to be approximately 4 mm₃ in this study.

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‘Pulsed Gas MAG’ welding process was proposed as a new approach to control metal transfer and reduce a spatter, in which a small quantity Ar gas pulses were periodically injected into CO₂ shielding gas so that gas composition of arc atmosphere was largely modified in a very short time. The change in the arc atmosphere may causes a change in arc force and a pulse shaped increase in welding current, which could produce a driving force for the droplet to release. However, it has not been known yet which one of these is the main factor for molten droplet release in PGMAG welding. Thus, in this study, it was aimed to clarify this point. As a result of the experiment, It was shown that, if only the gas compositional change occurred, synchronous metal transfer was achieved even under the condition of constant welding current. On the other hand, in pulsed welding without the gas change in which the current waveform was simulated as that of PGMAG, the metal transfer was not synchronized with the current change. In addition, as a result of spectroscopy observation of arc plasma, it was observed that the outer edge of the arc plasma was formed by argon plasma when the arc shape was a flare type. It is considered that the temperature of this part is higher than that of the arc center, and this suggests that the arc force acting on the bottom surface of a droplet may decrease because the argon plasma formed at the outer edge of the arc becomes the main current path. In conclusion, it was experimentally shown that a reduction in arc force due to the change in current path has a major influence on droplet transfer control in this process, rather than the current rise.

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Welding is widely used in the manufacture of steel structures, but weld deformations due to the heat input affects the strength of the structures. Therefore, it’s necessary to be straightened depending on circumstances. However, because straightening is a very costly process, numerical analysis is required to minimize weld deformations. Weld deformations can be simulated by nonlinear thermal elastic-plastic analysis based on the finite element method (FEM). Moreover, the idealized explicit FEM, which is capable of high-speed calculation, has been used to analyze real structures. However, the element sizes relative to the weld length becomes extremely fine when the heat source length of multi-pass welding problem for large steel structures is defined in actual dimensions, resulting in enormous calculation time even if the FEM is used. One way to counter this problem is to increase the heat source length. However, if the heat source length is excessive, analysis accuracy may be reduced due to excessive expansion or shrinkage in the heat input range. Thus, the effect of the heat source length on the analysis accuracy was examined in this study. In addition, the FEM was performed on the welded assembly of a hot dip galvanizing bath, which is one of the large steel structures, and the accuracy of the analysis was verified by comparing the analytical results with measured values. As a result, the maximum achievable temperature decreases with increasing heat source length and that weld deformation is underestimated. Therefore, it’s suggested that optimization of current density according to the length of heat source is necessary to achieve both accuracy of analysis and reduction of calculation time in the analysis of large steel structures.

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In large-scale construction, welding is indispensable but often leads to the deformations due to weld heat. These deformations can weaken joints, compromise structural integrity, reduce assembly efficiency, increase costs and affect the appearance. Predicting and controlling these deformations are important for maintaining the quality and safety of structures. However, predicting and controlling these deformations is difficult due to the influence of various factors such as heat input conditions, material properties and structural geometry. In particular, out-of-plane deformations often cause significant problems in the welding of large structures. This study investigates the impact of various heat input parameters and plate dimensions on out-of-plane deformations using thermal-elastic-plastic analysis with an Idealized explicit finite element method (FEM). As an index to evaluate the out-of-plane deformation, the inherent deformation calculated by integrating the inherent strain in the longitudinal and transverse directions was applied, and its validity was verified. Simplified predictions of out-of-plane displacement distribution based on inherent deformations were compared with detailed FEM results, demonstrating good agreement for simple plates. However, specific conditions with large heat input and long plates revealed reverse bending modes, indicating the limitations of inherent strain-based predictions. Therefore, this study categorized deformation modes into longitudinal bending-dominated, transverse bending-dominated and large deformation modes, examining and discussing how various parameters affect these deformation modes. It was found that larger heat input and longer or thinner plates likely to cause large deformation modes. These findings enhance the understanding of weld-induced deformation mechanisms and lead to the development of effective prediction and control strategies to mitigate these deformations in practical applications.

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FEM thermal elastic-plastic analysis, which is commonly used to analyze welding deformation and residual stress, is known to be very time-consuming because it sequentially analyzes local melting and elastic-plastic phenomena of the material from the start of welding to complete cooling. In contrast, the thermal shrinkage technique, a simpler numerical method for predicting weld deformation, models only the thermal shrinkage that occurs during the cooling process of the weld and predicts the final welding deformation in a one-step elastic-plastic analysis. However, it has some issues, such as the fact that it can only be applied to angular distortion. Therefore, modified thermal shrinkage technique is proposed in this study, which can reproduce not only angular distortion but also other deformations in the analysis by setting multiple input parameters. The proposed method is applied to bead-on-plate welding, and it is confirmed that the shrinkage area and shrinkage temperature can be calculated to reproduce well all transverse shrinkage and angular distortion under multiple heat input conditions. Using the shrinkage area and shrinkage temperature calculated from bead-on-plate welding, the modified thermal shrinkage technique was applied to a multi-layer weld with six passes, and it was confirmed that modified thermal shrinkage technique could reproduce well the transverse shrinkage and angular distortion in all passes. In addition to these results, the computation time is more than 250 times faster than that of FEM thermal elastic-plastic analysis, demonstrating the usefulness of the modified thermal shrinkage technique.

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Trimming processes are required as the upset length increases for removing bulge of weldment in gas pressure welding. If the upset length is reduced, the above processes are not required, and the welding process is simplified. On the other hand, a reduction in upset length is expected to result in a reduction weld strength. In this paper, gas pressure welding specimens were prepared under low upset length conditions using the constant pressure method, and weld strength was evaluated by tensile testing and bending testing. Rail gas pressure welding tests using the variable pressure method were conducted. As a result, an effective welding method was found by low-upset length. A numerical analysis model of rail gas pressure welds was used to evaluate the weld temperature and plastic strain of low-upset length rail gas pressure welds using the constant pressure and variable pressure methods, and the variable pressure method was effective in improving the weld temperature and plastic strain.

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The heat-affected zone (HAZ) formed during the welding of power plant piping is known to be susceptible to fracture caused by creep damage at high temperatures. In recent years, power plants are expected to experience larger load fluctuations due to the introduction of clean energy, and there is a growing demand for monitoring technologies for creep-fatigue in piping. When monitoring, it is crucial to understand the stress state acting on the piping from relatively easily obtainable data such as displacement and temperature. In this study, we developed a digital twin system that integrates FEM analysis with time-series deformation data to estimate the creep deformation behavior of welded joints in piping structures and evaluate their integrity. The system is capable of simultaneously estimating material constants representing the variability of welded joints, as well as load and boundary conditions. We applied this system to estimate the mechanical state during creep-fatigue testing of high-temperature steel pipes. First, we verified the validity of the proposed method. The results showed that the method accurately reproduced the experimentally applied load in both cases: when estimating only the load and when simultaneously estimating the load and creep coefficient. Additionally, we investigated the convergence of the proposed method and found that estimated load converged to values close to the experimentally applied load for different initial values. These findings confirm that the proposed method is a useful digital twin system for evaluating the integrity of welded joints in piping structures.

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With the increasing adoption of renewable energy, thermal power plants are expected to shift from baseload power sources to load-adjusting power sources. This change in operating environment may lead to temperature and strain variations during load fluctuations, including start-stop cycles, which can cause creep-fatigue damage in addition to creep damage due to internal pressure in large-diameter pipes of thermal power plants. As the damage mechanism may change with the operating conditions, it is crucial to develop a digital twin system that monitors temperature and strain during operation and estimates the remaining life of the components. In this study, we developed a system integrating data assimilation techniques with finite element method (FEM) to construct a digital twin for estimating the creep-fatigue life of high-temperature piping structures. We conducted numerical twin experiments using the developed system to estimate the state of a welded pipe structure subjected to internal pressure and bending loads, and verified the validity of the system. Additionally, we performed state estimation with measurement errors to demonstrate the robustness of the system against disturbances. When assuming errors in the measurement locations, the estimation errors increased compared to the case using simulated experimental data without errors. However, the system was able to estimate boundary conditions following the trend of the phenomenon. This trend was similar for both load estimation and creep-fatigue life estimation. When assuming errors in the measurement range, a similar trend was observed as in the case of measurement location errors, but with larger estimation errors. When random noise of up to 10% of the maximum strain was added to the pseudo-measurement data, the creep-fatigue life estimation was not significantly affected. However, when biased noise was added, the impact was relatively significant.

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This study investigates the influence of tool shape on stirring performance and wear in Friction Stir Welding (FSW) of steel materials using particle-based numerical simulation. Two tool shapes were examined: a conventional cylindrical probe and a spherical probe, which is expected to maintain equivalent joint quality while minimizing the stirring area and potentially extending tool life. The simulation method, based on particle dynamics, was employed to analyze material behavior during FSW and evaluate the effects of tool rotation speed and welding speed on stirring performance and tool wear for both tool shapes. Stirring performance was assessed by quantifying material dispersion in the welded joint, while tool wear was evaluated based on the adhesive wear theory. Results showed that stirring intensity was higher on the advancing side (AS) than the retreating side (RS) for both tool shapes, consistent with experimental observations from previous studies. The probe tip shape was found to significantly affect stirring performance. Increasing rotation speed enhanced stirring, while increasing welding speed reduced it. Tool wear was most pronounced during the initial plunge phase. During quasi-steady state, wear was greater on the RS of the tool where flow stress is lower, and at the tool edges where relative speed is high and hardness decreases due to temperature rise. The spherical tool exhibited less wear compared to the cylindrical tool across all tested conditions. The effects of welding parameters on tool wear aligned well with trends reported in literature, with higher rotation speeds increasing wear and higher welding speeds decreasing it. The spherical tool consistently demonstrated lower wear, particularly in the probe region, suggesting its potential as a longer-lasting alternative to conventional cylindrical tools in FSW of steel materials.

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This study examines the deformation caused by gas heating. Gas heating is a technique used in shipbuilding and other industries for plate bending and the straightening of welding-induced distortions. To examine the influence of various parameters on deformation during gas heating, dimensional analysis and Thermo-elastic-plastic analysis using Idealized Explicit FEM were employed. As a result, six dimensionless parameters were derived, including heat input, heating speed relative to thermal conductivity, shape of heat source, heat transfer, and specimen geometry. The validity of these parameters was verified by comparing temperature distributions obtained through thermal conductivity analysis. This study revealed that the heat source shape parameter significantly affects both transverse shrinkage and angular distortion, especially in gas heating conditions where the heat input distribution is wider than its distribution in welding. The parameter of heating speed showed a more significant effect on angular distortion in gas heating compared to welding. It was found that transverse shrinkage in gas heating is strongly affected by the heat source shape parameter and heat distribution parameter. Furthermore, angular distortion could be effectively organized using the heat distribution parameter for both welding and gas heating conditions. These findings contribute to a better understanding and prediction of deformation in gas heating processes, enhancing accuracy and efficiency in construction of steel structures.

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This study focuses on the development of a Digital Twin system for predicting thermal-elastic-plastic fields during gas heating processes. In large-scale structures, assembly welding often induces out-of-plane deformations, requiring skilled workers to apply corrective heating. Considering the decreasing number of skilled workers, it is crucial to automate these processes. A Digital Twin system that integrates FEM analysis with temperature and displacement measurements is proposed to accurately predict and optimize gas heating conditions. This system estimates unknown parameters such as heat distribution and temperature-dependent yield stress, achieving high accuracy in replicating actual phenomena. Using SS400 material, we conducted gas heating experiments to validate our system's effectiveness. Results showed that our system accurately reproduced experimental temperature and displacement fields. Moreover, the system identified optimal heating conditions that minimized deformation to within ±1 mm, demonstrating its potential for practical applications in the fabrication of complex structures. This Digital Twin approach represents a significant advancement in automating the gas heating process, reducing dependency on skilled workers, and contributing to precise deformation control.

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In this study, an AI-based line heating system is proposed for automating the production of arbitrarily shaped steel plates for shipbuilding applications. Line heating is a bending fabrication technique that uses gas burners and cooling water to locally heat and cool the plate to form arbitrary curved surfaces. Traditionally, the process of plate forming with complex curvatures, especially at the bow and stern of ships, has heavily relied on skilled workers using line heating technology. The proposed AI line heating system utilizes finite element method (FEM) and artificial intelligence to calculate the heating plan required to achieve the desired shape. The system involves two algorithms. The first algorithm calculates the bending scheme by approximating the target shape with discrete heating lines based on the principal curvature directions. The second algorithm uses Bayesian optimization and FE analysis to calculate a heating plan that is used to shrink the steel plate in-plane, thereby forming complex non-developable surfaces. Additionally, this system can calculate a heating plan to minimize the amount of heating work required for any shape and thickness of the plate. It was also demonstrated that using curvature as a measure of shape evaluation allows for the calculation of heating plan that forms highly accurate final curved surfaces. This study suggests that the AI line heating system can replace skilled workers with robotics in forming hull surfaces, capable of generating a heating plan for any shape and thickness.

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In this study, the mechanization and automation of ship hull surface shaping were aimed by integrating finite element method (FEM) with artificial intelligence (AI) technologies. In the bending process of thick plates, such as those used in shipbuilding, localized heating and cooling with gas burners and cooling water are employed to form arbitrary curved steel plates. Traditionally, the process of shaping complex ship hull surfaces, especially at the bow and stern, relied on skilled workers. A system was developed that combines FEM analysis and AI technologies, particularly Bayesian optimization, to accurately simulate thermal deformation and mechanically calculate the optimal heating plan. Experimental tests were conducted using this system along with a specially developed small robot. However, a difference was identified between the analytical space and the actual space, and despite being able to create the target shape in the analytical space, various factors in the actual space prevented the desired shape from being achieved by following the calculated heating plan. Therefore, methods to reduce the gap between analytical and actual spaces were introduced in this study, allowing surfaces that closely match the target shapes to be achieved in shipyard testing.

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Submerged arc welding is often used in the construction of large steel structures such as bridges where weather-resistant steel materials are used because of its high welding efficiency. However, end cracking, which is solidification cracking, may occur at the weld termination. To prevent such cracking, a tab plate is attached to the weld termination to restrain the end of the base metal, but depending on the type of restraint method, end cracking may not be prevented. In this study, the effects of two types of restraint methods in the tab plate on the occurrence of termination cracking in one-pass submerged arc welding were investigated from both experimental and analysis points of view. Experimental results showed that cracking occurred in the cascade restraint method with a large restraint in the groove, which was initially considered to have a low risk of end cracking. However, simple restraints with small restraint in the groove did not cause end cracking. After confirming that the temperature history, penetration shape, and transverse shrinkage agreed well with the experimental results, hot cracking analysis was used to evaluate end cracking, and it was confirmed that the analysis could reproduce well the trend of cracking risk and the location of cracking that occurred at the weld termination for different restraint methods. In addition, by comparing the amount of bevel shrinkage at the weld termination during cooling, it was confirmed that the amount of bevel shrinkage of specimen B, in which the restraint in the bevel is large, is smaller than that of specimen A, in which the restraint in the bevel is small.

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It is known that the crystal structure within the molten pool affects the initiation and propagation of solidification cracking. Cracking can occur where crystals meet or where crystals are adjacent to each other. To evaluate cracking behavior more precisely, analysis that takes into account the crystal structure associated with melting and solidification is required. In this study, a simple method was developed to visualize the growth direction of columnar crystals during the solidification process of weld metal from the temperature distribution, temperature gradient and their histories in FEM heat conduction analysis. In addition, by determining the location of the liquid phase in the weld transient and reflecting it in FEM thermal-elastic-plastic analysis, a solidification cracking analysis that takes into account the crystal structure and the existence of the liquid phase between crystals was proposed. As a result, it was confirmed that the distribution of columnar crystals growing in the molten pool and the strain behavior acting on the liquid phase region between crystals during welding can be well visualized. The proposed method was applied to a two-dimensional analysis to investigate the effect of crystal size on the existence of liquid phase and the amount of strain generated in the liquid phase region, and it was confirmed that the plastic strain increment in BTR generated in the liquid phase region was larger under conditions with larger columnar crystal diameter. The effect of welding speed on crystal growth and strain behavior in the molten zone were investigated by three-dimensional analysis, and it was confirmed that the growth of columnar crystals similar to the actual phenomenon could be well represented by varying the welding speed. The strain behavior acting on the liquid phase region during welding transient was examined, and it was confirmed that large strain occurred at the low temperature side and expanded to the high temperature side.

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Hot cracking is one of the most serious welding defects that can significantly reduce the strength of structures, and it is important to prevent hot cracking. In the Japanese shipbuilding industry, one-sided submerged arc welding with multiple electrodes is used to improve the production efficiency of welding large steel plate joints for ships, and it has been reported that hot cracking may occur at the termination. From the viewpoint of the mechanism of hot cracking, if tensile strain does not act within the Brittleness Temperature Range (BTR), the possibility of cracking is negligible. In other words, any hot cracking problem can be solved if compressive strain can be applied within the BTR during welding. In this study, a new “parallel heating method” is proposed to prevent hot cracking by using the thermal expansion caused by simultaneous additional heating to the welding torch. In the analysis of bead-on-plate welding, the effect of the parallel heating method in reducing high-temperature strain and its appropriate conditions were investigated. It was confirmed that the proposed method reduced high-temperature strain not only at the steady state but also at the beginning and end of the weld. It was also confirmed that the effect of reducing high-temperature strain varies depending on the position of the additional torch, and that there is an appropriate heating position depending on the target weld. In the analysis of multi electrode single side submerged arc welding, it was confirmed that the proposed method has little effect on the weld penetration shape, and it was shown that the proposed method can ensure weldability. It was also confirmed that the proposed method has a significant reduction effect on high-temperature strain generated by localized opening deformation, demonstrating the usefulness of the proposed method.

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Hot cracking is a phenomenon that occurs when the tensile strain acting on the liquid film is large within the Brittleness Temperature Range (hereinafter referred to as BTR), where solid and liquid coexist in the weld zone. Since the liquid phase has less strength than the solid phase, strain tends to be concentrated in the liquid phase. Therefore, in addition to considering the strength difference between the solid and liquid phases in the analysis, it is important to visualize the direction of crystal growth and the meeting angle to evaluate hot cracking. In addition, solidification shrinkage strain occurs during solidification of molten metal due to the density difference between the solid and liquid phases, and the effect of this strain cannot be ignored. Therefore, the authors have developed a weld hot cracking analysis method that can consider metallurgical factors such as solidification shrinkage strain and crystal growth direction, as well as mechanical factors such as strain, deformation and stress. In this study, the proposed method is applied to butt welds to investigate the effects of various factors on the high temperature strain. The effects of the bevel angle on the solidification morphology and the high-temperature strain of the weld zone were examined, and it was shown that hot cracking is more likely to occur in narrow bevel welds with small bevel angles. Furthermore, the effect of the relationship between temperature and solid fraction on the high-temperature strain of the weld zone during cooling was examined, and it was confirmed that the plastic strain increment in BTR was larger where the variation of solid fraction with temperature was large, and that the plastic strain increment in BTR was smaller where the variation of solidus ratio with temperature was small.

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This paper deals with ductility of weld heat affected zone of steel for building structure. The test materials which were applied thermal cycle simulating the heat affected zone of the multilayer CO2 gas shielded arc weld joint were prepared. The heat affected zone subjected to single thermal cycle and the one subjected to reheat in the two-phase region were tested. Tensile tests for smooth round-bar specimens and circumferentially notched round-bar specimens and FEM analysis simulating the tests were carried out to confirm the ductility of base metal and each heat affected zone. As a result, it was confirmed that the ductility of the coarse-grained heat affected zone and that of the coarse-grained heat affected zone subjected to two-phase zone reheating were lower than those of base metal and the other heat affected zone. The microstructure observation was carried out, and it was confirmed that the coarse-grained heat affected zone has a coarse-grained bainitic microstructure and that the coarse-grained heat affected zone subjected to two-phase zone reheating has the microstructure in which the acicular cementite disappeared partly by the reheating in the coarse-grained bainite microstructure. It was also confirmed that there was a difference in ductile fracture behavior by the fracture section observation, and it was clarified that the difference in ductile fracture behavior caused by the difference in microstructure of the material caused the difference in ductility of the material.

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This study discusses the temperature dependency of critical Weibull stress distribution around ductile-to-brittle transition temperature with consideration of stable ductile crack growth prior to cleavage fracture. The 3-point bend specimen was subjected to brittle fracture test at different test temperature, that were brittle temperature (-80ºC), and ductile-to-brittle transition temperature (-20ºC) where ductile crack growth prior to cleavage fracture occurred. From test results at -80ºC, the critical Weibull stress distribution was identified as shape parameter m=37.6 and scale parameter 𝜎_u=1786 MPa, that were independent of crack depth (crack-tip plastic constraint). Assuming Weibull shape parameter m was also independent of temperature, the critical Weibull stress distribution (Weibull scale parameter 𝜎_u) was estimated under ductile-to-brittle transition temperature (-20ºC) by simulating ductile crack growth based on the ductile damage model. The critical Weibull stress under ductile-to-brittle transition temperature was presented a higher distribution than that of brittle temperature (-80ºC). This result indicates that the scale parameter 𝜎_u (critical Weibull stress distribution) depends on temperature under ductile-to-brittle transition region where ductile crack growth occurs prior to cleavage fracture.

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As the use of hydrogen is attracting attention towards the realization of a low-carbon society, pipelines for transporting hydrogen gas are being considered as a method of transporting hydrogen in large quantities and at low cost. However, the effects of hydrogen embrittlement on pipeline steel are not clear. In this study, the effect of hydrogen on ductile crack initiation and growth resistance was investigated in high-strength linepipe steel. Fracture toughness tests showed that the steel exhibited stable ductile crack growth behavior under hydrogen environment without unstable brittle fracture, but the ductile crack growth resistance was significantly lower than that in the absence of hydrogen. In order to evaluate the effect of hydrogen on the mechanical properties controlling ductile crack growth resistance, tensile tests were conducted using smooth and circumferential notched round-bar specimens (R1 and R2 specimens) with and without hydrogen charging. The hydrogen-charged specimens exhibited a smaller reduction in area, indicating that hydrogen reduced the ductility of the high-strength linepipe steel. The effect of hydrogen on a stress triaxiality dependent critical strain for ductile failure was evaluated from the test results and elastic-plastic FEM analysis, and it was found that a stress triaxiality dependent critical strain for ductile failure were reduced by hydrogen. Fracture surface and cross-section of the round-bar specimens were observed to investigate the effects of hydrogen on critical strain for ductile fracture and damage behavior, and it was considered that the hydrogen decreased critical strain for void initiation. Consequently, degradation of ductile crack growth resistance due to hydrogen was caused by decrease critical strain for void initiation.

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It has been about 25 years since petroleum refining reactors used in hydrocracking and hydrodesulfurization process have been made of 2.25Cr-1Mo-V steel, but it is unclear whether the impact properties required for weld metals in the materials and fabrication standard such as API 934-A provide sufficient toughness for hydrogen assisted cracking resistance in the welded joints of reactor under hydrogen service. In this study, simulating the hydrogen embrittlement influence factors predicted under each reactor operating conditions such as startup, steady-state operation and shutdown into 1T-C(T) specimens, rising load test and holding load test, the threshold stress intensity factors for the hydrogen assisted crack initiation, K_IH and the quasi-cleavage crack growth rate da⁄dt were compared using the high and low toughness welded joint fabricated under two different PWHT conditions within the 2.25Cr-1Mo-V steel reactor fabricating specifications. Applying the fracture mechanics properties obtained from these hydrogen assisted crack resistance tests to crack propagation analyses assuming an actual reactor, we evaluated two different types of critical cracks in the high and low toughness welded joint. The former is the critical crack for hydrogen assisted crack initiation during the reactor hydrogen pressurization at room temperature, and the latter is the critical crack for the remaining life, which is the subcritical crack growth period from hydrogen assisted crack initiation to fast fracture or unstable fracture in the reactor cyclic service.

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The purpose of this study is to develop a ductile damage model for predicting ductile crack growth behavior of structural component subjected to large cyclic deformation. Work hardening rule with isotropic hardening stagnation is proposed based on the effective damage concept proposed by the authors. Damage evolution rule with damage acceleration under tensile straining and no damage acceleration under compressive straining just before fracture is also proposed. The ductile damage model for monotonic loading proposed by the authors is extended for cyclic loading by introducing the proposed work hardening rule and the damage evolution rule. The proposed work hardening rule can predict not only Bauschinger effect but also work hardening stagnation and permanent softening behavior with no additional parameters from conventional isotropic / kinematic combined hardening rule. Ductile crack growth predicted by simulation based on the ductile damage model with conventional damage evolution rule is larger than experimental results. On the other hand, simulation based on the ductile damage model with the proposed damage evolution rule can predict ductile crack growth behavior and load carrying capacity under not only constant but also variable strain amplitude. Consequently, the proposed ductile damage model for cyclic loading is found to be able to predict ductile crack growth behavior of structural component subjected to large cyclic deformation.

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The integrity of offshore structures such as wind turbines should be managed against corrosion fatigue. In this study, steel for welded structures, SM490A, was used for corrosion fatigue tests under welding residual stress and for constant displacement corrosion tests under four-point bending in synthetic seawater. Corrosion fatigue life of the specimens with compressive residual stress was longer than that with tensile residual stress. Effects of welding residual stress and constant surface stress on corrosion pit growth were not recognized by scattered pit size distribution, but a pit growth model was formulated in the following equations, where d_p,m is the mean corrosion pit depth (μm), A and B are fitting parameters, Δσ is the nominal stress range (MPa), and t is the time (month). d_p,m = A t ^B,　A = 0.0953Δσ + 3.26,　B = －0.0023Δσ + 0.892 The reason why corrosion fatigue characteristics are affected by residual stress is considered to be that residual stress affects the critical stress intensity factor range for corrosion fatigue crack initiation, the time required for corrosion fatigue crack initiation from a corrosion pit, and the time required for corrosion fatigue crack propagation.

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Linear Friction Welding (LFW) was applied to medium-high carbon steel S55C, which had difficulty for arc welding process, and fatigue strength of welded joints were evaluated. In LFW process, peak temperature can be easily controlled by applied pressure and fatigue strength of welded joints for two level of peak temperature are compared. As a result of fatigue test, the harmful welding defects are not detected in the joints. Sound welded joints can be fabricated by LFW. Because, the fatigue crack initiated at the surface of interface between joint area and base material, not in the joint area itself. A remarkable difference was recognized for the hardness distribution between two level of peak temperature, and low peak temperature joint has low and almost flat distribution. Low peak temperature joint came to have almost equivalent fatigue strength to that of a base material. The reason why the fatigue strength of LWF joints is maintained at the same level as the base material is because compressive residual stress is generated on the joint surface, which is the initiation site of fatigue crack. This compressive residual stress is thought to be due to the tensile plastic deformation of the surface due to uneven temperature distribution within the cross section during the air cooling process after welding.

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Fatigue damage in welded steel structures such as bridges has become a serious social problem due to cyclic loading and especially has become severe situation at the joints of horizontal members and horizontal stiffeners which joint type is out-of-plane gusset welded joint. The fatigue fracture initiation of out-of-plane gusset welded joint often occurs weld toe where stress concentration is high. As a method to increase the fatigue strength of the weld toe, there is a grinder treatment which smooths the weld toe to reduce the stress concentration and a hammer peening treatment which applies plastic deformation to the weld toe to introduce compressive residual stress. However, these treatments lead to a decrease in construction efficiency because additional treatments are necessary after welding. In this study, a new welding technique as a “ split boxing “ is developed to improve fatigue strength without decrease of the construction efficiency.The mechanism of improving fatigue strength is also discussed in the developed new welding technique,” split boxing “. The procedure of the developed welding is that the welding line is divided into short and long side of the gusset. The welding line at the long side of the gusset is extended. The effect of improving fatigue strength of out-of-plane gusset welded joints by applying the developed welding method is described from the viewpoints of the suppressing the initiation of fatigue cracks and the delay of fatigue crack propagation rate.

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In recent years, several research projects have focused on life extension techniques utilizing fatigue crack closure. This paper describes the experimental work for the fatigue life extension utilizing corrosion products and quantitative measurements of the corrosion volume with the 3D shape measurement tool. In the experiment of life extension, the SENT specimens, made from the rolled steels for welded structure SM490A, are adopted for the investigation. Fatigue tests are conducted under the different stress ratios R = -1.0 / 0.1. A corrosion accelerator, 35% HNO₃ liquid, is injected into the crack faces of the specimen with a crack length of corrosion a_co. = 0.4 / 3.0mm. Finally, the tests are resumed for the final fracture. Regarding the investigation of the accumulated corrosion products, precise measurements for the opposite fractured surfaces are performed. The volume of corrosion products is obtained by utilizing the notion of FRASTA to exclude the effect of plastic deformation along the crack faces. The obtained results show considerable life extensions containing crack arrest by accumulated corrosion products on crack faces under the different stress ratios R and the crack length of corrosion a_co., which suggest the applicability of this technique regardless of R and a_co.. Furthermore, a positive correlation is confirmed between the accumulated corrosion products on crack faces and the level of the life extension.

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