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In recent years, ship hulls have very complicated shapes in order to reduce frictional resistance and wave resistance during navigation. In particular, in the bow and stern, curved skin plates with complex shapes are used. Line heating is used to produce such complex shapes. Line heating is a bending technique using plastic deformation due to heating. The relationship between the heat input and the deformation is nonlinear, which may lead to difficulty in making a heating plan for forming the target shape. Thus, skilled workers are necessary in line heating, and the work time and dimensional accuracy depend on their skills. Another problem is the transfer of this technique to future generations. In order to overcome these problems, automation of the line heating process has been investigated urgently. On the other hand, artificial intelligence (AI) technology has been rapidly developed in recent years. An AI system can deal with nonlinear relationships and ambiguous feature quantities, which are difficult to express mathematically. By using AI, automation of the planning of the heating line can be expected. The purpose of the present study is to obtain the optimal heat input conditions for forming an arbitrary shape in line heating. In order to accomplish this, we constructed an AI system that integrated deep layer reinforcement learning and line heating simulation. The proposed system was applied to the formation of fundamental shapes of line heating, including the bowl shape, the saddle shape, and the twisted shape. As a result, the proposed system was found to be able to generate heating plans for these shapes with fewer heating lines.

application/pdf, Welding International. 2021, 33 (Issue 1-3 (2019)), P.1-16

In recent years, ship hulls have become larger. And the welding has become more important technology in ship building. In other word, welding has become more important technology in shipbuilding. In Japanese shipbuilding, multiple electrode butt welding has been performed with large current and high speed to increase construction efficiency. However, the multiple electrode welding method may cause a hot cracking. X-ray or ultra sonic wave inspection and repair welding are necessary due to the generation of hot cracking. These will increase the production cost. Therefore, it is important to prevent hot cracking. It is known that hot cracking is generated by both metallurgical and mechanical factors. The authors propose a new evaluation method of hot cracking based on the modeling of both mechanical and metallurgical behaviors. In the developed method, from the metallurgical point of view, solidification growth direction is determined from the temperature gradient obtained by the heat conduction analysis to predict the position of hot cracking. Moreover, from the mechanical point of view, the possibility of the generation of hot cracking is assessed using plastic strain increment in Brittleness Temperature Range (BTR). In order to show the validity of the developed method, the developed method is applied to the analysis of hot cracking on automatic tandem butt welding through the comparison with experimental results. As a result, it is demonstrated that the hot cracking can be evaluated by using the developed method.

The effect of specimen thickness on the behavior of fatigue crack growth rate (FCGR) requires rigorous investigation because the thickness effect is not independent and could be a function of the specimen geometry, material properties, loading conditions, and environmental conditions. The purpose of this study is to numerically investigate the effect of specimen thickness and stress level on the behavior of FCGR at the specimen-free surface and mid-thickness in notched mild steel specimens. Elastic-plastic numerical calculations were performed based on FEM using the domain integral method to calculate the cyclic J-integral parameter, ΔJ, which was used for calculating FCGR. Different specimen thicknesses and various constant amplitude stress levels were used in the numerical calculations. Crack growth calculations were carried out using the node release technique in which the history of the former crack was considered. The calculated fatigue lives were firstly verified against the experiments taken from the literature. The results of numerical calculations showed that no significant thickness effect was noticed at the mid-thickness for calculations carried out below general yield while when the stress level is close to general yield the thinner specimens showed faster crack growth rates. Further, thickness has a remarkable effect on the crack growth rate at the free surface when the applied stress level is below and close to general yield where the thinner specimens gave faster crack growth rates. The crack-tip driving force ΔJ and the local strains and stresses induced ahead of the crack front could explain the mechanics and behavior of FCGR for the applied thicknesses and stress levels. The distributions of the local strains and stresses could also reveal the effect of the local material ahead of the crack front on the behavior of crack growth.

application/pdf, Welding in the World. 2017, 61 (3), P.517-527

application/pdf, Welding in the World. 2016, 60 (5), P.915-929

application/pdf, Welding in the World. 2015, 59 (6), P.839-850

application/pdf, Welding in the World. 2015, 59 (4), P.589-595

In this research, to evaluate the welding distortion of stiffened thin-plate structures, an analysis method was developed that considers large deformation theory in elastic analysis using inherent strain based on Idealized Explicit FEM with shell elements. The developed method was applied to the analysis of stiffened thin-plate structures. The following factors were considered in the analysis to investigate their influence on welding distortion: heat input and welding processes such as double-sided continuous fillet welding, single-sided continuous fillet welding, double-sided intermittent fillet welding, and staggered fillet welding. From the analysis, it was clear that a buckling-type deformation is generated when the heat input is large. Regarding the welding method, the largest deformation was observed in double-sided continuous fillet welding. In addition, to show the applicability of the developed method to large-scale problems, the developed method was applied to the analysis of the car deck of a pure car carrier ship. As a result, it was found that the developed method can analyze the welding deformation of a large-scale stiffened thin-plate model in practical computing time.

In this research, the influence of various factors on solidification cracking under high-speed gap welding was investigated by nonlinear thermal-elastic-plastic analysis using the finite element method (FEM). To investigate the formation of cracks, the plastic strain increment in the brittleness temperature range (BTR) was introduced. In addition, to perform thermal-elastic-plastic analysis of a practical three-dimensional model, Idealized Explicit FEM was employed. In the simulations, the penetration shape was compared between the experimental measurement and the analyses to evaluate the analysis accuracy. As a result, it was found that the penetration shapes agree with each other. The heat input, welding speed, and torch distance were changed and the distribution of the BTR plastic strain increment was also compared to show the influence of the welding conditions. From the investigations, it was found that the BTR plastic strain increment increases with heat input, welding speed and decreases with torch distance.

In this study, the authors developed a three-dimensional coupling analysis method for spot welding by using Idealized Explicit FEM, which can analyze large-scale welding residual stress and deformation problems in practical computing time. This proposed method was applied to a spot welding problem to show its analysis accuracy. It was found that the nugget sizes obtained by experimental measurements and the method are in good agreement. In addition, to investigate the effect of shunt current on the nugget size in multiple-spot welding, the method was applied to three-dimensional spot welding with four points of welding. As a result, it was shown that the influence of shunt current in multiple-spot welding can be evaluated by using the proposed method in realistic computing time.

To make clear the effect of strain occurred in weld metal of Type 316L stainless steel during laser beam welding (LBW) on center-line cracking in LBWs, side-bead test was adopted to evaluate the susceptibility by measurement of the crack length. The stainless steels varied with impurity elements such as P and S were used as the materials for the test. From the side-bead test, the crack length decrease with increase in the laser welding velocity. Compared with high and low amounts of impurity elements, the crack length occurred in welds metals with higher contents of impurity elements were remarkably longer than those in lower ones. To discuss the phenomena, the thermal-elastic-plastic FEM analysis considering with temperature dependent interface element has been conducted to simulate the side-bead test. From the calculation, the cracking behavior recognized in the actual side-bead test was relatively in accordance with calculation. On the basis of these results, the side-bead test was a useful test method to evaluate the crack susceptibility. In addition, it seems to be suggested that the susceptibility may be possible to predict by the FEM analysis due to use correct physical constants of the materials.

application/pdf, Computational Materials Science. 2014, 93, P.62-67

Due to the rapid improvement of digital cameras, especially the pixel resolution, digital image correlation (DIC) has been introduced to measure the deformation and strain of structures. Using digital cameras for the DIC technique is an easy and fast method for obtaining structural information, represented as all the pixel points in a photo. Because a wide range of structural deformation can be obtained with high accuracy, this method has the potential to be very useful. Currently, DIC can execute a measurement with high accuracy only when the out-of-plane displacement is small. When the out-of-plane displacement is large, the deformation causes the measurement error. Therefore, a stereo imaging method using two digital cameras is proposed in this study. The proposed method can measure not only in-plane deformation but also out-of-plane deformation with high accuracy without calibration of the errors caused by the out-of-plane displacement. In this paper, the measurement accuracy of the proposed method fo...

Displacement during welding provides important information to understand the mechanisms of welding deformation and residual stress. In particular, if welding deformation can be measured sequentially and the displacement distribution over full field can be measured such as the results obtained by finite element analysis, they can be valuable information. Therefore, in this study, a 3-dimensional (3D) deformation (in-plane and out-of-plane deformation) measurement method is developed using a digital camera, which requires no special equipment. This method is a non-contact method and it can sequentially measure over the entire photographed image. Furthermore, since image analysis is based on the technique of image matching, the method is applicable even when measuring deformation is large. In addition, since it is possible to use all pixels as measuring points, the number of available measuring points at one time is the same as the number of effective pixels of the camera. This is currently more than 15 mill...

In the construction of thin plate steel structures, including ships, welding is widely used to join parts. Welding inevitably causes deformation in thin plate structures, which may cause various problems. In the present study, an analysis method is developed to realize the prediction of deformation during the construction of large-scale structures based on the thermal elastic plastic analysis method. The developed method uses the idealized explicit finite element method (IEFEM), which is a high-speed thermal elastic plastic analysis method, and an algebraic multigrid method (AMG) is also introduced to the IEFEM in order to realize an efficient analysis of large-scale thin plate structures. In order to investigate the analysis accuracy and the performance of the developed method, the developed method is applied to the analysis of deformation on the welding of a simple stiffened structure. The developed method is then applied to the prediction of welding deformation in the construction of a ship block. The obtained results indicate that the developed method has approximately the same analysis accuracy as the conventional method, and the computational speed of the developed method is dramatically faster than that of the conventional method. The developed method can analyze the welding deformation in the construction of the ship block structure which consists of more than 10 million degrees-of-freedom and is difficult to solve by the conventional method.

A three-dimensional (3D) deformation (in plane and out of plane deformations) measurement method is developed using digital cameras, which require no special equipment. This method is a non-contact method, and it can sequentially measure over the entire photographed image. Furthermore, since image analysis is based on the technique of image matching, the method is applicable even when the deformation to be measured is large. In addition, since it is possible to use all pixels as measuring points, the number of available measuring points at one time is the same as the number of effective pixels of the camera. In this study, the proposed method is applied to the sequential measurement of displacement under strong lighting levels in arc welding. Through the comparison of the results measured by a 3D shape measurement system (LAT-3D) using a laser displacement gauge and digital caliper, the quantitative validity of the proposed method is also verified.

The technique that can measure the transient welding deformation directly is very important for investigating the mechanism of welding deformation. In this study, an in-situ displacement measurement method using digital camera is self developed. This system is non-contact type and it does not need any complicated optical systems but it can measure the in situ displacement over the full field high accuracy using digital image correlation technique. Therefore, it is considered to be useful and easy to apply to practical problems. The number of measuring points that can be obtained at a time is more than 10 million. It is the same as the available pixels of the digital camera. Furthermore, since the active light source is not necessary in this system, the influence of the fluctuation of the atmosphere caused by the high temperature area is small. In this study, the detail of the proposed system is given and it is applied to the transient in-plane deformation problem under very high brightness lightened by we...

The welding distortion of thin plate was selected as research subject, which causes loss of dimensional control, structural integrity and increases fabrication costs for straightening. To study the characteristic of the welding distortion on thin plate structures, experiments were conducted. The test model was a thin plate stiffened structure and a large twisting distortion was observed. The welding distortion of the same structure was analyzed as a large deformation problem using a thermal elastic plastic FEM and an elastic FEM based on the concept of inherent deformation. The computed results by both methods showed the twisting distortion which is a typical buckling type deformation and the magnitude of this distortion agreed well with the experimental measurement.

Recently, the efficiency of computer modeling and simulations has been improving dramatically. Consequently, the structural analysis of welding phenomena based on thermal–elastic–plastic finite element method (FEM) can be performed with a good accuracy in a short time. However, if the model to be analyzed is large and it has a complex shape, problems might occur with preprocessing, such as the mesh generation process which is very time consuming. To overcome these problems, meshless methods such as the meshless local Petrov-Galerkin (MLPG) method have been developed. In this study, the MLPG method is applied to heat conduction analysis, in the problem of bead-on-plate welding (a moving heat source problem), and the fundamental properties (influence of MLPG parameters and nodal point density, etc.) of the method are investigated to verify the applicability of the proposed method. As a result, it is clearly shown that when the number of nodal points is sufficiently high, the influence of the MLPG parameters is small. In addition, an adaptive analytical method using the MLPG is proposed, which includes the addition and elimination of nodal points. From the results of the MLPG method using the adaptive method, it is found that a very high accuracy can be obtained. In this method, the nodal point distribution can be set relatively freely. Thus, the use of the MLPG method in welding analysis is expected to grow in the future.