Masakazu Shibahara

Studies on in-situ full-field measurement for in-plane welding deformation using digital camera

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 welding arc. Through the comparison between the experiment and thermal elastic–plastic finite element analysis, the validity of the transient transverse shrinkage distribution which is measured by the proposed system is verified. The residual deformation is also investigated to check the accuracy and usefulness of the proposed system.

NUMERICAL PREDICTION OF WELDING HOT CRACKING USING THREE-DIMENSIONAL FEM WITH TEMPERATURE DEPENDENT INTERFACE ELEMENT

The Finite Element Method (FEM) is a powerful tool to predict welding residual stresses and distortions. However, it is impossible to analyse the hot cracking with a simple thermal-elastic-plastic FEM code, since the conventional FEM models only volumetric behaviour. Based on the interface element proposed for the cracking propagation problem, a three-dimensional FEM with a temperature dependent interface element has been developed. The proposed method is applied to the analysis of the pear-shaped bead cracking under narrow gap welding as one example of hot cracking in welding. Both the pear-shaped bead cracking and the surface cracking along the welding line can be simulated using the proposed method. Also, it is found that the heat input, the groove width, and the constraint under the solidification are the essential causes of the pear-shaped bead cracking.

Development of in situ measurement system for welding deformation using digital cameras

Abstract 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.

Large-scale non-linear analysis of residual stresses in multi-pass pipe welds by idealized explicit FEM

In this research, idealized explicit FEM (IEFEM), which can analyze residual stress of the practical structures in practical time, was applied to large-scale residual stress problem of X groove multi-pass pipe welding. The analysis model has 3,376,074 degrees of freedom and 108 welding passes. In the analysis, all the welding passes and welding pass grouping technique were considered to investigate the influence of welding pass grouping on residual stress distribution. As a result, it was found that the residual stress distribution can be different if pass grouping technique is used. In addition, to investigate the influence of the welding pass sequence on residual stress distribution, analyses which consider another in-layer welding sequence and build-up sequence were carried out. As a result, it was found that the influence of the welding sequence on residual stress distribution is small. The computing time was practical even though all the welding passes are modeled in a full pass model.

Full-field time-series measurement for three-dimensional deformation under welding

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 million points, and the measuring precision is expected to increase as the camera pixel resolution continues to increase. Therefore, this method is expected to have future potential. In this study, the proposed method is applied to the sequential measurement of displacement under the strong lighting levels in arc welding. By comparing the time history of transverse shrinkage, longitudinal shrinkage and angular distortion with the results of FEM thermal elastic-plastic analysis, the qualitative validity of the proposed method is verified. To investigate the measurement precision and usefulness of the method, a 3D shape measurement system (LAT-3D) using a laser displacement gauge and digital caliper are used. The distributions of residual transverse shrinkage and residual angular distortion are measured by the proposed method, LAT-3D and digital caliper. Through the comparison of the results measured by these methods, quantitative validity of the proposed method is also verified.

Prediction of Hot Cracks of T-Joints in Full-Penetration Welding Process

In this study, the influence of welding conditions on the formation of a pear-shaped bead crack is examined through experiments on full-penetration welding of T-joints. These problems are analyzed using the finite element method (FEM). The formation and growth of a pear-shaped bead crack in T-joints welded by full-penetration, high-current pulsed MAG (Metal Active Gas) welding is simulated using temperature-dependent interface elements, which are introduced in the thermal-elastic-plastic FEM analysis. The experimental results show that a pear-shaped bead crack does not occur when the heat input is greater than 2 500 J/mm, or when the penetration bead is formed. However, according to commonly accepted knowledge, both large heat input and a large penetration bead (deep penetration relative to the penetration width) lead to hot crack formation. To understand these experimental results, FEM simulations are conducted and the influences of heat input and the size of the penetration bead are investigated.

Numerical analysis of residual stress distribution on peening process

Various peening techniques are employed to prevent stress corrosion cracking or to extend the fatigue life of structures. In this study, to investigate the effect of shot peening on operation, an analysis method that predicts the stress distribution due to shot peening was proposed. Using the proposed method, the load distribution from the shot collisions was modeled, and it was integrated with a dynamic analysis method based on the idealized explicit FEM (IEFEM). The accuracy of the proposed method was confirmed by comparing the stress distribution from the collision of a single shot with the results analyzed using ABAQUS. A thermal elastic-plastic analysis method using IEFEM was applied to the analysis of the residual stress distribution of a multi-pass-welded pipe joint. The calculated residual stress distribution was compared with the measured residual stress distribution measured using X-ray diffraction (XRD). The results showed that the two welding residual distributions were in good agreement. Considering the calculated welding residual stress distribution, the modification of the stress distribution due to shot peening was predicted using the proposed method. A similar stress distribution was obtained using XRD for the case where a large number of collisions were considered.

Development of Ultra Large Scale Computation for Transient Welding Deformation and Stress Using Idealized Explicit FEM Accelerated by GPU

Numerical simulations such as Finite Element Method (FEM) are widely used as tool of structural analyses in both design and production. However, in the application of FEM to welding problems, the simulation scale is usually limited to the welding joint level. Only a few large-scale welding analyses are performed on existing research because welding problems are transient and show strong nonlinearity. In such cases, it is necessary to use static implicit FEM to achieve an accurate analysis, but the larger analysis scale requires larger memory consumption and computing time. Thus, we previously proposed idealized explicit FEM (IEFEM) to achieve shorter computing time and lower memory consumption.Since IEFEM is based on dynamic explicit FEM, it is not needed to solve the stiffness matrix of the whole system and it is possible to analyze by only performing the calculation for each degree of freedom (DOF) and element. Such characteristic indicates that IEFEM is suitable for parallelization. Then, in this study, we developed parallelized IEFEM using a graphics processing unit (GPU). The usefulness and validity of the developed method are considered by analyzing a 3-dimensional multi-pass moving heat source problem, which is very difficult to analyze with commercial FEM software because of its analytical scale. As a result, it is found that parallelized IEFEM accelerated by a GPU can analyze a large-scale problem having over 1,000,000 DOFs on a single PC.Copyright

Prediction of Residual Stress in Multi-Pass Welded Joint Using Idealized Explicit FEM

Heavy thick steel plate is used for pipes and also ship structures, and multi-pass welding is usually adopted for the welding. Because of the heavy thickness, residual stress plays an important role, particularly in crack propagation. Implicit Finite Element Method (FEM) is often used as a welding analysis method to examine the residual stress of the welded plate, but it is not easily applied to multi-pass welding problems with tens of thousands of degrees of freedom, because of the huge computational time and memory consumption. Alternatively, it is possible to simulate the residual stress in shorter time with lower memory consumption by using Idealized Explicit Finite Element Method developed by the authors. Moreover, the computational time can be shortened by using Idealized Explicit FEM using a Graphics Processing Unit (GPU). In this research, Idealized Explicit FEM parallelized using a GPU is applied to the analysis of the residual stresses of the multi-pass welding joint of a pipe structure made of heavy thick steel plate.As the result, the residual stress simulated by the Idealized Explicit FEM corresponds to the measured residual stress. Furthermore, it is found that the grouping method may affect to the residual stress distribution.Copyright

Study on residual stress in multi-pass welded joint using idealized explicit FEM

Heavy thick steel plate is used for pipes and also ship structures, and multi-pass welding is usually adopted for the welding. Because of the heavy thickness, residual stress plays an important role, particularly in crack propagation. Implicit Finite Element Method (FEM) is often used as a welding analysis method to examine the residual stress of the welded plate, but it is not easily applied to multi-pass welding problems with tens of thousands of degrees of freedom, because of the huge computational time and memory consumption. Alternatively, it is possible to simulate the residual stress in shorter time with lower memory consumption by using Idealized Explicit Finite Element Method developed by the authors. Moreover, the computational time can be shortened by using Idealized Explicit FEM using a Graphics Processing Unit (GPU). In this research, Idealized Explicit FEM parallelized using a GPU is applied to the analysis of the residual stresses of the multi-pass welding joint of a pipe structure made of heavy thick steel plate. As the result, the residual stress simulated by the Idealized Explicit FEM corresponds to the measured residual stress. Furthermore, it is found that the grouping method may affect to the residual stress distribution.