Akikazu Kitagawa

Lens distortion correction for digital image correlation by measuring rigid body displacement

A method of lens distortion correction is proposed in order to improve the measurement accuracy of digital image correlation for two-dimensional displacement measurement. The amounts of lens distortion are evaluated from displacement distributions obtained in a rigid body in-plane translation or rotation test. After detecting the lens distortion, its coefficient is determined using the method of least squares. Then, the corrected displacement distributions are obtained. The effectiveness of the proposed method is demonstrated by applying the correction method to an in-plane translation test and tension tests. The experimental results show that the proposed distortion correction method eliminates the effect of lens distortion from measured displacements.

Fundamental study of narrow-gap welding with oscillation laser beam

Abstract Laser welding with oscillation laser beams enables control of the heat input distribution. In this study, we attempted to develop a narrow-gap welding process with oscillation laser welding. This process is expected to prevent a lack of fusion because the entire bottom to the groove can be melted by the oscillation laser. As the first step of the study, bead-on-plate welding experiments with an oscillation laser beam were performed to investigate the relationship between the welding conditions and welding results. The experiments revealed that the oscillation conditions strongly affect the welding penetration shape. It was clarified that the wire melting phenomena in oscillation laser welding differed from those of straight (non-oscillation) laser welding. Additionally, narrow-gap laser welding experiments were performed to investigate the relationship between the oscillation conditions and gap width. The results confirmed the effectiveness of oscillation laser welding for narrow-gap welding.

Development of narrow gap multi-layer welding process using oscillation laser beam

Abstract The oscillation laser beam is considered to be effective as a heat source of narrow gap multi-layer welding because oscillation laser welding can control the penetration shape and prevent the lack of fusion. In this study, in order to establish a narrow gap welding process by oscillation laser beam, butt welding experiments of 50 mm thickness carbon steel plate were performed. By the appropriate control of the heat input area using the in-process sensor for recognizing the groove shape, narrow gap welding of a thick plate with groove which was cut by gas cutting was achieved. Properties of the welded joint had been confirmed by nondestructive testing, tensile test and side bend test. A two-dimensional numerical calculation model for welding deformation was developed. This calculation model was used for investigation of the optimal groove angle. The results of calculations were in quantitative agreement with the experimental results. Microstructure of the weld zone had multiple thermal histories. According to the hardness test results, maximum hardness of the heat affected zone of the upper layer has been lowered than that of the lower layer.

Development of gap sensing system for narrow gap laser welding

Abstract Narrow gap welding with an oscillation laser beam is one of the effective processes for thick plate welding. To put this welding process into practical manufacturing, a groove-sensing system using image processing for narrow gap welding with an oscillation laser beam is used. This developed system uses still images of the weld zone taken by a coaxial CMOS camera. It can recognize the position of the groove wall by analysing the brightness distribution in the still image. It can then control the oscillation width and the laser-irradiated area by calculating the groove width and the groove centre position. Some narrow gap welding experiments were performed to evaluate the performance of the developed system. The results revealed that the developed system is effective for narrow gap welding with an oscillation laser beam. Using this system, the narrow gap groove can be welded even if the groove width has changed during the welding process.

A Semi-Analytical Nonlinear Regression Approach for Weld Profile Prediction: A Case of Alternating Current Square Waveform Submerged Arc Welding of Heat Resistant Steel

The complexity in weld profile caused by abrupt change in polarity in square waveform welding is investigated through the development of a model capable to accurately predict weld profile. A semi-analytical model is conceived wherein characteristic attributes of a composite parabolic-elliptic function, which represent the weld profile, are obtained through nonlinear regression (NLR). The proposed model is demonstrated for its efficacy in the prediction of weld profile over a wide range of welding parameters, vis-a-vis, welding current, frequency, electrode negative (EN) ratio, and welding velocity. The investigation suggests that the center and outer cores of welding arc remains more active during positive and negative polarity, respectively, that leads to distinct macroscopic zones in weld cross section and thus, necessitates a composite profile for representation of weld profile. The intersection of the zones forms a metallurgical notch which the investigation offers a method to estimate and thus control. Unlike the convention continuous arc welding, the waveform arc welding caters welding at higher velocity without compromising the weld penetration and almost abolishing the metallurgical notch as well.

Development and application of 10 kW fiber laser welding system

We have been developing laser welding technologies for the past 20 years. Our practical applications started for thin stainless steel products in 1999. In this study, we developed a high-power laser welding system for thick plate products. A 10 kW fiber laser is used in this system. With this system, we can fabricate thick plate products with maximum thicknesses of 15 mm. We applied this laser welding system to some parts of chemical plant equipment. The cross section was a simple T-joint, and the welding length was about 4000 mm. The mechanical properties of the laser-welded joint satisfied the design requirements. An arc welding method was also applied to this fabrication, but too much time was needed to correct large distortions. The developed laser welding system dramatically decreases the cost and correction time.

New proposal for the solution of thick plates lap joint problem

The strength of laser welding with lap joints depends on its welding length and width of bead at plates interface. Therefore, the narrow bead width of laser lap welding joints at plates interface forces to apply multi-times welding to get required strength. The goal of this paper is to increase interface bead width on thick plate laser lap welding. The authors turn their attention to the penetration shapes of high power CO2 laser welding for achieve that. It is well known that penetration shapes are like a wine cup in the case of deep penetration welding using high power CO2 laser. The penetration shape of the wine cup is suggested that a large amount of laser power is absorbed at the surface of the test pieces. Then, the authors have studied how to change the heat source distribution on laser lap welding, and have succeeded to increase the bead width at the interface by a new developed process.

Thick-sandwich-panel fabrication for bridge deck structure and its joint strength

The purpose of this study is to evaluate the static strength of thick plate laser welded lap joint and the performance of thick sandwich panel for bridge deck structure. The strength of thin sheet laser welded lap joint whose thickness is less than 1.6 mm have been already reported, but the strength of thick plate laser welded lap joint whose thickness is more than 10 mm is not known. The authors believe this thick laser welded lap jont enable heavy industries to make a revolution on applying it for sandwich panel fabrication. Then, they have conducted mechanical tests of 10 mm + 6 mm thick laser welded lap joints to grasp their static strength. According to the obtained data, they fabricate a thick sandwich panel model of newly designed bridge deck structure for evaluation. The static lading test, cyclic loading test and collapsing test show that the thick sandwich panel has enough performance for the practical application.