Development of methods for prediction and control of welding stress and deformation besed on concept of inherent strain as their essential source

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To promote the automation of shipbuilding process, the prediction and the control of the welding deformation are very important problems to be solved. Also, to achieve highly reliable estimation of the safety of ships against the fatigue damage, the quantitative estimation of welding residual stress is necessary. For establishing a consistent view on the welding residual stress and the deformation, the problem should be examined on the bases of the concept of inherent strain, which is considered to be the source of both the residual stress and the deformation. For the quantitative comparison to identify the most influential factor, numerical methods such as the FEM is a very powerful tool.Thus, a FEM code was developed in the first year of this research program for the prediction and the control of the welding deformation. The characteristics of the inherent strain and the residual stress were clarified. Further, the effects of tack welding pitch and the type of tab-plate are investigated based on the FEM simulation.In the following two years, the effects of root gap due to the cutting error and the residual stress in the steel plate are examined. Through this study, it is found that these factors are more influential than the tack weld and the tab. Thus, it is necessary to control them to reduce the welding deformation.To estimate the residual stress in butt weld and fillet weld with small number of measuring points, a new measuring method was developed and its validity was examined through experiments. The proposed method is based on the concept of inherent strain and the fact that the distribution of the inherent strain can be accurately described as a function involving small number of parameters.