Welding deformation analysis based on improved equivalent strain method to cover external constraint during cooling stage

Abstract

The prediction and control of welding deformation at design stage has been an essential task in shipbuilding industry to ensure high fabrication quality as well as high productivity. The most widely used method is the thermal elasto-plastic analysis method due to its high accuracy in the simulation results. However, it has a disadvantage in computational time due to the consideration of non-linearity of material and temperature-dependent material properties. In order to overcome the difficulties, a few efficient approaches which are applicable to complicated welding process of ship hull blocks have been developed. In the present study, the existing equivalent stain method is improved to make up for its weaknesses. The improved inherent strain model is built considering more sophisticated three dimensional constraints which are embodied by six cubic elements attached on three sides of a core cubic element. From a few case studies, it is found that the inherent strain is mainly affected by the changes in restraints induced by changes of temperature-dependent material properties of the restraining elements. On the other hand, the degree of restraints is identified to be little influential to the inherent strain. Thus, the effect of temperature gradients over plate thickness and plate transverse direction normal to welding is reflected in the calculation of the inherent strain chart. A 3D contour of inherent strain is plotted versus temperature gradients of thickness direction and transverse direction for a maximum welding temperature value. A series of inherent strain charts are obtained varying the maximum welding temperature value. The welding deformation can be calculated by an elastic FE analysis using the inherent strain values taken from the inherent strain chart. The proposed method is verified by comparing the calculated welding deformation analysis results with the existing method, thermal elasto-plastic FE analysis, and experimental results. External restraints imposed normal to plate during cooling stage is identified to be effective to the reduction of angular distortion of butt-welded or fillet-welded plate. The external restraint is represented by vertical force on work piece at both sides and bending stress forms in transverse direction. The additional bending stress distribution across plate thickness is reflected into the improved inherent strain model and a set of inherent strain charts with different levels of bending stress are newly calculated. Welding deformation can be calculated from an elastic linear FE analysis using the inherent strain values taken from the chart and compared with those from a 3D thermal elatsto-plastic FE analysis.