Study Of Residual Stress For Ship Hull Plate In Cold Forming

Traditional line-heating forming combined with rolling forming model, which not only means the less efficiency but also has difficulty in automated production, is still used for the forming of3D ship hull plate in shipyards presently and contrary to the trend of shipbuilding technology. The ship plate machine based on reconfigurable loose mould forming mechanism with square press heads, could make the ship plate forming automatic and numerical controlled by cold forming as a new and promising kind of forming technology.The springback and cold-forming residual stress which has complex distribution rules and has substantial influence for the safety of formed parts, will be induced for the release of elastic deformation inevitably. On the one hand, the hull plate in service would be damaged in advance under the actions of the residual stress and working loads. On the other hand, the property of material will be enhanced as the effect of cold forming. Consequently, the research of prediction method and appropriate control technology for residual stress are import approaches to make the forming technology extensive and widespread. Based on the structural characteristics and forming mode of SKWB-400CNC ship plate machine, numerical simulation model is established and perfected to predict the residual stress of formed parts, and key issues for establishing the FE model are discussed.Cylindrical parts, spherical parts and saddle surface parts are taken as the research objects, and the processes of load and unload are simulated by the FE method. Based on the mechanism analysis of plate formed under pure bending, the theoretical equation of residual stress for plate in single curvature cold-forming is invented. The distribution regularities of the stress and strain is analyzed and the changing law of the residual stress’peak value caused by the thickness and curvature of cylindrical plate is got through the numerical simulation of multi-point forming mechanism with square press heads. Results of both numerical simulation and theoretical derivation indicate that the value of residual stress forecasted by simulation achieves considerable accuracy. The distribution rules of stress and strain are dissimilar for different forming shapes, and the peak values of residual stress are increased as the increase of the plate’s thickness and deflection.According to the characteristic of modular surface and the changed rules of residual stress, alternate forming technology is recommended to improve the forming precision and lower the residual stress. Results of the simulation show that the peak value of residual stresses would be reduced obviously and springback would be minimized effectively by alternate forming technology. As the increase of forming steps, the shape of the formed parts will tend to the target forming shape and the residual stress will distribute uniformly and homogeneously.