Research On The Grillage Ultimate Strength Considering Welding Initial Defects

As the upper surface of ship hull girder,the grillage bears relatively large load when the ship is in hogging or sagging state,and the evaluation of ultimate strength of the grillage is an important aspect in the process of ship structure design.As a common process in modern ship construction,welding can cause structural deformation,residual stress and other initial defects,and cut down on th mechanical properties such as structural strength and stability.So it is important to study the ultimate strength of deck grillage considering initial welding defects.Based on thermal elasto-plastic finite element method(TEP FEM),the welding deformation and residual stress is calculated.On this foundation,based on the non-linear Riks method,this article studies the similarities and differences on the ultimate strength of the grillage before and after welding,and the effects of deformation degree and residual stress on ultimate strength of the grillage are discussed respectively,and the ultimate strength of the grillage after deformation control during welding process is also calculated.In addition,the ultimate strength of opened deck grillage before and after welding is studied.The main works of this paper are as follows:(1)Background and significance of the research,and the research status of numerical simulation of welding and ultimate strength of ship structure at home and abroad.The theoretical basis of this paper is introduced,including the thermal elasto-plastic finite element method(TEP FEM)for numerical simulation of calculating welding process and the non-linear Riks method for ultimate strength calculation.(2)Taking deck grillage as the research object,the welding process with different welding sequence was simulated,and the ultimate strength of the grillage considering welding defects was compared with the ideal state.The results show that the greater the deformation degree of the welded frame,the lower the ultimate bearing capacity and overall stiffness of the frame.The welding deformation of the structure can be reduced by using specific welding sequence,and the ultimate strength of the frame can be improved in return.(3)The effects of deformation degree and residual stress on the ultimate strength of the plate were studied independently,and the ultimate strength of the grillage with vertical mechanical constraints during welding process was calculated.The ultimate capacity ability and overall stiffness of the grillage can be effectively improved when the degree of deformation of the frame is reduced;vertical mechanical constraints such as fixed pressure blocks applied during welding process can effectively reduce the welding deformation of the structure and significantly improve the ultimate capacity ability and overall stiffness of the grillage.(4)The effect of welding initial defects on opened deck grillage is studied.The results show that the opening weakens the ultimate strength of the deck grillage,but after considering the welding defects,the influence of the opening on the ultimate strength of the grillage is smaller.Compared with the complete grillage,the influence of welding deformation and residual stress on the ultimate strength of the opened grillage is smaller.