Research On Residual Stresses In Cutting And Welding Continuous Process Of Hull Members

The process of shipbuilding consists of a series of technologies,which are sequentially applied to hull components,and the residual stresses and deformations in hull structure should be considered as the result of the coupling effect among the multiple processes.In the aim of accurately controlling the residual stresses distribution and deformation in hull structure,it's no enough that just focus the residual stresses induced by individual process such as welding.Instead,a dynamic view should be introduced throughout the entire life cycle in shipbuilding,following the residual stress evolution during continuous process,and mastering the distribution of coupling residual stresses induced by multi-processes.In this study,the views of"distribution" and "evolution" were emphasized.The typical continuous process of thermal cutting-welding in shipbuilding was concerned,and some key issues in this process were emphatically studied,including the residual stresses distrituion,evolutions and coupling effects between cutting and welding.(1)In order to accurately control the residual stresses distribution and deformation in hull structure,the initial stresses field as welding should be well studied.As the critical process prior to welding,the initial residual stresses induced by thermal cutting have important effect on welding residual stresses distribution and deformation.Based on the combustion reaction equations in oxygen cutting and oxide composition analysis,a heat source model was proposed with the combination of preheating flame and heat generation of iron combustion reaction,which has been valiated by infrared imaging during oxygen cutting experiment.Based on this heat source model,a numerical method was presented to analyze the cutting thermodynamic process on the basis of thermal elastic-plastic FEM,involving the temperature and stresses distribution,which has been validated by stress measurements with the impacted indentation method.It can be concluded that the cutting residual stresses,with the values exceeding the tension yield,distribute uniformly along the seam.If the cutting reaches a certain length,the global distribution of cutting residual stresses could be represented by the stresses distribution along the transverse centerline.According to the stresses distribution features,the transverse centerline could be divided into three stress regions,and six parameters were proposed in the aim of the parametric expression for the residual stresses distribution.(2)The quantitative analysis was carried out to master the distribution features of cutting residual stresses.The previous studies usually focused on the stress value,but the stresses distribution features get special attention in this research,and mathematical relationships were established between oxygen cutting technological parameters and cutting residual stress distribution features.First of all,via the further selection of the six stress distribution features,two distribution features with more representation were found:the breadth of high tensile stress zone(B1),and tensile force on transverse section(Ft),as the dependent variables in the regression equation.And then the cutting technological parameters,as the independent variables in the regression equation,were classified and selected.According to the single factor analysis between B1,Ft and cutting velocity(v),boundary constraint(R),cutting length(L),workpiece breadth(B),cutting thickness(H),the effect of each technological parameter on stress distribution feature was studied,and the inverse functions have good effect to fit the relationships between B1,Ft and L,B,H,respectively,and R could be treated as the qualitative variable into the regression equation.Finally,according to the orthogonal test and plenty of numerical calculations,the multi-factor regression model that B1,Ft to L,B,H,R was presented,and with good result(error less than 10%)for predicting the cutting residual stresses distributions under the given condition.(3)Taking the continuous process as research object:the steel plate was cut in half and then welded together,the research was carried out on the coupling residual stresses(Sycw)distribution and evolution induced by cutting-welding continuous process,considering the effect of initial residual stress induced by cutting.Based on the regression model of initial stresses distributions,a high-efficiency numerical method was presented to analyze the cutting-welding thermodynamic continuous process,involving the coupling residual stresses distribution,which has been validated by cutting-welding experiment and stress measurements based on X-ray diffraction.The distribution of coupling residual stresses is mainly affected by the process which can generate a wider distribution of high stresses.If the cutting generate a wider distribution of high stresses(B1c)than welding(B1w),as a result,cutting residual stresses will have an obvious influence on coupling residual stresses distribution.By dividing both the time domain and stress field,the evolutions of coupling stresses were studied in each segment and period.According to the temperature history,the process of evolution was divided into three period:Ti?T3.According to the B1c and B1w,the transverse centerline was divided into three segments:?-?3.During Ti,the initial stress field will be redistributed:relaxed or superimposed depending on the distance to seam,and during T2 and T3,Sycw will vary with the temperature itself.The influence of initial stresses(due to cutting)on Sycw evolution was studied by comparing the evolutions between coupling stresses and welding stresses,and it could be described as:if there exists initial stresses,the variation gradient of Sycw will be higher than that without initial stresses(such as Syw).