Research On Spatial Distribution And Control Technology Of Residual Stresses In Butt Welding For Bridge Steel Thick Plates

Due to the sustainable development of transportation,more and more steel bridges have been constructed.The high-strength thick steel plate is also widely used to fulfill the bridge requirements of carrying and cross-capacity.Now,welding is the main way to connect the steel plate;however,residual stress induced by welding has a complex spatial distribution and large tensile stress around the weld.These will seriously affect the manufacture and installation precision,and reduce fatigue life and durability of bridge structure.The welding residual stress is usually assumed to be uniform distributed along the plate thickness.However,for the thick plate,this assumption will bring a large design error,which induces the safety risk for the bridge structure.Therefore,in this thesis,the spatial distribution and control technology of welding residual stress in bridge thick steel plate are studied systematically.(1)The application situation of bridge thick steel plate,the distribution and damage of welding residual stress in thick plate were described.The present research situation around the world was summarized,including the measurement methods of welding residual stress,measurement and simulation of welding residual stress and control technologies of welding residual stress.Based on the shortcomings in the current research work,research objectives and contents of this paper were established.(2)To eliminate the influence of the stress concentration on the test precision in blind-hole method and expand the application range of empirical equation,the Monte Carlo method was used to randomly sample the influencing factors in the empirical equation.The reasonable range of empirical equation was analyzed.Compared with the graded calibration values of Q345 qD steel,it could be seen that the assessment process was reasonable.(3)During the experiment,Q345 qD steel plates with various thicknesses(16,30,40,60,80,and 100 mm)were welded.Through-thickness residual stresses for these specimens were measured using a combination of local material removal and the hole-drilling method.In order to gain an overall understanding of the welding stress distributions,two-dimensional stress contours were generated by applying the measured data to the Surfer software.Commonly used empirical equations for residual stresses distribution were modified to expand their range of application based on the data obtained in present work with Q345 qD butt-welded specimens.The modified equations can predict the residual stresses more comprehensively and reliably than the original equations.(4)The three-dimensional finite element models of 16-and 30-mm-thick welded test plates were established in ABAQUS software.The welding processes were simulated based on thermal elastic-plastic finite element method.The evolution mechanism of residual stress field was analyzed,and the influences of plate thickness and material strain hardening parameter on residual stress field were revealed.The prediction ability of the model was verified by comparing with the measured results,which would be a foundation for further analysis of the effect of welding residual stress field on fatigue fracture performance.(5)Based on the distribution of simulated residual stress in the 30 mm thick test plate,within the linear elastic fracture mechanics,the weight function method and the finite element method were used to study the effects of welding residual stress on stress intensity factor of semi-elliptical surface crack.Different aspect ratios and relative depths were analyzed.(6)In order to study the residual stress field induced by shot peening treatment,a three-dimensional random-shots model was established.A new method for calculating shot peening coverage was proposed based on the image processing technology in MATLAB.And compared with the results of existing PEEQ method and empirical equation,it could be concluded that the new method of coverage calculation was reasonable and correctly.(7)The surface shot peening treatment was carried out for Q345 qD stress-free specimen and welded specimens.The variation of residual stress on the welded specimen surfaces before and after shot peening were studied.The surface residual stress and roughness of stress-free specimen were analyzed by experimental measurement and simulation method.Based on the three-dimensional random-shots model,the evolution law of the residual stress field and roughness under different shot peening coverage was studied to find an effective way to improve the residual stress distribution and the surface morphology of the weld.