| Orthotropic steel bridge decks connect steel materials into a whole by welding.The welded part is usually considered as the fatigue weak link of the structure.This is because the part is susceptible to local stress concentration and tensile welding residual stress.There may be various defects,such as initial cracks,undercuts,and slag inclusions.In order to improve its fatigue characteristics and prolong its service life,scholars at home and abroad have adopted different post-weld improvement methods.Among them,as a relatively novel postweld treatment method,ultrasonic impact has been recognized by many researchers.It has been proved that the improvement of welded toe geometry and the introduction of compressive residual stress are the main reasons for ultrasonic impact to improve the fatigue mechanism.However,there is still a lack of in-depth and systematic research on the details of the doublesided welded structure of the longitudinal rib top plate of the orthotropic steel bridge deck using this post-weld treatment method.Therefore,this thesis takes the new double-sided welding structure details of the U-rib top plate of orthotropic steel bridge deck as the main research object,uses numerical simulation to study the mechanism of ultrasonic impact to improve the fatigue performance of the outer welded toe of the top plate,and analyzes different ultrasonic waves.The impact of the impact process parameters on the impact results.Finally,the ultrasonic impact model of double-sided welding of U-rib top plate with initial crack defect was established,and the stress field of the welding model after the initial crack was inserted and the crack opening displacement and stress intensity factor change law after ultrasonic impact treatment were analyzed.Strengthening effect,the main contents include:(1)The diseases and causes of fatigue cracking of orthotropic steel bridge slabs are introduced.At the same time,the problems existing in the welding process are explained,common methods to deal with the problems are given,and the characteristics of various research methods are summarized.Based on the analysis,the ultrasonic impact treatment method adopted in this paper is put forward.According to the summary analysis of the research status,the main research questions of the paper are put forward,and the main research content and goals are determined.(2)Explain the characteristics of implicit static welding analysis and dynamic impact analysis simulation of two plates involved in ANSYS analysis in this article.The part of implicit welding simulation theory includes basic law of welding heat transfer,welding heat transfer control equation,welding stress field elastic theory and elastoplastic theory and temperature field control differential equation,welding stress field plastic theory’s yield criterion,flow criterion,The strengthening criterion is introduced,and the finite element simulation method of inserting the initial crack in the welding process is described.The characteristics of different welding initial cracks and the fracture criterion are explained.On this basis,different stress intensity factor calculation methods are given.Related calculations provide theoretical basis.(3)The thermo-elastoplastic finite element method was used to analyze the welding of the new double-sided welding structure model of the U-rib top plate.The simulation results of the welding temperature field and the weld pool shape and welding stress field were verified with the experimental data in this paper,and the results were in good agreement.On this basis,the ultrasonic impact treatment method is used to impact the outer weld toe of the U-rib top plate,and the parameter analysis of the impact pin is carried out.The simulation results show that the shape of the impact pit after impact is related to the shape of the impact pin.The treatment only has an effect on a local area,and has little effect at a far distance.After the treatment,the longitudinal residual stress reduction effect of the weld toe is obviously greater than the transverse residual stress reduction effect.(4)Using the method of fracture mechanics,different initial crack defects were inserted into the outer weld toe of the U-rib top plate,and the stress redistribution model after the initial crack insertion was impacted by the ultrasonic impact method,and the relative displacement of the crack surface before and after the impact was compared.,And the stress intensity factors at the surface points and deepest points of the crack.The research results show that ultrasonic impact treatment can increase the fatigue life of the double-sided welded structure of the Urib top plate by 1.9-10 times.And it can completely close the cracks with a depth of less than 2 mm. |
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