Research On Fatigue Performance Of Welded Joints Of Steel Structure Reinforced By Ultrasonic Impact

Welding is the most widely used connection technology in engineering.Due to the process characteristics of welding itself,the existence of stress concentration and residual stress cannot be avoided,which affects the fatigue life of the structure.With the development of the times,the requirements for the quality of engineering structures are getting higher and higher,especially in key areas such as aerospace,automobiles,railways and bridges,which require higher fatigue strength and stress corrosion resistance,so effective technological measures must be adopted to increase the fatigue life of the overall structure.Ultrasonic impact treatment can effectively improve the fatigue performance of welded structures and is currently the most promising metal surface strengthening technology.In this paper,two kinds of typical welding structure details,equal thickness butt welds and load-carrying fillet welds,with the widely used Q345 qD steel and 2205 duplex stainless steel as test materials,sufficient number of fatigue test results are obtained.Based on the nominal stress method,the reinforcement effect of ultrasonic impact treatment on different steel grades and welded structures is explored.According to the fatigue test results of butt joints under the same stress amplitude,the fatigue life of the treated specimens can be increased by 2-6 times,and the condition fatigue limit of different structural details is characterized by the fatigue strength under 2 million cycles(FAT-200).For Q345 qD steel butt joints,the value of FAT-200 is 157.5MPa under the welding state,194.2MPa under the impact state,23.3% higher than that under the welding state;For 2205 duplex stainless steel butt joint,the value of FAT-200 is 147.7MPa in the as welded state and 194.1MPa in the impact state,which is 31.7% higher than that in the welded state.According to the fatigue test results of the cross stress fillet weld,it is found that this kind of welding structure details cause damage from the root of the weld,and only the surface treatment can not improve its fatigue life.Residual stress is an important factor affecting the fatigue life of welded structure.In this paper,Q345 qD steel welded plate is taken as the research object,and the change of residual stress field before and after ultrasonic impact treatment is analyzed by blind hole method.The test results of as welded specimen along the direction parallel to the weld showed that the average value of the maximum residual stress on both sides of the weld was only 8.39 MPa,about 4.0% of the maximum principal stress.It can be considered that the residual stress field on both sides of the weld basically distributed symmetrically.The test results along the direction perpendicular to the weld show that the residual stress value is higher in the area30 mm away from the weld center line,and the closer to the weld,the higher the residual tensile stress value,the highest can exceed the yield strength of the steel.The test results of the residual stress of the single side impact treatment sample show that the ultrasonic impact treatment has an obvious effect on the elimination of the residual stress on both sides of the weld of the welded structure,which can basically reach more than 200%,and successfully introduce compressive stress.In addition,the other mechanisms of ultrasonic impact treatment to improve the fatigue performance are analyzed,mainly including the improvement of stress concentration at the weld toe,the typical appearance characteristics of fatigue fracture,the comparative analysis of macro morphology before and after impact,and the change analysis of surface hardness.The mechanism of ultrasonic impact treatment to improve the performance of welding structure is deeply understood.