| Ultrasonic impact treatment(UIT)has the advantages of convenient operation,low noise,high efficiency,low cost,energy saving and so on.It is one of the latest fatigue life extension technologies in the world.This technology can overcome the problems of the joint fatigue strength reduction caused by the stress concentration of the welding structure,the stress concentration of the weld toe notch and the residual stress of high value welding.It has been applied to the structure of offshore platforms,ships,bridges and construction machinery,and successfully expanded to surface engineering,aluminum alloy structural orthopedics,auxiliary 3D printing,etc.To date,most of the researches on UIT fatigue life extension behavior are focused on small-scale fatigue specimens,and the influence of welding residual stress is not fully considered.There are few researches on the influence of ultrasonic impact amplitude on the fatigue performance of welded structures,and few reports on the change rule of residual stress caused by different impact strength and the theoretical mechanism of its influence on the fatigue performance.Therefore,this paper studies the influence of welding residual stress on the fatigue behavior and evolution of UIT stress field of welded structure,and the influence of UIT amplitude on the fatigue life of welded joint.The influence of welding residual stress on the fatigue behavior of UIT on welded structure was studied by means of stress relief method and variable average stress method.The results show that for butt joint and non-load carrying cruciform joint,UIT can improve the fatigue performance of welded joints.UIT after stress relief can significantly reduce the release of residual compressive stress in the process of fatigue loading,and thus improve the fatigue life significantly and transfer the fatigue failure site from weld toe to base metal,weld seam or weld root.Finite element method was used to investigate the evolution of stress field by UIT under service condition.The process of welding,UIT and loading-unlaoding process of butt welded joint and non-load cruciform joint were modelled.By comparing with specimens without welding residual stresses,after applying stress of 267 MPa,the relaxation of compressive residual stresses induced by UIT is larger for welded specimen.The compressive residual stresses are relaxed more sigficiantly in butt welded joint than in cruciform joint.The relaxation of compressive residual streses mainly depends on the mean welding residual stress along depth and the residual stress value in the range of 1.5-4 mm beneath the surface.The influence of UIT strength on the fatigue properties of welded joints was studied by the combination of experiment and finite element method.The fatigue tests of butt joint and cross joint with UIT amplitudes of 18 ?m and 25 ?m were carried out,and the fatigue fracture analysis,microstructure observation and hardness test were carried out.The results show that under the same UIT condition,the increase of amplitude can effectively improve the fatigue strength of welded joints.For butt joint,with the decrease of stress range,the effect of fatigue life extension is better with the increase of amplitude. |
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