| The theory and applications and of ultrasonic peening treatment was studied in this article.In this paper, a newly designed ultrasonic peening treatment (UPT) system was developed to meet the operational requirements of construction site. An ultrasonic peening tool, a peening needle that turned any angle and a portable ultrasonic power supply were included in this completed sets of equipments. This compact half-wavelength peening system compared with the original full-wavelength one advanced in reducing the size of the device and improved the adaptability of equipment for field operations which, in the same time, did not change the output power. It expanded the applications of the UPT equipment.Fatigue design code of welded structures continued to use fatigue limit theory in applications, while the testing and engineering examples proved that to be dangerous in when the welded structure was serving in the ultra-long life fatigue range. The fatigue behavior of two commonly used in engineering structural steel Q235 and Q345 in the ultra-long life regime was discussed and possibility of life extension using UPT was studied in this paper. The fracture morphology of UPT and original materials were also compared. Studies had shown that: For the welded joint fatigue, regardless of whether amplitude of the fatigue load may change, the fatigue limit did not exist. The fatigue life of welded joints should be designed according to different post-weld treatment separately, and fatigue design curve to should be amended accordingly. The fracture mode of welded joints were still an surface and/or subsurface inclusions caused crack while the UPT specimens had a small peening-cause-crack fracture mode.Commercial finite element software ANSYS was employed in the study of the welding and subsequent UPT process. A 3D model had been established and the welding thermal field and coupled stress field had been analyzed. Based on the welding analysis, a UPT process had also been simulated. The simulation results and experimental ones agreed with each other very closely which gave the further study a numerical reference.The application of UPT on existing structures is carried out now in the United States, Ukraine, Russia and Canada and other countries while studies and comparisons on the behavior of the non-load bearing and load bearing structure were barely performed. In this article, the fatigue behavior of UPT on non-load bearing and loading bearing structures were compared, and the numerical simulation method, for the first time, was used the to analyze a quantitative difference between the two. Research showed that regardless of the kinds of UPT, the knuckle radius of the specimens was improved in the weld toe areas while UPT on the bearing load structures can better improve the fatigue strength than that on the non-load bearing ones.In addition, the anti-stress corrosion performance of the UPT 304 stainless steel specimens was also studied with a boiling magnesium chloride stress corrosion test in this paper. The test results showed that the anti-corrosion lifespan of UPT base materials and weld specimens had been prolonged 10 to 13 times respectively. The reason was that the resultant stress from the high value tensile residual stress and UPT induced compressive residual stress became a very small tensile stress or even a compressed one in the end which can not reach the value of critical corrosion stress. The crack morphology of both specimens showed that corrosion crack was a typical kind of brittle one. The fracture surface morphology contained two modes of fractures which were cleavage and intergranular fracture and no other modes were observed.The study of the polarization curve showed improvement of UPT on stress corrosion resistance of 304 stainless streel.
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