A CASE STUDY OF THE USE OF ULTRASONIC PEENING FOR UPGRADING A WELDED STEEL STRUCTURE

Fatigue tests were carried out to establish the fatigue life improvement that could be obtained from the high-frequency ultrasonic peening technique known as ultrasonic impact treatment (U IT) of longitudinal fillet welded joints in steel viaducts in an urban railway system that was being upgraded for increased capacity after a period of service. The work involved comparative fatigue tests on as-welded and weld toe peened specimens consisting of 30 mm-thick steel plates with longitudinal non-load carrying fillet welded stiffeners. To take account of previous service, the test specimens were fatigue tested for a proportion of their expected lives before the peening was applied, to simulate that service. Two conditions were investigated, peening applied to the unloaded specimen and peening applied while the specimen was held in tension, simulating the dead load stress in the actual structure. In each case, subsequent fatigue testing was carried out under axial tension-tension loading with a constant minimum stress of 120 N/mm(2) to represent the dead-load stress. The treated specimens all failed in the fillet weld throat and in an attempt to identify the factors that control the resulting fatigue life for this failure mode the present and relevant published results were re-analysed in terms of the structural stress on the weld throat, as determined by finite element analysis