Ultrasonic Impact Peening Treatment of Welds and Its Effect on Fatigue Resistance in Air and Seawater

Abstract

Abstract Welding residual stresses and stress concentrations are main factors whichdetermine the load carrying capacity of welded structures under multicyclicloading conditions as typically experienced by ships and offshore structures. To improve fatigue performance the E. O. Paton Institute recently introducedUltrasonic Impact Peening Treatment. This local treatment of the weld toe iscarried out by means of specially-developed equipment consisting of amagnetoconstriction convertor, an ultrasonic wave transmitter and a specialtool with holder. The tool tip using either a single 16mm ball element ormultiple needles, vibrates at 27 KHz with maximum axial amplitudes up to 30microns. The tool holder isolates the operator from the vibration and there islittle audible sound during the treatment. The process:Introduces compressive residual stresses (up to 900 MPa at and near thesurface) into areas of high stress concentration.Reduces stress concentration associated with weld local geometry.Creates plastic deformation strain hardening in a surface layer (up to 0.7mmin depth). This paper briefly describes the fatigue test results obtained from plateand tubular connections in air and sea water. The cyclic endurance increasesfrom 2 to 10 times and the fatigue limit increases from 27% to 200% dependingon the type of joint, base material strength and load ratio. Introduction Many structural weldments - offshore platforms, cranes, bridges, antennas, and other structures - are subject to the action of large numbers of cyclicloads during service. The development of fatigue fractures amounts toapproximately 30% of the total premature failures in these structures. Thefatigue limit of butt welds in different strength classes of steel establishedby the criterion of fatigue crack initiation is reduced to 32% to 46% of thefatigue limit of the corresponding parent metal with a rolled surface. Similarly, the fatigue limit of lap joints with longitudinal fillet welds undercyclic stress conditions constitutes only 20% to 40% of the fatigue limit ofthe corresponding parent metal [1, 2].