Recently the authors have published data in this area, both for as welded and defective welds [5-8]. However in these papers no results are presented of residual stress distributions and the crack propagation phase was not analysed in detail. The results presented in this paper cover the effect of hammer peening, on the fatigue performance of non-loaded carrying cruciform joints loaded in bending in the main plate and made from a medium strength (class 400MPa yield) carbon steel. The hammer peening process is studied in some detail, and results are presented on the impact forces, distribution of residual stresses induced by the process. EXPERIMENTAL DETAILS Material and Specimens The material used in this study is a medium strength structural steel of the 400 MPa yield class (St 52-3, DIN 17100 specification), with a weld metal in an overmatching condition. The welds were made by the covered electrode process. For hammer peening, a small portable pneumatic hammer was used (Fig. 1), fitted with a special hard metal tool, instrumented with four strain gauges bonded in full bridge (Fig. 1) to measure the impact forces and stresses during the hammer peening working cycles. The tool diameter was approximately 8.5mm, the air pressure was 3.5 bar and four passes, along the transverse direction of the specimen, were applied [4]. The frequency was 3000 blows/minute (bumps/minute). Fatigue and Hardness Tests The fatigue tests were carried out under constant amplitude loading in a + 250 kN capacity servohydraulic fatigue test machine. The frequency was 10-15 Hz, and the stress ratio, R=0.1. Vickers micro hardness data with 25gf loads were obtained along the longitudinal directions of the plate, close to the upper and lower surfaces. The variation of hardness along the thickness of the specimen at the weld toe and in the crack propagation direction was also obtained. The main objective of these tests was to compare the hardness distributions for the as welded and hammer peened specimens.
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