This paper describes an investigation of methods forfatigue strength improvement of welded joints, such a TIGdressing, hammer peening and burr grinding. The main results maybe summarised as follows.1 A fatigue strength improvement is achieved by all treatmentmethods considered in the present research study. Most notably,the effect of burr grinding is not very dependent on the effectivestress range and produces a strong fatigue strength improvement effect.2 The fatigue strength improvement effect produced by TIGdressing is only found in the short crack region, and its fatiguestrength improvement effect can be evaluated throughconsideration of the stress concentration reducing effect due toweld toe shape improvement.3 The fatigue strength improvement effect produced by hammerpeening strongly depends on the stress range. Little such effect isfound in the N~ fatigue crack initiation region adopted in thepresent research study. In the N~ fatigue crack propagation region,however, a strong effect is evident. This is attributable to a stressconcentration reducing effect due to weld toe shape improvementand a fatigue strength improvement due to the effect of the com-pressive residual stress introduced down to some specific depth(around 4 mm according to the measurement results obtained in thepresent research study) under this treatment. The ditch introducedby hammer peening treatment acts as a crack initiation source. Toensure that the effect due to hammer peening is more fullyexhibited, it is necessary to apply burr grinding treatment to theweld toe before hammer peening as well as to prevent ditchformation by use of a hammer with a smaller tip radius.4 The fatigue strength improvement effect produced by burrgrinding is only found in the short crack Nc region. It depends forits effect on a stress concentration reducing effect due to weld toeimprovement, although the effect thus produced is strong. Toensure that the stress concentration reducing effect is more fullyexhibited, it is desirable to minimise base metal denting. Thistreatment also offers scope for some reduction in residual stressthrough its ability to machine off a large region using a burrgrinder tip with an extremely large radius of curvature, and thisaspect warrants further investigation in future.
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