Methods for fatigue strength improvement by weld toe treatment

This paper describes an investigation of methods for fatigue strength improvement of welded joints, such a TIG dressing, hammer peening and burr grinding. The main results may be summarised as follows. 1 A fatigue strength improvement is achieved by all treatment methods considered in the present research study. Most notably, the effect of burr grinding is not very dependent on the effective stress range and produces a strong fatigue strength improvement effect. 2 The fatigue strength improvement effect produced by TIG dressing is only found in the short crack region, and its fatigue strength improvement effect can be evaluated through consideration of the stress concentration reducing effect due to weld toe shape improvement. 3 The fatigue strength improvement effect produced by hammer peening strongly depends on the stress range. Little such effect is found in the N_cfatigue crack initiation region adopted in the present research study. In the N_pfatigue crack propagation region, however, a strong effect is evident. This is attributable to a stress concentration reducing effect due to weld toe shape improvement and a fatigue strength improvement due to the effect of the compressive residual stress introduced down to some specific depth (around 4 mm according to the measurement results obtained in the present research study) under this treatment. The ditch introduced by hammer peening treatment acts as a crack initiation source. To ensure that the effect due to hammer peening is more fully exhibited, it is necessary to apply burr grinding treatment to the weld toe before hammer peening as well as to prevent ditch formation by use of a hammer with a smaller tip radius. 4 The fatigue strength improvement effect produced by burr grinding is only found in the short crack N_cregion. It depends for its effect on a stress concentration reducing effect due to weld toe improvement, although the effect thus produced is strong. To ensure that the stress concentration reducing effect is more fully exhibited, it is desirable to minimise base metal denting. This treatment also offers scope for some reduction in residual stress through its ability to machine off a large region using a burr grinder tip with an extremely large radius of curvature, and this aspect warrants further investigation in future.