General

(1) In cases where the fatigue cracks would initiate at the weld toe, the capacity of welded joints can be improved. In practice such methods are normally used at the most highly stressed welds or for improving welds having low strength.

(2) In cases where specified improvement techniques have been employed, an improvement of 30% for fatigue fracture at a weld toe can be obtained. The highest improvement is achieved by the combination of two methods like machining (or grinding) and hammer peening where the double improvement of the individual methods can be reached.

(3) The following methods are considered here:

- Machining or grinding

- Dressing by TIG or plasma

- Peening (shot peening, needle peening or hammer peening)

(4) For all methods the following aspects should be considered:

- A suitable work procedure must be available.

- Before applying the measures for improvement one must assure that no surface cracks are present in the critical locations. This should be done by dye penetrant or other suitable NDT methods.

- For improvement at the mid and long fatigue life region the capacity can be improved by 30% measured by stress range.

- In the short life region where the local stresses exceed the yield strength the initiation period is a small fraction (irrespective of the notch case) and the improvement is thus small. Hence, there will be no improvement in design at 105 cycles. (The Aa-N curve is thus rotated with fixed values at 105).

- Potential fatigue fracture locations other than that being improved must be considered: e.g. if the weld toe area is improved, then locations like the weld throat or internal cracks (partial penetration), might be the limiting factor for the fatigue life and the usefulness of improvement methods should be considered.

- Under freely corroding conditions in water, the improvement is often lost. Methods involving compressive residual stresses (peening) are less susceptible. Corrosion protection is therefore needed if the improvement is to be achieved.

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