Effect of blast-cleaning on the fatigue strength of welded and non-welded constructional details

Blast-cleaning, which is closely related to shot peening, is used to achieve a surface preparation grade of Sa 3 prior to the application of corrosion protection coatings. The blast-cleaning treatment with sharp-edged material is carried out with defined parameters (pressure, distance and angle). As a result of the blast-cleaning treatment, residual compressive stresses are induced near the surface, whereby the surface layer becomes work-hardened. Findings from the literature show that welded components made of mild structural steels (S235, S355) exhibit an improvement in fatigue strength. Constructional details with fillet and butt welds, whose potential crack location is the weld toe, can benefit from this. Improvement factors for post-treatment methods, such as TIG dressing, burr-grinding or high-frequency mechanical impact methods, have already been considered in standards or guidelines (IIW, FKM, EN 1993). In contrast, there are still no generally applicable improvement factors depending on the constructional detail when using blast-cleaning. The fatigue strength verification has become a design driver, particularly for structures subjected to high cyclic loads, which has a significant impact on manufacturing costs due to the amount of materials used. In particular for wind turbines, which are becoming larger and larger, and which are subjected to longer operating lifetimes, there are high cost pressures. The aim of the paper is to show whether the fatigue strength of welded and non-welded constructional details can be improved or whether a reduction in fatigue strength must be accepted by increasing the surface roughness. For this purpose, fatigue tests were carried out at constant amplitude loading (CAL) in the finite life region of the S-N curve and statistically evaluated according to EN 1993-1-9. It will also be shown that critical compressive preloads might influence the fatigue strength.