Improvement of fatigue strength in heavy steel offshore-constructions through arc brazing

The possibility of arc brazing for attachments with the filler material CuAl7 instead of welding in heavy steel construction was investigated. The notch effect under cyclic loading was significantly reduced for arc brazed bushings and therefore the fatigue strength improved. In comparison to samples with welded attachments, the fatigue strength of samples with arc brazed attachments could be increased up to a fatigue class of FAT112. As a result of tests series, where only one parameter differed, the following influences could be concluded: An increasing stress ratio leads to decreased fatigue strength. No reduction of the fatigue strength was observed for thicker plates. For plates of high strength steel the same increased fatigue strength was confirmed on with arc brazed attachments as for mild steels. Microstructural analysis tests showed, that the dimension of the heat affected zone was reduced through reduced heat input. The appearance of liquid metal penetration w as observed over the interface between filler material and base metal, which obviously had no negative effect to fatigue strength. Measurements of residual stresses in the weld / braze toe area showed that tensile residual stresses have not significantly been reduced. Numerical analysis showed that compared to a welded detail, the stress concentrations on the weld toe of the arc brazed fillets drop significantly due to the lower E-modulus of the alloy and therefore attract less forces due to the reduced stiffness leading to more favourable behavior under fatigue loading. The testing of the load bearing capacity of arc brazed bushings showed that high tensile stresses, which are comparable to the strength of the base metal, can be reached before fracture and that this detail is therefore suitable for applications in steel construction.