Effective design concepts for welded mixed connections in steel structuresMixed connections made of normal strength and high strength steels enable optimal material use to the forces acting on them. This leads to ecological and economic advantages through reduced material consumption and smaller weld geometries. Normative rules for butt-welded mixed connections of normal strength and high strength steels do not exist, therefore a design approach for such connections is to be developed in the research project "Effective design concepts for mixed connections in steel structures". In addition, the design concepts already developed for butt-welds and fillet-welds are being extended up to S960, in order to integrate the results into the new version of EN 1993-1-12. For this purpose, extensive experimental investigations of the load-carrying capacity are carried out by varying the base material, welding consumable, heat input, plate thickness and weld geometry. With the results obtained and further accompanying investigations, the influences of the varying parameters on the load-carrying capacity of the welded connections will be identified. First results of that project are reported in the following.

Bei hochbeanspruchten Stahlkonstruktionen wie Windenergieanlagen oder Brücken ist der Einsatz höchstfester Baustähle interessant, allerdings wird bei diesen Bauwerken meist der Ermüdungsnachweis maßgebend. Die in den zu verwendenden Regelwerken angegebenen Kerbfälle sind unabhängig von der Werkstofffestigkeit, sodass der Einsatz hoch‐ oder höchstfester Stähle bei einer Dominanz des Betriebsfestigkeitsnachweises keine Vorteile bei diesen Konstruktionen bietet. Durch den Einsatz einer Schweißnahtnachbehandlung mit höherfrequenten Hämmerverfahren (HFH) kann die Ermüdungsfestigkeit geschweißter Verbindungen jedoch entscheidend erhöht werden. Dabei zeigen alle bisherigen Untersuchungen eine zunehmende Effektivität der HFH‐Verfahren mit steigender Werkstofffestigkeit. Die neue DASt‐Richtlinie 026 regelt die Ermüdungsbemessung von Kerbdetails aus den Stahlsorten S235–S700 für die Anwendung von HFH‐Verfahren. Die Erweiterung auf höchstfeste Stähle bis S960 steht noch aus. Ebenso ist der Umgang mit geometrischen Imperfektionen außerhalb der Grenzwerte der Schweißnahtbewertungsgruppe B gemäß ISO 5817 bislang nicht geregelt. Im laufenden AiF‐FOSTA‐Forschungsprojekt 21410 BG (P 1505) sollen die Grundlagen für eine Erweiterung der DASt‐Richtlinie 026 auf höchstfeste Stähle bis S960 gelegt und der Einfluss von erhöhten Fertigungsimperfektionen auf die Ermüdungsfestigkeit HFH‐behandelter Schweißnähte gezielt untersucht werden. Im Folgenden wird das Vorhaben vorgestellt und erste Projektergebnisse gezeigt.

Never before has an optical method been used to determine the arc temperature inside a submerged arc welding cavern with frequencies up to 5 kHz. To be able to do that, a combination of high-speed imaging and spatially resolved high-speed spectroscopy with up to 5000 fps has been performed. A DCEP (direct current electrode positive) process with 600, 800, 900 and 1000 A, and an AC (alternating current) process at 800 A were included in this research. The Bartels method has been used to calculate these temperatures for the first time. It generated temperatures from approximately 7000–9000 K. Also, it was found that a decrease of arc temperature for rising currents appeared until 900 A. After that, it is reversed for currents higher than 900 A.

The possibility of arc brazing of non‐loaded attachments with the filler material CuAl7 instead of welding on cyclically stressed steel constructions was investigated. The notch effect under cyclic loading was significantly reduced for non‐loaded arc brazed plates on base material of S355 and therefore the fatigue strength improved. The specimens did not show sensitivity to changes of the thickness and size of the non‐loaded arc brazed plates during fatigue testing. Furthermore, no negative thickness effect by increasing the thickness of the base material was observed. Microstructural analysis showed that the dimension of the heat affected zone was reduced through reduced heat input and that liquid metal penetration (LMP) occurs in the base metal, which has no negative effect on fatigue strength. Measurements of residual stresses in the weld / braze toe area showed that tensile residual stresses have not significantly been reduced, which can be attributed to comparable reactional forces of the copper alloy during cooling through a higher coefficient of thermal expansion. 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 Young's Modulus of the alloy. The static testing of the load bearing capacity of arc brazed attachments showed that high tensile and shear stresses, which are comparable to the strength of the base metal, can be reached before fracture.