P 1205 - Characterisation of the load-bearing behaviour of resistance spot welded joints of ultra-high-strength steels under cyclic loading and development of a calculation concept for fatigue strength

P 1203 - Design of crane structures made of laser beam and laser hybrid welded ultra-high and ultra-high strength steels under consideration of the metallurgical notch Motivated by the utilisation of the highest material strengths currently available on the market, also in joints, and against the background of the application in highly stressed areas of load-bearing structures, such as those occurring in tensile-loaded areas of the telescopic boom of mobile cranes, the research project aims at the safe and reliable use of laser beam and laser hybrid welded joints made of ultra-high-strength and ultra-high-strength fine-grain structural steels. Four fine-grain structural steels of different strength classes, one ultra-high strength (S960) and three ultra-high strength (S1100 and S1300), which are produced using different manufacturing processes (quenched and tempered and thermomechanically rolled), form the basis for this investigation. The laser beam welding process variants, laser beam without filler metal and laser-MSG hybrid, are used to produce three construction details, butt joints without and with sheet thickness jumps and welded-on longitudinal stiffeners. As a result of the test specimen welds, assessment group B or B90 is achieved regardless of the material. Significant weld seam irregularities are mainly characterised by excessive seam elevation (top and bottom side) and penetration notches (especially laser hybrid). Laser measurements of the finished samples show very small (laser beam) to moderate angular distortions (laser hybrid). Hardness measurements show high hardness values and gradients from the weld metal to the visible HAZ and, depending on the base material and as a result of the subsequent heat input, low hardness values, the so-called hardness sag, in the tempering zone. The exception here is the S1100M tested, which has hardness values at the base material level in the tempering zone. In notched bar impact bending tests for the notch position in the weld metal, only low notch impact energy values are determined; for the notch position in the HAZ, the scatter of the determined notch impact energy values increases and a crack deviation into other microstructural areas is frequently detected in the fracture surface analysis. The fatigue strength tests show that the laser-welded butt welds consistently exhibit a higher fatigue strength compared to the laser-hybrid-welded butt welds, both under constant and variable load amplitudes, clearly in the case of the butt welds without plate thickness jumps and at least moderately in the case of plate thickness jumps. The very high fatigue strength of the laser-welded butt joint made of S1100QL is striking in comparison to the other laser-welded series. The fracture surface analysis shows no to few weld seam irregularities for both the laser beam and the laser hybrid welds. The sample components made of S1300QL, which were selected based on the telescopic boom of mobile cranes, show a high degree of conformity with the corresponding laser butt-welded notch detail series. The linear accumulation of damage leads to the recommendation of a permissible damage sum of 0.5 for collective maximum values in the area of fatigue strength and for selected modifications, resulting in conservative results in approx. 97 % of cases. The applicability of the local fatigue strength assessment using the notch stress concept is shown using the example of S1100QL butt welds by comparing the recommended FAT class FAT 500 (k = 5.0) for the reference radius rref = 0.05 mm. However, compared to the FAT classes currently recommended in the regulations, a reliable estimate is not consistently made, which is why the generally valid use must be questioned on an application-specific basis. In addition, the application of the notch strain concept with the reference radii rref = 1.00 mm and rref = 0.05 mm and the cyclic material behaviour of the S1100QL is discussed on the basis of the scattering of results. For the combination of rref = 0.05 mm and the damage parameter PSWT, the overall lowest scatter is in the range of 1.32 ? 1 : TP ? 1.36. However, it is evident that the consideration of the scatter as the sole criterion of the estimation quality is not sufficient. For the classification of the fatigue strength results according to the nominal stress concept in the relevant design codes, a base material-independent consideration of the various construction details differentiated according to the welding process (laser beam without SZW or laser hybrid), based among other things on the different weld seam geometry, is expedient. Instead of the upper starting point of the design fatigue line at N = 10,000 cycles, which is selected in some codes, the definition is based on a limit upper stress, which is selected as a function of the strength class of the base material in the amount of the largest experimentally determined upper stress below the respective deformation limit. As a result, a linear extension of the Wöhler curve up to this limit stress range or a service life of well below 10,000 cycles is possible and experimentally verified, which extends the application of the nominal stress concept to a service life range that is relevant in practice. The economic efficiency analysis presented compares the analysed laser beam welding variants with the conventional MSG manufacturing variants from an economic point of view. This shows that the more detailed the analysis is to be, the more concretely the selected boundary conditions must be defined, which is why two different levels of detail are selected. In principle, a reduction in the proportion of labour costs and throughput times can be determined for the laser welding variants for two selected reference examples. In large component production, which is characterised by long lead times, the acquisition costs are a decisive factor in determining the equipment cost rates. The laser hybrid variant proves to be particularly cost-efficient due to the low-cost weld seam preparation, high welding speeds and single-pass welding. The methodological approach of setting a technical target value (e.g. fatigue strength) in direct correlation to the costs shows that, among other things, a high fatigue strength at N = 2-106 cycles in the as-welded condition cannot be achieved with all process variants. In general, it should be noted that the specific application not only plays a decisive role in cost estimation, but also provides the framework conditions for the production and use of ultra-high and ultra-high-strength fine-grained structural steels and their welded joints. Publication: 2021 Authors: Prof. Dr.-Ing. T. Melz, Prof. Dr.-Ing. T. Ummenhofer

P 1208 - Consistent design model for production-optimised composite dowel bars - Basis for DASt guideline and transfer to Eurocode 4 This report summarises the investigations carried out on the load-bearing and deformation behaviour of composite dowel bars in the field of structural engineering. The competitiveness of this economical and also technically sensible composite material is limited in comparison to conventional shear stud dowels by the fact that only a national and time-limited approval is currently available. In contrast, shear connectors are firmly anchored in European regulations. In order to contribute to the integration of bonded dowel bars into the European standards, the participating research institutions focussed on the investigation of bonded dowel bars in cracked and non-cracked concrete, under longitudinal shear, tensile and combined shear-tensile loading as well as in installation positions close to the surface. New failure mechanisms were observed and recorded using appropriate design formulae. First, existing studies on composite dowel bars, as well as existing research results and design models, were reviewed and summarised. Subsequently, experimental and numerical investigations were carried out on the load-bearing capacity of bonded dowels in non-cracked concrete. The results of the small-part tests were used to derive approaches for the load-bearing behaviour under shear, tensile and combined shear-tensile loading. Component tests with three different test arrangements were used to verify the models, thus covering a wide range of possible constellations of bonded anchors under combined loading in building construction. The load-bearing capacity of the composite anchors in the beam always turned out to be better than predicted on the basis of the small-part tests. The design models therefore provide a reliable estimate of the real load-bearing capacities. Based on this, test specimens with bonded dowels were produced and tested in transversely and longitudinally cracked concrete. It was shown that longitudinal cracks have no load-reducing effect on the shear load-bearing capacity of bonded dowel bars and therefore do not need to be taken into account in the design for predominantly static loads. In contrast, a decrease in the shear load-bearing capacity was observed for transverse cracks in the concrete. Furthermore, the load-bearing behaviour of bonded dowels in installation positions close to the surface was investigated. This led to the failure case of edge breakage, which has not yet been taken into account in the current type approval and was observed in small part and beam tests. Statistically verified design equations were derived on the basis of these findings. The existing model of the shear load-bearing capacity was extended on the basis of the new findings, while the failure mode of shearing of the concrete dowel was reviewed and categorised as not relevant for the calculation. Publication: January 2021 Authors: Prof. Dr.-Ing. M. Feldmann, K. Wolters, M.Sc., Prof. Dr.-Ing. W. Kurz, Y. Broschart, M.Sc., Prof. Dr.-Ing. Hegger, Dr.-Ing. M. Claßen, G. Christou, M.Sc.

P 1209 - Wheel load induced fatigue of crane runway girders for heavy metallurgical cranes With the replacement of DIN 18800 and DIN 4132 and the introduction of the Eurocodes for steel construction regulated by building law, it has become apparent that the fatigue analyses for highly stressed crane runway girders, such as those found in steelworks, have undergone significant changes. Compared to the previous design, stricter notch classifications, particularly in the area of wheel load application, must be taken into account. These changes can go as far as doubling the thickness of the cross-sectional components, which would mean, for example, that web plates of up to 80 mm would also have to be taken into account, whose K-seam connections to the flange would be associated with considerable problems in terms of production and monitoring and would also no longer be justifiable from an economic point of view. Against this background, the present research project derived fatigue strengths for the crane runway-specific fatigue detail "top flange-web plate connection" in EC3-1-9 Table 8.10, in particular for the heavy metallurgical crane systems concerned, and prepared them as recommendations for a rule amendment or supplement. The focus here is specifically on the detail of the wheel load introduction with KN weld design, for which no differentiation is currently made with regard to the design quality or the manufacturing process. The essential core of the methodological approach is that existing crane runway girders from practice are recorded, documented and evaluated with regard to wheel load-induced fatigue ("best practice" experience). This allows a direct quantitative statement to be made about the fatigue safety of the detail to be analysed. In addition, numerical comparative calculations are carried out to derive synthetic notch cases and existing results from previous research work are included, although these alone were not sufficient. Finally, a concept for the successful and timely implementation of the results in current standardisation is presented. FOSTA - Research Association for Steel Application e. V. Publication: April 2019 Institute for Steel Construction, RWTH Aachen University Prof. Dr.-Ing. Markus Feldmann Sandro Citarelli M.Sc.

P 1200 - Innovations for optimising the use of weathering-resistant structural steel in steel and composite bridge construction Compared to unalloyed structural steel, weathering-resistant structural steel is characterised by significantly higher corrosion resistance. As a result of natural corrosion processes, a firmly adhering, dense coating forms on the surface of the unprotected weathering-resistant structural steel, which slows down further corrosion. Due to the slowed corrosion reaction of the weather-resistant structural steel, use under natural corrosion conditions is also possible and permissible without an additional corrosion protection system. As many steel and composite bridges in Germany show signs of damage due to age and design, there is a great need for durable and low-maintenance replacement structures. Due to its naturally occurring, maintenance-free corrosion protection, corrosion-resistant structural steel can fulfil this requirement as a construction material in bridge building. Despite slightly higher material costs and necessary wall thickness surcharges, weathering-resistant structural steel offers an economical and sustainable alternative. The economic advantages of weathering-resistant structural steel over unalloyed structural steel lie in the elimination of the costs and all follow-up costs for the initial coating and standard, repeated renewal of a conventional organic corrosion protection system, as well as the associated lower environmental impact. The formation of the compact, firmly adhering top layer, which is responsible for the significantly slower corrosion behaviour of weather-resistant structural steel, is influenced by sulphur dioxide during the natural corrosion reaction. As a result of industrial structural change, the concentration of sulphur dioxide in today's atmosphere in Germany is very low, which has changed the formation of a corrosion-inhibiting oxidic surface layer under natural atmospheric weathering. The investigations showed that in the changed low sulphur dioxide atmosphere, compact and inconspicuous oxide layers still form on weather-resistant steel, which do not require any changes to the construction with weather-resistant structural steel. The visual detection of rarely occurring fatigue cracks, which is necessary in the context of bridge inspections, is difficult or even impossible for weather-resistant structural steel due to the dense and firmly adhering oxide top layer, as the corrosion-inhibiting top layer should not be removed from the component surface due to its anti-corrosion properties. This research project therefore developed a method for the geometry-independent, non-contact detection of cracks for bridges made of weatherproof steel and verified it on real bridge structures. The investigations have shown that this method is suitable for practical use and can be successfully applied to steel structures without a coating, such as uncoated weather-resistant structural steel, but also to a superficial surface layer. Cracks and scratches can be recognised on flat surfaces from a depth of 0.3 mm. Publication date: January 2021 Authors: Prof. Dr.-Ing. D. Ungermann, P. Hatke M.Sc., Prof. Dr.-Ing. habil. F. Walther, Dr.-Ing. M. Knyazeva, Dr.-Ing. S. Friedrich, Dipl.-Ing. (FH), SFI P. Lebelt