P 1403 - Detection of preload losses in bolts based on electromechanical impedance spectra

P 1404 - Process extension of resistance element welding for steel-intensive three-sheet hybrid mixed joints with two ultra-high-strength steel grades in the centre and base layer Against the backdrop of dwindling resources, climate policy requirements and simultaneously high customer demands in terms of comfort and safety, the automotive industry is forced to reduce the environmental impact of its vehicle fleets. In addition, the trend towards autonomous driving is increasing the number of technical devices to be moved in cars. As the vehicle body accounts for a considerable proportion of the total weight of the vehicle, various lightweight construction measures are being realised at this point. Mixed construction with form-hardened steels has established itself here, whereby a steel-intensive bodyshell is combined with various aluminium outer skin components. Both steel and aluminium-intensive lightweight construction can be realised using conventional joining technologies, as existing mechanical joining systems are capable of producing similar and dissimilar joints. Thermal joining processes enable cost-efficient same-type joints between two or three body panels. Dissimilar joints are often produced by mechanical joining processes, as process-reliable joints are possible in combination with adhesives. Due to thermal, electrical and chemical incompatibilities between steel and aluminium, conventional resistance spot welding (WPS) is not an efficient joining solution for dissimilar joints. Thermal-mechanical joining processes are increasingly coming into focus, as they utilise the advantages of mechanical and thermal joining technology to realise dissimilar mixed joints. Resistance element welding (RW) offers great potential for creating steel-aluminium combinations. The object of the research project is the fundamental investigation of two-stage resistance element welding for the production of three-sheet hybrid mixed joints. The investigations include fundamental experimental and simulative analyses with regard to different material-thickness combinations. Sample rivets are produced on the basis of an auxiliary joint geometry designed using numerical simulations, suitable welding parameters are determined by means of welding tests and the joint formation is evaluated. Adequate joint strengths are identified in investigations into the mechanical load-bearing capacity. A demonstrator component is realised to prove the feasibility of three-layer WES connections. The knowledge gained contributes to a deeper understanding of the process of two-stage WES of three-ply hybrid joints and opens up a wide range of applications in the steel processing industry, especially for small to medium-sized companies, with enormous potential for weight savings in car body construction. The results provide users with a basis for the design, economical production and improvement of three-layer WES joints, The aim of the research project was achieved Publication: 2024 Authors: G. Meschut, M. Heitmann, V. Haak

P 1408 - Experimental investigation of wind pressure distribution in open and closed circular cylindrical silo structures and tanks Silos and tanks are used in industry for the storage of bulk materials and liquids. With a circular cross-section, the vertical side walls are loaded rotationally symmetrically by the filling material, resulting in a uniform tensile stress in the circumferential direction. This makes it possible to produce thin-walled silos and tanks in steel construction with a wall thickness of just a few millimetres. When empty or partially filled, however, there is a risk of stability failure due to the asymmetrical wind load in the circumferential direction, so that the wind is often the decisive design situation. The existing approaches in the Eurocode for describing the wind pressure distribution do not adequately cover some design situations, particularly for silos and tanks within a group. As part of this research project, extensive and systematic wind tunnel tests were carried out on models of circular cylindrical silos and tanks with different H/D (height/diameter) ratios. The aim was to further develop the existing design models in order to create a safe and at the same time economical design basis. The experimental investigations carried out are divided into two parts. In the first part, tests were carried out on individual models with eight different H/D ratios (H/D = 0.3 - 4.0). The effects of the three-dimensional flow around the cylinder and the change in pressure distribution as a function of the cylinder slenderness were investigated. The suction stresses in particular vary greatly as a function of H/D and increase with increasing cylinder slenderness. The maximum stresses (both pressure and suction) occur at around 70 % of the cylinder height. In addition to the external pressure distributions, the internal pressure distributions were determined for cylinders open at the top. It was shown that a greater negative pressure (퐶푝푖 ≈ -0.8) occurs with slim silos (H/D ≥ 1) than the value of -0.6 recommended in the Eurocode. In the second part, pressure measurements were carried out on single and double-row vessel groups consisting of two to six cylinders. The tests were carried out on the models with H/D = 0.5 and 2.0. The direction of flow β and the distance between the cylinders S/D (gap width/diameter) were varied. In total, tests were carried out on more than 200 container groups. The main results are, on the one hand, a significant increase in the positive pressure range on the side facing the wind with decreasing S/D, particularly on the centre cylinder in a row of three with a vertical direction of flow (β = 90°). The magnitude of the compressive stresses increases with the cylinder slenderness H/D. On the other hand, for the slender silo models (H/D = 2.0), a considerable increase in the lateral suction stresses on the side of the upstream cylinder with free flow around it at β = 120° - 135° and S/D = 0.1 - 0.25 was determined. The main findings were processed in the form of design recommendations and a load model was developed to describe the wind pressure distribution of individual silos and tanks as well as container groups. The objective of the research project was achieved. Publication: September 2023 Authors: M.Sc. N. Wieczorek, Dr.-Ing. J. Unglaub, Univ.-Prof. Dr. sc. techn. K. Thiele

P 1410 - Extension of DIN EN 1993-1-10 to offshore conditions for notch cases on hollow section connections for jacket construction To date, there are no standardised rules for the design of jacket foundations in offshore applications with regard to material selection and toughness requirements. The aim of this project is therefore to transfer the concept of toughness verification established in civil engineering to jacket foundations in offshore applications on the basis of rules for material selection. Using fracture mechanics concepts for brittle fracture resistant component design, a reference geometry is first identified, crack assumptions are made and a crack propagation calculation is carried out based on the determined load spectrum. This results in the fracture mechanics limit state analysis, which is used to derive toughness requirements. This also takes into account the influences of strain rate, cold forming and the specific offshore operating conditions. The fracture mechanics concept of material selection established in the field of building and bridge construction is thus successfully transferred to jacket foundations. In addition, the project is the first to develop a method for deriving toughness requirements for the component in a specific application with the aid of a damage mechanics model. The modified Bai-Wierzbicki model is used, which can model plasticity, ductile damage and brittle failure as a function of stress state, strain rate and temperature. To ensure comparability between fracture mechanics and damage mechanics, the same boundary conditions are considered. Damage mechanics also derives toughness requirements, confirms the results of fracture mechanics and demonstrates its conservative calculation method. Publication date: November 2022 Authors: Prof. Dr.-Ing. Kühn, P. Krahl, M.Eng, Univ.-Prof. Dr.-Ing. S. Münstermann, D. Lenz, M.Sc.

P 1413 - Shape optimisation of welded-on lamellae under fatigue loading A weight-optimised design to avoid an unnecessarily increased consumption of resources is becoming more and more important in terms of economic efficiency and especially against the background of significantly increased procurement prices. Localised reinforcement measures, so-called reinforcement lamellas, are used for this purpose in the construction industry and also in other steel processing sectors. These locally increase the cross-section to match the external stress. In the case of alternating stresses, such as those caused by traffic passing over steel bridges, these structures must be designed to withstand fatigue failure. Here, the very notch-sharp end of the lamella leads to a poor notch case classification, which represents the resistance side in the fatigue analysis. The aim of the research project was therefore to make the end of the lamella significantly less notchy by optimising its shape and production. To this end, various disc shapes and designs were analysed numerically in a first step as part of the preliminary investigations. Experimental investigations were then carried out on selected variants. By varying the disc shape and different designs with, for example, tapered weld seams, the two most promising shapes and designs were then selected for the main tests as part of the preliminary investigations. Influencing factors such as the material quality, the design and the installation situation were analysed experimentally as part of the investigations. In addition, the transferability of the small component tests to larger dimensions was checked by means of large component tests. The design of the tapered lamellae with tapered weld seams proved to be very notch resistant and led to a significant increase in fatigue strength in both the untreated and post-treated states compared to conventional standard moulds. In addition, the parameter range of the experimental tests was extended by a comprehensive numerical parameter study and the design of the lamella shapes was additionally optimised by accompanying numerical models. As part of the project, a design recommendation for tapered weld seams with tapered lamellae was developed on the basis of the nominal and notch stress concept. In order to enable applicability in practice, additional design recommendations were developed that contain quality requirements with regard to the design. The aim of the research project was achieved. Publication: February 2024 Authors: A. Dürr, J. Roth, M. Mensinger, N. Rausch, M. Wilnhammer