P 1408 - Experimental investigation of wind pressure distribution in open and closed circular cylindrical silo structures and tanks

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 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 1403 - Detection of preload losses in bolts based on electromechanical impedance spectra Bolts in HV joints are preloaded as planned, whereby the operational safety of these bolted joints is significantly influenced by the level of preload force. However, generating and maintaining the required preload force before and during operational loading is not always sufficiently successful. A loss of preload force due to automatic loosening during the operational loading of bolted connections can be caused by two different causes, namely loosening due to settling or creeping or automatic loosening of the bolt and/or nut. Preloaded bolts relax due to the settling of the surfaces of the clamping package (levelling of surface roughness, creep of the coating). Creep occurs when the interfacial pressure of the pressurised surfaces is exceeded. These effects are time-dependent and lead to a reduction in the preload force. However, failure is also frequently due to loosening and/or automatic loosening during operation, particularly in the case of bolted connections subject to vibration. This is synonymous with the complete or partial loss of the preload force, which in many cases results in a fatigue fracture of the bolt. In this research project, a method was therefore developed that enables cost-effective, continuous monitoring of the bolt force using electromechanical impedance spectra as part of structural health monitoring (SHM), for example. As an introduction to the topic, Chapter 2 provides a brief overview of the state of the art, the current methods for measuring preload forces in bolted joints are discussed (Chapter 2.1) and the possibilities and objectives for the preload of bolts based on the currently applicable standards and guidelines are presented (Chapter 2.2). Building on this, the main features of measurements based on electromechanical impedance spectra are presented in chapter 2.3. In particular, it explains what electromechanical impedance spectra are, how they are generated and what is required for the measurement of electromechanical impedance spectra. This part of the report is aimed in particular at readers who have had little or no previous contact with electromechanical impedance spectra. Furthermore, a brief overview of the test specimens used in the project is given using an example of an M16x50 test specimen equipped with measurement technology. Subsequently, the factors that are important for the measurement of electromechanical impedance spectra are discussed in relation to the present application using exemplary FE calculations; the influences of preload force, temperature, sensor damage and adhesive layer on the electromechanical impedance spectra are explained. Building on this, Chapter 0 presents the (physical) background to the measurement of electromechanical impedance spectra. The relationships between the formation of electromechanical impedance spectra and the influences of preload force, temperature and sensor damage, which were already presented in the previous chapter 2.3, are explained in detail. By deriving causalities, it can be shown in which parameter ranges the influences of preload force, temperature, sensor damage and adhesive layer on the impedance spectra are valid. Chapter 2.5 explains the statistical analysis of the generated data in accordance with Military Handbook 1823, focusing primarily on the procedure and objective of the statistical analysis and the choice of method for the analysis. Chapter 3 first explains the basics of the experimental investigations (Section 3.1) and, building on this, the results of the preliminary investigations (Section 3.2). The spectra used to analyse preload force, temperature and sensor damage were determined on the basis of the preliminary tests. The series measurements are then presented and analysed (section 3.3). Firstly, it is shown how a loss of preload force and its magnitude can be reliably recognised on the basis of the evaluated measurements at a constant temperature. The compensation of the temperature influence is carried out in chapter 3.4. Finally, a statistical analysis is used to show the magnitude of the preload losses, which can be recognised in at least 90 % of cases with a confidence level of 95 %. Chapter 3.5 presents the tests for detecting damage in the measurement chain and develops a method that enables the detection of typical damage. The additional tests carried out on HV screws M72x380 are briefly presented in Chapter 3.6. The results of the ultrasonic measurements carried out are then discussed in Chapter 3.7. Publication: February 2022 Authors: Univ.-Prof. Dr.-Ing. D. Pak, Dr.-Ing. D. Sahm, Univ.-Prof. Dr.-Ing. C.-P. Fritzen, A.-L. Dreisbach, 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