P 1123 - Hollow section truss structures with bonded strut connections

P 1121 - Development of a method for the design of bonded joints in structures subject to vibration in agricultural machinery and plant engineering In constructions made of thicker steel sheets (approx. 3-8 mm) in agricultural machinery and plant engineering, adhesive bonding technology has so far only been used to a limited extent, although it is already an established joining method in other areas of application. Bonded joints are generally characterised by excellent behaviour under vibration load, which was also demonstrated on thicker steel sheets in a previous SME study by FOSTA e.V. in random sample tests. The main objectives of the project are - Experimental investigation of the strength behaviour of bonded joints made from thicker thicker sheets using higher-strength steels. Comparison of the strength behaviour of bonded and welded joints, - Development of a procedure for the design of bonded joints under cyclic loading as an cyclic loading as an alternative to welded joints. For this purpose, the joint and seam forms of welded construction details commonly used in agricultural machinery and plant engineering are analysed at the beginning and evaluated with regard to the feasibility of adhesive bonding solutions. A bracket connection is selected as a bonded sample component for subsequent investigations. At the beginning of the research project, the strength behaviour is examined in quasi-static, Wöhler and fatigue strength tests using the example of a lap joint suitable for adhesive bonding technology without and after exposure to media or corrosion. It is shown that bonded joints under quasi-static loading fall significantly short of the strength of the welded joints investigated. However, the strength of the bonded joints can be increased by using higher-strength adhesives. The potential of bonded joints lies in the high-cycle fatigue range. Due to the significantly lower notch effect, the fatigue behaviour above approx. 105 to 106 load cycles is much more favourable compared to welded joints. The influence of medial and corrosion effects must always be taken into account, i.e. seam seals are required in zones subject to greater stress in this respect. The high resistance to fatigue identified at the lap joint is also evident in the sample components analysed, provided the joint zones are sufficiently rigid. The lightweight construction potential of bonded high-strength steels is clearly demonstrated by the fact that a reduction in sheet thickness has a significantly lower impact on the service life compared to welded joints. To ensure handling strength in production, mechanical processes accessible from one and two sides (blind rivets, etc.) are analysed and evaluated. Finally, recommendations for the methodological procedure for the development of structural adhesive bonds for agricultural machinery and plant engineering are formulated from the project results. Based on the data obtained, a concept for determining the component S-N curve and the component fatigue strength using nominal shear stresses in accordance with the FKM guideline is presented. Publication: 2019 Authors: Prof. Dr.-Ing. G. Kötting, Prof. Dr.-Ing. G. Meschut

P 1125 - Increasing technological properties by cryo-treatment of tool steels As part of the project, the effects of cryogenic treatment on the microstructure, mechanical properties and wear resistance of the high-alloy tool steels X38CrMoV5-3 (electroslag remelting process (ESR)), X153CrMoV12 (powder metallurgy (PM) and fusion metallurgy (SM)) and ~X190CrVMo20-4 (PM) were investigated. The tempering parameters of the samples for the mechanical tests and those of the wear samples were selected on the basis of the tempering curves so that a similar hardness was present for both the conventionally heat-treated and the cryo-treated samples. Although a finer, martensitic microstructure and a more homogeneous distribution of secondary carbides in the X38CrMoV5-3 was achieved by including a cryogenic treatment in the conventional heat treatment or an increased amount and a more homogeneous distribution of secondary and nano carbides in the X153CrMoV12 and ~X190CrVMo20-4 steels could be attributed to the cryogenic treatment, no differences were found in the mechanical properties and in various wear tests (sandpaper method, wear cup and cavitation erosion). The wear resistance of the high-alloy tool steels depends to a large extent on the primary carbides and the macro hardness, less on the secondary or nano carbides. Both cryo-treatment and tempering three times at high temperatures achieved an almost complete conversion of the retained austenite into martensite in the small samples. Cryogenic treatment of tool components with large wall thicknesses would have a positive effect, as a uniform and complete transformation of the retained austenite is achieved over the entire cross-section. This allows the user to better assess the cryogenic treatment technology and thus utilise it in a more targeted manner, thereby achieving improved results or saving costs. Publication: 2021 Authors: Dr.-Ing. A. Schulz, Dr.-Ing. F. Nürnberger, Dipl.-Ing. T. Ümit

P 1126 - Determination of the load-bearing capacity of novel steel shell structures under realistic consideration of welding distortion Recently, stiffened shell structures (so-called monocoque) have been increasingly used in steel construction. Due to the slender construction methods, buckling is a limit state that must be taken into account. A geometrically and materially non-linear analysis of the imperfect shell structure (GMNIA) has the highest theoretically achievable degree of reality. Its verification quality depends crucially on whether the imperfections introduced into the calculation model describe the effect of the imperfections of real structures equivalently. In principle, it is now possible to reliably predict the effects of the welding process using welding simulation. The analytical-numerical hybrid model developed at the LFT offers a practice-relevant methodology for the welding simulation of large components. From a design perspective, an interface to the welding simulation is to be realised and subsequently verified. The aim of the project was to provide a robust and reliable calculation method for a numerically supported GMNIA verification of welded shell structures. In contrast to previous approaches, this is the first time that this has been done directly using hybrid approaches to capture the influences of the welding process. The analytical numerical hybrid model was used to calculate the welding imperfections. In order to be able to use the model to calculate the welding imperfections that occur when welding curved structures in space, the underlying analytical model had to be extended. Based on numerical variation calculations, an acquisition function was derived that takes into account the curvature in space and the resulting reduced stiffness. The hybrid model has already been successfully applied to a large number of differently complex components. However, the focus was on welding distortions. Little attention has been paid to the localised stress state in the near-seam area. However, it influences the load-bearing behaviour of welded components. Both the verification of the stresses calculated using the hybrid model with the results from thermomechanical calculations and the validation with radiographic measurements showed good agreement. Only a new meshing strategy was developed to improve this. Since in most cases the stiffeners are welded on both sides, an influence function was derived based on the source method, which takes into account the intermediate time between the two welds and records its effect on the plastic strains. Publication: August 2020 Authors: Prof. Dr.-Ing. habil. H. Pasternak, Dr.-Ing. Ch.Stapelfeld, Dr.-Ing. Launert, Prof. Dr.-Ing. habil. V. G. Michailov, Dr.-Ing. N. Doynov, C. A. Kaya

P1127 - Materials science-based development of simulation strategies for the use of adiabatic cutting in sheet metal part production In order to reduce the amount of material used and at the same time pursue the concept of lightweight construction, high-strength steels with tensile strengths greater than 1000 MPa are increasingly being used in automotive engineering. Practically every sheet metal component undergoes a trimming operation during the manufacturing process. However, conventional shear cutting or fine blanking cannot be used for ultra-high-strength steels, or only at great expense in terms of pre- and post-treatment. Here, adiabatic shear cutting offers an alternative to conventional shear cutting or cost-intensive laser trimming. It has already been shown that this process has a high quality in terms of straightness, roundness and surface finish of the shear-cut edge. However, for the industrial use of adiabatic shear cutting, there is a lack of fundamental investigations for relevant high-strength steels, such as frequently used press-hardening steels. Furthermore, the microstructural processes occurring during adiabatic shear cutting and their effect on technological properties have not been described for these steels. Basic approaches for estimating the necessary process energy, process speed and a classification regarding the suitability of the materials to be used are also lacking. The aim of the project is to answer these open questions and to close the gap to industrial application by means of a fundamental characterisation of the process. Furthermore, a methodology for the simulation of the strongly temperature- and strain rate-dependent process is required. Extensive material characterisation tests (temperatures up to 1000 °C and strain rate ?̇ up to 1000 1/s) were carried out to achieve the objectives. Tensile tests with notch geometry were used to determine the failure behaviour. A total of three different steel grades were analysed. Based on these investigations, numerical and analytical models were developed to estimate the locally generated temperature and the process energy required to perform the separation operation. Adiabatic shear cutting tests were carried out to produce cylindrical components and various demonstrators (rectangular plates with rounded edges). Furthermore, it was shown that two of the investigated steels (20MnB5 and C75S), despite almost similar mechanical properties at room temperature under quasi-static load, showed significant differences during adiabatic cutting. In particular, it was shown that the strain rate sensitivity of the steels has a major influence on the initiation and formation of adiabatic shear bands, but at the same time the required process energy is also strongly influenced by this factor. Using finite element simulations, the resulting characteristic shape of the shear-cut surfaces could be predicted with a high degree of accuracy. The methods developed can be used for process design. Publication: May 2020 Authors: Prof. Dr.-Ing. A. E. Tekkaya, Dr.-Ing. T. Clausmeyer, Dipl.-Ing. D. Staupendahl, F. Schmitz M.Sc., Univ.-Prof. Dr.-Ing. habil. M. F.-X. Wagner, Dipl.-Ing. S. Winter