P 1326 - Inspection, monitoring and documentation of steel structures

P 1327 - Optimised design of combined steel sheet pile walls for the installation process and the final state The overall objective of the research project is the optimisation of design methods for combined steel sheet pile walls used in quay wall construction to secure the ground jumps. The following objectives are defined for this purpose: - The design is carried out without taking the installation process into account. The influence on the final position and thus also on the design must be checked (objective 1). - Furthermore, the surrounding soil is not taken into account as resistance in the combined verification against buckling and lateral-torsional buckling, so that the load-bearing behaviour in the final state is underestimated. Approaches must be developed to take soft soils and stratified subsoil structures into account. (Objective 2). - For the flanges of the load-bearing piles, global internal forces and local influences from the flange bending are currently only taken into account in a generalised manner by reducing the yield strength. Possible load-bearing capacity reserves with separate consideration are to be examined (objective 3). - When determining the load-bearing capacity, either only plastic or only elastic resistance values are assigned to the intermediate piles. The use of partially plastic resistances should be investigated (objective 4). Firstly, the basics were determined in the research project in a parameter identification. For this purpose, common quay wall projects were analysed with regard to the subsoil stratification of load-bearing piles. The valid design methods were also compiled. This was followed by a metrological investigation of the installation during heavy pile driving in order to identify the dynamic influences of the installation method on the final position. The results show the influence of pile driving during installation and serve as a basis for numerical analyses. The numerical replacement model for the installation of the soldier pile was used to identify individual influencing variables on the position in the final position and recommendations were made for accurate installation. The model can be used to predict the final position after installation. To take into account the bedding of soldier piles in cohesive and layered soils, model tests as well as full-scale tests were carried out and spring stiffnesses were derived. Generally valid approaches were then determined with the aid of a numerical model. Together, these serve as the basis for further investigations into the buckling and lateral-torsional buckling of I-shaped soldier piles in layered soils. The lateral-torsional buckling investigations took into account common quay wall structures and soil layers in North German deep-sea harbours. Extensive parameter analyses were carried out for these and the buckling length coefficients were evaluated. Approximation functions were derived and statistically evaluated. With the help of these results, it is now possible to check against flexural buckling and lateral-torsional buckling with indirect consideration of the resistance in the soil. Investigations show an optimisation of the verification for a typical quay wall of up to 40 %. Furthermore, limit criteria for the necessity of the verification were developed. The results are to be incorporated into the recommendations of the "Embankments" working committee. In addition, parameter analyses were carried out on partially welded bearing piles. This was used to derive formulae for determining the effective torsional moment of inertia. Until now, it was not possible to take partial welding fully into account in the design. The formulae determined were proposed in TC250/SC3/WG18 and incorporated into the draft standard prEN1993 Part 5. Recommendations for determining the resistance were given for the design of load-bearing piles under local stress with an additional global stress. In addition to the water pressure, the resistance model also takes into account other important geometric parameters as well as the material of the bearing and intermediate piles. In order to consider partially plastic conditions in the design of Z-slabs subjected to bending, 4-point bending tests were carried out and an FE model was validated. Parameter analyses were carried out on various profiles from practice, a resistance model was derived and partial safety factors were determined. The results for the partially plastic design were incorporated into the Eurocode 3 Part 5 standard. The experimental and numerical investigations have already been presented in standardisation meetings. Up to 15 % higher load capacities can be taken into account. Publication: 2021 Authors: Prof. Dr.-Ing. U. Kuhlmann, A. Enders, M.Eng, Prof. Dr.-Ing. J. Grabe, J. Beuße, M.Sc.

P 1328 - Sustainability of cold-formed steel profiles in the construction industry Thin-walled, cold-formed steel profiles are used in a wide range of areas from steel hall construction to shelving and wine growing. Due to the wide range of applications, the spectrum of individual, user-specific profile shapes, some with perforations, is very large. DIN EN 1993-1-3 is available for dimensioning. However, this was mainly developed for C- and Z-shaped purlins and for typical trapezoidal profile problems, so that it can only be used to a limited extent for the large number of other types of cold-formed profiles. For profiles with perforations, a design is usually even completely impossible. In addition, the application of DIN EN 1993-1-3 is extremely time-consuming, complicated and provides conservative results even for simple cross-section shapes due to the highly complex, overlapping stability shapes. The aim of the research project was therefore to develop a new, Eurocode-compliant design concept, dispensing with the complex method of the effective width of DIN EN 1993-1-3 and incorporating modern finite strip software for eigenvalue analysis. The user-friendly "Direct Strength Method" (DSM) developed at Cornell University (USA), which is part of the American standardisation AISI, essentially forms the basis for calculating the cross-sectional load-bearing capacity. In order to develop and verify this combined design method in accordance with DIN EN 1993-1-3, including the DSM, the load-bearing behaviour of various cold-formed steel profiles was investigated in extensive experimental tests. In order to analyse the interaction of different stability phenomena and to determine the load-bearing capacity, tests were carried out under compressive, bending and combined compressive and bending loads on various C, Z and omega profiles. The test matrix was supplemented by extensive numerical parameter studies, whose models were calibrated to the experimental tests, in order to confirm the validity of the developed design concept. The analytical consideration of the results showed that the consideration of a displacement of the compressive force resultants cannot be neglected in the design of short profiles, so that an approach was developed to take these effects into account. It was shown that a calculation of the bearing load with the new combined approach provides reliable results. In addition, perforated profiles can be designed by applying equivalent thicknesses in the branching load calculation using finite strip programmes according to [1]. Overall, the combined design method provides a simple, time-saving, practical, efficient and therefore very good alternative to the existing methods according to DIN EN 1993-1-3. Publication: 2022 Authors: Prof. Dr.-Ing. D. Ungermann, T. Lemański, M.Sc.

P 1323 - Extension of the constitution diagrams for high Mn-containing steels in dissimilar welded joints by hazard zones The primary aim of the research project was to determine the hazard ranges for the newly developed Constitution of High Manganese Steel Welds (COHMS) constitution diagram in order to enable low-hazard welding of high-manganese steels in dissimilar joints with ferritic joining partners. Three economically relevant austenitic iron-manganese steels (Fe-Mn) with different alloying concepts, two ferritic body steels and various welding consumables were used as test materials. This allowed all microstructural areas in the COHMS diagram to be covered. Seam irregularities, such as impermissible hardening due to unwanted martensite formation and weld-related hot or cold cracking, can occur in the mixed weld metal of weld seams of these joining partners. In order to determine the location of the risk areas, dissimilar weld metals had to be produced with defined mixtures and then tested using suitable test methods such as 4-point bending, shear and impact tensile tests and hardness tests (hardening crack tendency) as well as programmed deformation crack (PVR) and cast pin tear (CPT) tests (hot crack tendency). Testing these dissimilar weld metals required a modification of the test sequence for the PVR test and the construction of a suitable crucible induction melting furnace for the CPT test. The construction of this induction melting furnace went through several iteration loops and was significantly delayed due to the pandemic. As the final prototype could not be finalised on the hardware side until the end of the project, it was no longer possible to carry out CPT hot tear tests with the mixed welding materials. In addition, the furnace is currently being supplemented by a SIMATIC S7-1200 controller, which serves to automate the entire process. The final induction melting furnace currently enables the casting of a maximum of 33 g of metallic sample material in an inert atmosphere in gravity casting or suction casting in addition to the casting of hot crack samples. Tensile, compression and impact test specimens can be produced from these cast samples. Although the solidification cracking tendency of dissimilar weld metal could not be determined using the CPT test, the results of the other work packages in the research project made it possible to extend the COHMS diagram to include the hazard ranges. It was also possible to determine a low-hazard area. The objective of the research project was achieved. Publication: March 2025 Authors: F. Urban, N. Kauss, M. Kuhlmann, M. Zinke

P 1322 - Development of a design concept for multi-axis stressed tools made of carbide-rich tool steel for cold work applications For cold work applications, carbide-rich, high-alloy tool steels are very often used as cutting, punching, cold forming or pressing tools. In order to increase the efficiency and productivity of forming processes, the service life of cold work tools must be successively increased. The specific development of carbide-rich, highly wear-resistant alloys has continuously increased the service life or tool life of cold-working tools over the last few decades, meaning that several million load cycles (or more) can often be achieved during the operation of a tool. The broad requirements profile for tool steels has therefore been extended by an additional, decisive factor: the cyclic strength or fatigue strength of high-carbide tool steels. Toolmaking and tool users in Germany in particular are strongly organised as small and medium-sized enterprises (SMEs) and would benefit from increased tool life quantities, a fundamental, statistical understanding of the factors influencing fatigue strength and improved, targeted design and dimensioning of cold work tools. On the one hand, the attractiveness and high quality of your products can be further increased and, on the other hand, new, innovative potential for SMEs in the new and further development of specific cold work tools can be created. For the manufacturers of tool steels in Germany, dynamic opportunities and new potentials also arise in the field of targeted alloy development of tool steels with improved cyclic strength and wear properties. Overall, a significant contribution can be made here to maintaining and increasing the competitiveness of German SMEs in particular on European and international markets. German SMEs in industrial plant and tool construction pool a large part of the experience and knowledge on the design and construction of tools and tool steels, although these are not usually based on standardised norms or design works, e.g. the FKM guidelines [1, 2, 3] or DIN EN ISO 4957 [4], as is typical in mechanical engineering, but are mostly based on many years of experience and practical test series. In addition, there are no design-relevant design parameters or cyclic (and static) characteristic values for tool steels in standards or guidelines. Tool steel is the most frequently used material group in the construction of cold work tools and is subject to a very broad requirements profile. The requirements are dominated by high hardness, wear resistance and strength (set via the heat treatment and the hard phase content and type), sufficiently good toughness and, under certain circumstances for specific applications, also good hot hardness, tempering resistance and corrosion resistance. In practice, experience has shown that a compromise must always be made in order to realise the right balance between these different, sometimes conflicting requirements for each individual application. For toolmakers and designers, the chemical composition of the alloy, the production route (ingot casting, continuous casting, powder metallurgy), design adjustments to the geometry and the subsequent heat treatment provide a wide range of options for setting specific microstructures and tool properties, which largely determine the tool life. Due to the continuous further developments in the field of tool steels, wear or corrosion are now often no longer life-determining and, in the case of desired increases in tool life, the focus for SMEs and steel manufacturers is increasingly on the previously rather insignificant criterion of fatigue strength, whereby tools are often exposed to non-proportional multi-axial fatigue loads (changing main stress directions) due to their operation. There is currently a lack of fundamental understanding of which factors, e.g. multiaxiality, microstructure and load type, specifically influence the fatigue strength of tool steels. There is a lack of experimental investigations and statistical validation of fatigue strength values. Proper tool design according to fatigue criteria would result in more efficient material utilisation and optimised service life estimation and prediction for German tool manufacturers and designers, as well as allowing targeted material development according to fatigue criteria. In the future, a statistically verified, comprehensive database of fatigue strength values for carbide-rich tool steels could be transferred to corresponding standards, calculation and design works and used for scientific tool design. This research project therefore focusses on two fundamental questions: <ol> - What is the general quantitative effect of stress and microstructure variations on the fatigue strength of high-carbide tool steels? - On the basis of which material characteristics and using which failure hypotheses should cold work tools be designed with regard to non-proportional multi-axial fatigue loading? </ol> A cold-work tool steel and a high-speed tool steel are tested in the underlying test matrix under uniaxial fatigue loads and systematic microstructure and load variation. To understand the fracture mechanics of fatigue, the threshold value, long crack propagation in the Paris region and the fracture toughness of these materials are also analysed. The cyclic material parameters obtained in this way serve as the basis for the development of a simulative design approach for non-proportional multi-axis, synchronously loaded tools with the quadratic failure hypothesis (QVH). For non-proportional multiaxial loads (e.g. different forms of vibration loads or different static mean stresses, frequency or phase shifts), in contrast to proportional multiaxial loading, both magnitude-varying principal stresses and changing principal stress directions would have to be explicitly considered by an extended failure hypothesis, e.g. QVH [5-8, 9-12] or SIH [13-20]. To validate the new design approach, the results of the QVH are compared with those of the shear stress intensity hypothesis (SIH), the FKM guideline [1, 3] as well as the biaxial and triaxial normal stress hypothesis (NH), shear stress hypothesis (SH) and shape change energy hypothesis (GEH) according to von Mises. In parallel, experimental validation is carried out under non-proportional, multi-axial, synchronous fatigue loading on a 2-axis test rig (axial loading combined with superimposed torsional loading). As experimental reference tests, one cylindrical laboratory geometry and one tool-like, complex-notched geometry per material are tested in statistically verified Wöhler tests. The validation results can be used to further develop the design approach and increase the accuracy of the predictions. The results of the research project show that the primary factor influencing the fatigue strength of carbide-rich tool steels is the defect size (size of defects in the fracture mechanics sense). The size of the microstructural, crack-inducing defects such as carbides or inclusions must be focussed on as the decisive factor for increasing the fatigue strength of tool steels. The properties of the microstructure are mainly set during material production (and heat treatment). The carbide size is the dominant factor in cast microstructures and the size of the non-metallic inclusions in materials produced by powder metallurgy. Tool steels produced by powder metallurgy exhibit significantly higher fatigue strengths due to the much smaller, round carbides, which are very homogeneously distributed, whereby the location of the crack initiation now changes to the larger non-metallic inclusion. The degree of deformation and purity are therefore decisive for the fatigue strength, as they directly influence the size and distribution of the carbides and inclusions. The secondary influencing factor is presumably the matrix hardness and the tertiary the defect morphology, whereby these three factors influence each other and there is a complex interaction between them, which should be investigated in more detail in the future. Surface nitriding with a pure diffusion layer can significantly improve the fatigue strength of carbide-rich tool steels, but this depends on the defect size and the original matrix hardness. In particular, the fatigue strengths of PM tool steels (with comparatively very small non-metallic inclusions as crack-inducing defects) and their scattering of test values can be effectively improved by the high residual compressive stresses introduced during nitriding with a diffusion layer. The results of the fracture mechanics tests make it clear that the threshold value for long crack growth and fracture toughness in cast tool steels is greater than in chemically identical, powder metallurgically produced variants, irrespective of the stress ratio R. At the same time, the gradient m of the Paris straight line for stable long crack growth is smaller in cast variants. This means that only the crack formation phase and possibly microcrack growth play a role for fatigue in tool steels. The significantly higher fatigue strength of PM materials is solely due to a drastically reduced and delayed crack formation phase. The experimental results of the multiaxial validation tests show strong deviations from the simulation results of the QVH. By modifying the orientation-dependent (anisotropic) fatigue strength calculations of the QVH, it was possible to achieve a significant improvement in accuracy for both investigated mill tensile steels, at least for the cylindrical laboratory geometry. For this purpose, an extrapolation in the Haigh diagram up to the stress ratio R = - infinity was used. In most cases, the prediction calculation according to FKM shows smaller or even the smallest errors compared to the experimental validation. However, larger prediction errors of up to approx. 80 % still occur for thevii tool-like geometries for all failure hypotheses considered. Predicting the multiaxial fatigue strength of complex-notched, real tools remains a major challenge for all hypotheses considered, which is why improving accuracy should be the focus of future projects. With regard to the standardisation relevance of the research project, it can be summarised that the determined material parameters (static, uniaxial cyclic, multiaxial cyclic) were both statistically extensively validated and evaluated and conscientiously tested with suitable testing machines in accordance with applicable standards. It is therefore intended to incorporate this database of cyclic characteristic values into current guidelines and standards (FKM guideline, DIN EN ISO tool steels) in order to enable and facilitate the use of this database in the future, especially for SMEs. The aim of the research project was achieved. At the same time, new research hypotheses and questions were developed from the basic principles and findings obtained here, which were realised as part of the application for a follow-up project. The follow-up project FOSTA P 1608 or IGF project no. 22271 N was approved and started on 1 March 2022. Publication: August 2022 Authors: Univ.-Prof. Dr.-Ing. Ch. Broeckmann, M.Sc. L. Scholl