P 1035 - Increasing the load-bearing capacity of welded I-beams made of high-strength structural steel through improved approaches

P 1036 - Introduction of undercuts in highly stressed components by multidirectional forging using the example of steel pistons In this AiF project (IGF No. 18162 N), undercut forging was investigated using the example of piston pin bores for steel pistons. In contrast to the conventional forging process for steel pistons, multi-directional forming takes place in the final forming stage, in which the piston pin bores, which represent an undercut, are preformed. To this end, the sequence of stages was first developed virtually with the aid of material flow simulations. A tool concept was then developed to enable undercut forging using a multi-directional tool. With this tool, punches move laterally into the workpiece and deform it further. This tool was designed and manufactured. Various forging tests were then successfully carried out and analysed. The economic efficiency of the process was then assessed and an application guide for the development of undercut forging processes was developed. In this project, the bolt holes in a steel piston could be pre-punched in the forging process for the first time. The advantages of this process are the material savings, improved component properties of the forged part and a faster and simplified reworking process compared to the conventional forging process. The material saving was 5% compared to the standard forging process. The adapted fibre flow in the bore area is likely to be of great importance for the component properties. From an economic point of view, the possibility of accelerated reworking through undercut forging is of the highest relevance. The design of the undercut forging process differs from conventional closed-die forging processes only in the final forming stage. It turned out that the complete punching of the component is associated with too much effort in terms of process technology, so that the pre-punching variant was favoured. The clamping forces required for multi-directional undercut forging are a particular challenge for undercut forging tools. The variants of a force-fit (using springs) and a form-fit closure (using toothing) were investigated. Due to the lower design and production costs, the design of the non-positive locking variant was pursued further. Nevertheless, the positive locking variant promises advantages, as previously impossible forging geometries can be realised, so that future investigations in this direction appear promising. The tests showed that both the retracting and extending movements of the lateral punches require high forces. In particular, the return movement of the plungers requires correspondingly 20 kN must be taken into account and the tools must be dimensioned accordingly. The evaluation of the forging results showed that the forgings were well formed in the undercut area. The fibre flow of the pistons in the bore area turned out to be very favourable and confirmed the simulations carried out previously. Due to a reduced notch effect, more favourable fatigue strength properties can be expected for bores produced using undercut forging. The undercut forging process was used in this project for the insertion of piston pin bores. In general, the process is suitable for introducing a wide variety of undercut geometries into workpieces during forging processes. Publication: 2018 Authors: M. Stonis, J. Ross

P 1033 - Fatigue analysis of rails based on fracture mechanics criteria In this research project, approaches for the fracture-mechanical description of crack growth in the rail base are developed. For this purpose, finite element simulations are used to show how the stresses from residual stresses, temperature, train passage and structural deformation are distributed in the rail base. The magnitude of the loads to be applied from structural deformation is determined. It can be shown that the stress components can be categorised into three groups according to their distribution in the rail footing, which can subsequently also be found in the determination of the K-factors. Finally, the relationship between torsion and displacement of the bridge head is analysed in more detail. The residual stresses of the rails are determined experimentally. It is shown that the expected residual compressive stresses at the edge of the rail footing also occur in reality. Tensile residual stresses prevail in the centre of the rail base. However, the degree of residual tensile stresses varied greatly depending on the rail grade tested. The large-scale fracture mechanics tests on notched rails show that notched rails of the same quality behave similarly and that fracture mechanics concepts based on cracks in the rail base can therefore in principle describe the cracking of rails. can describe the cracking of rails. However, it is also established that the large-scale tests do not allow any conclusions to be drawn about the actual life expectancy of a railway rail in the track. The notch introduced modifies the test specimens in such a way that the results obtained with them allow conclusions to be drawn about the fracture mechanics model to be used. It is not possible to draw conclusions about the resistance to crack initiation of the individual test specimens. In small-scale fracture mechanics tests, the fracture toughness of the rail steels is determined at the "rail base centre" and "rail base edge" extraction points. It turns out that a purely elastic evaluation of the tests is not expedient. An evaluation for elastic-plastic materials is necessary. It cannot be conclusively clarified whether this necessity is due solely to the material properties of the rail steel or also to the very narrow samples. In order to visualise crack growth using fracture mechanics methods, various approaches are different approaches are shown and the possibility of simulating seasonal dependencies and the interaction of different loads is presented. It is shown that the simulation of individual cracks is very suitable for demonstrating the influence of individual parameters on crack growth. Publication: January 2021 Authors: Prof. Dr.-Ing. M. Mensinger, M.Sc. S. Pessel, Prof. Dr.-Ing. S. Freudenstein, Dr.-Ing. Ch. Simon, Prof. Dr.-Ing. Fischer, M.Sc. N. Schramm, Prof. Dr.-Ing. P. Langenberg

P 1037 - Reduction of springback when cutting rollformed profiles Roll forming is an important process for the production of profiles from rolled starting material. When the profiles are separated after the forming process, residual shear and longitudinal stresses are released which cause the profile ends to collapse or spring open at the separation point. This so-called head bounce leads to deviations in dimensional accuracy, which must be compensated for by additional processing steps. This results in increased system times and therefore higher machine and labour costs. As part of this research project, the state of the art in roll forming companies is first recorded with the help of an anonymised questionnaire. Subsequently, a finite element model will be developed, which enables an exact mapping and prediction of the head crack. With the help of the numerical model, a comprehensive parameter study can be carried out on the basis of the questionnaires. Numerical investigations can be used to identify countermeasures to reduce head bounce and their practical feasibility can be tested experimentally. The results obtained can be used to derive design guidelines for the roll forming process with regard to minimising residual stresses. In the course of the project, the influencing factors of sheet thickness, material strength, forming per station, side rollers, pressure overlay, profile shape, stand spacing, profile height, roll forming strategy, bending radius and roller diameter were determined. The deformation of the profile during cutting allows conclusions to be drawn about the acting stress components. Based on this, several effective countermeasures for reducing the head bounce and their possible applications were developed. Overbending and rebending proved to be universally applicable, as it reduces residual shear and longitudinal stresses. By superimposing compressive or longitudinal stresses, it was possible to reduce one component of the causal stresses in each case. Superimposing longitudinal stresses proved to be effective when both ends of the profile cracked, while superimposing compressive stresses should be applied when the profile collapsed and cracked. Publication: 2017 Authors: P. Groche, M. Moneke

P 1032 - Characterisation and modelling of mechanical joints with one-sided accessibility for profile-intensive lightweight construction under crash load The legal regulations to reduce greenhouse gas emissions are prompting the automotive industry to implement consistent lightweight construction. The reduction of CO2 emissions to an average of 95g CO2/km in passenger car traffic by 2020 has been agreed. The vehicle body, which accounts for over 30% of the total weight, offers the greatest potential for implementing lightweight construction, which can be achieved by using high-strength steels in thin sheet thicknesses and other lightweight materials such as aluminium or fibre-reinforced plastics. Due to the profile-intensive mixed construction method, the requirement for accessibility of the joints on both sides cannot be realised throughout the design. Joining technology is increasingly being used for joining tasks that can only be accessed from one side. Methods such as direct screwing (flow-hole moulding screws) and bolt setting are then used here. The aim of the research project is the experimental investigation and numerical modelling of the strength, load-bearing behaviour, deformation and failure behaviour of mechanical joints with one-sided accessibility for profile-intensive lightweight construction under crash load. Using the LWF-KS-2 test technique, characteristic values for the replacement modelling of these connections for crash simulation are to be determined for the joining methods of bolt setting and flow-hole and thread-forming screws (FLS). A substitute model is to be further developed and calibrated for these connections on the basis of the existing substitute model for self-pierce rivet connections. This modelling is to be validated for use in crash simulations using simulations of a sample component test. As part of the research project, tensile tests for a quasi-static and an impact strain rate were carried out for the test materials as an initial test. These form the basis for the subsequent simulation. Following the sampling of the joints planned in this project, characteristic joint values were determined for different set bolt and FLS joints. In order to assess the influence of the position of the joining point in a component and thus the stiffness of the joining parts on the joint behaviour, tests were carried out under shear and peel loading with variation of the specimen geometry. By developing a substitute model and characterisation methodology for joints in profile-intensive lightweight construction, such as flow-hole forming screws and bolting, steel users in design and calculation are able to make reliable statements about the load-bearing capacity of joined steel structures under crash load. This enables the efficient and economical development of joined components made of sten steels for profile-intensive lightweight construction and improves the competitiveness of steel compared to other lightweight construction materials. Publication: 2017 Authors: Dr.-Ing. S. Sommer, Prof. Dr.-Ing. G. Meschut