P 1008 - Application-oriented simulation of thermal straightening

P 1007 - Method for characterising the failure behaviour of high-strength sheet steel materials in a wide range of stress states using uniaxial tensile testing machines The main objective of the project was to develop a new method for characterising the failure behaviour of high-strength sheet steel materials. This method should be based on shear tensile tests on new shear tensile specimens using a new testing module in uniaxial tensile testing machines as well as on hydraulic cupping tests and enable failure characterisation in a wider range of stress states compared to the conventional forming limit curve. The core of the developed approach is the new shear tensile specimen, which is characterised by a localisation of deformation leading to crack initiation inside the material and not at a free edge, regardless of the direction of an external uniaxial load in the specimen plane. A new test module was designed and manufactured for the shear tensile tests on the new shear tensile specimens, which applies a uniaxial test load into the following stresses in the material to be tested: a uniaxial tensile load with a superposition of hydrostatic tension, five different tensile shear loads with a dominant tensile or shear component and a compressive shear load with a dominant shear component. The degree of failure deformation of a dual-phase sheet steel material was experimentally determined using shear tensile tests with five different specimen orientations to the test load using an optical measuring system. In addition, the prevailing stress triaxiality at the material point of crack initiation was determined by numerically modelling the shear tensile tests performed. By combining the respective mean value of the experimentally determined degrees of failure deformation and the respective characteristic value of the numerically determined stress triaxiality, a support point for the fit of the material-specific failure curve was obtained for each tested material stress. In addition to these five support points, a support point from hydraulic cupping tests was also included. From the relative arrangement of the six support points, it can be seen that the dependence of the degree of failure deformation on the stress triaxiality is fundamentally captured using the developed approach. Although the work carried out confirms the fundamental suitability of the approach for characterising the failure behaviour of high-strength sheet steel materials, it also reveals possibilities for its further development. Individual focal points of this further development are an optimisation of the sample geometry to suppress edge cracks, an investigation into the influence of sample production on test results, improvements in recording the time and position of crack initiation and an investigation into the transferability of test results to the numerical simulation of forming processes. In the foreseeable future, these focal points are to be addressed more intensively in newIF UM research projects in order to promote acceptance of the newly developed method in the simulation-based design of sheet metal forming processes in the research landscape and in industrial practice. The research project P 1007 of the Forschungsvereinigung Stahlanwendung e.V., Düsseldorf, was carried out by the Institute of Forming Technology and Machines, University of Hanover, with technical support and financial assistance from FOSTA. Publication: 2015 Authors: B.A. Behrens, M. Vucetic, I. Peshekhodov

P 1010 - Pre-hole-free joining of steel-intensive lightweight structures by resistance element welding with prefabrication In recent years, due to legal requirements, the automotive industry has increasingly focussed on lightweight construction measures, particularly in the body structure through the load-appropriate use of various lightweight materials. A mixed construction method, in which high-strength and ultra-high-strength, press-hardened sheet steel parts are combined with aluminium materials, is particularly suitable for large-scale production in order to create lightweight, yet rigid and crash-resistant body structures in shell construction. The thermal-mechanical joining processes developed in recent years for series production and not exclusively for this application offer a high application potential and the first series applications have already been realised. Due to the novelty of resistance element welding, the process-related properties and characteristics have so far been insufficiently investigated, so that there is a lack of sufficient knowledge on the welding, strength and failure behaviour of corresponding joints and material combinations. The aim of the research project was to develop and qualify resistance element welding with prefabrication of an aluminium or FRP composite partner as an economical joining process for joining these lightweight construction materials with high-strength and ultra-high-strength steel materials. As resistance element welding is still a relatively new joining process, the requirements were initially adapted on the basis of the investigations carried out to date. This involved analysing the special process-related features of resistance element welding of prefabricated composite partners and characterising the properties of the resulting joints. By analysing the joining process and the joint formation, taking into account the properties of the rivet elements, it was possible to gain important insights into the design of the rivet elements and processes as well as their reliable use. The evaluation of the performance characteristics in terms of joint and handling strength was also part of the project. As a result, industrial users were provided with information on the use of a flexible joining process for the realisation of future lightweight structures. The implementation of the optimisation derived from the results obtained marks an important contribution to the development of resistance element welding and the economic production of future lightweight structures in mixed steel construction. Publication: December 2020 Authors: Prof. Dr.-Ing. G. Meschut , Dipl.-Wirt.-Ing. V. Janzen

P 1005 - Investigations into the load-bearing behaviour and service life of bonded joints in steel structures under cyclic loading The use of innovative adhesive bonding technology increases the productivity, quality and efficiency of German steel construction. It also represents an alternative to conventional joining methods, which have fundamental disadvantages for steel structures. For example, the bonding joining process can avoid welding-related component distortion and cross-sectional weakening due to bolt holes. Due to the high degree of prefabrication, steel construction is predestined for the use of adhesive bonding, as load-bearing, serviceable and durable bonded joints can be produced under workshop conditions. However, it is not possible to verify such bonded steel joints on the basis of current standards. Time-consuming and expensive approval procedures would be necessary, which is why small and medium-sized enterprises (SMEs) decide against adhesive bonding technology. The inclusion of bonded steel joints in standardisation would provide a remedy here. For this reason, the first step is to develop basic principles for the verification of load-bearing capacity using two application cases from steel façade construction, which relate exclusively to short-term, one-off and quasi-static loads. A further aim of the project is to supplement these findings by investigating the long-term load-bearing behaviour of bonded joints under cyclic loading. The results of the analyses provide input parameters for the development of verification concepts for bonded steel joints. Publication: 2018 Authors: Prof Pasternak, Prof Dilger

P 1011 - Use of modified spray arc processes to optimise welding-related stresses on components made of high-strength fine-grain structural steels Due to current lightweight construction efforts and demands for energy and resource efficiency, high-strength fine-grain structural steels with yield strengths ≥ 690 MPa are increasingly being used in modern steel structures. These are primarily processed with yield strengths of up to 960 MPa and joined using MAG welding processes. Modern inverter technology has recently enabled numerous company developments (e.g. "DeepARC", "forceArc"), which have led to modified spray arc welding (mod. SLB) with similar properties. In addition to the well-known economic advantages of these mod. SLB, the reduced possible seam opening angles and the lower heat input are expected to have a positive effect on welding-related stresses. Initial residual stress analyses in previous weldability tests with mod. SLB have already shown lower residual tensile stresses in the weld seam in free-shrinking laboratory samples compared to conventionally welded samples. However, the operationally stable design of high-strength welded constructions and the utilisation of high yield strengths require sufficient quantification of welding-related stresses in mod. SLB under defined, component-relevant shrinkage restraint. This concerns both the forces and stresses in the local weld seam area and the overall stress as well as the influence of heat conduction, component geometry, degree of clamping and seam opening angle. There was hardly any knowledge of this in the industry, especially among SMEs, and the aim of this research project was therefore to develop this knowledge. The comparative investigations with conventional arc and mod. SLB with adapted seam configuration were carried out with the aid of special testing equipment that can be used to model real component stiffnesses. With both process variants, weld seams could be realised with the selected parameters while fulfilling the requirements for the mechanical-technological properties. During the analyses, a significant reduction in internal stresses due to reduced heat input was observed when using a seam configuration with a lowered seam opening angle. In particular, the narrower seam gaps proved to be effective in reducing multi-axial stress states due to bending moments in the welds close to the component. The knowledge gained and recommendations made were presented to the relevant standardisation committees in order to facilitate simpler design and safe processing of high-strength fine-grain structural steels in the medium term, especially for SMEs. Standardisation can reduce qualification and approval costs, which are currently often necessary when using high-strength steels. Furthermore, the results will enable companies to utilise the technical and economic advantages of mod. SLB processes and to achieve significant savings in filler metal, welding and set-up time as well as optimised results with regard to the load-bearing capacity of components, taking into account the inherent stresses, and to increase productivity. Publication: 2017 Authors: T. Kannengießer, D. Schröpfer