P 1298 - Joining of shear-cut metal-coated lightweight composites

P 1297 - Investigation of linear polarisation as an electrochemical method for the evaluation of new thin-film pretreatments for corrosion protection in the industrial sector One aim of the investigations in this project was to find out what contributions linear polarisation can make to the evaluation of the corrosion protection effect of pre-treatments. To this end, four thin-film pre-treatments and, as a benchmark, zinc phosphating (trication process) and untreated substrates were characterised using linear polarisation and the results compared with classic corrosion protection tests carried out on KTL-coated sheet metal. Zinc phosphating offers the best corrosion protection on cold-rolled steel (CRS). Here, the current density in the cathodic branch correlates well with the cathodic infiltration at the scribe after the salt spray test. No such correlation was found for the galvanised substrates (electrogalvanised and zinc-magnesium coating). Zinc phosphating is rated worst on ZE in the salt spray test. The ranking of thin-film pre-treatments is also partially reversed on ZE. On ZM, some thin-film pre-treatments are at least on a par with classic zinc phosphating. The salt spray test and the VDA alternating test VDA 233-102 show clear differences in the corrosion mechanism, especially on galvanised substrates. The results of the tested systems (substrate-VBH-KTL) can only be transferred to other systems to a limited extent. The user must therefore check the requirements of each system before use and decide whether the use of thin-film VBH is suitable in his case. Publication: 2018 Authors: Prof. Dr.-Ing. T. Bauernhansl

P 1299 - Design method for bonded joints with KTL-coated joining parts and polymer fibre-reinforced materials Cathodic dip coating (CDC) has been state of the art in automotive engineering for years to protect the car body from corrosion. Fibre-reinforced plastics (FRP), other temperature-sensitive materials or even metallic materials are often bonded to the already painted surface after cathodic dip coating. In principle, the cathodic dip coating provides an excellent adhesive base. However, the problem is that such bonded joints represent the strength-limiting factor for the joint in certain load cases. For this reason, it is not possible for SMEs to reliably design an adhesive bond between KTL-coated and temperature-sensitive components according to the current state of the art. This is where the present research project comes in. In this research project, the basic requirements for achieving reproducible cathodic dip coating strengths were first determined. Tests were carried out with different adhesive systems and CDP coatings and substrates in order to identify suitable materials. The effects of various influencing factors on the failure behaviour of the bonded joints, depending on the adhesive and substrate, were then investigated. The strength and stiffness of the respective adhesive system and the geometric design of the adhesive layer played a key role here. Based on this, it was examined whether the knowledge gained can also be transferred to FRP mixed joints. In the next step, a formulaic relationship was derived on the basis of the data collected in the project to mathematically describe the transition from cohesive adhesive failure to cathodic dip coating failure. This led to the hypothesis that a high proportion of normal stresses leads significantly to cathodic dip coating failure. Based on the calculated stresses, it became clear that it is not the absolute values of the individual stresses that cause cathodic dip coating failure, but rather the ratio of normal to shear stresses that is decisive. It was shown that for bonds with greater overlap lengths and adhesive layer thicknesses, the proportion of normal stresses increases in relation to the shear stresses, which correlates with an increased tendency towards cathodic dip coating failure. Finally, it was shown that the cause-effect mechanisms determined in the project can also be transferred to samples similar to components. Publication: 2021 Authors: Prof. Dr.-Ing. G. Meschut

P 1296 - Adhesive strengthening of fatigue damage for steel structures In steel construction, unforeseen fatigue damage can occur in cyclically stressed structures, making repair measures necessary. Adhesive bonding technology represents a promising alternative to conventional strengthening methods. The aim of the research project was therefore to develop and analyse concepts for the adhesive repair of fatigue damage by using bonded steel patches for steel structures, taking into account defined site-specific operating conditions. As part of the completed StressPatches research project, the strengthening of typical steel construction notch details using bonded steel patches was investigated. For this purpose, the boundary conditions and strain rates from real bridge loads, component tolerances and loads on the bond from the environment were first determined. Based on a specification sheet, more than ten adhesives were identified and provided by the industry, which were analysed in initial tests with regard to workability and final properties. Based on these results, two cold-curing epoxy resin adhesives and a polyurethane adhesive were selected for further investigation and validated in tests on pre-treatment options, curing behaviour under ambient conditions and accelerated curing. The refurbished notch details with patches were then tested experimentally using quasi-static and cyclic small component, component and large component tests. Analytical and numerical investigations were used to identify parameters, recalculate and optimise the tests and to derive a (preliminary) design concept. During the production of the samples, it became clear that assembly aids will be required to support the joining of the components, which will need to be developed in the future. Furthermore, the potential of using fibre optic sensors for structural monitoring was demonstrated. Based on the adhesives in focus, various manufacturing concepts and process sequences were then considered and discussed, and the options were also taken into account in the economic feasibility study. In view of all the results, a pre-dimensioning concept was drawn up. It was shown that the bonding of steel sheets to a bridge structure has the potential to significantly limit the development of possible damage in the form of cracks. These results are now available to SMEs in the engineering services and planning sectors. At the same time, there is a need for further research to enable implementation on real structures. To this end, a further research application has been submitted and already approved, as well as a ZIM project. Publication: 2022 Authors: Prof. Dr.-Ing. Ch. Schuler, Prof. Dr.-Ing. M. Feldmann, Prof. Dr.-Ing. K. Dilger

P 1294 - Novel joining technology for the production of hybrid component structures with a short flange made of high-strength steel and aluminium High-strength steels are increasingly being used in lightweight vehicle construction and combined with components made of aluminium alloys. Mechanical joining processes have been further developed for this combination of materials, but these have fundamental technological limitations and are not as widely used as resistance welding technology, especially in the SME sector. Especially when joining mould-hardened components with thin aluminium sheets, high joining forces and small tolerance fields are required for a stable joining process. In order to realise a cost-attractive lightweight construction, a process for mixed joints of high-strength and ultra-high-strength steels with aluminium sheets is to be developed in which simple, cost-effective joining elements and short flanges can be implemented. In addition, this process should also be flexibly applicable under different production boundary conditions, such as those often required by SMEs and where short flanges < 10 mm are possible. The technology is intended to enable SMEs to use their expertise in resistance welding technology to produce steel-aluminium joints without having to switch to riveting or screwing technology, which would involve considerable investment in plant and staff. The joining elements were made from welding wire and form the material transition from steel to aluminium at the joint level without intermetallic phases impairing the joint properties. The connection can be made by a very short welding process < 30 ms, which introduces so little heat that additional bonding is possible. The joint properties were determined both on suitable test specimens with a wire element and on special multi-point specimens that simulate the complex stresses in the production process and in operation. In addition to users from the component and supplier industry, manufacturers of welding systems, most of which are small and medium-sized, should benefit from the results. Publication: 2021 Authors: Prof. Dr.-Ing. S. Jüttner