P 918 – Proof of equivalence of novel corrosion protection coatings for steel crash barriers (S 024/10184/11)

Road restraint systems are crucial for traffic safety on roads. Of course, the longevity of products is fundamental to maintain the safety function, therefore a lifetime of at least 25 years is being demanded. For steel guardrails the corrosion protection plays a major role with regard to that. Following the specifications given in Germany by the Technischen Lieferbedingungen für Stahlschutzplanken (TL-SP 99), these parts shall be manufactured from steel by profiling and subsequent hot dip galvanization, according to DIN EN ISO 1461.
Using new corrosion protection coatings according to DIN EN 10346 (continuously hot-dip coated steel products) or DIN 50997 (Zinc-aluminium coatings applied by thin film galvanizing) would offer a number of immediate advantages due to the different coatings or production process (e.g. sustainability advantages, economic advantages and safety-related advantages).
As part of this research project, various corrosion protection coatings for steel crash barriers were analysed and compared in terms of their equivalence.
Natural weathering in-field applications have been installed on two selected highway sections and being evaluated over a period of five years.
The results from magnet inductive coating thickness measurements after five years infield natural weathering show no significant reduction for none of the different corrosion protection coatings. more information

P 1263 – Determination of the Intrinsic Threshold and ist Validation as a Material Parameter (IGF-No 20530 N)

The application of fracture mechanics methods for damage-tolerant component design is state of the art for cyclic loads in the range of the Paris-line. However, relevant from a practical viewpoint are often lower loads in the threshold range of the crack growth curve. Due to the large influence of crack closure effects, the determination thereof is very time-consuming and also error-prone. It is expedient to base the design on effective – i.e. crack closure corrected – crack propagation data. An important material parameter is also the intrinsic threshold, below which no crack propagation is possible.
In this research project, the influence of various test methods and environmental conditions on the determination of the threshold against fatigue crack propagation ΔKth was investigated. Statistically validated recommendations for the experimental determination of the intrinsic threshold value ΔKth,eff were derived. This serves as input parameter of an further elaborated procedure for component design based on effective crack propagation data, determined at a load ratio of R ≈ 0.8. more information

P 1272 – Experimental and numerical investigation of the damping characteristics of adhesive bonded structures under dynamic stress (IGF-No 20080 N)

A large number of structures from areas of steel construction, such as bridge structures, supporting structures for wind turbines and mast and observation tower constructions, as well as from areas of (oscillating) mechanical engineering and automotive construction, are exposed to dynamic excitation during their lifetime. These excitations often result from environmental influences from the the regular operation of a machine or an automobile and lead to structural vibrations in the construction, which result in increased mechanical stresses. Adhesives, based on epoxy resins and polyurethanes can be modified in a way that, in addition to excellent strength, they also offer good damping characteristics. The viscoelastic material behavior of the adhesive results in material damping, which contributes to structural damping. So far, however, it has not been clarified how dynamically stressed structures can be optimized with adhesive bonded joints in order to exploit the advantageous damping properties of viscoelastic adhesives in dynamic coordination. Within the framework of the completed research project, the performance of adhesively bonded joints was therefore investigated and modelled with special consideration of the damping properties. Based on mechanical and dynamic characterisation tests, a 2C-PU and a 2C-EP adhesive were selected for further investigations. In the context of vibration stress, there is a need to investigate the damping behaviour for the application areas of steel construction, automotive construction and vibration machinery and plant construction.
Within the framework of the project, a new methodology was developed to take into account damping adhesive layer properties of dynamically stressed structures. The focus is placed on both experimental and numerical identification and characterisation of the damping properties of bonded joints. The development progress achieved can lead to an increase in efficiency and cost-effectiveness in the areas of steel, automotive as well as vibration machines and plant engineering.
The aim of the research project was to develop a concept for the reliable calculation and assessment of the damping properties of dynamically stressed bonded lap and plug-in joints. more information

P 1291 – Workpiece-side impact on and modelling of tribological conditions within hot stamping ((IGF-No 20339 N)

The occurrence of friction and wear represents a current and pressing challenge in the hot stamping process, as the use of lubricants is not possible due to elevated process temperatures > 850 °C. The tribological conditions in the contact zone between the tool and workpiece are a critical component in the process, which has a significant influence on both the tool life and the quality of the final product. While the previous project P1228 “Influence of tool-side tribological conditions on hot stamping” investigated the factors influencing the hot stamping process on the tool side, previous esearch work has shown that the coating used on the workpiece side has a significant influence on the friction and wear mechanisms that occur.
The objective of the research project is to determine functional correlations between austenitizing parameters and coating formation as well as the tribological application behavior of coating materials during press hardening. By understanding the interrelationships, measures will be determined which increase the overall lifetime of hot stamping tools and thus the economic efficiency of the process. more information

P 1305 – Development of a learning phase transformation and dilatometer model for the virtual process design of press hardening processes (IGF-No 20071 BG)

Hot stamping of ultra-high-strength steels is a well-established process for manufacturing safety-relevant car body parts such as b-pillars or front bumpers. Besides the conventional process route, there are additional variants that aim to manufacture parts with tailored properties. All these processes have in common, that the mechanical properties are mainly influenced by the phase transformations of the sheet material during the processing.
Therefore, a suitable process design of conventional and so-called partial hot stamping processes requires fundamental knowledge about the underlying phase transformation kinetics. As a result, most of the time, it is necessary to conduct extensive and timeconsuming experimental investigations, for example with a deformation dilatometer.
Thus, a physically-based model will be developed in the framework of this research project to reduce the experimental effort required for process design. more information

P 1308 – Improved toughness requirements for high strength steels in pressure vessel applications (IGF-No 19942 N)

The aim of this work was to define new toughness requirements for HSLA steels, based on damage mechanical methods, in order to create the possibility to increasingly apply higher strength steels and to exploit their potential.
In order to achieve this goal, it was first necessary to refine and extend existing models. In this work, particular attention was paid to the aspects of temperaturedependent material parameters and feasibility with respect to nominal parameters.
The model adaptations for the simulation of the nominal parameters refer primarily to the flow curve. An already existing method was chosen to derive flow curves based on yield strength, tensile strength and uniform elongation. In this way, important and easily determined parameters, which are largely given by standards, are used as a basis. The procedure is thus as simple and efficient as possible.
The second important aspect for the damage-mechanical model is the temperature dependence of the model parameters. First, an efficient and precise concept was developed based on the choice of nominal flow curves, which ensures an adjustment of the flow function with temperature. The results obtained are significantly better than those obtained with the model previously available at IEHK. Furthermore, this continuity allowed to keep the ductile material parameters constant with changing temperature.
In addition, an innovative stress state-based method was developed in this work to accurately represent the temperature transition in the Charpy impact test, which is essential for toughness and based on the change of fracture mechanism. The combination of these steps allows the simulation of nominal toughness requirements in Charpy impact tests and their transferability to components. more information

P 1314 – Half Monocoque Structures (IGF-No 20211 N)

(Steel-) Sandwich elements have load-bearing properties that allow loads to be transferred in and out of their plane, in addition to their outstanding space-enclosing function and thermal insulation properties. Single-story structures, for example small warehouses, gas station stores and car washes, can therefore be built with sandwich elements only, dispensing with additional load-bearing frame substructures. There is a lack of verification procedures for this, especially for load transfer, which would enable this economical construction method.
The aim of the research project was to validate and if necessary, further develop existing calculation approaches for load-bearing capacity and serviceability and the detailed solutions for load introduction and load distribution on the basis of experimental investigations. The knowledge gained from the holistic view of the construction method should then serve as the basis for obtaining a building authority approval. more information

P 1338 – Electron beam welding of duplex stainless steels with nickel-based filler material and without post-weld heat treatment (IGF-No 20622 N)

The ferritic-austenitic duplex stainless steels are widely used in several industrial fields such as in plant engineering. Thick-walled (t >12 mm) components made of duplex stainless steels e.g. longitudinally-welded pipes are frequently welded by means of electron beam due to economic and technological reasons. However, the process characteristics of the electron beam welding lead to an unbalanced microstructure which may contain 90 % ferrite. This consequently entails a low toughness and a decreased corrosion resistance of the fusion zone. In order to restore the microstructure, the current way involves a solution treatment subsequent to the welding process. The research project aimed to develop an electron beam welding strategy, which results in a balanced microstructure in the fusion zone. In this way, the cost-intensive heat treatment may be dispensed with. more information

P 1350 – One-sided resistance element welding for the steel-intensive multi material design (IGF-No 20560 N)

Against the backdrop of dwindling resources and the achievement of climate policy targets for reducing CO2 emissions by means of statutory requirements, vehicle manufacturers are striving to significantly reduce the fuel and energy consumption of their new vehicles despite increasing customer demands for safety, comfort, quality and durability. In many cases, the vehicle body has the greatest potential for weight savings. The means of reducing weight can include both material selection and design measures. The use of ultra-high-strength steels or press-hardened steels in body-inwhite construction has increased steadily in recent years. Due to their high strength, ultra-high-strength steels in particular allow the use of thinner sheet thicknesses and thus contribute to weight reduction. In addition to conventional automotive sheet-metal shell construction with ultrahigh-strength steel grades, there is also the concept of formed lightweight construction with hollow sections made from hot-formable steels with high torsional stiffness. In this type of body construction, the joining technology must ensure sufficiently high joint strength with one-sided accessibility to the joint. Due to its high cost-effectiveness, resistance welding is a very widespread joining technique, especially among SMEs from the sheet metal and profile processing sectors.
The research project dealt with the investigation and qualification of resistance element welding (REW) with one-sided accessibility to the joint. In a numerical and experimental analysis of the one-sided welding process, the resulting joints were investigated with regard to their properties and the process limits were determined. For this purpose, the process fundamentals were worked out in such a way that the one-sided joining process and the mechanical and technological properties of the joints were characterized. The development and optimization of the required equipment technology and auxiliary joining elements as well as the determination of significant influencing factors and optimum process parameters enable the user to fully exploit the potentials of single-sided resistance element welding. more information

P 1365 – Modeling of the wear behaviour of coated tools in cold massive forming with experimental and numerical methods by example of flat rolling (IGF-No 20238 N)

Hot and cold forming leads to high tool wear due to thermal and mechanical alternating stresses. One way to reduce wear is to coat the forming tools. However, the effort required for the experimental development of new wear-protective coatings is immense for SMEs, as there is currently no known methodology for quantifying the wear resistance and adhesive strength of metal coatings on forming tools.
The aim of the research project was to develop an experimental and numerical methodology for evaluating wear-resistant coatings on forming tools under the specific process parameters of cold rolling steel strip. To achieve this, process models were built at macro and meso levels to calculate the load spectra, and methods were developed to determine the adhesion strength and fatigue of the coating on the forming tool. The commonly used hard chrome with coating thicknesses of 20 µm and 50 µm and the potentially also useful electroless nickel with a coating thickness of 20 µm were selected as roll coatings. more information

P 1403 – Detection of preload losses in bolting assemblies based on electromechanical impedance spectra (IGF-No 20844 N)

Bolts in HV connections are pretensioned, whereby the operational safety of these bolted connections is significantly influenced by the level of the pretensioning force. However, the generation and preservation of the required pre-tensioning force before and during operational loading is not always sufficiently successful. A loss of pre-tensioning force due to automatic loosening during the operational loading of bolted joints can be caused by two different reasons, namely by loosening because of settling or creep or by automatic loosening of the bolt and/or nut. Pretensioned bolts relax due to settlement of the surfaces of the clamping package (levelling of surface roughness, creep of coating).
Creep occurs when the interfacial pressure of the pressure-loaded surfaces is exceeded. These effects are time-dependent and lead to a reduction of the preload force. However, especially in the case of bolted joints subject to vibrations, failure is also often due to loosening and/or self-loosening during operation. This is equivalent to the complete or partial loss of the pretensioning force, which in many cases results in a fatigue failure of the bolt.
In this research project, a method was developed that enables cost-effective, continuous monitoring of the bolt force, e.g. within the framework of structural health monitoring (SHM). The monitoring of the bolt force is carried out with the help of electromechanical impedance spectra. more information

The research projects were funded by the Federal Ministry of Economic Affairs and Climate Action as part of the "Industrial Collective Research" programme on the basis of a resolution of the German Bundestag and by the Foundation for Steel Application.