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Institute
Characterization of NiTi Shape Memory Damping Elements designed for Automotive Safety Systems
(2014)
Actuator elements made of NiTi shape memory material are more and more known in industry because of their unique properties. Due to the martensitic phase change, they can revert to their original shape by heating when subjected to an appropriate treatment. This thermal shape memory effect (SME) can show a significant shape change combined with a considerable force. Therefore such elements can be used to solve many technical tasks in the field of actuating elements and mechatronics and will play an increasing role in the next years, especially within the automotive technology, energy management, power, and mechanical engineering as well as medical technology. Beside this thermal SME, these materials also show a mechanical SME, characterized by a superelastic plateau with reversible elongations in the range of 8%. This behavior is based on the building of stress-induced martensite of loaded austenite material at constant temperature and facilitates a lot of applications especially in the medical field. Both SMEs are attended by energy dissipation during the martensitic phase change. This paper describes the first results obtained on different actuator and superelastic NiTi wires concerning their use as damping elements in automotive safety systems. In a first step, the damping behavior of small NiTi wires up to 0.5 mm diameter was examined at testing speeds varying between 0.1 and 50 mm/s upon an adapted tensile testing machine. In order to realize higher testing speeds, a drop impact testing machine was designed, which allows testing speeds up to 4000 mm/s. After introducing this new type of testing machine, the first results of vertical-shock tests of superelastic and electrically activated actuator wires are presented. The characterization of these high dynamic phase change parameters represents the basis for new applications for shape memory damping elements, especially in automotive safety systems.
The corrosion resistance of stainless steels is massively influenced by the condition of their surface. The surface quality includes the topography of the surface, the structure and composition of the passive layer, and the surface near structure of the base material. These factors are influenced by final physical/chemical surface treatments. The presented work shows significantly lower corrosion resistance for mechanical machined specimens than for etched specimens. It also turns out that the rougher the surface, the lower the corrosion resistance gets. However, there is no general finding which shows if blasted or grinded surfaces are more appropriate, but a dependency on process parameters and the characteristics on corrosive exposure in terms of corrosion behavior. The results show that not only the surface roughness Ra has an influence on corrosion behavior but also the shape of peaks and valleys which are evolved by surface treatments. Imperfections in the base material, like sulfidic inclusions lead to a weaker passive layer, respectively, to a decrease of the corrosion resistance. By using special passivating techniques the corrosion resistance of stainless steels can be increased to a higher level in comparison to common passivation.
Investigation of magnetic effects on austenitic stainless steels after low temperature carburization
(2018)
This work aims at investigating the magnetic effects of austenitc stainless steels which can occur after a low temperature carburisation depending on the alloy. Samples were prepared of different alloys and subjected to a multiple low temperature carburisation to obtain different treatment conditions for each alloy. The layer characterisation was carried out by light microscope and also by hardening profiles and shows that the layer develops with each additional treatment cycle. A lattice expansion could be detected in all treated samples by X-ray diffraction. Magnetisability was measured using Feritscope and SQUID measurements. Not all alloys showed magnetisability after treatment. In addition to MFM measurements, experiments with Ferrofluid were also used to visualize the magnetic areas. These studies show that only about half of the formed layer becomes magnetisable and has a domain-like structure.
In Anlehnung an das Tempcore-Verfahren wurde an wärmebehandeltem Stabstahl das Zugverfestigungsverhalten des Kernes, der Außenhaut sowie dem gesamten Stab experimentell und numerisch ermittelt. Es zeigte sich, dass die Dehnungen am Kern und am äußeren Rand gleich sind und der Einfluss des Kerngefüges entscheidend für den Beginn der Einschnürung in der Außenhaut ist. Eine Verbesserung der Eigenschaften des Kerngefüges kann somit die Bruchempfindlichkeit des gesamten Stabes reduzieren.
Thermochemical surface hardening is used to overcome the weak mechanical performance of austenitic and duplex stainless steels. Both low-temperature carburizing and nitrocarburizing can improve the hardness, wear, galling, and cavitation resistance, while maintaining their good corrosion resistance. Therefore, it is crucial to not form chromium-rich precipitates during hardening as these can deteriorate the passivity of the alloy. The hardening parameters, the chemical composition of the steel, and the manufacturing route of a component determine whether precipitates are formed. This article gives an overview of suitable alloys for low-temperature surface hardening and the performance under corrosive loading.
The project aims for the development of a new material system from high tensile stainless steel wires as net material with environmentally compatible antifouling properties for off-shore fish farm cages. Therefore, current net materials from textiles (polyamide) shall be partially replaced by high strength stainless steel in order to have a more environmentally compatible system which meets the more severe mechanical loads (waves, storms, predators (sharks)). With a new antifouling strategy current issues like reduced ecological damage (e.g. due to copper disposal), lower maintenance costs (e.g. cleaning) and reduced durability shall be resolved.
Mechanical properties after stretching testings were calcu-lated and experimentally determined via Tempcore method for bar core, bar surface and whole bar cross section. It was displayed on the base of experiments and imitating simulation that deformation in core and surface areas of a bar are equal and therefore influence of structural parameters in the core area is principally decisive for initiating of neck forming in the surface area. The results showed that resistance to destruction of martensite surface layer has rather less effect on bar properties in general in comparison with previous investigations. It is concluded that improvement of core structure quality can help to lower brittleness of the whole bar. It was also proved that used techniques provide good concordance between the obtained results and experimental data. Therefore, the additivity rule for structural components can be used successfully for determination of whole bar parameters, taking into account thickness of surface layer that can be measured easily using hardness sensor. It will simplify practically quality control of products.