Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
- nein (1)
While the increase in use of renewable energy sources is a necessity in times of climate change the use of gas turbines as back-up requires them to be run in a much more flexible manner in order to compensate for side effects like sudden fluctuations of energy generation. The significant changes of stress and temperature levels in turbine blades due to start-up and shut down can cause crack initiation and growth in the blades‘ alloy. The aim of this research project is to identify a model for lifetime prediction for gas turbine components made of a Nickel base superalloy under high temperature with a Focus on stress concentration points such as cooling holes.
The electrospun fibers with micrometer diameter show ribbon-like morphology, the physical features are further studied by Scanning Electron Microscopy (SEM), Fourier Transformed Infrared Spectroscopy (FTIR) and Raman and XPS Spectroscopy is used to study the changes after irradiation. Partial electron beam treatment of the electrospun mats lead to localized fluorescence with a broad band emission. The change in the optical response of the fluorescent area in the mats is measured under different chemical environments to evaluate their suitability as an optical chemical sensor in polar and non polar environments.
Oxidation of a Fe-13Cr alloy under water vapor at 600 °C produced a zone of nano-sized precipitation underneath the outside scale formed by iron oxides and Fe‒Cr spinel. The majority of the spinel layer shows a mixed orientation relationship to the ferritic matrix {100}α || {100}sp & <011>α || <001>sp. However, also the discovered precipitated particles are characterized by the same crystallographic orientation relationship to the respective ferritic parent grain. The habit of the precipitates is best described by a lath morphology with their main axis parallel to <100> of ferrite.
Energy dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD) in an scanning electron microscope (SEM) have been applied to characterize the oxide layer in the micrometer scale. The clearly smaller precipitates were subsequently investigated by transmission electron microscopy (TEM). Specimens have been prepared by focused ion-beam (FIB) milling at an area previously characterized by EBSD. They cover the ferritic base material, but mainly the precipitation zone and the Fe‒Cr spinel layer. Energy filtered selected area diffraction (SAD) in the conventional (C)TEM and high-angle annular darkfield (HAADF) imaging in the scanning (S)TEM mode were employed in the characterization of the specimens.
Evaluation of nano-sized internal oxidation in a Fe-13Cr alloy exposed to water vapor atmosphere
(2016)
The topotactic formation of nano-sized precipitates immediately under the internal oxide layer in a Fe-13Cr alloy exposed to oxidizing water vapor atmosphere was recently reported. The precipitates were identified as lath-shaped Fe-Cr spinel exhibiting a crystallographic orientation relationship with the ferritic matrix. The authors proposed that these precipitates could act as a precursor to the formation of the spinel layer observed in the adjacent part of the oxide scale.The occurrence of internal oxidation in Fe–Cr alloys subjected to similar conditions had been previously identified and directly correlated to the presence of water vapor. In the present work, we attempt to rationalize the processes occurring during the oxidation of this alloy through thermodynamic and kinetic analyses based on the CALPHAD approach, using the geometrical representation of phase equilibria and concepts developed to describe internal oxidation. The influence of water vapor on the mechanism and kinetics of formation of the nano-sized precipitates and its role in the overall oxidation process is also considered.
The investigation of creep behavior of the single crystal superalloy CMSX-4 at 1288°C is important for the understanding of deformations mechanisms under hot isostatic pressing (HIP). This treatment is used to increase the lifetime of single crystal superalloys by reducing the number of cavities. The understanding of the internal processes during this treatment is still limited due to the material being not single-phase at room temperature, the extreme conditions of HIP and the complexity of the material.
The result of predicting the pore shrinkage rate using classical crystal plasticity was not satisfying. A more complex model has been implemented, now taking the heterogeneity of dislocation sources into account. By introducing a dislocation density, one can describe the transport, nucleation and interaction of dislocations.
Commercial grade-1 titanium samples (Ti, 99.5% purity) were treated using three alternative methods, i.e., fs-laser processing in air, thermal heat treatment in an oven, or anodization in an electrochemical bath, all resulting in the formation of differently conditioned superficial oxide layers. The laser processing was carried out by a Ti:sapphire laser (pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz) in a regime of generating laser-induced periodic surface structures (LIPSS). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of several square-millimetres large surface areas covered homogeneously by these nanostructures. The thermal processing in the oven was done at two different temperatures, while the electrochemical anodization was performed at room temperature, aiming to generate different polymorphs of titanium oxide at similar oxide layer thickness. The irradiated surface regions were characterized by optical and scanning electron microscopy, and micro Raman spectroscopy. The tribological performance of the differently treated titanium surfaces was characterized in the regime of mixed friction by reciprocating sliding tests against a sphere of hardened steel in un-additivated paraffin oil and fully formulated engine oil as lubricants. The specific tribological performance of the differently treated surfaces is discussed on the basis of possible physical and chemical mechanisms.
Compared to its intrinsic strength, the practical strength of glasses is substantially
lowered by microscopic flaws. Their sub critical growth from the surface is strongly
affected by ambient humidity.
A new device was designed to enable the observation of sub critical crack growth
(SCCG) in vacuum and controlled atmosphere. It allows measurements in transparent and
semi-transparent materials in double cantilever beam (DCB) geometry. The aim of the
present work is to study the effect of crystals on crack growth.
Analytical model for ball-lensed couplers to calculate the insertion loss for each optical mode. The basis for the calculation are commercial perfluorinated polymer optical (PFGI-POFs) and silica based fibres (SOFs). The modal power distribution for given misalignments are presented as well as the reflection-based losses at all optical interfaces. Additionally, the ball-lensed interconnection between PFGI-POF and SOF was theoretically demonstrated.
The duplex stainless steel 1.4062 (X2CrNiN22-2) is used as alternative material to austenitic stainless steels in the construction industry. The corrosion resistance of welded seams is influenced by the base material, the weld filler material, the welding process and also by the final surface treatment. The scale layer next to the weld seam can be removed by grinding, pickling, electro-polishing or blasting depending on the requested corrosion resistance. Blasted surfaces are often used in the industrial practice due to the faster and cheaper manufacturing process compared to pickled or electro-polished surfaces. Furthermore blasting with corundum-grain is more effective than blasting with glass-beads which also lower the process costs. In recent years, stainless steel surfaces showed an unusually high susceptibility to pitting corrosion after grinding with corundum. For this reason, it is now also questioned critically whether the corrosion resistance is influenced by the applied blasting agent. This question was specifically investigated by comparing grinded, pickled, corundum-grain- and glass-bead-blasted welding seams. Results of the SEM analyses of the blasting agents and the blasted surfaces will be presented and correlated with the results of different corrosion tests (KorroPad-testing and pitting potentials).
Sustainable energy and clean engine fuels receive more and more attention. Petrodiesel could be substituted by biofuels such as biodiesel produced from renewable resources. Biodiesel is derived by transesterification of oils with alcohols for example vegetable oils, animal fats or food waste. Its chemical composition of unsaturated fatty methyl acids causes biodiesels susceptibility to oxidation. Especially, the polymer-biodiesel performance under long-term conditions has been considered only in few researches.
Our study addresses a long-term storage scenario of a polymeric fuel tank containing biodiesel or diesel.
High-density polyethylene (PE-HD) is a widely applied plastic for fuel storage tank applications. But such tanks, filled with diesel or biodiesel, provide excellent environmental conditions for growth of a broad spectrum of fungi and various bacteria1. This can result in fuel contamination, engine plugging or possible (bio-) degradation2. Our research focusses on the initial attachment phase of Bacillus sp. and Pseudomonas aeruginosa isolated from a „dieselpest“2 on two distinctly PE-HD materials, typical for fuel storage tank applications, pre-damaged thermally and by UV-irradiation.
UCST-type thermoresponsive polymers that phase separate from solution upon cooling present a tremendous potential not only in aqueous media where they can be used in drug delivery, diagnostic and microfluidic applications, but also in water/alcohol mixtures, where they can be used for instance in sensing systems for alcohol-soluble drugs. However, only a few thermoresponsive polymers have been reported that present an UCST in a relevant temperature range and “green” solvents such as water or ethanol. In this context, acrylamide-based monomers can be very useful building blocks for designing novel UCST-type polymers because of their hydrophilic nature (with the right side chain) and propensity to form hydrogen bonds.
We want to present our latest results on the copolymer poly(acrylamide-co-acrylonitrile) (P(AAm-co-AN)) that present a UCST in water as well as on two homopolymers based on an acrylamide derivative of 2,6-diaminopyridine, namely poly(N-(6-aminopyridin-2-yl)acrylamide) (PNAPAAm) and poly(N-(6-acetamidopyridin-2-yl)acrylamide) (PNAcAPAAm) that show UCST-type thermoresponsiveness in water/alcohol mixtures.
Our focus for P(AAm-co-AN)) is its aggregation behaviour above and below its phase transition temperature as the size of thermoresponsive polymeric systems is of prime importance for biomedical applications (as size dependent processes take place in the body) and is linked to the optical properties of a material that matter in materials science.
In the case of PNAPAAm and PNAcAPAAm, we focused on the co-solvency/co-non solvency effect on the phase transition temperature in water/alcohol mixture. Indeed, polymers with UCST behavior below 60°C in water/alcohol mixtures are extremely promising for the preparation of “smart” materials for sensing.
Different environmental samples reveal that methanogenic Archaea are part of a multi-species biofilm on corroding metallic structures (Fig. 1). Studies on microbial influenced corrosion (MIC) focus mainly on sulphate reducing Bacteria (SRB), leading to the assumption that they are exclusively responsible for metal corrosion. In fact, methanogenic Archaea are known to be involved in metal corrosion as well (e.g. Methanococcus maripaludis DSM 2067). In some cases SRB and methanogenic Archaea have comparable high corrosion rates. However, the underlying mechanisms causing corrosion are still unknown. The goal of this study is to analyse two environmental isolates (M. maripaludis DSM 2067, M. maripaludis KA1) and two human-related isolates (Methanobrevibacter oralis and Methanobrevibacter smithii) for their ability to deteriorate/transform metals, which are relevant for technical and clinical applications. Moreover, the studies will provide essential information on the interaction mechanisms of human-related Archaea, which are frequently found in peri-implantitis, with dental material such as implants, crowns and bridges leading to their degradation/ transformation.
Influence of corundum grinding processes on the corrosion behaviour of austenitic stainless steels
(2017)
The scientific poster shows by means of corrosion testing in combination with surface analytical methods to which degree corundum grinding influences the corrosion behaviour of stainless steel surfaces. The austenitic stainless steel AISI 304 from a consistent heat was used for all investigations to guarantee the same chemical composition and thus no additional influences caused by the material itself. A wide range of different grinding parameters such as pressure, cooling medium and grain size of the abrasive was varied. Thus, comparison of the corrosion behaviour of different surface states and the evaluation of an optimum grinding process using corundum abrasives were possible. The results will contribute to present discussions and give novel impulses for companies in the metalworking industry.
Conventional methods of crack detection only provide a snapshot of the fatigue evolution at a specific location and in the moment of examination. The crack luminescence method realizes a clear visibility of the occurring cracks in loaded components during ongoing operation. Several different experiments show that due to the sensitive coating even the early stage of the crack formation can be detected what makes the crack luminescence helpful to determine the incipient crack opening behavior depending on load alternation. Due to the emitting of light under UV-radiation the crack gets clearly visible what makes continuous monitoring and automated crack detection possible. This can reduce costs and time needed for maintenance and inspection.
Electroplated magnetostrictive actuator layer enabling the external diagnosis of strain sensors
(2017)
Embedded fibre-opticaloptical strain sensors are suitable for structural health monitoring. In order to validate long-term performance (e.g.sensors integrated in airplane wings or wind power turbine blades) an external diagnostics is required.
A strain-detecting region of the optical fibre, the fibre Bragg grating (FBG, is encased by a magnetostrictive layer system serving as actuator. For the validation of the correct sensor function, an external magnetic field introduces mechanical strain resulting in a defined shift of the Bragg wavelength. The layer system is realized by acombined PVD /ECD process optimized regarding magnetostrictive and mechanical properties. The long -term stability of layer adhesion and actuator function has been verified.
Pharmaceutical cocrystals are homogenous crystalline structures made up of two or more components in a definite stoichiometric ratio, where at least one of the components in the crystal lattice is an active pharmaceutical ingredient (API).1 Pharmaceutical cocrystals have opened the opportunity for engineering solid-state forms designed to have tailored properties to enhance drug product bioavailability and stability, as well a enhance processability of the solid material inputs in drug product manufacture. In this work the cocrystallization of bexarotene, an approved API by the U.S. Food and Drug Administration that belongs to Biopharmaceutics Classification System Class II (low solubility–high permeability), with pyridine carboxamide isomers (picolinamide, nicotinamide and isonicotinamide) was successfully undertaken. The synthesis was achieved by liquid assisted grinding (LAG) and the solids obtained were characterized by differential scanning calorimetry (DSC), infrared spectroscopy (FTIR-ATR), powder X-ray diffraction (XRPD), single crystal X-ray diffraction (SXD), and polarized light thermomicroscopy (PLTM). For bexarotene: picolinamide and bexarotene:isonicotinamide, 1:1 cocrystals were obtained directly from milling. SXD data of bexarotene:isonicotinamide cocrystal reveal acid∙∙∙aromatic nitrogen supramolecular heterosynthon and that the homosynthon amide∙∙∙amide in isonicotinamide is preserved. The bexarotene:nicotinamide mixtures prepared by ball milling give rise to simple binary solid–liquid phase diagram with an eutectic point, well described by the Schröder-van Laar equation. Melt crystallization of the 1:1 mixture gives rise to a cocrystal for which a complex phase behaviour is observed.
1. Introduction
Alumina coatings are one possibility to increase the corrosion resistance, lifetime and application range of thermally loaded steel components, e.g. in modern power plants where the use of the Oxy-fuel technology corrosive fuel gas (H2O-CO2-O2-SO2 at 650 °C) affects the steel parts. In previous investigations the efficacy of protective alumina coatings on steel P 92 under those conditions was demonstrated. A shutdown and re-start of power plants or parts of them causes thermal stresses of the components which can cause detrimental effects like microstructural changes in the steel itself, changes in its oxidation behavior, delamination or microstructural changes in the coating. All those effects can lead to failure of the components, resulting in lifetime reduction.
2. Objectives
As a first step, we concentrate on the influence of thermal cycling tests and observe the impact on the microstructure of the coating and the interface in laboratory air. These investigations will help understanding the processes which occur, show directions of potentially necessary changes of the coating due to improved thermal stress behaviour.
3. Materials & methods
P 92 is a ferritic-martensitic steel, containing 9% Cr which forms protective Cr-oxide-rich scales in dry environments and non-protective ones in water-containing environments. Coupons of P 92, having ground surfaces, were dip-coated via a sol-gel process and subjected to thermal cycling for 500 h (1000 cycles) in laboratory air in a temperature range between room temperature and 660° C. The resulting mass loss was determined by weighing.
Samples for TEM investigations were produced as cross sections normal to the sample surface by FIB preparation (Quanta 3D, (FEI)). The TEM/STEM investigations were performed using a JEM2200FS (JEOL) operated at 200 kV. The microstructure of the coating and the interface after cycling tests was characterized via TEM, HREM, and STEM images, electron diffraction as well as EDX and EFTEM methods.
4. Results
At steep edges in the surface profile the coating was imperfect and cracks have formed during the thermal cycling. Flat surface regions are well-covered. The whole interface region between the steel and the coating shows a dense Cr-oxide-rich zone, which can form protective regions in case of local failure. The Cr-oxide zone is followed by a region of mixed oxides, containing Cr, Mn, Fe, and Al in variable composition, to which a porous δ-Al2O3 zone is joined.
5. Conclusions
• Alumina coatings promote the formation of dense, Cr-rich zones at the interface, which makes the system self-healing.
• These zones are stable during thermal stresses, even in regions with cracked coatings.
• They cause reduction of outward diffusion and mass loss during thermal cycling.
Typical transport packages used in Germany are equipped with wooden impact limiting devices. In this paper we give an overview of the latest status regarding the development of a finite element material model for the crush of spruce wood. Although the crush of wood – mainly in longitudinal direction – is a phenomenon governed by macroscopic fracture and failure of wood fibres we smear fracture and failure mechanisms over the continuous voume. In first step we altered an existing LS-DYNA material model for foams, which considers an ellipse shaped yield surface written in terms of the first two stress invariants. The evolution of the yield surface in the existing model depends on the volumetric strain only. For the use with spruce wood, we modified the existing material model to consider the deviatoric strain for the evolution of the yield surface as well. This is in accordance with the results of crush tests with spruce wood specimens, where the crushing deformation was rather deviatoric for uniaxial stress states and rather volumetric for multiaxial stress states We rate the basic idea of this approach to be reasonable, though other problems exist regarding the shape of the yield surface and the assumption of isotropic material properties. Therefore we developed a new transversal isotropic material model with two main directions, which considers different yield curves according to the multiaxiality of the stress state via a multi-surface yield criterion and a non-associated flow rule. The results show the ability to reproduce the basic strength characteristics of spruce wood. Nevertheless, problems with regularization etc. show that additional investigations are necessary.
Acoustics is an important aspect for large rooms especially in schools, kindergartens and offices. Foam tiles made from melamine resins are often used for acoustic insulation because they show good fire resistance and good thermal insulation properties. But they can emit formaldehyde. In Germany there are cases with a resulting indoor air concentration of higher than 100 µg/m³. In this study samples from two schools and one office were investigated. Additionally, new tiles were purchased and tested to evaluate the general potential as formaldehyde source.