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- монокристалл (1)
- рентгеноструктурный анализ (1)
Organisationseinheit der BAM
- 5 Werkstofftechnik (1137)
- 5.1 Mikrostruktur Design und Degradation (338)
- 5.2 Metallische Hochtemperaturwerkstoffe (277)
- 5.4 Multimateriale Fertigungsprozesse (216)
- 5.6 Glas (151)
- 5.3 Polymere Verbundwerkstoffe (131)
- 5.5 Materialmodellierung (115)
- 8 Zerstörungsfreie Prüfung (107)
- 9 Komponentensicherheit (98)
- 7 Bauwerkssicherheit (91)
Paper des Monats
- ja (11)
The hydrogen tightness of high-pressure hydrogen storage is a Basic criterion for long-term storage. The H2 permeation coefficients of epoxy resin and a glass lacquer were determined to enable the geometric optimization of a glass capillary storage. It was found that the curing conditions have no significant influence on the H2 permeation coefficient of resin.
The H2 permeation coefficient of epoxy resin is only about three orders of Magnitude greater than that of borosilicate glass. This suggests that the initial pressure of 700 bar takes about 2.5 years to be halved in capillary array storage. Therefore, a high-pressure hydrogen storage tank based on glass capillaries is ideally suited for long-term storage in mobile applications.
Performing mechanical tests at high temperatures is a nontrivial issue: Compared to room temperature testing, additional phenomena like time-dependent Deformation processes and oxidation effects raise the complexity of the material’s response, while more sophisticated test setups and additional control parameters increase the number of potential sources of error. To a large extent, these complications can be overcome by carefully following all recommendations given in the respective high temperature testing standards, but more comprehensive background information helps to identify points of specific importance in particular test campaigns. In this chapter, an overview is given on general high temperature testing issues like the appropriate choice of experimental equipment and key aspects of temperature measurement. In subsequent sections, the major static and dynamic high temperature test methods are reviewed and their Special features, as compared to testing at room temperature, are highlighted based on example data sets. Influences of specimen size and environmental effects are shortly outlined in a concluding section. In the whole chapter, a focus is set on testing of “classical” metallic high temperature materials, but many considerations are equally valid for testing of intermetallics, composites, and high temperature ceramics.
A novel method to generalize kinetic data of viscous crack healing in glasses is proposed. The method assumes that crack healing progress is proportional to the healing time, t, and indirect proportional to viscosity, n. This way, crack length and crack width data, normalized to the initial crack length and plotted versus t/n, allow to compare crack healing progress for different cracks and healing temperatures in a master curve. Crack healing experiments conducted in this study demonstrate the applicability of this method for a commercial microscope slide glass.
The possibility to produce dense monolithic ceramic parts with additive manufacturing is at the moment restricted to small parts with low wall thickness. Up to now, the additive manufacturing of voluminous ceramic parts is realized by powder bed based processes which, however, generate parts with residual porosity. Via infiltration these parts can be processed to dense parts like for example SiC but this is not possible for all ceramics like for example Si3N4. There is a lack of methods for the additive manufacturing of dense voluminous parts for most ceramics.
We have developed a new additive manufacturing technology, the Laser Induced Slip casting (LIS), based on the layerwise deposition of slurries and their local drying by laser radiation. Laser Induced Slip casting generates ceramic green bodies which can be sintered to dense ceramic components like traditional formed ceramic powder compacts. We will introduce the LIS technology, green bodies and sintered parts will be shown and their microstructure and mechanical properties will be discussed.
Soiling of photovoltaic (PV) systems compromises their performance causing a significant power loss and demanding periodical cleaning actions. This phenomenon raises great concerns in the solar energy field, thus leading to notable research efforts over the last decades. Soiling is caused by a dual action of dust deposition and biofouling. However, surprisingly, the microbiological contribution to PV soiling is often overlooked or underestimated. In this study, a variety of qPCR-based methods have been developed to quantify the microbial load of fungi, bacteria and phototrophs on PV panels. These protocols were evaluated by comparison with culturedependent methods, and were implemented with real solar plants for two years. The results show that the developed molecular methods are highly sensitive and reliable to monitor the microbial component of the soiling. Fungal biomass was clearly dominant in all analysed PV modules, while bacteria and phototrophs showed much lower abundance. Light microscopy and qPCR results revealed that melanised microcolonial fungi and phototrophs are the main biofilm-forming microorganisms on the studied solar panels. In particular, the fungal qPCR protocol is proposed as a useful tool for monitoring of PV soiling, and investigating the microbial contribution to specific soiling cases.
Helical springs with a rectangular cross-section have been machined from sintered and grinded hollow cylinders with high geometrical precision and good reproducibility. Such springs made from tetragonal zirconia polycrystal (TZP) ceramic show excellent edge quality because of high fracture toughness and bending strength of the starting material. Hence, springs with desired geometric dimension and tailored spring constant can be manufactured for highly demanding applications at high temperatures and in harsh environments.
Prior to any practical use, application limits of springs under mechanical and thermal load have to be analyzed. Therefore, different displacement experiments were carried out on the helical TZP springs.
- Dynamic displacement tests at various temperatures from -15°C to +60°C using a piezo actor to load/unload springs with frequencies between 1 and 40 Hz: Springs remained undamaged and the spring constants were not altered, even after more than one million cycles of compression loading.
- Long-time displacement measurements under static tensile loading at room temperature with a high-precision interferometer test facility: Significant spring elongation under constant strain was surprisingly proved over a period of many hours already at room temperature.
- Creeping experiments for 48 h under static compression load at different temperatures up to 1000 °C: After cooling down and load removing no permanent length reduction of springs was observed for test temperatures up to 700 °C. However, reshaping of TZP springs by plastic deformation is possible at higher temperatures and opens up additional possibilities for spring design and manufacturing.
The present work provides a comparative study on the interface and adhesion properties of surface modified single glass fibers embedded in an acrylate matrix. To facilitate a covalent bonding at the fibermatrix interface, the fibers are functionalized with selected organosilanes that comprise either passive (unsaturated C¼C bonds of methacrylate moieties) or photoactive functionalities (photocleavable bis(acyl)phosphane oxide groups). Immobilization of the functional silanes is carried out by a classic silanization reaction involving a condensation reaction across the surface hydroxyl groups of the inorganic glass fibers. The change of the physico-chemical properties of the fibers due to desizing and subsequent surface modification is monitored by X-ray photoelectron spectroscopy and zeta potential measurements. In addition, scanning electron microscopy is used to follow the changes in surface morphology. After the modification step, the desized and modified single fibers are embedded in a photocurable acrylate resin formulation. By performing single fiber pull-out tests, maximum pull-out force, friction strength and apparent interfacial shear strength are determined as a function of the coupled silanes. The results reveal that the attached organosilanes lead to a significant increase in adhesion strength, whilst the performance of the photo-cleavable organosilane is superior to the passive methacryl-functional derivative.
In diesem Forschungsvorhaben wurde ein Verfahren entwickelt, um die Genauigkeit der Lebensdauerbewertung für Abgasturbolader (ATL)-Verdichterräder aus der ausscheidungs-härtbaren Aluminiumlegierung EN AW-2618A zu verbessern. Dies umfasst insbesondere die Berücksichtigung der Werkstoffalterung, mit der beim Einsatz dieser Bauteile unter entsprechender Betriebsbeanspruchung (Temperatur, Zeit, Spannung) gerechnet werden muss. Zunächst wurde eine solide experimentelle Datenbasis für den untersuchten Werkstoff geschaffen. Dies umfasste sowohl eine mechanische als auch eine mikrostrukturelle Charakterisierung des Ausgangszustands (T61) sowie von ausgelagerten Zuständen bis zu 25.000 h bei betriebsrelevanten Temperaturen (160 °C, 180 °C, 190 °C). Bei den Untersuchungen zur Mikrostruktur wurde der Fokus auf die Entwicklung der Radien der stäbchen-förmigen aushärtenden Phase gelegt, die während der Alterung vergröbern. Der Zusammenhang zwischen Auslagerungstemperatur, -zeit und ggfs. mechanischer Belastung wurde erstmals in diesem Umfang quantifiziert und entsprechende Gesetzmäßigkeiten zur Beschreibung der Vergröberung ermittelt. Vergleichend dazu wurde der Einfluss der Alterung auf die Härte, die Festigkeit, das Kriechverhalten und die Ermüdungslebensdauer (LCF, TMF) bestimmt, so dass eine Korrelation zur Mikrostruktur vorliegt.
Die Ergebnisse der Kriechversuche wurden zur Kalibrierung eines modifizierten Sinus-hyperbolicus-Kriechmodells verwendet. Ein zeit- und temperaturabhängiges Verformungsmodell nach Chaboche, welches die wesentlichen Phänomene der Hochtemperaturverformung und der Wechselplastizität beschreibt, wurde erweitert und berücksichtigt nun die Werkstoffalterung, indem die Festigkeit nicht nur als Funktion der Temperatur, sondern auch des Alterungszustands (d. h. des mittleren Radius der Ausscheidungen) dargestellt wird. Für die Erweiterung des Modells zur Bewertung der Ermüdungslebensdauer wurde analog verfahren. Die berechnete Lebensdauer der im Verlauf des Vorhabens durchgeführten LCF- und TMF-Versuche stimmt gut mit den experimentellen Ergebnissen überein. Lediglich ein LCF-Versuch mit einer Haltezeit von 900 s im Zug weicht deutlich vom Mittelwert ab und zeigt die Grenzen des zeitunabhängigen Schädigungsmodells auf. Versuche mit kürzeren Haltezeiten bis zu 60 s liegen im Streuband, ebenso die verschiedenen Alterungszustände. Eine Schädigungsakkumulation unter Berücksichtigung der fortschreitenden Alterung über der Lebensdauer wurde entwickelt und implementiert. Neben der Berücksichtigung der Alterung in der Verformungs- und Lebensdaueranalyse besteht auch die Möglichkeit, komplexe Belastungszyklen in einem Postprozessing zu bewerten, indem Rainflow-Klassierungen typischer komplexer Belastungszyklen vorgenommen werden.
Die Methodik und die Modelle wurden in die Finite-Elemente-Programme Abaqus und Ansys implementiert und stehen für die rechnerische Bewertung von Bauteilen zur Verfügung.
Cyclic fatigue behavior of glass fiber reinforced epoxy resin at ambient and elevated temperatures
(2018)
The fatigue behavior of ±45° glass fiber reinforced epoxy resin under cyclic mechanical and constant thermal loading is investigated in this study. Tests at three different temperature levels in the range 296 K to 343 K have been performed in order to create S-N curves for each temperature level. The specimen damage is measured in-situ using optical grayscale analysis. The characteristic damage state (CDS) is evaluated for each specimen. It is shown that the point of CDS is suitable as a failure criterion to compare the resulting S-N curves. With micromechanical formulations, the temperature-dependent matrix effort is calculated for each stress-temperature level. In terms of matrix effort, the longest fatigue life is reached at high temperatures, while, in terms of stress, the lowest fatigue life is reached at the highest temperatures.
Cyclic fatigue behavior of glass fiber reinforced epoxy resin at ambient and elevated temperatures
(2018)
The fatigue behavior of ±45° glass fiber reinforced epoxy resin under cyclic mechanical and constant thermal loading is investigated in this study. Tests at three different temperature levels in the range 296 K to 343 K have been performed in order to create S-N curves for each temperature level. The specimen damage is measured in-situ using optical grayscale analysis. The characteristic damage state (CDS) is evaluated for each specimen. It is shown that the point of CDS is suitable as a failure criterion to compare the resulting S-N curves. With micromechanical formulations, the temperature-dependent matrix effort is calculated for each stress-temperature level. In terms of matrix effort, the longest fatigue life is reached at high temperatures, while, in terms of stress, the lowest fatigue life is reached at the highest temperatures.
3D structural investigations are described by X-ray laminography studies of sandwich shell segments, made of a PVC foam core, covered by non-crimp fabric glass fibre composite lay-ups processed by vacuum assisted resin infusion of epoxy. The specific scope of this study is to image transversal flaws within the foam core (joints) and of single ply overlaps. Test flaws were purposely implemented in order to simulate typical failure under cyclic load. In a dedicated test rig for shell structures, the flaw evolution/propagation is monitored by thermography and optical 3D inspection of deformation. Due to the unfavourable preconditions for classical computed tomography as of large aspect ratio, the samples were investigated by coplanar translational laminography. Its limited range of observation angles of ± 45°, results in anisotropic artefacts about the normal to the sample surface, but the typical flaws are well visualized in the as-prepared state, in a state of early damage, and in the repaired state.
Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in the bulk of IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Um den sicheren Betrieb von Windkraftanlagen zu gewährleisten, werden Rotorblätter nach der Fertigung und nach Reparatur mit verschiedenen zerstörungsfreien Verfahren auf Schaden untersucht. Bei der Fertigung von Blattschalen, die in einer Sandwich-Schalenbauweise gefertigt werden, entstehen Imperfektionen, die unter Belastung zu großen Rissen führen können. Zur Prüfung von Blattschalen wird unter anderem Ultraschallprüfung in Kontakttechnik eingesetzt. Um den Wartungsaufwand von Ultraschallanlagen zu reduzieren und um manche Kompositstrukturen vor Koppelmittel zu schützen, wird die Anwendung von Luftultraschall erforscht. Insbesondere große Fortschritte gibt es im Bereich der Entwicklung neuer Wandler.
In diesem Beitrag berichten wir über die Luftultraschallprüfung von Schalen für Rotorblätter. Es wurden typische Sandwichschalenstrukturen hergestellt und mit einem eigenentwickelten Schalenprüfstand unter simulierter Betriebsbeanspruchung belastet. Die in den Schalenprüfkörpern entstandenen Schäden wurden außerhalb des Prüfstandes mit Luftultraschall in Durchschallung untersucht. Es wurden fokussierende Wandler auf Basis von Ferroelektreten entwickelt und für diese Prüfungen eingesetzt. Mit zusätzlicher elektrischer Vorspannung konnte die Empfindlichkeit der Empfänger deutlich erhöht werden. Die nach dem Lastwechsel entstandenen Schäden in den Rotorblattschalen waren eindeutig zu detektieren.
Um den sicheren Betrieb von Windkraftanlagen zu gewährleisten, werden Rotorblätter nach der Fertigung und nach Reparatur mit verschiedenen zerstörungsfreien Verfahren auf Schaden untersucht. Bei der Fertigung von Blattschalen, die in einer Sandwich-Schalenbauweise gefertigt werden, entstehen Imperfektionen, die unter Belastung zu großen Rissen führen können. Zur Prüfung von Blattschalen wird unter anderem Ultraschallprüfung in Kontakttechnik eingesetzt. Um den Wartungsaufwand von Ultraschallanlagen zu reduzieren und um manche Kompositstrukturen vor Koppelmittel zu schützen, wird die Anwendung von Luftultraschall erforscht. Insbesondere große Fortschritte gibt es im Bereich der Entwicklung neuer Wandler.
In diesem Beitrag berichten wir über die Luftultraschallprüfung von Schalen für Rotorblätter. Es wurden typische Sandwichschalenstrukturen hergestellt und mit einem eigenentwickelten Schalenprüfstand unter simulierter Betriebsbeanspruchung belastet. Die in den Schalenprüfkörpern entstandenen Schäden wurden außerhalb des Prüfstandes mit Luftultraschall in Durchschallung untersucht. Es wurden fokussierende Wandler auf Basis von Ferroelektreten entwickelt und für diese Prüfungen eingesetzt. Mit zusätzlicher elektrischer Vorspannung konnte die Empfindlichkeit der Empfänger deutlich erhöht werden. Die nach dem Lastwechsel entstandenen Schäden in den Rotorblattschalen waren eindeutig zu detektieren.
3D structural investigations are described by X-ray laminography studies of sandwich shell segments, made of a PVC foam core, covered by non-crimp fabric glass fibre composite lay-ups processed by vacuum assisted resin infusion of epoxy. The specific scope of this study is to image transversal flaws within the foam core (joints) and of single ply overlaps. Test flaws were purposely implemented in order to simulate typical failure under cyclic load. In a dedicated test rig for shell structures, the flaw evolution/propagation is monitored by thermography and optical 3D inspection of deformation. Due to the unfavourable preconditions for classical computed tomography as of large aspect ratio, the samples were investigated by coplanar translational laminography. Its limited range of observation angles of ± 45°, results in anisotropic artefacts about the normal to the sample surface, but the typical flaws are well visualized in the as-prepared state, in a state of early damage, and in the repaired state.
Motivation:
> Zunehmender Einsatz von Pulveraktivkohle (PAK) zur
Spurenstoffelimination in kommunalen Kläranlagen
> Bisher keine analytische Methode zur Quantifizierung von PAK in
Belebungsbecken, bzw. in Gegenwart von Belebtschlamm
> Anwendung der Thermogravimetrischen Analyse (TGA) zur
Feststoffcharakterisierung auf Umweltfragestellungen
Ergebnisse:
> Quantifizierung von PAK in Belebtschlamm ist möglich und kann
über unterschiedliche zweistufige TGA-Methoden erfolgen
> Zersetzungsgasanalyse zeigt veränderte und verzögerte
Freisetzung aliphatischer Produkte des Belebtschlamms
Advanced wastewater treatment with powdered activated carbon (PAC) leads to a spread of PAC into different purification stages of wastewater treatment plants (WWTP) due to recirculation and filter back-wash. Currently, no methods for quantification of PAC in activated sludge are available. In this study, PAC containing activated sludge from four WWTP were examined by two-step thermogravimetric analysis (TGA) with heating up to 600°C in N2 and subsequently in synthetic air. Direct quantification of PAC according to temperature specific weight losses was possible for one WWTP. Quantification by combining specific mass losses was found to be an alternative direct method, with a detection limit of 1.2% PAC in dry sample mass.
Additionally, evolved gas analysis (EGA) by infrared-spectroscopy (FTIR) during TGA revealed interaction mechanisms between PAC and activated sludge. Aliphatic compounds from activated sludge were identified as major substances influenced by PAC. In derivative thermogravimetry (DTG), a typical double peak at approximately 300°C was found to be related to carbonylic species with increased evolution of acetic acid in aged activated sludge. TGA and EGA are promising tools to understand, control and optimise the application of PAC in advanced wastewater treatment.
The presence, fate and effects of microplastics (MP) in terrestrial systems are largely unknown. The few existing studies investigated either agricultural or industrial sites. Several techniques were used for analysis, primarly spectroscopic methods such as FTIR or Raman. Sample pretreatments like density separations are common to reduce matrix. A lack of harmonised and standardised sampling instructions for microplastic investigations in the terrestrial area was identified as particular critical, because different studies are barely comparable.
The aim of the project is to develop a proposal for a harmonized procedure for sampling, sample preparation and the detection of microplastics in terrestrial matrices for total content determination. By detecting specific degradation products the thermal extraction desorption gas chromatography mass spectrometry (TED-GC-MS) allows a direct determination of mass content of MP in environmental samples.
Powder based Additive Manufacturing (AM) processes are widely used for metallic and polymeric materials, but rarely commercially used for ceramic materials, especially for technical ceramics. This seemingly contradicting observation is explained by the fact that in powder based AM, a dry flowable powder needs to be used. Technical ceramics powders are in fact typically very fine and poorly flowable, which makes them not suitable for AM. The layerwise slurry deposition (LSD) is an innovative process for the deposition of powder layers with a high packing density for powder based AM. In the LSD process, a ceramic slurry is deposited to form thin powder layers, rather than using a dry powder This allows the use of fine powders and achieves high packing density (55-60%) in the layers after drying. When coupled with a printing head or with a laser source, the LSD enables novel AM technologies which are similar to *Denotes Presenter 42nd International Conference & Exposition on Advanced Ceramics & Composites 127 Abstracts the 3D printing or selective laser sintering, but taking advantage of having a highly dense powder bed. The LSD -3D printing, in particular, offers the potential of producing large (> 100 mm) and high quality ceramic parts, with microstructure and properties similar to traditional processing. This presentation will give an overview of the milestones in the development of this technology, with focus on the latest results applied both to silicate and to technical ceramics.
The knowledge of the temperature-induced changes of the superconductor volume and of the thermomechanical behavior of the different coil and tooling materials is required for predicting the coil geometry and the stress distribution in the coil after the Nb3Sn reaction heat treatment. In this paper, we have measured the Young’s and shear moduli of the HL-LHC 11 T Nb3Sn dipole magnet coil and reaction tool constituents during in situ heat cycles with the dynamic resonance method. The thermal expansion behaviors of the coil components and of a free standing Nb3Sn wire were compared based on dilation experiments.
A glass of the composition 37BaO·16CaO·47SiO2 wt% produced on an industrial scale is crystallized at 970 °C for times ranging from 15 min to 2 h. The crystallization at the immediate surface as well as the crystal growth into the bulk are analyzed using scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD) as well as X-ray diffraction in the Θ–2Θ setup (XRD). The immediate surface shows the oriented nucleation of walstromite as well as the formation of wollastonite and an unknown phase of the composition BaCaSi3O8. All three phases also grow into the bulk where walstromite ultimately dominates the kinetic selection and grows throughout the bulk due to a lack of bulk nucleation. Walstromite shows systematic orientation changes as well as twinning during growth. A critical analysis of the XRD-patterns acquired from various crystallized samples indicates that their evaluation is problematic and that phases detected by XRD in this system should be verified by another method such as EDXS.
An improved diffusion model is proposed for pore annihilation during HIP of single-crystal nickel-base superalloys. The model assumes the pore dissolution by emission of vacancies and their sink to the low angle boundaries. Calculation, considering distribution of the pore sizes, predicts the kinetics of pore annihilation similar to the experimental one.
Sand erosion of solar glass: Specific energy uptake, total transmittance, and module efficiency
(2018)
Surface roughness, R Z , normal transmittance, Τ N , total transmittance, Τ T , and photovoltaic (PV) module efficiency, η S , were measured for commercial solar glass plates and PV test modules identically sandblasted with different loads of quartz sand (200 – 400 μ m), impact inclination angles, and sand particle speed. Measured data are presented versus the specific energy uptake during sand blasting, E (J/m2). Cracks, adhering particles, and scratch ‐ like textures probably caused by plastic flow phenomena could be observed after sand blasting. Their characteristic size was much smaller than that of sand particles.
After blasting and subsequent cleaning, the glass surface was still covered with adhering glass particles. These particles, cracks, and scratch ‐ like textures could not be removed by cleaning. For sand blasting with α = 30° inclination angle and E = 30 000 J/m2, normal transmittance, total transmittance, and relative module efficiency decreased by 29%, 2% and ∽ 2%, respectively. This finding indicates that diffusive transmission of light substantially contributes to PV module efficiency and that the module efficiency decrease caused by sand erosion can be better estimated from total than by normal transmittance measurements.
A study was made on the effect of creep loading on the precipitate radii evolution of the aluminum alloy 2618A.
The overageing process of the alloy was investigated under load at a temperature of 190 °C with stresses between 79 and 181 MPa and compared to stress free isothermal ageing. The precipitates responsible for strength were characterized using dark-field transmission electron microscopy (DFTEM). This allows the experimental Determination of radii distributions of the rod-shaped Al2CuMg precipitates and the evaluation regarding their mean precipitate radius. It was found that the mean precipitate radius enables the comparison of the different microstructural conditions of crept and uncrept samples. The mean precipitate radii of the samples experiencing creep are significantly higher than those of undeformed samples. It was shown that the acquired radii distributions are viable to determine averaged particle radii for comparison of the aged samples. A ripening process including pipe diffusion along dislocations describes the data on coarsening very well for the creep samples.
Dry pressing of ceramic materials requires homogeneously soft granules with good flowability to allow rapid die filling and to avoid packing defects. Spray-drying granulation appears to be the best method for obtaining granules with high flowability in industrial scale. But, strength reducing internal microstructural defects caused by spray-dried granules with hollow and hard shells are often observed using nano and/or multi-component starting powders.
Using the example of a ZTA composite, the potential of slurry optimization, ultrasound atomization and infrared drying for better granule properties and compaction behavior were investigated. Starting granules produced in a conventional spray dryer (Niro, Denmark) with a two fluid nozzle showed typical defects like large central pores and dimples.
The early step of slurry preparation already possesses an essential optimization possibility in the form of stability adjustments. Granule compaction was clearly improved upon a specific reduction in slurry stability. The second optimization opportunity to improve the granule quality was the atomization step. Implementation of an ultrasound atomizing unit into the conventional spray dryer positively affected granule size distribution and therefor flowability and as well granule yield. But, a combination of both process optimizations delivered the best sinter bodies with highest density and strength due to further reduction in maximum size and fraction of pores.
As last step of a spray drying process, the drying is the focus of further investigations. A current setup implying a spray dryer prototype utilizes stacked infrared heater in a countercurrent setup delivering a further increase in granule yield and enduring spraying process stability.
As the well-known damage mechanisms slow crack growth (SCG) and environmental stress cracking (ESC) are the major causes for possible failure of polyolefin-based materials, especially for PE-HD, they are highly relevant and need to be considered thoroughly. Furthermore, due to slight but perceptible differences in damaging effect, a differentiation between SCG and ESC is expedient. SCG appears in “inert” or “neutral” media without a decisive influence of the surrounding medium whereas ESC occurs in “active” media, which influence the failure behavior and time to failure crucially.
To characterize the inherent resistance of the material against those damage mechanisms, the well-established Full-Notch Creep Test (FNCT) is used. In this study, the FNCT – usually applied according to ISO 16770 [3] using a few universal model liquid media and mainly for pipe materials – is extended by investigations with appropriate parameters of selected relevant PE-HD container materials also in real media, such as the topical fuels diesel and biodiesel. The investigations were performed using a novel FNCT-device with 12 individual sub-stations, each equipped with individual electronic stress and temperature control and continuous online monitoring of the specimen elongation.
Especially, mechanical stress and temperature were varied systematically during FNCT and time to failure values, time-dependent elongation data as well as detailed fracture surface analysis by laser scanning microscopy (LSM) were combined for the first time (Fig. 1). Particularly, the fracture surface analysis provides a sound basis to characterize failure behavior, mainly regarding the balance between brittle crack propagation and ductile deformation. Therefore, fracture surface analysis is an essential tool for a decent assessment of SCG and ESC by FNCT measurements.
Die Mikrostruktur von ausscheidungsgehärteten Aluminiumlegierungen ist von herausragender Bedeutung für ihre Festigkeit, da nur Werkstoffe mit gezielt eingestellter Mikrostruktur für Radialverdichterräder ausreichende Festigkeiten erreichen. Diese optimierte Mikrostruktur ändert sich jedoch während des Betriebs, denn die Bauteile werden bei Temperaturen eingesetzt, die nahe der Aushärtetemperatur liegen oder sogar darüber hinausgehen und folglich ist mit Alterung der Mikrostruktur und damit einhergehender Degradation der Eigenschaften zu rechnen. Diese Effekte konnten bisher in der Lebensdauervorhersage nicht berücksichtigt werden, da ihre diesbezüglichen Auswirkungen lediglich qualitativ bekannt waren. In diesem Forschungsvorhaben wurde daher für die Legierung EN AW-2618A einerseits eine sorgfältige und umfassende Charakterisierung der Gefügeentwicklung für anwendungsrelevante Temperaturen und Zeiten bis zu 25.000 h vorgenommen und andererseits eine solide Datenbasis bezüglich der mechanischen Eigenschaften (Zugfestigkeit, Kriechwiderstand, LCF-, TMF-Verhalten) und ihren Änderungen geschaffen. Darauf aufbauend wurde ein Verfahren entwickelt, um die Genauigkeit der Lebensdauerbewertung für Abgasturbolader (ATL)-Verdichterräder aus EN AW-2618A zu verbessern. Ein zeit- und temperaturabhängiges Verformungsmodell nach Chaboche, welches die wesentlichen Phänomene der Hochtemperaturverformung und der Wechselplastizität beschreibt, wurde erweitert und berücksichtigt nun die Werkstoffalterung, indem die Festigkeit nicht nur als Funktion der Temperatur, sondern auch des Alterungszustands (d. h. des mittleren Radius der Ausscheidungen) dargestellt wird. Für die Erweiterung des Modells zur Bewertung der Ermüdungslebensdauer unter Rainflow-klassierten Betriebslastkollektiven wurde analog verfahren. Die berechnete Lebensdauer der im Verlauf des Vorhabens durchgeführten LCF- und TMF-Versuche stimmt gut mit den experimentellen Ergebnissen überein.
Dielectric strength testing of ceramics is simple and yet challenging. The execution of a breakthrough voltage measurement of a given sample is fast and straightforward. ASTM D149 describes the standardized procedure. But, there are versatile effects of test conditions and sample properties that affect the result of such a measurement. As one example, ASTM D149 allows different sizes of test electrodes and does not unambiguously prescribe the condition of the electrodes. Thus, different electrode configurations are used in the field. We conducted several test series on alumina samples to comprehensively quantify the effect of test conditions and sample properties on dielectric strength results. In our study, testing of alumina substrates using different electrode configurations resulted in differences of mean values of up to 20%. Further test series on alumina focused on the effect of voltage ramp rate. The results are complemented by calculations of failure probability at different voltage levels and corresponding withstand voltage tests. We conclude that a communication and comparison of single dielectric strength values is insufficient and may be misleading. A meaningful comparison of dielectric strength studies from different sources requires a thorough consideration of test conditions.
Influence of deposition hatch length on residual stress in selective laser melted Inconel 718
(2018)
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
Up to now, the mechanisms of surface nucleation and surface-induced texture formation are far from being understood. Corresponding phenomena are discussed hypothetically or even controversial, and related studies are restricted to very few glasses. In this talk the state of the art on mechanisms of surface nucleation are summarized. On one hand, mechanical damaged surfaces show high nucleation activity, at which the nucleation occurs at convex tips and edges preferentially. On the other hand, solid foreign particles are dominant nucleation sites at low damaged surfaces. They enable nucleation at temperatures even far above Tg. The nucleation activity of the particles is substantially controlled by their thermal and chemical durability. But no systematic studies on initially oriented crystal growth or nucleation from defined active nucleation sites have been pursued, so far. Therefore, the main objective of a just started project is to advance the basic understanding of the mechanisms of surface-induced microstructure formation in glass ceramics. We shall answer the question whether preferred orientation of surface crystals is the result of oriented nucleation or caused by other orientation selection mechanisms acting during early crystal growth. In both cases, crystal orientation may be caused by the orientation of the glass surface itself or the anisotropy and orientation of active surface nucleation defects. As a first attempt we focused on possible reorientation of separately growing surface crystals during early crystal growth. First results show clear evidence that separately growing crystals can reorient themselves as they are going to impinge each other.
Sintered bioactive glass scaffolds of defined shape and porosity, e.g. made via additive manufacturing, must provide sufficient bioactivity and sinterability. As higher bioactivity is often linked to high corrosion and crystallization tendency, a certain compromise between sintering ability and bioactivity is therefore required. Groh et al. developed a fluoride-containing bioactive glass (F3), which allows fiber drawing and shows a bioactivity well comparable to that of Bioglass®45S5.
To study whether and to what extent the sinterability of F3 glass powder is controlled by particle size, coarse and fine F3 glass powders (300-310µm and 0-32µm) were prepared by crushing, sieving and milling. Sintering, degassing and phase transformation during heating were studied with heating microscopy, vacuum hot extraction (VHE), DTA, XRD, and SEM.
For the coarse glass powder, sintering proceeds slowly and is limited by surface crystallization of primary Na2CaSi2O6 crystals. Although the crystallization onset of Na2CaSi2O6 is shifted to lower temperature, full densification is attained for the fine powder. This finding indicate that certain porosity might be tuned via particle size variation. Above 900°C, intensive foaming is evident for the fine powder. VHE studies revealed that carbon species are the main foaming source.
Borosilicate glasses (16Na2O–10B2O3–74SiO2, NBS) with water contents up to 22 mol% H2O were prepared to study the effect of water on structural relaxation using DTA, viscometry and internal friction measurements. The results show that the glass transition temperature Tg of DTA and the isokom temperature T12, of viscometry are in excellent agreement, confirming the equivalence of enthalpy and viscous relaxation for NBS glass. Combining Tg data with water speciation data demonstrates that OH groups are mainly responsible for the decrease of Tg with increasing hydration, while molecular water plays only a minor role. Internal friction spectra at 7.125 Hz confirm the decisive influence of water on mechanical relaxation. The temperature range of α-relaxation (glass transition) strongly decreases while two β-relaxation peaks (sub-Tg) progressively appear with increasing water content. A high temperature β-relaxation peak, attributed to the presence of OH groups, shifts from 670 to 450 K as total water content increases from 0.01 to 5 wt%. A low temperature β-relaxation peak, attributed to molecular water, appears at 380 K and 330 K in glasses containing 3 and 5 wt% H2O, respectively. These findings suggest that relaxation mechanism of different hydrous species at low temperature may contribute to fatigue of stressed glasses.
Les aubes de turbines à gaz utilisées en particulier pour les turboréacteurs de l’aéronautique sont élaborées par fonderie en superalliage monocristallin à base de nickel. Le procédé de fonderie, ainsi que les traitements thermique d’homogénéisation réalisés à très haute température, induisent la présence de pores au sein des pièces qui affectent les propriétés mécaniques et la durée de vie des aubes. Afin de réduire cette porosité les motoristes effectuent un traitement de compression isostatique à chaud (CIC) au cours duquel la porosité diminue par fermeture des pores. Afin de mieux comprendre les mécanismes impliqués au cours du traitement de CIC, nous avons lancé un programme de recherche dans le cadre du projet ERA-Net MICROPORE. La modélisation par champ de phase des mécanismes en jeu est présentée au cours de ce colloque. Nous présentons dans cette affiche un des volets de la caractérisation expérimentale du projet.
Des échantillons de superalliage CMSX4 sont observés après traitement de mise en solution et CIC sous 103 MPa à 1288°C pour différentes durées. Les pores présents sont caractérisés par microscopie électronique à balayage (MEB) afin de suivre l’évolution du taux de porosité au cours du traitement. Une caractérisation plus détaillée de pores partiellement refermés est menée par MEB et grâce à la diffraction des électrons rétrodiffusés (EBSD). Une vision tridimensionnelle de ces défauts est obtenue par des coupes métallographiques effectuées par découpe ionique (FIB).
Le projet ERA – Net MICROPORE est financé en Allemagne par la DFG (projects EP 136/1-1 and FE933/2-1) et en France par l’ANR (projects ANR15-MERA-000-03 and ANR15-MERA-0003-04).
In the present study, microstructure and texture development during single and multiple pass friction stir processing (FSP) of a strain hardenable wrought AlMg alloy (AA5086) was investigated. Subtle differences were observed while comparing with heat treatable alloys in the nucleation mechanism of the recrystallized microstructure observed in the nugget zone. Strain induced boundary migration was the dominant mechanism of microstructure evolution in the alloy, which influenced the crystallographic texture development by weakening it. Micro-texture measurements reveal variations in the crystallographic texture along the thickness of the sample. Recrystallization texture components were observed in the nugget zone indicative of a pronounced static recrystallization in the alloy as compared to the heat treatable alloys. Bulk texture measurements within the nugget zone of the optimally processed sample reveal a relatively dominant C component of shear texture. Average grain size in the nugget zone remained the same and the bulk crystallographic texture components were retained during multiple-pass FSP. The lower strain energies involved and the enhanced recovery processes due to the high temperature materials processing of the alloy during FSP resulted in a stable microstructure and texture. In summary, FSP could be promoted as a competent and suitable secondary processing technique for the bulk production of ultra-fine-grained materials in strain hardenable aluminium alloys.
Tooth wear induced by abrasive particles is a key process affecting dental function and life expectancy in mammals. Abrasive particles may be plant endogenous opal phytoliths, exogene wind-blown quartz dust or rain borne mineral particles ingested by mammals. Nano-indentation hardness of abrasive particles and dental tissues is a significant yet not fully established parameter of this tribological system. We provide consistent nano-indentation hardness data for some of the major antagonists in the dental tribosystem (tooth enamel, tooth dentine and opaline phytoliths from silica controlled cultivation). All indentation data were gathered from native tissues under stable and controlled conditions and thus maximize comparability to natural systems. Here we show that native (hydrated) wild boar enamel exceeds any hardness measures known for dry herbivore tooth enamel by at least 3 GPa. The native tooth enamel is not necessarily softer then environmental quartz grit, although there is little overlap. The native hardness of the tooth enamel exceeds that of any silica phytolith hardness recently published. Further, we find that native reed phytoliths equal native suine dentine in hardness, but does not exceed native suine enamel. We also find that native suine enamel is significantly harder than dry enamel and dry phytoliths are harder than native phytoliths. Our data challenge the claim that the culprit of tooth wear may be the food we chew, but suggest instead that wear may relates more to exogenous than endogenous abrasives.
The EU recommendation for a definition of nanomaterial (2011/696/EU) should allow the identification of a particulate nanomaterial based on the number-based metric criterion according to which at least 50% of the constituent particles have the smallest dimension between 1 and 100 nm. Within the European Project NanoDefine (www.nanodefine.eu) a two-tier approach has been developed, whereby firstly a screening method is applied for the rough classification as a nanomaterial or non-nanomaterial, and for borderline cases a confirmatory method (imaging methods or field flow fractionation) must be considered.
One of the measurement methods well suited to particulate powder is the determination of volume-specific surface area (VSSA) by means of gas adsorption as well as skeletal density. The value of 60 m2/cm3 corresponding to spherical, monodisperse particles with a diameter of 100 nm constitutes the threshold for decisioning if the material is a nanomaterial or non-nanomaterial. The correct identification of a nanomaterial by VSSA method (positive test) is accepted by the EU recommendation.
However, the application of the VSSA method is associated also by some limitations. The threshold of 60 m2/cm3 is dependent on the particle shape. For particles containing micro-pores or having a microporous coating, false positive results will be produced.
Furthermore, broad particle size distributions – as typically for ceramic materials – as well as multi-modal size distributions make necessary to adjust the threshold.
Based on examples of commercially available ceramic powders, the applicability of the VSSA approach will be tested (in relation with SEM and TEM measurements) in order to expand the actual knowledge and
improve the method.
Im Vortrag werden das Messprinzip des Zentrifugen-Sedimentationsverfahrens erläutert und die Anforderungen der zugrundeliegenden Normen diskutiert. Es schließen sich umfangreiche Ausführungen zur praktischen Durchführung der Messung, insbesondere auch zu den vorbereitenden Arbeiten, sowie zur Auswertung der Rohdaten an. Nach einem Vergleich der Ergebnisse mit anderen Messverfahren, wird das Verfahren in einer Zusammenfassung bewertet.
The EC’s recommendation for a definition of nanomaterial (2011/696/EU) should allow the identification of a particulate nanomaterial based on the number-based metric criterion according to which at least 50% of the constituent particles have the smallest dimension between 1 and 100 nm. However, it has been recently demonstrated that the implementation of this definition for regulatory purposes is conditioned by the large deviations between the results obtained by different sizing methods or due to practical reasons such as high costs and time-consuming (SEM, TEM).
Within the European project NanoDefine (www.nanodefine.eu) a two-tier approach has been developed, whereby firstly a screening method is applied for the rough classification as a nano-/non-nanomaterial, and for borderline cases a confirmatory method (imaging methods or field flow fractionation) must be considered.
One of the measurement methods well suited to particulate powder is the determination of volume-specific surface area (VSSA) by means of gas adsorption as well as skeletal density. The value of 60 m2/cm3 corresponding to spherical, monodisperse particles with a diameter of 100 nm constitutes the threshold for decisioning if the material is a nano- or non-nanomaterial. The correct identification of a nanomaterial by VSSA method is accepted by the EU recommendation.
However, the application of the VSSA method is associated also by some limitations. The threshold of 60 m2/cm3 is dependent on the particle shape, so that it changes considerably with the number of nano-dimensions of the particles. For particles containing micro-pores or having a microporous coating false positive results will be produced. Furthermore, broad particle size distributions make necessary to adjust the threshold. Based on examples of commercially available ceramic powders, the applicability of the VSSA approach will be tested (in relation with SEM and TEM measurements) in order to expand the actual knowledge and to improve this good available and agglomeration tolerant method.
The influence of the support structure on residual stress and distortion in SLM Inconel 718 parts
(2018)
The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.
Wollastonite (CaSiO 3 ) – diopside (CaMgSi 2 O 6 ) glass-ceramic scaffolds have been successfully fabricated using two different additive manufacturing techniques: powder-based 3D printing (3DP) and digital light processing (DLP), coupled with the sinter-crystallization of glass powders with two different compositions. The adopted manufacturing process depended on the balance between viscous flow sintering and crystallization of the glass particles, in turn in fluenced by the powder size and the sensitivity of CaO – MgO – SiO 2 glasses to surface nucleation. 3DP used coarser glass powders and was more appropriate for low temperature firing (800 – 900 °C), leading to samples with limited crystallization. On the contrary, DLP used finer glass powders, leading to highly crystallized glass-ceramic samples. Despite the differences in manufacturing technology and crystallization, all samples featured very good strength-to-density ratios, which bene fit theiruse for bone tissue engineering applications. The bioactivity of 3D-printed glass-ceramics after immersion in simulated body fluid and the similarities, in terms of ionic releases and hydroxyapatite formation with already validated bioactive glass-ceramics, were preliminarily assessed.
The Layerwise Slurry Deposition is a technology for the deposition of highly packed powder layers. A powder bed is achieved by depositing and drying layers of a ceramic suspension by means of a doctor blade. This deposition technique was combined with the binder jetting technology to develop a novel Additive Manufacturing technology, named LSD-print. The LSD-print was applied to a porcelain ceramic. It is shown that it was possible to produce parts with high definition, good surface finish and at the same time having physical and mechanical properties close to those of traditionally processed porcelain, e.g. by slip casting.
This technology shows high future potential for being integrated alongside traditional production of porce-lain, as it is easily scalable to large areas while maintaining a good definition. Both the Layerwise Slurry Deposition method and the binder jetting technologies are readily scalable to areas as large as > 1 m2.
A shape evolution approach based on the thermally activated self-organization of 3D printed parts into minimal surface area structures is presented. With this strategy, the present communication opposes currently established additive manufacturing strategies aiming to stipulate each individual volumetric element (voxel) of a part. Instead, a 3D structure is roughly defined in a 3D printing process, with all its advantages, and an externally triggered self-organization allows the formation of structural elements with a definition greatly exceeding the volumetric resolution of the printing process. For enabling the self-organization of printed objects by viscous flow of material, functionally graded structures are printed as rigid frame and melting filler. This approach uniquely combines the freedom in design, provided by 3D printing, with the mathematical formulation of minimal surface structures and the knowledge of the physical potentials governing self-organization, to overcome the paradigm which strictly orrelates the geometrical definition of 3D printed parts to the volumetric resolution of the printing process. Moreover, a transient liquid phase allows local programming of functionalities, such as the alignment of functional particles, by means of electric or magnetic fields.
Tricalcium phosphate (b-TCP) can be used as bone graft, exhibiting suitable bioabsorption and osteoconduction properties. The presence of silica may induce the formation of a hydroxyapatite layer, enhancing the integration between implant and bone tissue. Preceramic polymers present silicon in their composition, being a source of SiO2 after thermal treatment. Using the versatility of 3D printing, b-TCP and a polysiloxane were combined to manufacture a bulkb-TCP with a silica coating. For the additive manufacturing process, PMMA powder was used as passive binder for the b-TCP particles, and polymethylsilsesquioxane (MK), dissolved in an organic solvent, was used both as a printing binder (ink) and as the source of SiO2 for the coating. Five distinct coating compositions were printed with increasing amounts of MK. The structures were then submitted to heat treatment at 1180 °C for 4 h. XRD and FTIR showed no chemical reaction between the calcium phosphate and silica. SEM allowed observation of a silicon-based ating on the structure surface. Mechanical strength of the sintered porous structures was within the range of that of trabecular bones.
A shell test bench was developed at BAM 5.3 which allows for static and fatigue testing of curved fiber-reinforced plastic (FRP) structures, during which in-situ the damage state can be non-destructively inspected by thermography and strain-field measurement techniques. Sandwich shell specimens with typical wind turbine blade manufacturing defects were designed and tested. The tested imperfections show a fairly significant reduction (up to 90%) of the shell test specimens‘ lifetime, depending on the type of imperfection. Using the in-situ NDT methods incorporated in the shell test bench, the location and cycle time of the initial defects and the damage evolution was investigated.
A shell test bench was developed at BAM 5.3 which allows for static and fatigue testing of curved fiber-reinforced plastic (FRP) structures, during which in-situ the damage state can be non-destructively inspected by thermography and strain-field measurement techniques. Sandwich shell specimens with typical wind turbine blade manufacturing defects were designed and tested. The tested imperfections show a fairly significant reduction (up to 90%) of the shell test specimens‘ lifetime, depending on the type of imperfection. Using the in-situ NDT methods incorporated in the shell test bench, the location and cycle time of the initial defects and the damage evolution was investigated.
The described investigation of carbon-fibre-reinforced plastics (CFRP) documents that damage evolution can be observed by means of X-ray refractography. Comparative investigations with synchrotron technique on CFRP and grey-scale analysis on glass fibre-reinforced-plastics (GFRP) confirm these results. Moreover it was found that the fracture mechanical properties of the matrix system influence damage nucleation and propagation in the laminate during static and fatigue loads. Single-step fatigue tests were carried out on Laminates with RIM135 and LY556 matrix systems made from non-crimped fabric (NCF) or twill weave in different fibre orientations. The damage to the LY556 laminates was characterized by laminate cracks growing rapidly over the whole specimen width, whereas the damage on the RIM135 laminates was characterized by an earlier onset of micro-cracking followed by laminate cracks. The specimens were fatigued up to 108 (very high cycle fatigue (VHCF) regime) load cycles. S-N-curves of damage initiation were drawn and boundaries were identified for endurance within the VHCF regime. A phenomenology based model focusing on matrix stress was applied to reproduce the first inter-fibre failure (IFF) under static and fatigue loads.
Fiber-reinforced-polymers (FRPs) are in current research focus in the lightweight construction industry, because of their extraordinary characteristics (stiffness and strength-to-density relation). The structure of polymer matrix and the interaction with reinforcement are crucial for optimization of the mechanical and thermal properties of FRPs. Due to the macromolecular chain structure, the mechanical properties of a polymer strongly vary with temperature: Below the glass transition, the chain segments of a polymer are “frozen”. Regarding fracture, the total changed energy during fracture if only dissipates for the generation of the new surfaces. However, in the region of the glass transition, the polymer chain segments start to get “unfrozen”, and the energy is not only required for generating new surfaces, but also for irreversibly deformation. This irreversible deformation is affected by the global temperature and the local temperature near the crack tip, which is affected by the local strain rate and crack propagation velocity.
Hence, in this research project, the irreversible deformation of neat and reinforced polymers will be controlled by changing the global temperature as well as the local temperature. With using different fracture experiments, the amount of energy required for creating new surfaces and for the irreversible deformation will be separated. The fracture tests include the conventional tensile test, the macroscopic peel test and the single fiber peel – off test.
Temperature-induced, stress-induced martensitic phase transitions and martensite reorientation process in Ni rich (50.9 at.%) NiTi pseudoelastic NiTi wires with ultra-fine grained (UFG) microstructure were studied by electrical resistance measurements. Measurements of the electrical resistance as a function of temperature at different constant mechanical loads accompanied by complementary experiments with variable loads at constant temperature were performed. Results show that the transformation mechanisms in UFG microstructures exhibit a higher level of complexity when compared with those characterizing the behavior of other microstructures (e.g., recrystallized or larger grains size). It was found that a threshold stress level below 150 MPa delimits the transition from a homogeneous (low stress) to localized but reversible Lüders type transformation (high stress) when the transformations are induced under constant applied stress and that reorientation processes require stresses of 100 MPa in the present UFG wires. Even though the strain evolutions do not always show two distinct yielding events during cooling or heating, electrical resistance measurements proved that a two-step transformation involving R-phase and B19' martensite was always present in the extended range of temperatures and stresses investigated here.
Calciumcobaltit und Calciummanganat gehören zu den vielversprechendsten thermoelektrischen Oxiden im Temperaturbereich zwischen 600 °C und 800 °C an Luft. Mittels thermoelektrischer Generatoren kann ein Temperaturgradient direkt in elektrische Leistung umgewandelt werden. Für die kostengünstige Pulverherstellung von Funktionsmaterialien wird im industriellen Maßstab meist die Festphasenreaktion (bzw. Kalzinierung) verwendet. Da es sich dabei um einen Hochtemperaturprozess handelt, ist diese Kalzinierung sehr energieintensiv. In der Literatur werden sehr unterschiedliche Prozessbedingungen zur Pulversynthese thermoelektrischer Oxide genutzt. Soweit dem Autor bekannt, ist keine systematische Untersuchung des Einflusses der Pulversynthesebedingungen auf die thermoelektrischen Eigenschaften publiziert.
Deshalb wurde eine systematische Untersuchung des Einflusses der Pulversynthesebedingungen (Temperatur, Haltezeit, Partikelgröße, Wiederholungen) auf die thermoelektrischen Eigenschaften von Calciumcobaltit und Calciummanganat durchgeführt.
Es konnte gezeigt werden, dass sich ein höherer Energieeintrag während der Kalzinierung negativ auf die thermoelektrischen Eigenschaften auswirkt.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Different imaging techniques were employed to monitor Full Notch Creep Test (FNCT) experiments addressing environmental stress cracking in more detail. The FNCT is a well-established test method to assess slow crack growth and environmental stress cracking of polymer materials, especially polyethylene. The standard test procedure, as specified in ISO 16770, provides a simple comparative measure of the resistance to crack growth of a certain material based on the overall time to failure when loaded with a well-defined mechanical stress and immersed in a liquid medium promoting crack propagation.
Destructive techniques which require a direct view on the free fracture surface, such as light microscopy and laser scanning microscopy, are compared to non-destructive techniques, i.e. scanning acoustic microscopy and xray micro computed tomography. All methods allow the determination of an effective crack length. Based on a series of FNCT specimens progressively damaged for varied Durations under standard test conditions, the estimation of crack propagation rates is also enabled. Despite systematic deviations related to the respective Imaging techniques, this nevertheless provides a valuable tool for the detailed evaluation of the FNCT and its further development.
The creep behavior of single crystals of the nickel-based superalloy CMSX-4 was investigated at 1288 °C, which is the temperature of the hot isostatic pressing treatment applied to this superalloy in the industry. It was found that at this super-solvus temperature, where no gammaPrime-strengthening occurs, the superalloy is very soft and rapidly deforms under stresses between 4 and 16 MPa. The creep resistance was found to be very anisotropic, e.g., the creep rate of [001] crystals was about 11 times higher than that of a [111] crystal. The specimens of different orientations also showed a very different necking behavior. The reduction of the cross-sectional area psi of [001] crystals reached nearly 100 pct, while for a [111] crystal psi = 62 pct. The EBSD analysis of deformed specimens showed that despite such a large local strain the [001] crystals did not recrystallize, while a less deformed [111] crystal totally recrystallized within the necking zone. The recrystallization degree was found to be correlated with deformation behavior as well as with dwell time at high temperature. From the analysis of the obtained results (creep anisotropy, stress dependence of the creep rate, traces of shear deformation, and TEM observations), it was concluded that the main strain contribution resulted from <01-1>{111} octahedral slip.
Die Festigkeit von Gläsern wird durch die Oberflächenqualität beeinflusst. Kommt es neben dem Auftreten von Defekten zusätzlich zum Risswachstum ausgehend hiervon, wird die Festigkeit minimiert. Das Wachstum hängt dabei maßgeblich von der Luftfeuchtigkeit ab. Dieses Ermüdungsverhalten von Gläsern besser zu verstehen und dabei die Mechanismen und den Einfluss von im Volumen eingebauten Wasser auf das unterkritische Risswachstum zu untersuchen, ist Ziel der Arbeiten. Als Teilprojekt im Rahmen des DFG Schwerpunktprogramms SPP 1594 „Ultrastrong glasses“ soll der Einfluss des im Volumen eingebauten Wassers auf die Rissspitze untersucht werden. Zusammen mit der Leibniz Universität Hannover und der TU Clausthal werden hierfür hochwasserhaltige Gläser (bis zu 8 Gew%) bei 8 kbar über die Flüssigphase synthetisiert, die makroskopisch den hohen Wasseranteil nachstellen. Die Charakterisierung erfolgt hinsichtlich des Wassereinbaus, der mechanischen Eigenschaften und des Risswachstums. Die Arbeiten in Berlin beziehen sich hierbei auf die Messungen des unterkritischen Risswachstums in Luft und Vakuum, sowie Verlustwinkelmessungen.
Erste Ergebnisse zeigen Unterschiede im korrosionsbeeinflussten (langsames) und inerten (schnelles und im Vakuum stattfindendes) Risswachstumsverhalten der untersuchten Gläsern. Die Rissgeschwindigkeit beim Übergang vom korrosionsbeeinflussten zum inerten Risswachstum ist hin¬gegen für alle Gläser ähnlich und folglich ein kinetisch durch den äußeren Wassertransport an die Rissspitze bestimmter Prozess. Der Widerstand gegen Risswachstum steigt mit Tg und zusätzlich kann anhand der Verlustwinkelmessungen ein Zusammenhang zwischen der Netzwerk- und der β-Relaxation ermittelt werden. Je höher der Wassergehalt im Glas ist, desto niedriger wird Tg und einfacher das Risswachstum, welches sich durch längere Risse kennzeichnet. Besonders stark tritt dieser Effekt bei einem Überschuss an molekularem Wasser auf.
Premature failure of glass under load is caused by sub-critical crack growth (SCCG) originate from microscopic flaws at the surface. While SCCG is related to the humidity of the ambient atmosphere, leading to stress corrosion phenomena at the crack tip, the detailed mechanism and the effect of different network formers are still not fully understood. For more clarity, various soda silicate glasses with a second network former were investigated by double cantilever beam technique: Na2O*Al2O3*SiO2 (NAS), Na2O*B2O3*SiO2 (NBS), Na2O*PbO*SiO2 (NPbS).
Three effects on the crack growth velocity, v, versus stress intensity, KI, curves were found out. The slope in region I, which is limited by corrosion, increases in the order NAS < NBS ≲ NPbS. The velocity range of region II reflecting the transition between corrosion effected and inert crack growth (region III), varies within one order of magnitude between the glasses. The KI region of inert crack growth strongly scatters between 0.4 and 0.9 MPam1/2. For comparison, crack growth at different humidity in commercial soda lime silicate glass (NCS) was measured.
Statistical analysis of Vickers induced subcritical crack growth in soda-lime silicate glasses
(2018)
Studies on Vickers induced subcritical crack growth are controversially discussed since the stresses that drive the crack growth are distributed three dimensionally within the material and cannot be retraced by available methods. Hence, empirical approaches are used to calculate mechanical material parameters such as the stress intensity factor KI. However, the results of these approaches show large deviations from those measured by standardized techniques such as double cantilever beam (DCB) or double cleavage drilled compression (DCDC). Yet, small specimen sizes and low specimen quantities can prevent the execution of DCB and DCDC measurements. Here we present an approach that is based on a statistical analysis of Vickers induced radial cracks. For this purpose more than 150 single radial cracks were analyzed. The cracks were generated in a commercial soda-lime silicate glass. The experiments were performed in a glovebox purged with dry nitrogen gas to minimize the influence of atmospheric water on crack growth. The temporally resolved evolution of the radial cracks was monitored in-situ using an inverted microscope equipped with a camera system directly below the Vickers indenter. An automated image analysis software was used to determine the crack length over time. The data show that the crack propagation and thereby the crack velocities are not uniformly but statistically distributed. These findings allow, using the statistical mean value of the distributions in combination with DCB data, a precise formulation of KI for each measured crack length.
The presence of water in the surrounding atmosphere of a propagating crack has a major influence on the subcritical crack growth. While these external phenomena are well understood, there is still a lack of knowledge on the influence of structurally bound water on crack propagation. Thus, our recent study aims on the analysis of crack propagation in water bearing soda-lime silicate glasses with up to 8 wt.% water. The samples were synthesized in an internally heated pressure vessel at 0.5 GPa. Since this preparation route limits the sample sizes, standard test geometries allowing for the determination of stress intensity factors, such as double cantilever beam, are not feasible. Thus, radial cracks in the hydrous glasses were initiated by Vickers indentation and crack growth was simultaneously captured with a camera system. An automated image analysis algorithm was used for the analysis of the crack length of each single video frame. To minimize influences by atmospheric water, all experiments were conducted in a glovebox purged with dry N2. About 150 cracks per glass composition were analyzed to provide statistical significance of the Vickers-induced SCCG. The results show that structurally bound water has a major influence on SCCG by means of crack lengths, growth rates and time of crack initiation.
Der Vortrag gibt eine Einführung in die Methode der Vakuumheißextraktion und beschreibt die Anwendungsmöglichkeiten der an der BAMN betriebenen Anlage.
Giant magneto-resistive (GMR) sensor based metal magnetic memory (MMM) technique is proposed for mapping of deformation-induced self-magnetic leakage fields (SMLFs) in carbon steel. The specimens were subjected to different amounts of tensile deformation and the deformation-induced SMLFs were measured using a GMR sensor after unloading the specimens. 3D-nonlinear finite element modeling was performed to predict stress–strain state in a steel specimen under tensile load. The experimentally obtained SMLF images were correlated with the finite element model predicted stress–strain states. Studies reveal that the MMM technique can detect the plastic deformation with signal-to-noise ratio better than 20 dB. The technique enables the mapping of plastic deformation in carbon steels for the evaluation of the severity of deformation. The study also reveals that deformation-induced SMLF is influenced by the presence of initial surface residual stress, introduced by shot peening. The intensity of SMLF signal is found to increase with increase in tensile load and decrease with shot peening.
Dissolved water decisively influences numerous thermally activated relaxation phenomena in glasses like stress relaxation, sub-critical crack growth, internal friction, viscosity, sintering, and crystallization. Thermoanalytical methods can essentially help for better understanding of these phenomena. The lecture introduces the Vacuum Hot Extraction method (VHE) and illustrates its possibilities for measuring water content, degassing and mobility. As another thermoanalytical method, the Dynamic Mechanical Themoanalysis (DMA), allowing to study the effect of dissolved water on the internal friction in glasses, is introduced.
Wasser in Silicatglas
(2018)
Glass powders are promising candidates for manufacturing a broad diversity of sintered materials like sintered glass-ceramics, glass matrix composites or glass bonded ceramics with tailored mechanical, thermal, electrical and optical properties and complex shape. Its wide and precise adjustability makes this class of materials a key component for advanced technologies. Processing of glass or composite powders often allow even more flexibility in materials design. At the same time, however, processing can have substantial effects on the glass powder surface and sinterability. Thus, mechanical damage and surface contamination can strongly enhance surface crystallization, which may retard or even fully prevent densification. Whereas sintering and concurrent crystallization have been widely studied, partially as cooperative effort of the TC7 of the ICG, and although glass powder sintering is predominantly applied for glasses of low crystallization tendency, sintering is also limited by gas bubble formation or foaming. The latter phenomenon is much less understood and can occur even for slow crystallizing glass powders. The lecture illustrates possible consequences of glass powder processing on glass sintering, crystallization and foaming.
In Germany spent nuclear fuel and high level radioactive waste is stored in interim storage containers with double lid systems. Those lids are equipped with metal seals (e.g. Helicoflex®) that ensure the safe enclosure of the inventory. The used metal seals consist of three components as can be seen in the cross-sectional view in Figure 1. The innermost part is a helical spring that is surrounded by an inner jacket made of stainless steel. The outer jacket that is made of a softer material which in case of assembly in the aforementioned storage containers is silver or aluminum (i.e. Al 99.5). During application the seal is compressed and due to the restoring force of the helical spring, the outer jacket is plastically deformed and adapts to the sealing surface. Hence, leakage paths are closed and the sealing function is generated. In Germany the above-mentioned containers are licensed for up to 40 years of interim storage, which in case extended storage becomes necessary before a final repository is available will have to be extended to even longer periods. Therefore, the evaluation of the long-term behavior of the seals is necessary, taking into account storage conditions, decay heat and possible mechanical loads as well.
At Bundesanstalt für Materialforschung und –prüfung (BAM) long-term investigations are being conducted in which seals are assembled in test flanges and aged at temperatures ranging from room temperature to 150°C for accelerated aging. The aged seals are tested semi-annually (after the first 6 months in which the seals are tested more frequently) regarding the sealing performance, the remaining seal force, and the useable resilience upon decompression. Results of these investigations have been published over the past years (e.g. Grelle, Wolff, Probst, Jaunich, & Völzke, 2017; Völzke, Wolff, Probst, Nagelschmidt, & Schulz, 2014). It was found that the seal force and the useable resilience decrease with time and temperature, which is in agreement with the result of other studies (Sassoulas et al., 2006; Wataru et al., 2016) as well. Geometry change of the outer jacket has been identified as the main reason for this seal behavior. At the prevailing operating temperatures and stresses the aluminum is subjected to creep deformation leading to a thinning of the outer jacket. Since the seal groove depth remains unchanged the helical spring expands, which in turn leads to a decrease of the generated spring and seal force.
Although the main reason for the change of seal parameters over time and temperature is known, a detailed characterization of the seal behavior and a reliable prediction of the parameter development for aging times that exceed the experimental time frame have not been possible, yet.
For deeper understanding of the aging processes, an Investigation program, which is covered in this contribution, is conducted at Bundesanstalt für Materialforschung und –prüfung (BAM) that focusses on the behavior of the aluminum jacket and its influence on the long-term sealing performance. The program investigates properties of material samples as well as the behavior of the seal as a component.
Original sheet material of the same aluminum that is used for manufacturing of the seals is investigated in compression creep tests. For this, a DMA (dynamic mechanical analysis) machine is employed (here used for static tests) that allows for a measurement of the specimens deformation under forces of up to 500 N. The advantage of this method is that the original material can be tested in the same shape as used for the seals which is 0.5 mm thick sheet material. For investigation of tensile creep standard specimens are used, that were machined from surrogate material of the same composition and annealing condition.
Furthermore, aluminum seals that are cut into smaller segments are assembled in flanges and placed in heating chambers at temperatures ranging from 23°C to 150°C. After different periods of time from 3 days to 300 days the segments are taken out of the flanges and are investigated, thus giving information on different states of aging. Measurements of the development of the seal contact width and the aluminum jacket thickness are done with an optical microscope. Further investigations on the segments will include metallography and hardness measurements.
From the detailed material and component behavior including the results of the long-term seal force and useable resilience investigations a better understanding of the overall seal behavior can be gained. The aim is to contribute to the development of material models and analytical approaches for the prediction of the sealing behavior in dependence of time and temperature.
Common air-coupled transducers for non-destructive testing consist of a piezocomposite material and several matching layers. Better acoustical matching to air is achieved by transducers based on charged cellular polypropylene (PP). This material has about hundred times lower acoustic impedance than any piezocomposite, having about the same piezoelectric coefficient. The piezoelectric properties of cellular PP are caused by the polarization of air cells. Alternatively, a ferroelectret receiver can be understood as a capacitive microphone with internal polarization creating permanent internal voltage. The sensitivity of the receiver can be increased by applying additional bias voltage. We present an ultrasonic receiver based on cellular PP including a high-voltage module providing bias voltage up to 2 kV. The application of bias voltage increased the signal by 12 to 15 dB with only 1 dB increase of the noise.
This receiver was combined with a cellular PP transmitter in through transmission to inspect several test specimens consisting of glass-fiber-reinforced polymer face sheets and a porous closed-cell PVC core. These test specimens were inspected before and after load. Fatigue cracks in the porous PVC core and some fatigue damage in the face sheets were detected. These test specimens were originally developed to emulate a rotor blade segment of a wind power plant. Similar composite materials are used in lightweight aircrafts for the general aviation. The other inspected test specimen was a composite consisted of glass-fiber-reinforced polymer face sheets and a wooden core. The structure of the wooden core could be detected only with cellular PP transducers, while commercial air-coupled transducers lacked the necessary sensitivity. Measured on a 4-mm thick carbon-fiber-reinforced polymer plate, cellular PP transducers with additional bias voltage achieved a 32 dB higher signal-to-noise ratio than commercial air-coupled transducers.
Common air-coupled transducers for non-destructive testing consist of a piezocomposite material and several matching layers. Better acoustical matching to air is achieved by transducers based on charged cellular polypropylene (PP). This material has about hundred times lower acoustic impedance than any piezocomposite, having about the same piezoelectric coefficient. The piezoelectric properties of cellular PP are caused by the polarization of air cells. Alternatively, a ferroelectret receiver can be understood as a capacitive microphone with internal polarization creating permanent internal voltage. The sensitivity of the receiver can be increased by applying additional bias voltage. We present an ultrasonic receiver based on cellular PP including a high-voltage module providing bias voltage up to 2 kV. The application of bias voltage increased the signal by 12 to 15 dB with only 1 dB increase of the noise.
This receiver was combined with a cellular PP transmitter in through transmission to inspect several test specimens consisting of glass-fiber-reinforced polymer face sheets and a porous closed-cell PVC core. These test specimens were inspected before and after load. Fatigue cracks in the porous PVC core and some fatigue damage in the face sheets were detected. These test specimens were originally developed to emulate a rotor blade segment of a wind power plant. Similar composite materials are used in lightweight aircrafts for the general aviation. The other inspected test specimen was a composite consisted of glass-fiber-reinforced polymer face sheets and a wooden core. The structure of the wooden core could be detected only with cellular PP transducers, while commercial air-coupled transducers lacked the necessary sensitivity. Measured on a 4-mm thick carbon-fiber-reinforced polymer plate, cellular PP transducers with additional bias voltage achieved a 32 dB higher signal-to-noise ratio than commercial air-coupled transducers.
Virtual-lab-based determination of a macroscopic yield function for additively manufactured parts
(2018)
This work presents a method for the yield function determination of additively manufactured parts of S316L steel. A crystal plasticity model is calibrated with test results and used afterwards to perform so-called virtual experiments, that account for the specific process-related microstructure including crystallographic and morphological textures. These simulations are undertaken on a representative volume element (RVE), that is generated from EBSD/CT-Scans on in-house additively manufactured specimen, considering grain structure and crystal orientations. The results of the virtual experiments are used to determine an anisotropic Barlat yield function, that can be used in a macroscopical continuum-sense afterwards. This scale-bridging approach enables the calculation of large-scale parts, that would be numerically too expensive to be simulated by a crystal plasticity model.
Here we demonstrate that by applying exclusively Martian resources a processing route involving suspensions of mineral particles called slurries or slips can be established for manufacturing ceramics on Mars. We developed water-based slurries without the use of additives that had a 51 wt. % solid load resembling commercial porcelain slurries in respect to the particle size distribution and rheological properties. These slurries were used to slip cast discs, rings and vases that were sintered at temperatures between 1000 and 1130 °C using different sintering schedules, the latter were set-up according the results of hot-stage microscopic characterization. The microstructure, porosity and the mechanical properties were characterized by SEM, X-ray Computer tomography and Weibull analysis. Our wet processing of minerals yields ceramics with complex shapes that show similar mechanical properties to porcelain and could serve as a technology for future Mars colonization. The best quality parts with completely vitrificated matrix supporting a few idiomorphic crystals are obtained at 1130 °C with 10 h dwell time with volume and linear shrinkage as much as ~62% and ~17% and a characteristic compressive strength of 51 MPa.
The employment of renewable energy resources, which are naturally intermittent, for electricity generation has altered the working conditions of conventional power plants from continuous (baseload) to cyclic or flexible operation. For a long time, 9-12% Cr ferritic-martensitic stainless steels have been widely used in power plants due to their favorable characteristics such as high creep strength at high temperatures and oxidation and corrosion resistance. The components of power plants are subjected to long term cyclic loadings including fatigue and creep-fatigue at high temperatures. As ferritic-martensitic steels are known to exhibit cyclic softening when subjected to such loading scenarios, it is crucial to study the material response in such conditions. Since it is impossible to test the material behavior exactly as the operation conditions of power plants, due to technical difficulties and cost issues, it is necessary to develop physically-based material models that can predict the material behavior in more realistic situations.
In recent years, many material models have been proposed to describe the behavior of 9-12% Cr ferritic-martensitic stainless steels, which follow phenomenological or physically-based approaches. Phenomenological models provide a stress-strain relation based on empirical observations although they usually lack physical background. To alleviate this drawback and to allow for more flexibility and wider ranges of strain-rate and temperature, physically-based models are suggested. In this approach, microstructural evolution, dislocation movement and/or kinetics of plastic deformation processes are included. The physically-based models allow for a better extrapolation from the experiment results to other operation conditions and their material constants can be interpreted physically. Compared to the former approach, the material behavior can be described more accurately and flexibly and the number of material constants is less in general. In the presented work, the existing micromechanical models developed for P92 steel are compared and extended to allow for new dislocation-based strengthening/cyclic mechanisms. Their performance is assessed in the light of mechanical test data from creep-fatigue and thermo-mechanical fatigue experiments and detailed characterizations of the microstructure evolution in the fatigued material.
For years there have been more and more reports on the presence of drugs in the aquatic environment. Due to the demographic change, the consumption of pharmaceuticals has risen sharply. After taking the drugs, they are partly metabolized in the human body. However, the metabolism is not complete so that both the metabolites and non-metabolized amounts of the parent compounds are excreted. These compounds reach the waste water and afterwards the sewage treatment plants. In sewage treatment plants transformation products can be formed by the oxidative conditions during wastewater treatment processes. The transformation products may have a higher toxicity than the actual environmental pollutants and are often only partly removed during the waste water treatment. Since a lot of these compounds are still unknown, the transformation products are not detected by target analysis used in sewage treatment plants and are often released undetected in the aquatic ecosystems. The released substances may be subject to additional transformation processes in the environment. Pharmaceuticals produced in high amounts can be already detected in the μg/L range in water bodies worldwide.
Metformin and its major transformation product guanylurea are one of the main representatives. Metformin is the drug of choice for treating type 2 diabetes. The drug therapy for diabetes mellitus has increased significantly in recent years. In the year 2015 1500 tons of metformin were prescribed in Germany (for statutory insured persons). Metformin is not metabolized in the human body and is excreted unchanged therefore concentrations between 57 μg/L and 129 μg/L are found in German waste water treatment plants influents.
In this work the transformation of the antidiabetic drug metformin is investigated. The degradation of metformin is initialize by commercial water treatment techniques like UV-radiation or noncommercial techniques like heterogenous photocatalysis based on titanium dioxide. The degradation of metformin and resulting transformation products are analyzed by LC-MS/MS and LC-HRMS.
A constitutive model for the mechanical behaviour of single crystal Ni-base superalloys under high temperature conditions has been developed in the framework of a Cooretec project in cooperation with Siemens AG, MTU Aero Engines AG and University Bayreuth. In addition to the conventional material properties e.g. elastic constants, the model requires the parameters of the initial microstructure as an input. Thus, the γ’-precipitate size and the channel width of the γ-matrix were obtained from SEM micrographs. The model uses the slip system theory and describes the movement, multiplication and annihilation of dislocations in the channels. Furthermore, the cutting of precipitates is another mechanism contributing to the plastic flow. The evolution of the morphology due to rafting and its effects on the deformation have been implemented according to. The kinematic hardening is introduced as a stress tensor to realistically represent the strain hardening of arbitrary oriented single crystals.
The mechanical behaviour of single crystal specimens has been experimentally investigated in tension tests at different strain rates and in creep tests under various loads. The constitutive model has been calibrated based on the experimental data for temperatures of 950°C and 850°C and the [001] and [111] crystallographic orientations.
Finally, a micromechanical model was created to simulate the creep response of additive manufactured polycrystalline structures. An EBSD image is taken to obtain the grain geometry and their respective orientation. The grain boundaries are discretised using cohesive elements, whereas the single crystal model was applied to each grain in the representative volume. The polycrystal model is generated using Dream3D, NetGen and other software previously developed at the BAM.
Environmental conditions are known to influence sub-critical crack growth (SCCG) that are released from microscopic flaws at the glass surface, leading to stress corrosion phenomena at the crack tip. The processes at the crack-tip are complex and water has been identified as a key component governing SCCG at low crack velocities (region I). In particular, the influence of humidity accelerating crack propagation is well studied for industrial soda-lime silicate glasses, which are practically free (< 1000 ppm) of dissolved water. To shed light on the corrosion process, the situation at the crack-tip is reversed in the present study as dissolved water in larger fractions is present in the glass and crack propagation is triggered in dry environment. For this purpose, water-bearing silicate glasses of up to 8 wt% total water were synthesized in an internally heated pressure vessel at 0.5 GPa and compared to dry glasses of standard glass manufacturing. SCCG was measured using the double cantilever beam technique and by Vickers indentation. For dry glasses, three trends in the crack growth velocity versus stress intensity curve were found. The slope in region I limited by environmental corrosion increases in the order sodium aluminosilicate < sodium borosilicate ≲ sodium lead silicate. The velocity range of region II reflecting the transition between corrosion affected and inert crack growth (region III), varies within one order of magnitude among the glasses. The KI region of inert crack growth strongly scatters between 0.4 and 0.9 MPam1/2. For hydrous glasses, it is found that those of low Tg are more prone to SCCG. As water strongly decreases Tg, it promotes SCCG. First results indicate that molecular water has a dominating influence on SCCG.
Effective hydrogen storage capacities are prerequisite for an efficient energy provision using fuel cells. Since glass has low intrinsic hydrogen permeability, it is a promising material for hydrogen storage containers as well as hydrogen diffusion barriers. Previous studies on oxidic glasses suggest a correlation between the glass composition and hydrogen permeation that was derived mainly from silica glass. In the present study, we concentrate on the relationship between thermodynamic (i.e., configurational entropy) and topologic (i.e., free volume, network polymerization) parameters. Experimental data were gathered well below the glass transition temperature, excluding significant effects caused by structural relaxation and chemical dissolution of hydrogen. The results of seven analysed glasses on the SiO2-NaAlO2 joint showed that the hydrogen permeability in fully polymerized glasses cannot solely be derived from the total free volume of the glass structure. Hence, evidence is provided that the size distribution of free volume contributes to hydrogen solubility and diffusion. Additionally, the results indicate that the configurational heat capacity ΔCp at Tg affects the hydrogen permeability of the investigated glasses.
Glass-fibre-reinforced plastics (GFRP) are commonly used for the construction of pressure vessels and tanks for example in automotive and aerospace industries. Especially for pressure vessels used for the storage of natural gas or hydrogen with operation pressures up to 700 bar, an early material fatigue detection is of great interest. Measuring the distributed strain profile of GFRP structures helps to understand and detect material fatigue. In this article, we demonstrate the great potential of swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of pressure vessels made from GFRP. A resin transfer molding (RTM) process was used to manufacture GFRP-tube specimens. Commercial optical fibres with polyimid coating were glued to the surface externally in circumferential and axial direction. A cyclic load of up to 150 bar was applied to the samples using a servo-hydraulic test bench. Comparing the loaded and unloaded test conditions, we determined up to 2 % elongation in circumferential direction. We demonstrate reliable distributed strain measurements with sub-centimetre spatial resolution. By monitoring these high-resolution strain profiles, we were able to detect local material degradation that manifested itself as localized strain changes. Crucially, the material Degradation could be detected already after 75 % of dat the fatigue life before a crack appeared that led to leakage.
The first documentation of fuel biodeterioration dates back to the late 19th century. However, extensive studies concerning the microbial fuel contamination started in 1980’s. Polymeric fuel storage tanks containing diesel and biodiesel provide environmental conditions for microbial growth. Several studies demonstrated that bacteria, which were found in contaminated fuel systems, can use fuels as macronutrient; but such bacteria can also cause microbiologically influenced corrosion and fouling.
The aim of this study is to investigate the initial attachment behavior of bacteria, isolated from a diesel contamination, on neat and photooxidized high-density polyethylene (PE-HD). Two common PE-HD’s, less- and biodiesel-stabilized, were radiated to UV light representing a tank exposed to sunlight. The effect of photooxidiation on PE-HD’s surface were characterized chemically by Fourier-transform infrared spectroscopy (FTIR). The attached bacteria Pseudomonas aeruginosa and Bacillus subtilis on the polymer surface were evaluated by fluorescence microscopy and colony-forming unit tests (CFU).
PE-HD as a polymeric fuel storage tank material: Photooxidation, fuel sorption and long-term storage
(2018)
High-density polyethylene (PE-HD) is a commodity thermoplastic polymer which is typically used for packing of dangerous goods. Its good resistance against photooxidation, fuels, chemicals and other environmental factors in addition to low production costs makes PE-HD attractive for fuel storage applications. Typical engine fuels stored in polymer tanks are petrol, diesel and biodiesel that receives increasing attention as proper alternative to fossil fuels. One of the major problems with biodiesel is its susceptibility to oxidize due to its chemical composition of unsaturated fatty acids which also can cause polymer degradation.
The aim of this study is to investigate the influence of different environmental factors, UV radiation and commonly stored fuels, on the mechanical, physical and chemical properties of two types of PE-HD polymers (stabilized and non-stabilized). The influence on the mechanical properties was tested by Charpy and tensile tests, chemical and physical properties were evaluated by Fourier-transform infrared spectroscopy (FTIR) and by dynamical mechanical analysis (DMA) tests. Samples were characterized after varying exposure time of UV radiation and after fully and partially immersion in biodiesel. In addition, similar experiments were conducted using diesel for comparison.
We invented the first non-metallic, self-adhesive and dry biosignalling electrode. The PEDOT polymer electrode changes its aggregate state and conductivity by a light curing procedure. The electrode can be applied as a gel underneath hair without shaving. With the aid of blue light, the electrode can be hardened within a few seconds at the desired location on the scalp. The cured polymer electrode is highly conductive and can be applied on a very small location. Unlike other EEG electrodes, our electrode does not lose conductivity upon drying. Furthermore, our electrode strongly bonds to Skin and does not require any additional adhesive. Short circuits due to an outflow of gel are prevented with this technique. Therefore, the PEDOT polymer electrode is extremely well suited for applications that, up to now, have been challenging, such as non-invasive EEG recordings from awake and freely moving animals, EEG recordings from preterm babies in the neonatal intensive care unit or long-term recordings in the case of sleep monitoring or epilepsy diagnostics. We addressed two technical questions in this work. First, is the EEG recorded with polymer electrodes comparable to a standard EEG? Second, is it possible to record full-band EEGs with our electrodes?
Additive Manufacturing (AM) through the Selective Laser Melting (SLM) route offers ample scope for producing geometrically complex parts compared to the conventional subtractive manufacturing strategies. Nevertheless, the residual stresses which develop during the fabrication can limit application of the SLM components by reducing the load bearing capacity and by inducing unwanted distortion, depending on the boundary conditions specified during manufacturing. The present study aims at characterizing the residual stress states in the SLM parts using different diffraction methods. The material used is the nickel based superalloy Inconel 718. Microstructure as well as the surface and bulk residual stresses were characterized. For the residual stress analysis, X-ray, synchrotron and neutron diffraction methods were used. The measurements were performed at BAM, at the EDDI beamline of -BESSY II synchrotronand the E3 line -BER II neutron reactor- of the Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. The results reveal significant differences in the residual stress states for the different characterization techniques employed, which indicates the dependence of the residual state on the penetration depth in the sample. For the surface residual stresses, longitudinal and transverse stress components from X-ray and synchrotron agree well and the obtained values were around the yield strength of the material. Furthermore, synchrotron mapping disclosed gradients along the width and length of the sample for the longitudinal and transverse stress components. On the other hand, lower residual stresses were found in the bulk of the material measured using neutron diffraction. The longitudinal component was tensile and decreased towards the boundary of the sample. In contrast, the normal component was nearly constant and compressive in nature. The transversal component was almost negligible. The results indicate that a stress re-distribution takes place during the deposition of the consecutive layers. Further investigations are planned to study the phenomenon in detail.
The creep behavior of single-crystals of the nickel-base superalloy CMSX-4 was investigated at 1288°C, which is the temperature of the hot isostatic pressing (HIP) treatment applied to this superalloy in the industry. It was found that at this super-solvus temperature, where no Gamma’-strengthening occurs, the superalloy is very soft and rapidly deforms under stresses between 4 and 16 MPa. The creep resistance was found to be very anisotropic, e.g. the creep rate of [001] crystals was about 11 times higher than that of a [111] crystal. The specimens of different orientations also showed a very different necking behavior. The reduction of the cross-section area psi of [001] crystals reached nearly 100%, while for a [111] crystal psi=62%. The EBSD analysis of deformed specimens showed that despite such a large local strain the [001] crystals didn’t not recrystallize, while a less deformed [111] crystal totally recrystallized within the necking zone. From the shape of deformed specimens and TEM investigations it was concluded that the main strain contribution resulted from <011> {111} octahedral slip.
Efficient energy provision using fuel cells requires effective hydrogen storage capacities. Glass is a material of low intrinsic hydrogen permeability and is therefore a promising material for hydrogen storage containers or diffusion barriers. Pioneer work on oxidic glasses seems to indicate a correlation between glass composition and hydrogen permeation, which was mainly derived from the behavior of silica glass. In this study, we focus on the relationship between topologic (free volume; network polymerization) and thermodynamic (configurational entropy) glass parameters. Experiments were performed well below the glass transition temperature, which excludes significant structural relaxation and chemical dissolution of hydrogen. The compositional dependence of seven glasses on the SiO2-NaAlO2 join pointed out that in fully polymerized glasses the H2 permeability cannot be solely derived from the total free volume of the glass structure. Hence, evidence is provided that the size distribution of free volume contributes to hydrogen diffusion and solubility. Additionally, results indicate that hydrogen permeability of the glasses is affected by the configurational heat capacity ΔCp at Tg.
Glass-ceramics are noted for their unusual combination of properties and manifold commercialized products for consumer and specialized markets. Evolution of novel glass and ceramic processing routes, a plethora of new compositions, and unique exotic nano- and microstructures over the past 60 years led us to review the Definition of glass-ceramics. Well-established and emerging processing methods, such as co-firing, additive manufacturing, and laser patterning are analyzed concerning the core requirements of processing glass-ceramics and the Performance of the final products. In this communication, we propose a revised, updated definition of glass-ceramics, which reads “Glass-ceramics are inorganic, non-metallic materials prepared by controlled crystallization of glasses via different processing methods. They contain at least one type of functional crystalline phase and a residual glass. The volume fraction crystallized may vary from ppm to almost 100%”.
Glasses with compositions of 21Gd2O3-63MoO3-(16-x)B2O3-xTeO2 (mol%) (x= 0, 2, 4, 8) were prepared using a conventional melt quenching technique, and the crystallization behavior of ferroelastic β′-Gd2 MoO4)3 Crystals was examined to clarify the mechanism of self-powdering phenomenon and to design bulk crystallized glasses. It was found that the self-powdering phenomenon appeared significantly during the crystallization at temperatures near the crystallization peak temperature, but the phenomenon is suppressed in the crystallization at temperatures much higher than the glass transition temperature. It was also found that the substitution of TeO2 for B2O3 in the base glasses suppresses the self-powdering phenomenon and consequently bulk crystallized glasses were obtained in the glass with x=8 mol%. The densities at room temperature of the base glasses are d =4.755–4.906 g/cm3, being much higher than the value of d=4.555 g/cm3 for β′-Gd2(MoO4)3 crystal. It is proposed that the stresses in the inside of crystals induced by large density differences (i.e., large molar volume differences) between the glassy phase and crystals might be relaxed effectively in the glasses containing TeO2 with weak TeeO bonds and fragile character.
Glass powders are promising candidates for manufacturing a broad diversity of sintered materials like sintered glass-ceramics, glass matrix composites or glass bonded ceramics with properties and complex shape. Powder processing, however, can substantially affect sinterability, e.g. by promoting surface crystallization. On the other hand, densification can be hindered by gas bubble formation for slow crystallizing glass powders. Against this background, we studied sintering and foaming of silicate glass powders with different crystallization tendency for wet milling and dry milling in air, Ar, N2, and CO2 by means of heating microscopy, DTA, Vacuum Hot Extraction (VHE), SEM, IR spectroscopy, XPS, and ToF-SIMS. In any case, foaming activity increased significantly with progressive milling. For moderately milled glass powders, subsequent storage in air could also promote foaming. Contrarily, foaming could be substantially reduced by milling in water and 10 wt% HCl. Although all powder compacts were uniaxially pressed and sintered in air, foaming was significantly affected by different milling atmosphere and was found most pronounced for milling in CO2 atmosphere. Conformingly, VHE studies revealed that foaming is mainly driven by carbonaceous species, even for powders milled in other gases. Current results of this study thus indicate that foaming is caused by carbonaceous species trapped on the glass powder surface.
The sound numerical prediction of welding-induced thermal stresses, residual stresses, and distortions strongly depends on the accurate description of a welded material’s thermomechanical deformation behaviour. In this work, we provide experimental data on the viscoplastic deformation behaviour of a grade-s960ql steel up to a temperature of 1000 ◦C. In addition, a multi-phase viscoplastic material model is proposed, which accounts for the experimentally observed isothermal deformation behaviour of grade-s960ql steel base and austenitised material, as well as for athermal contributions that originate from solid-state phase transformations. The multi-phase viscoplastic and a classic rateindependent isotropic hardening material model were applied in the numerical simulations of both-ends-fixed bar Satoh tests and a single-pass gas metal arc weld. The influence of material modelling choices on the agreement between numerical simulation and experimental results is discussed, and recommendations for further work are given.
In Germany spent nuclear fuel (SNF) and high level radioactive waste (HLW) are stored in interim storage containers with double lid systems. Those lids are equipped with metal seals (e.g. Helicoflex®) that ensure the safe enclosure of the inventory. Being licensed for up to 40 years of interim storage the evaluation of the long-term behavior of the seals is necessary, taking into account storage conditions, decay heat and possible mechanical loads.