5 Werkstofftechnik
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- 5 Werkstofftechnik (188)
- 5.1 Mikrostruktur Design und Degradation (60)
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Eingeladener Vortrag
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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.
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.
The increasing pollution of terrestrial and aquatic ecosystems with plastic debris leads to the accumulation of microscopic plastic particles of still unknown amount. To monitor the degree of contamination analytical methods are urgently needed, which help to quantify microplastics (MP). Currently, time-costly purified materials enriched on filters are investigated both by micro-infrared spectroscopy and/or micro-Raman. Although yielding precise results, these techniques are time consuming, and are restricted to the analysis of a small part of the sample in the order of few micrograms. To overcome these problems, here we tested a macroscopic dimensioned NIR process-spectroscopic method in combination with chemometrics. For calibration, artificial MP/soil mixtures containing defined ratios of polyethylene, polyethylene terephthalate, polypropylene, and polystyrene with diameters < 125 µm were prepared and measured by a process FT-NIR spectrometer equipped with a fiber optic reflection probe. The resulting spectra were processed by chemometric models including support vector machine regression (SVR), and partial least squares discriminant analysis (PLS-DA). Validation of models by MP mixtures, MP-free soils and real-world samples, e.g. and fermenter residue, suggest a reliable detection and a possible classification of MP at levels above 0.5 to 1.0 mass% depending on the polymer. The benefit of the combined NIRS chemometric approach lies in the rapid assessment whether soil contains MP, without any chemical pre-treatment. The method can be used with larger sample volumes and even allows for an online prediction and thus meets the demand of a high-throughput method.
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.
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.
Thermoelectric materials can convert waste heat directly into electrical power by using the Seebeck effect. Calcium cobaltite Ca3Co4O9 is a promising p-type oxide thermoelectric material for applications between 600 °C and 900 °C in air. The properties and morphology of Ca3Co4O9 are strongly anisotropic because of its crystal structure of alternating layers of CoO2 and Ca2CoO3. By aligning the plate-like grains, the anisotropic properties can be assigned to the component. Pressure-assisted sintering (PAS), as known from large-scale production of low temperature co-fired ceramics, was used to sinter multilayers of Ca3Co4O9 green tape at 900 °C with different pressures and dwell times. In-situ shrinkage measurements, microstructural investigations and electric measurements were performed. Pressure-less sintered multilayers have a 2.5 times higher electrical conductivity at room temperature than dry pressed test bars with randomly oriented particles. The combination of tape casting and PAS induces a pronounced alignment of the anisotropic grains. Relative density increases from 57 % after free sintering for 24 h to 94 % after 2 h of PAS with 10 MPa axial load. By applying a uniaxial pressure of 10 MPa during sintering, the electrical conductivity (at 25°C) improves by a factor of 15 compared to test bars with randomly oriented particles. The high temperature thermoelectric properties show the same dependencies. The smaller the applied axial load, the lower the relative densities, and the lower the electrical conductivity. Longer dwell times may increase the density and the electrical conductivity significantly if the microstructure is less densified as in the case of a small axial load like 2 MPa. At higher applied pressures the dwell time has no significant influence on the thermoelectric properties. This study shows that PAS is a proper technique to produce dense Ca3Co4O9 panels with good thermoelectric properties similar to hot-pressed tablets, even in large-scale production.
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.