BAM Dissertationsreihe
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- 2014 (6) (entfernen)
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Schlagworte
- 3D Ray Tracing Method (1)
- 3D Ray Tracing Verfahren (1)
- Actuation (1)
- Austenitic Weld (1)
- Biolog Phenotype MicroArrays (1)
- Black Yeast-Like Fungi (1)
- Gesteinsbesiedelnde Pilze (1)
- IEC 60243-1 (1)
- Inhomogeneous Materials (1)
- Model Organisms (1)
121
Shape memory polymers (SMPs) can change their shape on application of a suitable stimulus. To enable such behavior, a ‘programming’ procedure fixes a deformation, yielding a stable temporary shape. In thermoresponsive SMPs, subsequent heating triggers entropy-elastic recovery of the initial shape. An additional shape change on cooling, i.e. thermoreversible two-way actuation, can be stimulated by a crystallization phenomenon. In this thesis, cyclic thermomechanical measurements systematically determined (1) the shape memory and (2) the actuation behavior under constant load as well as under stress-free conditions. Chemically cross-linked, star-shaped polyhedral oligomeric silsesquioxane polyurethane (SPOSS-PU) hybrid polymer networks and physically cross-linked poly(ester urethane) (PEU) block copolymers were investigated around the melting and crystallization temperatures of their polyester soft segments. (1) The SPOSS-PUs showed excellent shape fixities and recoverabilities of almost 100% at high cross-linking density, while PEUs exhibited pronounced shape memory properties at increased soft segment content. Furthermore, two-fold programmed SPOSS-PU specimens were able to recover their initial shape in two thermally separated events. Even a neck, which formed during deformation of SPOSS-PUs with high soft segment content, was reversed. (2) In PEUs, globally oriented crystallization on cooling drove expansion of the sample, in particular at high soft segment content and after application of a strong deformation. Melting reversed that orientation; the PEU sample contracted and thereby completed the thermoreversible actuation cycle. Under load, multiple polymorphic phase transitions enabled two successive expansion and contraction steps, while under stress-free conditions various geometric shape changes, including the increase and decrease of PEU sample length and thickness as well as twisting and untwisting could be experimentally witnessed. Such actuation technology allows for entirely new applications, enabling mimicry of reversible, bidirectional and repeated organic movements.
120
A novel optical detection method for partial discharge in HV/EHV cable terminations has been proposed. Optical sensor fibres integrated into the HV equipment provide high sensitivity as well as immunity to electromagnetic interference and enable therefore on-line monitoring in electromagnetically noisy environment. The availability of optically transparent silicone rubbers that meet strict dielectric and mechanical criteria is a crucial prerequisite for the implementation of this method. The optically transparent silicone rubbers can be applied for the fabrication of a modern rubber stress cone as well as for the development of a new optical sensing element sensitive to PD activities. In this thesis, AC dielectric strength behaviour and mechanical properties of three types of commercially available silicone rubbers were investigated. One of the characterized silicone rubbers was a translucent type whereas the two others were optically transparent types, however with different chemical curing reactions. The measurements of tensile strength and elongation at break were carried out according to the ISO 37 standard. For investigation of the dielectric strength Eb behaviour of the virgin and modified silicone rubbers, a new methodology was developed. It is, at the same time, highly reliable and efficient, saves time and reduces material consumption in comparison to previously reported methodologies. The key component of this methodology is a specifically developed test facility. Furthermore, the methodology comprises determinations for easy preparation and handling of high-quality test specimens. This test method provides various advantages over other methods that have previously been used for measurement of the fundamental quantity Eb value of silicone rubbers. Both technical and economic demands are satisfied. The new facility also enables cost-effective routine tests in material research laboratories. The high quality of the obtained test results was verified by statistical analysis based on the 2-parameter Weibull distribution function. The investigations revealed that the virgin translucent silicone rubber has a large elastic region with an acceptable plastic deformation and also provides an AC 50 Hz dielectric strength of approximately 24 kV/mm for 0.5 mm thickness. These values enable considering the tested translucent silicone as replacement material for an opaque elastomer that is currently used for a rubber stress cone of HV cable accessories Unfortunately, its optical transmittance is poor compared to optically clear transparent silicone rubbers. On the other hand, the mechanical properties of virgin transparent silicone rubbers do not comply with those demanded from push-on stress cones. In particular, their elongation at break is considered too low for that application. However they provide the AC dielectric strength values in either 28 kV/mm or 29 kV/mm for 0.5 mm thickness, which are higher than those of the translucent type. Moreover, it was found that the post-curing process does not provide a positive impact on the ultimate elongation of silicone rubbers. Hence, the elongation at break of virgin transparent silicone rubbers must be improved before they can be used as insulating material for a rubber stress cone. In addition, the influence of mechanical tensile stress on the dielectric strength of the virgin translucent silicone rubber was investigated. The results show that mechanical tensile stress does not negatively influence on dielectric strength of such silicone rubber, so it can be well-operated under combined electrical and mechanical stresses. Beside the improvement of optical PD detection performance in the translucent silicone insulation materials, the influence of fluorescent dye’s modification was investigated. The results indicate that the commercially available fluorescent dyes of 0.02 wt. % mixed into the translucent silicone polymer do not negatively influence on the Eb value of such silicone material. So an optically compatible silicone rubber is perfectly suitable for the fabrication of novel fluorescent silicone optical fibres, which can be integrated into the modified transparent rubber stress cones of HV cable terminations. The final outcomes of this investigation are experimentally substantiated recommendations for future revision of IEC 60243-1, especially the chapter dealing with the determination of AC dielectric strength of silicone rubbers. Recommendations and suggestions for further investigations are addressed in the final chapter of this thesis.
119
Black fungi are recently described microorganisms and amongst the most stress-tolerant eukaryotes currently known. They are a taxonomically diverse, but morphologically similar group of filamentous fungi that share two distinct signature characteristics, i.e. melanisation of the cell wall and compact colony morphology, which confer them passive, constituent extremotolerance. Albeit morphologically undifferentiated, black fungi show extensive phylogenetic and ecological diversity. Due to their persistence in unfavourable niches, they are ubiquitous on deserts and in glaciers and are permanent settlers of rock and other atmosphere-exposed material surfaces as well as man-made environments like salterns, humidifiers and dishwashers, and thus widespread in temperate regions worldwide. Some members are devastating opportunistic pathogens of invertebrates or vertebrates, including humans; others show symbiotic potentials with co-occurring microorganisms in extreme ecosystems. Beside their interest for fundamental biology, black fungi are important for several applied applications, e.g. in biotechnology, astrobiology, bioremediation and material preservation. Despite recent advances in the study of these fungi, many biological questions remain to be clarified regarding the molecular mechanisms underlying persistence, their physiology and nutritional modes, and their specific interactions with putative symbiotic partners. Models for pathogenic and halotolerant black fungi are established; however, no model was yet available for rock- and material-inhabiting ones. This thesis introduces the strain Knufia petricola A95 as a suitable model to study rockinhabiting lifestyle. For this purpose, the strain was characterised at the physiological and molecular levels by phenotype microarrays, growth experiments and genome analyses as well as further methods. Cell- wall mutants of K. petricola A95 isolated during the course of this study were described and included in the comparative analysis to investigate effect of melanisation on physiology and stress tolerance. Direct comparisons were also performed between the model strain and the phylogenetically distant but ecologically, biogeographically and morphologically highly similar rock inhabitant Coniosporium apollinis. Preliminary observations of a model biofilm of K. petricola A95 and the photosynthetic cyanobacterium Nostoc punctiforme ATCC 29133 are introduced to study symbiotic interactions of rock-inhabiting microorganisms. Data presented here are a contribution to the understanding of ecophysiology and extremotolerance of rock-inhabiting black fungi.
117
Thermal and Dynamic Glass Transition in Ultrathin Films of Homopolymers and a Miscible Polymer Blend
(2014)
Nowadays nanoscale thin polymer films are widely used in many fields like coatings, membranes, sensors, electronic devices and so on. Meanwhile, a lot of research work has evidenced the fact that many physical properties (glass transition, crystallization, dewetting, physical aging, etc.) of ultrathin polymer films show strong deviations from their bulk behavior. Since the aforementioned properties of polymer are closely related to their application and functionality, the discrepancies motivated us to obtain a more complete understanding of how nanoscale confinement affects the physical properties of polymer. The research work presented in this thesis is focused on understanding how the free surface (air- polymer interface), the polymer-substrate interface and the film thickness influence the glass transition temperature (Tg) and the related segmental dynamics (α-relaxation process) in both homopolymers and miscible polymer blends of thin films. Complementary experimental techniques including Differential Scanning Calorimetry (DSC), Capacitive Scanning Dilatometry (CSD), Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS) have been used to investigate the glass transition of thin polymer films from both the thermodynamic and the kinetic point of view. In the thesis the film thickness dependence of Tg and segmental dynamics of different thin polymer films have been investigated. For ultrathin polycarbonate (PC) films capped between two aluminum (Al) layers an increase of both the glass transition temperature (Tg) and Vogel temperature (T0) with decreasing film thickness (d) was observed when the thickness became lower than 20 nm. The segmental relaxation time at a fixed temperature was found to increase for the ultrathin PC film of 19 nm measured by BDS, whereas no thickness dependency of the segmental dynamics was detected within the experimental error limit for the PC films supported on silicon dioxide (SiO2) (10-192 nm) in the SHS measurements. These properties are discussed in terms of the thin film geometry and the relevant interfacial interaction between the polymer and the substrate. In the case of thin polystyrene (PS) films with high molecular weight (Mw), Tg is decreasing with reducing film thickness while the segmental dynamics is independent of film thickness. Moreover, the effects of the Mw and the annealing protocol performed on thin PS films on their Tg and segmental dynamics is studied. In the part of thin poly(vinyl methyl ether) (PVME) films, no thickness dependence of the segmental dynamics was observed in the SHS measurements. The last part of the thesis was concentrated on the thin films of a miscible polymer blend, PS/PVME with the weight fraction of 50/50. It was observed that the segmental dynamics became faster with reducing the film thickness. This phenomenon is explained in terms of surface enrichment of PVME in the polymer blend system where PVME has a lower surface energy than PS. The segmental dynamics of the PVME-enriched free surface layer are faster than the bulk dynamics. Such free surface effect becomes so predominant with reducing the film thickness that it affects the segmental dynamics of the whole films detected by SHS using differential AC chip-based calorimetry. X-ray photoelectron spectroscopy (XPS) was used to probe the surface composition in order to confirm such surface enrichment phenomena.
116
Ultrasonic guided waves offer a wide range of applications in fields such as non-destructive testing, structural health monitoring or material characterization. They can be excited in thin-walled structures and propagate over comparably long distances. Due to their complex and dispersive propagation behavior, numerical methods are often required in order to analyze the guided wave modes that can be excited in a given structure and to simulate their interaction with defects. In the work presented in this thesis, highly efficient numerical methods have been developed that are specifically optimized for guided wave problems. The formulation is based on the Scaled Boundary Finite Element Method (SBFEM). The SBFEM is a semi-analytical method which evolved from the concept of Finite Elements but requires the discretization of the boundary of the computational domain only. To compute dispersion curves and mode shapes of guided waves, only the cross-section of the waveguide is discretized in the Finite Element sense, while the direction of propagation is described analytically. The wavenumbers of guided wave modes and the corresponding mode shapes are obtained as the eigenvalues and eigenvectors of a frequency-dependent Hamiltonian matrix. For the discretization, higher-order spectral elements are employed, leading to very low computational costs compared to traditional Finite Elements. Particular formulations are presented for plate structures as well as axisymmetric waveguides, where only the throughthickness direction has to be discretized. For the cases where the waveguide is embedded in or coupled to a quasi-infinite medium, a dashpot boundary condition is proposed in order to account for the effect of waves being transmitted into the surrounding medium. Though this approach is not exact, it leads to sufficiently accurate results for practical applications, while the computational costs are typically reduced by several orders of magnitude compared to other Finite Element based approaches. As a particular application, an experimental set-up for material characterization is discussed, where the elastic constants of the waveguide’s material are obtained from the analysis of waves propagating through the waveguide. A novel solution procedure is proposed in this work, where each mode of interest is traced over the required frequency range. The solutions are obtained by means of inverse iteration. To demonstrate the potential of the SBFEM for non-destructive testing applications, the interaction of guided wave modes with cracks in plates is simulated in the time domain for several examples. Particularly for the modeling of cracked structures, the SBFEM is very well suited, since the side-faces of the crack do not require discretization and the stress-singularity at the crack tip does not introduce additional difficulties. Hence, the computational costs can be reduced by typically a factor 100 compared to traditional Finite Elements and the meshing is straightforward.
112
Austenitic welds and dissimilar welds are extensively used in primary circuit pipes and pressure vessels in nuclear power plants, chemical industries and fossil fuelled power plants because of their high fracture toughness, resistance to corrosion and creep at elevated temperatures. However, cracks may initiate in these weld materials during fabrication process or stress operations in service. Thus, it is very important to evaluate the structural integrity of these materials using highly reliable non- destructive testing (NDT) methods. Ultrasonic non-destructive inspection of austenitic welds and dissimilar weld components is complicated because of anisotropic columnar grain structure leading to beam splitting and beam deflection. Simulation tools play an important role in developing advanced reliable ultrasonic testing (UT) techniques and optimizing experimental parameters for inspection of austenitic welds and dissimilar weld components. The main aim of the thesis is to develop a 3D ray tracing model for quantitative evaluation of ultrasonic wave propagation in an inhomogeneous anisotropic austeniticweld material. Inhomogenity in the anisotropic weld material is represented by discretizing into several homogeneous layers. According to ray tracing model, ultrasonic ray paths are traced during its energy propagation through various discretized layers of the material and at each interface the problem of reflection and transmission is solved. The influence of anisotropy on ultrasonic reflection and transmission behaviour in an anisotropic austenitic weld material are quantitatively analyzed in three dimensions. Theultrasonic beam directivity in columnar grained austenitic steel material is determined three dimensionally using Lamb’s reciprocity theorem. The developed ray tracing model evaluates the transducer excited ultrasonic fields accurately by taking into account the directivity of the transducer, divergence of the ray bundle, density of rays and phase relations as well as transmission coefficients. The ray tracing model is able to determine the ultrasonic wave fields generated by a point source as well as finite dimension array transducers.