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Organisationseinheit der BAM
Glass is an amorphous material. When compared to steel, both its density and weight is three times lower. Its high theoretical strength makes it stand out as a premier material for a variety of applications. One such application is acting as a pressure resistant vessel for gas storage. Because glass has a high theoretical strength this makes it potentially suitable to withstand much higher pressures than steel or composite vessels. As a result of its brittle character, glass breaks when reaching a critical stress level. Therefore, the stress distribution during pressure load needs to be homogeneous without local stress peaks. At those peaks an initial crack will occur and the material will break. This PhD thesis is primarily concerned with the determination of the strength of several structures made of single hollow glass fibers during inner pressure treatment. Therefore, different kinds of hollow glass structures with varying parameters of shape and dimension were examined concerning their strength by determining the burst pressure. The burst pressure method was compared to the tensile test method, which poses the common test method for examining the strength of a material. The conclusion reached was that both test methods lead to comparable results and therefore, the burst pressure method poses an adequate tool for examining the strength of a hollow material against inner pressure. Another tool used in this thesis is the Finite Elements Method (FEM) simulation of internal stress and expansion of glass structures during pressure treatment. FEM was used to validate the burst pressure test results. A few selected material parameters needed to be incorporated, most notably the Young’s Modulus. Therefore, the expansion of single glass fibers was measured with light microscope during pressure load. Within the parameters of expansion, wall thickness and applied pressure, the Young’s Modulus was calculated with the Barlow’s Formula. According to the results, different two-dimensional models from single fibers to complex structures with up to 1000 single fibers were constructed and simulated with the CFD software Comsol Multiphysics. The expansion as well as the principal stress during pressure load was calculated. Different dimensions as well as different geometries of the glasses were considered to find a structure with the highest possible free volume and at the same time as less stress peaks as possible. This calculation was made in order to determine the best structure for gas storage. For this purpose the calculations were done with different dimensions of round single fibers right up to hexagonal structures consisting of more than one thousand round single fibers, which resulted in constant expansion of the structure. Furthermore, the problem of occurring interspaces between round single fibers, regarding their burst pressure-decreasing influence, was approached. Closing these interspaces with glass or other materials to avoid unsolicited pressure load led to increased strength of the structure and low storage capacities due to the increased weight and less free inner volume. The behavior of hexagonal fibers was determined as single fiber as well as in bundled condition. The walls between two hexagonal single fibers with applied inner pressure showed homogeneously distributed stress. Merely the outer walls without counter pressure showed high deformation and high structural stress. Based on that knowledge, several structures were modeled varying in different aspects. The fibers with hexagonal shape showed optimal stress distribution and high storage capacities because of high free inner volume, provided that these fibers are surrounded by additional fibers with identical inner pressure. Reducing the wall thickness for even higher free inner volume led to similar distribution but higher stress and expansion. To overcome the problem with the high stress at the outer fibers, the influence of outer fibers with different shape and dimension was simulated as well as the influence of solid glass fibers at the outer layer of the structure. The results showed that a structure with hexagonal thin-walled fibers should be surrounded by round fibers with higher wall thickness. This way the high stress peaks at the outer fibers are lowered. The examined practical strength of glass is about 100 to 1000 times lower than the theoretical strength. This is caused by defects, which may occur at the glass surface by handling or inside the material by defective production. Since the modeled results are based on the theoretical strength, the optimal wall thickness with a good compromise of strength and free inner volume needs to be found in practical tests. If further handling of the structures is necessary, an outer layer of solid fibers works as a protection layer against damages at the outer hollow glass fibers and increases the strength. Additionally, the influence of collapsing fibers inside a structure on the remaining system has been modeled as well as the influence of defects like holes or cracks at the surface or manufacturing induced defects inside the material. Any kind of defect leads to areas of high stress, whereby failure occurrence will be encouraged. In order to approve the theoretical results, the simulated structures were compared to the previously manufactured and tested ones. Due to the burst pressure test results, the tested structures showed low strength compared to the theoretical strength. This was primarily caused by the existence of defects in the material and on the surface of the glass structures. Therefore, the production process needs to be optimized in order to prevent such defects. Furthermore, an additional protection against outer influence like air humidity or the physical contact to other materials is required.
For almost 150 years it is known that hydrogen has a deleterious effect on the mechanical properties of metallic components. Nowadays, the problem of hydrogen assisted degradation is highly relevant in energy related fields due to the massive use of steel as a structural component in these applications and its sensitivity to hydrogen. Since the discovery of hydrogen assisted cracking (HAC), researchers studied intensively and suggested possible explanations and mechanisms in order to define how hydrogen is affecting the material. In general, it is considered that hydrogen changes the mechanical properties more in terms of ductility (deformation capacities) than in strength (load capacities). Hydrogen concentration is one of three crucial factors in the degradation process, together with the microstructure of the material and the internal/external mechanical load. The relatively high concentration of hydrogen resulting in this loss of ductility can originate during production or before service (e.g. welding processes) and during service (i.e. catholically protected systems to eliminate corrosion processes in sour environments).
In parallel to the theoretical work, tremendous efforts were, and are still, invested in searching for a proper method to elucidate, map and quantify the hydrogen in the microstructure, which is the basis for this work. For steels, the focus is mainly on the observations of diffusion processes and the interaction of hydrogen with the microstructure
in regions with high local stresses/strains (for example around evolving cracks). The challenge for reaching this goal arises from the fact that accurate indication of hydrogen by means of position, unlike heavier atoms, can be made only by mass spectrometry or by interaction with another element (e.g. silver decoration, special coating and resonant nuclear reaction by nitrogen). In addition to this, the difficulty recording the hydrogen behavior while it rapidly diffuses through the material, leaving only the unpredicted failure, should be taken into account.
Although using powerful characterization methods, models and computational simulations, the key to defining the mechanisms behind HAC is still under debate and not fully understood. The relationship between material and hydrogen is determined by three factors, i.e., the material structure and microstructure – determining the physical properties, the mechanical load applied on the material and the hydrogen concentration. It is well known that in order to have a complete definition of HAC these three factors must be examined locally with the minimal scale and the maximal resolution reachable. The major gap is the lack in such a characterization method or a technique by which one has the ability to detect and observe the hydrogen in the metallic microstructure. The commonly used techniques nowadays are capable of characterization of the microstructure without the ability to observe the hydrogen distribution. Global hydrogen concentration and localized hydrogen observation are possible by some techniques which are incapable of indicating a change in the structure or microstructure therefore a comprehensive overview can be gained only by combining several methods.
In the presented research, secondary ion mass spectrometry (SIMS) was adopted as the main tool to detect and locally map the hydrogen distribution in two types of duplex stainless steel grades: EN 1.4462 (standard 2205 duplex stainless steel) and EN 1.4162 (2101 lean duplex stainless steel). The term duplex stainless steel (DSS) refers to the austenitic-ferritic microstructure of the steel where the combination of physical and mechanical properties of the two phases is achieved. The DSS was selected as a case study for this work due to the wide use of this grade in many energy and the lack of knowledge on hydrogen behavior in two-phase containing microstructures. ToFSIMS was exploited in-situ and ex-situ in three experimental approaches during or following
an electrochemical charging procedure. This type of hydrogen charging was selected as it simulated a procedure of cathodic protection of most sub-water oil and gas extraction and delivery systems. The experimental procedures were:
1. Ex-situ charging followed by ToF-SIMS imaging for basic understanding of hydrogen distribution.
2. Ex-situ charging followed by in-situ mechanical loading to obtain information on hydrogen behavior around a propagating crack.
3. In-situ permeation of hydrogen through a steel membrane inside the ToF-SIMS to obtain information on diffusion behavior of hydrogen in a two-phase microstructure.
The comprehensive view of the effect of hydrogen on steel was gained by using supplementary methods, such as high resolution scanning electron microscopy (HR-SEM), focused ion beam (FIB) and electron back-scattered diffraction (EBSD). The state of the art in this work lies in applying both: in-situ experimental approaches and data treatment of the ToF-SIMS raw data. The data treatment includes the combination of data from several sources (data fusion).
The results for the ex-situ charging followed by static sample imaging and data fusion showed that when the analyzed surface is directly exposed to the electrolyte the degradation is pronounced differently in the ferrite, austenite and interface. The degradation mechanisms in the ferrite and austenite were reflected by the formation of cracks on the surface of both, where a high concentration of hydrogen was obtained. This result supports the assumption that hydrogen is attracted to highly deformed regions. The advantage of using in-situ charging/permeation in comparison to ex-situ charging is that the effect of hydrogen on the ferrite and austenite phases when the hydrogen is evolving from within the microstructure is realized, in comparison to when the analyzed surface is initially exposed directly to the electrolyte. In both experiments the ferrite was observed as a fast diffusion path for the hydrogen. The faster diffusion of hydrogen through the ferrite is expected due to the higher diffusion coefficient, however, a direct proof for the diffusion sequence in this scale was never shown. Most significant results were achieved by the ‘core’ experiments of this research. These experiments included the design of a novel dynamic mechanical loading device to apply an external load during SIMS imaging of a hydrogen precharged-notched sample. For the first time it was shown that plastic deformation induced by applying a mechanical load is resulting in a redistribution of hydrogen locally around the notch.
Glass has different outstanding material-specific properties which offer theoretically the application of thin-walled hollow fibers in the field of high pressure gas storage. Especially the storage of hydrogen as renewable and environmental friendly energy carrier is possible. Glass is an amorphous material which is characterized by a theoretical tensile strength much higher than this of other materials. However, in practice the tensile strength is decreased significantly by defects on the glass surface or in the material and its network structure. As part of this thesis the burst pressures of hollow glass fibers were determined. The burst pressure correlates very well with the tensile strength of hollow glass fibers. By using the Weibull statistic the results of different test series were evaluated in respect to failure probabilities and compared to each other. Thereby the influence of various parameters on the pressure resistance was investigated. Beside the influence of the chemical composition of the material the aging by environmental and their effect on the pressure resistance was investigated. Additionally hollow glass fibers were loaded dynamically and statically with different gases. Afterwards the burst pressure was determined and the effect of used gas on the pressure resistance could be determined as well as the impact of method and duration of loading. A further influence of the dimension of hollow glass fibers on their resistance against inner pressure load is the ratio between wall thickness and inner diameter which was investigated as well as the combination of different glasses and the utilization of their disparate coefficient of thermal expansion which lead to prestressing of the hollow glass fiber. Finally, the impact of the variation of several production parameters on the pressure resistance was determined experimentally as like as the influence of surface coatings. These shall protect the glass surface from subsequently procured defects and, hence, increase the pressure resistance. As essential part of current thesis the defect analysis of test samples of various series was done whereby the differentiation between material and production dependent defects was important. Not only a light microscope but a scanning electron microscope was used for the investigation, as well. Beside volume defects like bubbles or inclusions surface defects in the form of scratches or spalling can be detected and observed. A calculation of the failure-causing defect size from measured burst pressure is possible. Dependent on the dimension and determined burst pressure value of each single fiber defect sizes of less than one micron were calculated. Particularly the geometry of the test samples inappropriate for many examination methods and the fact that the calculated defect size occurs only under loaded conditions at actual burst pressure the local detection of corresponding defect rendered impossible. In the end, the present thesis shows the pressure resistance of hollow glass fibers and their potential to store safely gases under high pressure.