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Die Bau-Ausstellung zu Beginn des 20. Jahrhunderts oder „Die Schwierigkeit zu wohnen“, so lautet der Titel der Arbeit und legt damit ihren Fokus offen. Sie betrachtet ein um 1900 aufkommendes Ausstellungsphänomen in seinen ersten Dekaden; in einer Zeit, in der das Wohnen problematisiert wurde.
Der Praxis des Wohnens ist Anfang des 20. Jahrhunderts seine Selbstverständlichkeit abgesprochen worden. Die Bau-Ausstellung wird als ein Symptom dieses Verlusts interpretiert und zugleich als Versuch, ihm entgegenzuwirken. Statt sie innerdisziplinär als eine architekturgeschichtliche Episode der Moderne rein positivistisch zu beschreiben, legt die vorliegende Arbeit im Typus der Bau-Ausstellung eine weit reichende Problematik frei, die kulturphilosophische ebenso wie medientheoretische Dimensionen hat: Inwieweit lässt sich das Wohnen überhaupt ausstellen? Wie kann man eine alltägliche Praxis, die kaum eine Definition erlaubt, zeigen? Den Umgang mit dieser Aporie untersucht die Arbeit.
Promotion an der Bauhaus-Universität Weimar (Dr. phil.) und zugleich PhD an der University of British Columbia Vancouver.
2015 auf dem Server der University of British Columbia mit Embargo veröffentlicht. Das Embargo wird immer wieder verlängert. Aktuell:
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Die Dissertation über „Ambiguität im zeitgenössischen Film – Flugversuche“ folgt der Spur einer populären narrativen Tendenz im Kino – nämlich der Mehrdeutigkeit – und zeichnet ihr dramaturgisches Potential, wie ihre ethischen (bzw. mikropolitischen) Implikationen nach. Um typische Muster in der Wahrnehmung mehrdeutiger Filmerzählungen zu beschreiben, die bereits auf der vorbewussten Ebene der Affekte wirksam sind, greife ich auf Begriffe der Prozessphilosophie Alfred North Whitehead’s zurück und auf ihre neueren Reformulierungen bei Gilles Deleuze und Brian Massumi. Ausgehend von Alejandro González Iñárritu’s "Babel" (2006) begibt sich der Leser im ersten Teil auf einen virtuellen Rundflug durch ausgewählte Filmbeispiele mit einem kulturellen Ankerpunkt im heutigen Japan. Im zweiten Teil beschreibe und reflektiere ich mein methodisches Vorgehen in den ersten Phasen der Stoffentwicklung zu einem suggestiven Spielfilmprojekt, und kontextualisiere es mit Interviews zeitgenössischer Autorenfilmer, die ähnliche Erzählweisen entwickeln.
Inhaltliche Struktur:
Theoretischer Teil: Um Kulturtechniken des künstlerischen Ausdruckes, und damit den Bezug zwischen Kunst und Medientechnologie zu ergründen, wurde das Verhältnis zwischen dem Betrachter und dem Betrachteten erforscht. Begriffe wie Originalität, Repräsentation, Imitation sowie Aneignung und Rezeption kommen an historischen Beispielen zur Diskussion.
Praktischer Teil: Mit zwei Ausstellungen (Malerei, Installation) wurden verschiedene Modelle des Sehens problematisiert, um kulturelle und historische Variablen zu ermitteln, die visuelle Kultur begründen. Außerdem wurden die Unterschiede zwischen Beobachter und Betrachter/Zuschauer in Ausstellungssituationen hinterfragt.
Kurzfassung:
Medientechnologie hat sich selbst zu einer Kunstform entwickelt. Sie kann Künstler dabei unterstützen, ihre Ideen handwerklich umzusetzen (Beispiel: Camera Obscura) und zu reflektieren. Dabei geht es um mehr als um technische Hilfmittel für den künstlerischen Ausdruck. Wird die Beziehung zwischen Betrachter und Betrachtetem als eine Transaktion gesehen, in der beide Begriffe gleichermaßen aufs Spiel gesetzt werden, dann ist es notwendig eine Visualität zu bestimmen, an der der Betrachter (im Sinne von Jonathan Crary) aktiv teilnimmt. Das Sehen darf nicht als absolute Erfahrung verstanden werden, denn einzeln analysiert – ohne Einbeziehung anderer Sinneswahrnehmungen und deren mediale Erweiterungen – führt es zu keinem adäquaten Verständnis davon, wie Bedeutungen produziert werden. Daraus ergibt sich die Forschungsfrage, wie die künstlerische Reaktion auf die im 19. Jahrhundert aufstrebenden Fotografie die Ausdrucksformen beeinflusste und ob sich die gegenseitige Abhängigkeit von Malerei und Fotografie wissenschaftlich-künstlerisch rekonstruieren lässt.
Die theoretische Referenz für diese Forschung bildeten Theorieansätze nicht der Kunstgeschichte, sondern der „Visual Culture Studies“, konkret: Martin Jay („Scopic Regimes of Modernity“), Jonathan Crary („Techniques of the Observer“) und Rosalind Krauss („Originality“).
Dazu wurde die Rezeption der aufkommenden Fotografie in der künstlerischen Produktion beispielhaft untersucht, wobei mit Eugène Delacroix, Gustave Caillebotte sowie dem künstlerischen Dialog zwischen Henri Matisse und Brassai die Problematik herausgearbeitet wurde, mit der sich die bildende Kunst im Lauf jener Jahrzehnte, in denen die Fotografie sich etabliert hat, konfrontiert sah. Sie war eine medientechnische Neuheit mit älteren Wurzeln und diente der Malerei als Hilfsmittel (Camera Obscura) und hat durch ihr technische Potenzial dennoch das Verständnis von Kunst radikal verändert: sie löste das Ideal einer bestimmten Maltechnik ab, die sich mit dem Verbergen des Pinselstriches verband, und bedeutete in diesem gewissen Sinn das „Ende der Kunst“ (nach Paul Delaroche).
Bis in die heutige Kunstproduktion zieht sich die Frage, was nach dem Ende der Perfektionierung von Reproduktion durch ihre Übernahme mittels medientechnischer Apparatur, die nun selbst zum Ausdruck strebt, überhaupt noch ein „Kunstwerk“ ist oder sein kann. Technisch gestützte Formen der Aneignung und Imitation stellen das klassische Wertemodell der meisterlich ausgeführten Repräsentation von Wirklichkeit in Frage, obwohl „Originalität“ angesichts der Flut von Reproduktionen doch immer noch - oder gerade deswegen - eine Wertschätzung erfährt.
Daher wurden dann auch im künstlerischen Teil verschiedene Modelle des Sehens getestet, um kulturelle und historische Variablen zu ermitteln, die visuelle Kultur beeinflussen und festlegen. Dabei wurde vor allem auch die Rolle der Beobachter reflektiert - die anders als bei Unterhaltungsmedien - in Kunstkontexten eben keine passiven Zuschauer sind. Was den Künstler, und was den Betrachter von Kunst definiert, lässt sich gleichwohl weder auf ein bestimmte Sinnlichkeit noch eine bestimmte Medientechnologie reduzieren.
Wie schon nach dem Aufkommen der Fotografie werden Künstler weiterhin auf der Suche nach neuen Ausdrucksformen sein, um andere Weltwahrnehmungen und neue, andere Perspektiven menschlicher Existenz zu erlangen. Mediale Technologien haben sie dabei immer schon unterstützt. Aus meiner Untersuchung schließe ich, dass neue Technologien Kunst nicht definitv beeinflussen, wohl aber ihren Entstehungsprozess verändern und beeinflussen können. Die Frage, wie das geschieht und welche Effekte es zeitigt, bedarf einer stets erneuerten Verhandlung.
Daraus folgt die Schlussfolgerung, dass man von Künstlern verlangen kann oder sogar muss, sich vor der Wirklichkeit der Technologie nicht zurückzuziehen, sondern sich medientechnische Kompetenzen anzueignen und sich aktiv am Diskurs um neue Technologien zu beteiligen.
Die Ph.D.-Arbeit Soziales Grafikdesign – eine Frage der Haltung hinterfragt den Einfluss und die Wirkungsmacht, den visuelle Kommunikationsgestaltung auf den Betrachter und im weiteren Sinne auf die Gesellschaft im Ganzen haben können. Aufbauend auf den Erkenntnissen über die Wirkungsmacht wird untersucht, wie eine bewusste und sozial verantwortliche Haltung im Grafikdesign aussehen könnte.
Viele Grafikdesigner sind in der Werbebranche tätig und haben den Auftrag, ganz bewusst manipulativ und wirkungsstark Informationen visuell zu übersetzen, um den Betrachter zu einer Handlungsfolge zu animieren. Die bewusste Reflexion der eigenen Rolle als Designer in diesem Prozess führt immer wieder zu den Fragen nach der Verantwortlichkeit des Gestalters und auch dazu, wie der Gestalter die Potenziale der Kommunikationsgestaltung in anderen Bereichen, wie z. B. in städtischen Problemsituationen, für ein soziales Miteinander einsetzen kann.
In this study, the behavior of a widely graded soil prone to suffusion and necessity of homogeneity quantifi cation for such a soil in internal stability considerations are discussed. With the help of suffusion tests, the dependency of the particle washout to homogeneity of sample is shown. The validity of the great infl uence of homogeneity on suffusion processes by the presentation of arguments and evidences are established. It is emphasized that the internal stability of a widely graded soil cannot be directly correlated to the common geotechnical parameters such as dry density or permeability. The initiation and propagation of the suffusion processes are clearly a particle scale phenomenon, so the homogeneity of particle assemblies (micro-scale) has a decisive effect on particle rearrangement and washout processes. It is addressed that the guidelines for assessing internal stability lack a fundamental, scientifi c basis for quantifi cation of homogeneity. The observation of the segregation processes within the sample in an ascending layered order (for downwards fl ow) inspired the author to propose a new packing model for granular materials which are prone to internally instability.
It is shown that the particle arrangement, especially the arrangement of soil skeleton particles or the so-called primary fabric has the main role in suffusiv processes. Therefore, an experimental approach for identifi cation of the skeleton in the soil matrix is proposed. 3D models of Sequential Fill Tests using Discrete Element Method (DEM) and 3D models of granular packings for relative, stochastically and ideal homogeneous particle assemblies were generated, and simulations have been carried out.
Based on the numerical investigations and in dependency on the soil skeleton behavior, an approach for measurement of relevant scale, the so-called Representative Elementary Volume (REV) for homogeneity investigation is proposed. The development of a new testing method for quantifi cation of homogeneity is introduced (in-situ). An approach for quantifi cation of homogeneity in numerically or experimentally generated packings (samples) based on image processing method of MATLAB has been introduced. A generalized experimental method for assessment of internal stability for widely graded soils with dominant coarse matrix is developed, and a new suffusion criterion based on ideal homogeneous internally stable granular packing is designed.
My research emphasizes that in a widely graded soils with dominant coarse matrix, the soil fractions with diameters bigger than D60 build essentially the soil skeleton. The mass and spatial distribution of these fractions governs the internal stability, and the mass and distribution of the fi ll fractions are a secondary matter. For such a soil, the homogeneity of the skeleton must be cautiously measured and verified.
Problem definition and research objectives
1. The production of Portland cement clinker causes approx. 5% to 8% of the annual man-made CO2 emissions. This is due to the usage of mainly fossil fuel (approx. 40 % of the total CO2) and because of the decarbonation of limestone as a main component of the raw meal (approx. 60 % of the total CO2).
2. Various strategies are applied in order to reduce the green-house gas-emissions, such as optimizing the process of clinker production, the use of alternative fuel and the partial substitution of the clinker in blended cement by so-called SCM (supplementary cementitious materials). Hereby blast-furnace slag, fly ash and limestone are the most used materials.
3. Quaternary systems containing three SCM simultaneously besides Portland cement contribute to the reduction of CO2 emissions due to the decrease of the clinker content. In addition, such systems allow to use blast-furnace slag and fly ash in the most economical way and provide the possibility to account for shortages of SCM on the market.
4. Blast-furnace slag and fly ash show similarities in their principal chemical compositions such that similar hydrates are formed during their reaction in presence of Portland cement. Compared to ternary systems based on blast-furnace slag or fly ash besides limestone, quaternary systems that contain both, blast-furnace slag and fly ash, simultaneously besides limestone, are expected to perform similar in terms of phase assemblage and strength development.
5. The use of SCM as cement replacing materials is limited due to their generally slower reaction compared to neat cement which also leads to lower strength development, especially in the early stage of the hydration up to 28 d. To account for this it is necessary to study the reactivity of SCM such as blast-furnace slag and fly ash in detail in order to develop strategies to enhance the reactivity and thereby the strength development of SCM-containing systems.
6. The early hydration of clinker phases is studied in detail, mainly in diluted systems. It is unclear if processes that were found to control the reaction of such model systems are also prevailing in concentrated cement pastes under realistic water-to-solid ratios. Deeper insight to this aspect is needed to better understand interactions of neat Portland cement and SCM in the first hours of hydration.
State-of-the-art
7. Increasing Ca-concentrations lead to decreasing dissolution rates of C3S and C2S in diluted systems.
8. The hydration kinetics of C3S is controlled by the interplay of undersaturation with respect to C3S and oversaturation with respect to C-S-H.
9. Increasing Al-concentrations lead to a retardation of the hydration of C3S. It is unclear if the uptake of aluminum in C-S-H to form C-(A)-S-H which has a significantly lower growth rate than pure C-S-H or a retarding effect of Al on the dissolution of C3S causes this phenomenon.
10. The surface of limestone provides excellent conditions for the nucleation and growth of C-S-H such that significantly more C-S-H nuclei are formed in presence of limestone compared to other SCM.
11. The reactivity of blast-furnace slag and fly ash depends on the particle size as well as on the intrinsic reactivity of especially the amorphous phases.
12. An increase in network modifying oxides (e.g. CaO) in the chemical composition of amorphous (calcium)aluminosilicates leads to an increasingly depolymerized network which in turn causes increasing reactivity. The role of amphoteric oxides (Al2O3, Fe2O3) that can be present as network modifying oxides as well as network forming oxides is not completely solved.
13. CO2-containing AFm-phases are thermodynamically more stable than monosulfoaluminate. This indirectly stabilizes the voluminous ettringite which causes a higher total volume of hydrates and lower porosity whereby higher compressive strength is reached.
14. Only a few percent of limestone in blended cement reacts chemically dependent on the Al2O3 available for reaction. Al2O3 that is provided by the reaction of Portland cement but also by the dissolution of SCM, especially by fly ash, reacts to form hemicarboaluminate which is transformed to monocarboaluminate as the hydration proceeds.
Methodology
15. The influence of SCM on the early hydration of Portland cement in binary (including blast-furnace slag or fly ash or limestone or quartz) and ternary (including fly ash and limestone) systems was investigated applying isothermal calorimetry and analysis of the pore solution chemistry. Calculated saturation indices and solubility products of relevant phases were correlated with heat development. Based on the gained data it was reviewed if mechanisms that control the hydration of pure phases in diluted systems are also prevailing in cement pastes under realistic conditions.
16. The influence of the chemical composition of synthetic glasses on their dissolution at high pH was investigated in highly diluted systems using ion chromatography. Pozzolanity tests were conducted on pastes using simplified model systems and glass-blended Portland cements. The process of the glass dissolution was investigated by isothermal calorimetry and by thermogravimetry. Correlation of experimentally determined total bound water with bound water determined by mass balance calculations as a function of amount of glass reacted allowed to estimate the degree of glass reaction in the pastes. Further on selective dissolution experiments were carried out to crosscheck the results of the bound water/mass balance approach.
17. The reaction kinetics of quaternary pastes containing blast-furnace slag and fly ash simultaneously in the presence of limestone were investigated up to 28 d using isothermal calorimetry and chemical shrinkage measurements. In addition strength tests on mortar bars were carried out.
18. Pastes of quaternary blends were also investigated in terms of hydrate assemblage at ages of up to 182 d. Thermodynamic calculations regarding total volume of hydrates as a function of limestone and fly ash/blast-furnace slag content were conducted. The calculations were supported by thermogravimetric determination of bound water and portlandite content as well as qualitative X-ray diffraction. The results were correlated with strength tests on mortar bars.
19. The pore solutions of hydrated quaternary blends were extracted and investigated by means of ion chromatography at ages of up to 728 d. Based on the ion concentrations in the solutions saturation indices were calculated for relevant phases. In order to gain better insight to the blast-furnace slag reaction sulphate speciation was carried out at two blast furnace slag levels (20 and 30 wt.%) for selected samples up to 91 d of hydration and at 91 d for the whole matrix under investigation.
Main results
20. Investigations on the early hydration kinetics of binary systems showed a higher heat flow in presence of SCM compared to neat Portland cement. This is caused by the higher surface area that is available for the nucleation of hydrates and by the lower (over)saturation with respect to C-S-H. An increase in the Ca-concentration in the pore solution did not cause lower dissolution rates of C3S as was reported for pure phases in diluted systems. The highest dissolution was observed in the presence of limestone, i.e. at the highest Ca-concentration. The general trend of the reaction rate is inversely related to the degree of undersaturation with respect to C3S. The more undersaturated the faster the observed reaction. The presence of increasing Al-concentrations caused a retardation of the reaction which is in line with investigations on pure phases in diluted systems. Higher sulphate concentrations could be detected for the fly ash containing blend which possibly hindered ettringite precipitation and results in higher Al-concentrations. Correspondingly the low sulphate concentrations lead to lower Al-concentrations in the presence of quartz, blast-furnace slag and limestone compared to fly ash.
21. The early hydration kinetics of quaternary systems is significantly accelerated in the presence of limestone while fly ash leads to retardation. Compared to reference systems containing inert quartz, investigations by means of isothermal calorimetry and chemical shrinkage revealed an acceleration caused by blast-furnace slag. Additions of fly ash, limestone or mixtures thereof introduced another acceleration but differences are too small to be significant and clear distinguishing between the various SCM is not possible.
22. Investigations on the reactivity of synthetic glasses showed that increasing amounts of network modifying oxides caused an increase in reactivity and dissolution rates. The results reveal that Al2O3 acts mainly as network modifying oxide in all investigated glasses. Experimentally determined bound water (thermogravimetric experiments) in model systems and blended cements can be compared with bound water determined by mass balance calculations carried out as a function of the amount of glass reacted. This enables to estimate the degree of glass reaction.
23. The actual content of blast-furnace slag, fly ash or limestone does not exert significant influence on the development of compressive strength up to 7 d. At later ages thermodynamic calculations predict a degree of CaCO3 reaction of 2 to 5 wt.%. This leads to the formation of hemicarbonate and monocarbonate whereby ettringite is indirectly stabilized. As a result the total amount of solids is increased and compressive strength shows a slight maximum. Hereby the degree of CaCO3 reaction depends on the Al2O3 available for reaction which is not only provided by the dissolution of Portland cement but especially by the dissolution of the fly ash.
24. In general the presence of blast-furnace slag and fly ash in the presence of limestone exerts little influence on the hydrate assemblage. The substitution of some of the blast-furnace slag by fly ash leads to a slight decrease of portlandite and C-S-H and gives rise to the formation of more monocarbonate and hemicarbonate. Portlandite is consumed in a pozzolanic reaction with the fly ash whereby C-S-H is formed. However, the low reactivity of the fly ash causes a decrease in the amount of C-S-H formed. Thereby a lower total volume of hydrates is generated which is in line with slightly lower compressive strength in case of increasing fly ash content. The overall influence is small and all systems investigated reach strength class 42.5 N according to EN 196-1.
25. Corresponding to the investigations of the hydrate assemblage the pore solution chemistry of quaternary systems showed only small variations. Depending on the fly ash content the highest variations are observed for aluminium, i.e. increasing fly ash content leads to higher Al-concentrations. Another effect of increasing fly ash contents is an increasing undersaturation with respect to portlandite and a decreasing undersaturation with respect to strätlingite indicating the dissolution of portlandite.
26. The total concentration of sulfur in the pore solution is controlled by sulphate (SO42–) while the concentrations of sulphite (SO32–) and thiosulphate (S2O32–) were very low. Up to 2 d of hydration about 90 % of the total sulphur is present as SO42–. After 91 d this value is reduced to about 36 % while about 28 % are present as S2O32–. In general higher blast-furnace slag content leads to higher concentrations of sulphite and thiosulphate after 7 d.
Einfluss der Porosität von Beton auf den Ablauf einer schädigenden Alkali-Kieselsäure-Reaktion
(2016)
Der Dissertation liegt die Frage zugrunde, welchen Einfluss die Porosität von Beton auf den Ablauf einer schädigenden AKR hat. Insbesondere soll geklärt werden, ob der Einsatz von Gleitschalungsfertigern – anstelle von klassischen schienengeführten Betondeckenfertigern – und die damit verbundene verringerte Porosität der Betone den Ablauf einer schädigenden AKR begünstigt. Eine verringerte Porosität führt zu einer reduzierten Duktilität des Betons, so dass infolge AKR entstehende Zugspannungen schlechter abgebaut werden können. Weiterhin steht ein geringerer Expansionsraum für das entstehende AKR-Gel zur Verfügung. Diese Faktoren können den Ablauf einer AKR begünstigen. Allerdings können extern anstehende Alkalien schlechter in den Beton mit einer verringerten Porosität eindringen. Ferner wird die Diffusion der Alkalien zu den potenziell reaktiven Gesteinskörnungen verlangsamt.
Zur Beantwortung der aufgeworfenen Frage wird unter Einsatz einer neuartigen Prüfmethodologie und bei variierender Porosität untersucht, welche Schädigungsparameter maßgebend für den Ablauf und die Intensität einer schädigenden AKR sind. Die berücksichtigten Schädigungsparameter sind die mechanischen Eigenschaften des Betons, der zur Verfügung stehende Expansionsraum und die im Beton ablaufenden Transportvorgänge. Um den Einfluss der jeweiligen Schädigungsparameter spezifizieren zu können, gehen die Prüfungen einerseits von einem hohen internen und andererseits von einem hohen externen AKR-Schädigungspotenzial aus. In beiden Fällen erfolgen die Untersuchungen an langsam reagierenden alkaliempfindlichen Gesteinskörnungen. Die unterschiedlichen Porositäten ergeben sich hauptsächlich durch Variation des w/z-Wertes.
Bei dem hohen internen AKR-Schädigungspotenzial stehen die mechanischen Eigenschaften und der Expansionsraum im Vordergrund; außerdem ist der Einfluss der Zugabe eines LP-Bildners zu analysieren. Um ein hohes internes AKR-Schädigungspotenzial zu erreichen, kommt bei der Herstellung der Betonprobekörper ein Zement mit einem hohen Alkaligehalt zum Einsatz. Die Betonprobekörper werden der 40 °C-Nebelkammerlagerung und dem 60 °C-Betonversuch über Wasser unterzogen. Dabei findet eine neue Prüfmethodologie Anwendung, die der kontinuierlichen Messung der Dehnung und der ablaufenden Erhärtungs- und Rissbildungsprozesse dient. Diese Prüfmethodologie umfasst die Messung der Ultraschallgeschwindigkeit, die Schallemissionsanalyse und die µ-3D-Computertomografie.
Hingegen richtet sich der Fokus bei dem hohen externen AKR-Schädigungspotenzial auf die Transportvorgänge. Zur Provokation eines hohen externen AKR-Schädigungspotenzials werden die Probekörper der FIB-Klimawechsellagerung ausgesetzt.
The key objective of this research is to study fracture with a meshfree method, local maximum entropy approximations, and model fracture in thin shell structures with complex geometry and topology. This topic is of high relevance for real-world applications, for example in the automotive industry and in aerospace engineering. The shell structure can be described efficiently by meshless methods which are capable of describing complex shapes as a collection of points instead of a structured mesh. In order to find the appropriate numerical method to achieve this goal, the first part of the work was development of a method based on local maximum entropy (LME)
shape functions together with enrichment functions used in partition of unity methods to discretize problems in linear elastic fracture mechanics. We obtain improved accuracy relative to the standard extended finite element method (XFEM) at a comparable computational cost. In addition, we keep the advantages of the LME shape functions,such as smoothness and non-negativity. We show numerically that optimal convergence (same as in FEM) for energy norm and stress intensity factors can be obtained through the use of geometric (fixed area) enrichment with no special treatment of the nodes
near the crack such as blending or shifting.
As extension of this method to three dimensional problems and complex thin shell structures with arbitrary crack growth is cumbersome, we developed a phase field model for fracture using LME. Phase field models provide a powerful tool to tackle moving interface problems, and have been extensively used in physics and materials science. Phase methods are gaining popularity in a wide set of applications in applied science and engineering, recently a second order phase field approximation for brittle fracture has gathered significant interest in computational fracture such that sharp cracks discontinuities are modeled by a diffusive crack. By minimizing the system energy with respect to the mechanical displacements and the phase-field, subject to an irreversibility condition to avoid crack healing, this model can describe crack nucleation, propagation, branching and merging. One of the main advantages of the phase field modeling of fractures is the unified treatment of the interfacial tracking and mechanics, which potentially leads to simple, robust, scalable computer codes applicable to complex systems. In other words, this approximation reduces considerably the implementation complexity because the numerical tracking of the fracture is not needed, at the expense of a high computational cost. We present a fourth-order phase field model for fracture based on local maximum entropy (LME) approximations. The higher order continuity of the meshfree LME approximation allows to directly solve the fourth-order phase field equations without splitting the fourth-order differential equation into two second order differential equations. Notably, in contrast to previous discretizations that use at least a quadratic basis, only linear completeness is needed in the LME approximation. We show that the crack surface can be captured more accurately in the fourth-order model than the second-order model. Furthermore, less nodes are needed for the fourth-order model to resolve the crack path. Finally, we demonstrate the performance of the proposed meshfree fourth order phase-field formulation for 5 representative numerical examples. Computational results will be compared to analytical solutions within linear elastic fracture mechanics and experimental data for three-dimensional crack propagation.
In the last part of this research, we present a phase-field model for fracture in Kirchoff-Love thin shells using the local maximum-entropy (LME) meshfree method. Since the crack is a natural outcome of the analysis it does not require an explicit representation and tracking, which is advantageous over techniques as the extended finite element method that requires tracking of the crack paths. The geometric description of the shell is based on statistical learning techniques that allow dealing with general point set surfaces avoiding a global parametrization, which can be applied to tackle surfaces of complex geometry and topology. We show the flexibility and robustness of the present methodology for two examples: plate in tension and a set of open connected
pipes.
Piezoelectric materials are used in several applications as sensors and actuators where they experience high stress and electric field concentrations as a result of which they may fail due to fracture. Though there are many analytical and experimental works on piezoelectric fracture mechanics. There are very few studies about damage detection, which is an interesting way to prevent the failure of these ceramics.
An iterative method to treat the inverse problem of detecting cracks and voids in piezoelectric structures is proposed. Extended finite element method (XFEM) is employed for solving the inverse problem as it allows the use of a single regular mesh for large number of iterations with different flaw geometries.
Firstly, minimization of cost function is performed by Multilevel Coordinate Search (MCS) method. The XFEM-MCS methodology is applied to two dimensional electromechanical problems where flaws considered are straight cracks and elliptical voids. Then a numerical method based on combination of classical shape derivative and level set method for front propagation used in structural optimization is utilized to minimize the cost function. The results obtained show that the XFEM-level set methodology is effectively able to determine the number of voids in a piezoelectric structure and its corresponding locations.
The XFEM-level set methodology is improved to solve the inverse problem of detecting inclusion interfaces in a piezoelectric structure. The material interfaces are implicitly represented by level sets which are identified by applying regularisation using total variation penalty terms. The formulation is presented for three dimensional structures and inclusions made of different materials are detected by using multiple level sets. The results obtained prove that the iterative procedure proposed can determine the location and approximate shape of material subdomains in the presence of higher noise levels.
Piezoelectric nanostructures exhibit size dependent properties because of surface elasticity and surface piezoelectricity. Initially a study to understand the influence of surface elasticity on optimization of nano elastic beams is performed. The boundary of the nano structure is implicitly represented by a level set function, which is considered as the design variable in the optimization process. Two objective functions, minimizing the total potential energy of a nanostructure subjected to a material volume constraint and minimizing the least square error compared to a target
displacement, are chosen for the numerical examples. The numerical examples demonstrate the importance of size and aspect ratio in determining how surface effects impact the optimized topology of nanobeams.
Finally a conventional cantilever energy harvester with a piezoelectric nano layer is analysed. The presence of surface piezoelectricity in nano beams and nano plates leads to increase in electromechanical coupling coefficient. Topology optimization of these piezoelectric structures in an energy harvesting device to further increase energy conversion using appropriately modified XFEM-level set algorithm is performed .