Multi-scale modelling of thermal shock damage in refractory materials

Refractories are high-temperature resistant materials used extensively in many engineering structures and assemblies in a wide spectrum of applications ranging from metallurgical furnace linings to thermal barrier coatings. Such structures are often exposed to severe thermal loading conditions in the form of rapid temperature changes (thermal shock) and/or temperature cycles. The understanding and modelling of the failure processes are definitely necessary to achieve reliable life-time predictions of the existing structures and to develop design rules for improvement. Due to their high temperature resistance, alumina based refractory ceramics with a porous granular microstructure being far from homogenous are commonly used in the applications as mentioned above. In such heterogeneous material systems, local thermal expansion (CTE) mismatches, non-uniformities and anisotropy of the different constituents naturally lead to the appearance of internal stresses which are essentially the driving mechanisms for micro-cracking and damage. Under highly transient external thermal loading conditions, the resulting heterogeneous temperature distribution may lead to a complicated mechanical response along with a nonuniform mechanical and physical property degradation accompanied by irreversible geometry changes. The altered distribution of the mechanical properties dictates the macroscopic response when the external loading is further varied. Therefore, a strong coupling between the evolving microstructure and the macroscopic response arises. Moreover, microstructural configurational changes may trigger a significant interaction between the mechanical and thermal fields, for instance due to a reduced heat transport across a damaged interface. Therefore, an approach taking into account these mechanisms sufficiently well would render a versatile tool to improve the understanding of the influence of mechanical and thermal properties at the constituent level and their mutual interaction from a microstructural perspective. In this thesis, a concurrent multi-scale framework for the thermo-mechanical analysis of heterogeneous materials is proposed, with a particular focus on coarse grained refractory ceramics. The framework is essentially based on a rigorous extension of the well established FE2 computational homogenization technique, where the local macroscopic response is determined through the solution of a boundary value problem defined on a representative volume of the underlying microstructure. At first, the computational homogenization ideas are explored in the context of pure heat conduction processes in heterogeneous solids. Subsequently, the framework for coupled thermo-mechanical analyses is constructed by combining the first order mechanical homogenization with the dual procedure developed for heat conduction, within an operator-split (or staggered) solution algorithm which is composed of incrementally uncoupled nested (FE2) solution blocks for thermal and mechanical equilibrium subproblems. For predictive computations, the mechanical and thermophysical properties of individual phases and interfaces at the microstructural level are required, which is a distinctive characteristic of such a multi-scale approach. Due to the lack of material data, particularly for interfaces, direct numerical simulations (DNS) are exploited to identify the parameters inversely by using a limited set of molten aluminium thermal shock test results. On the basis of a microstructure composed of mutually noncontacting large grains embedded in a homogeneous matrix reflecting a compound of very fine grains, molten aluminium thermal shock tests are reproduced in full detail under realistic boundary conditions and a computational procedure is developed to determine the damage distribution along the specimen which is compared to experimental results. The failure mechanisms at the matrix-grain interface level are resolved by introducing thermo–mechanical cohesive zone elements not only capable of accounting for the mechanical decohesion but also including the reduced heat transport through themechanically damaged interfaces. Fine scalemicro-cracks within the matrix are smeared out by using a well-established continuum damage mechanics formulation which is free of any pathological localization and mesh sensitivity problems. Direct numerical simulation of thermal shock tests has also served for the investigation of short range effects (due to the local CTE mismatch) and long range effects (elastic fields accompanying the temperature gradient) on the resulting thermo-mechanical damage profile, through variations of different microstructural material parameters. In the last part of the thesis, predictive capabilities of the developed analysis framework are assessed by means of the two–scale analysis of a real size ladle refractory lining, based on the microstructural parameters identified through direct numerical simulations

SELF-REGULATION OF NOVICE MIDDLE SCHOOL MATHEMATICS TEACHERS IN THE PREPARATION PROCESS FOR TEACHING

This study analyses the self-regulation (goal setting and planning) of the novice middle school mathematics teachers during the preparation phase of teaching. The study is designed as a case study. The participants are six mathematics teachers with less than five-year teaching experience. The data were collected through interviews, observations and document analysis. In regard to the observations, the teaching of certain topics (e.g., basic elements of prisms) were observed to uncover the classroom behavior of the participants. In the semi-structured interviews carried out after the class observations, the participants were asked some questions about their goals and planning concerning the related teaching activities. The findings of the study indicate that the participants do not exhibit the goal setting and planning activities that are consistent with the conceptual learning covered in the mathematics education program. It is also found that they do not set clear goals and develop detailed planning in regard to teaching activities

The prevalence of asthma and allergy among university freshmen in Eskisehir, Turkey

AbstractThe aim of this study was to determine the current and cumulative prevalence of asthma, allergic rhinitis, atopic dermatitis and reactivities to allergen skin prick tests (SPT) among university freshmen.The data at the first stage were collated through the application of the European Community Respiratory Health Survey (-ECRHS- Stage I) questionnaire on 1603 students registering at various faculties and vocational colleges of Osmangazi University in Eskisehir, Turkey, in the academic year 1997–1998. At the second stage a physical examination as well as allergen SPTs were conducted on 151 students.Of the students within the study group, six (0·4%) had experienced an asthma attack within the previous 12 months, 11 (0·7%) had a past of asthma attacks and 123 (8·1%) reported wheezing attacks within the previous 12 months. The prevalence of asthma-like symptoms, rhinoconjunctivitis and dermatitis were found to be 17·0%, 10·0% and 5·9% respectively. Asthma and asthma-like symptoms were found to be significantly more prevalent among students who smoked. A positive SPT reaction to more than one allergen was found in 14·6% of the students. SPT positivity was 8·3% in asymptomatic students, 27·3% in asthmatic students, 14·5% in those with asthma-like symptoms, 28% in those with non-infectious rhinitis and 7·1% in those with dermatitis. In analysis of logistic regression, a history of atopy, as ascertained in the questionnaire, was seen to have a significant effect on SPT positivity.The rate of self-reported asthma and/or asthma-like symptoms among newly enrolled freshmen at the Osmangazi University was found to be lower than in other countries. Cigarette smoking was seen to increase such symptoms significantly, in comparison to non-smokers

A Thermo-mechanical cohesive zone model

In this paper, a cohesive zone formulation that is suitable for the thermo-mechanical analysis of heterogeneous solids and structural systems with contacting/interacting components, is presented. Well established traction-opening relations are adopted and combined with micromechanically motivated heat flux-opening relations reflecting the evolving heat transfer through the interfaces. The finite element approach for a coupled analysis within an operator-split solution framework is presented and demonstrated with an example problem

Exact metric around a wiggly cosmic string

The exact metric around a wiggly cosmic string is found by modifying the energy momentum-tensor of a straight infinitely thin cosmic string to include an electric current along the symmetry axis.Comment: 5 page

Geometry of the Field-Moment Spaces for Quadratic Bosonic Systems: Diabolically Degenerated Exceptional Points on Complex kk-Polytopes

$k$-Polytopes are a generalization of polyhedra in $k$ dimensions. Here, we show that complex $k$-polytopes naturally emerge in the higher-order field moments spaces of quadratic bosonic systems, thus revealing their geometric character. In particular, a complex-valued evolution matrix, governing the dynamics of $k$th-order field moments of a bosonic dimer, can describe a complex $k$-dimensional hypercube. The existence of such $k$-polytopes is accompanied by the presence of high-order diabolic points (DPs). Interestingly, when the field-moment space additionally exhibits exceptional points (EPs), the formation of $k$-polytopes may lead to the emergence of diabolically degenerated EPs, due to the interplay between DPs and EPs. Such intriguing spectral properties of complex polytopes may enable constructing photonic lattice systems with similar spectral features in real space. Our results can be exploited in various quantum protocols based on EPs, paving a new direction of research in this field.Comment: 9 page

Restoring Adiabatic State Transfer in Time-Modulated Non-Hermitian Systems

Non-Hermitian systems have attracted much interest in recent decades, driven partly by the existence of exotic spectral singularities, known as exceptional points (EPs), where the dimensionality of the system evolution operator is reduced. Among various intriguing applications, the discovery of EPs has suggested the potential for implementing a symmetric mode switch, when encircling them in a system parameter space. However, subsequent theoretical and experimental works have revealed that {\it dynamical} encirclement of EPs invariably results in asymmetric mode conversion; namely, the mode switching depends only on the winding direction but not on the initial state. This chirality arises from the failure of adiabaticity due to the complex spectrum of non-Hermitian systems. Although the chirality revealed has undoubtedly made a significant impact in the field, a realization of the originally sought symmetric adiabatic passage in non-Hermitian systems with EPs has since been elusive. In this work, we bridge this gap and theoretically demonstrate that adiabaticity, and therefore a symmetric state transfer, is achievable when dynamically winding around an EP. This becomes feasible by specifically choosing a trajectory in the system parameter space along which the corresponding evolution operator attains a real spectrum. Our findings, thus, offer a promise for advancing various wave manipulation protocols in both quantum and classical domains.Comment: 6 pages, 4 figure

Iridium(i) complexes bearing hemilabile coumarin-functionalised N-heterocyclic carbene ligands with application as alkyne hydrosilylation catalysts

A set of iridium(i) complexes of formula IrCl(¿C, ¿2-IRCouR')(cod) or IrCl(¿C, ¿2-BzIRCouR')(cod) (cod = 1, 5-cyclooctadiene; Cou = coumarin; I = imidazolin-2-carbene; BzI = benzimidazolin-2-carbene) have beeen prepared from the corresponding azolium salt and [Ir(µ-OMe)(cod)]2 in THF at room temperature. The crystalline structures of 4b and 5b show a distorted trigonal bipyramidal configuration in the solid state with a coordinated coumarin moiety. In contrast, an equilibrium between this pentacoordinated structure and the related square planar isomer is observed in solution as a consequence of the hemilability of the pyrone ring. Characterization of both species by NMR was achieved at the low and high temperature limits, respectively. In addition, the thermodynamic parameters of the equilibrium, ¿HR and ¿SR, were obtained by VT 1H NMR spectroscopy and fall in the range 22-33 kJ mol-1 and 72-113 J mol-1 K-1, respectively. Carbonylation of IrCl(¿C, ¿2-BzITolCou7, 8-Me2)(cod) resulted in the formation of a bis-CO derivative showing no hemilabile behaviour. The newly synthesised complexes efficiently catalyze the hydrosilylation of alkynes at room temperature with a preference for the ß-(Z) vinylsilane isomer. © The Royal Society of Chemistry

Regulation of pancreatic cancer aggressiveness by stromal stiffening

No abstract available

Course outlines and teaching styles at graphic design lessons at fine arts education departments

Türkiye’de Resim-iş eğitimi programlarındaki öğretim elemanları ile dersler, ders içerikleri ve işleniş biçimleri konularında yapılan görüşmeler sırasında, tüm fakültelerde uygulanması öngörülen Eğitim Fakülteleri ortak programının uygulama aşamasında bölümden bölüme değişiklikler gösterdiği fark edilmiştir. Bu farklılıkların ortaya konması ve mevcut durumun göz önüne serilerek yapılabileceklerin tartışmaya açılması fikri, bu araştırmanın yapılmasına sebep olmuştur. Araştırmanın amacı; Türkiye’deki Güzel Sanatlar Eğitimi Grafik Ana Sanat Dallarında 6 dönem boyunca okutulan derslerin mevcut durumunun ne olduğunun ortaya konulmasıdır. Veriler, bir anket aracılığı ile toplanmıştır. Ankette grafik ana sanat derslerinde var olan durum ve uygulamada karşılaşılan sorunlar ile ilgili sorular yer almaktadır. Araştırma sonucunda, elde edilen veriler değerlendirilmiş ve ortak uygulamaya yönelik alternatif bir içerik önerisi sunulmuştur.During our meetings regarding courses, course outlines and teaching styles with faculty members from Fine Arts Education departments at education faculties in Turkey, it was noticed that the outline that was expected to be followed universally by all faculties changed from department to department. Ascertaining these differences, and by doing so opening a discussion about the possible solutions to overcome them, led to this research. The purpose of this research is to examine the current situation of the courses that are taught over six semesters in Graphic Art Major Programs in Fine Art Education Departments. Data has been collected by conducting a questionnaire. The questionnaire consists of questions regarding the current situation of the graphic art courses and the problems that are confronted during their execution. As a result, the data obtained were evaluated and an alternate content for common application is proposed