Tesi sul tema "Finite-Temperature properties"

Thesis (M.S.)--Dept. of Physics. University of Missouri--Kansas City, 2007.
"A thesis in physics." Typescript. Advisor: Elizabeth P. Stoddard. Vita. Title from "catalog record" of the print edition Description based on contents viewed Dec. 18, 2007. Includes bibliographical references (leaf 23). Online version of the print edition.

Les propriétés et le comportement des matériaux dans des conditions extrêmes sont essentiels pour les systèmes énergétiques tels que les réacteurs de fission et de fusion. Cependant, prédire avec précision les propriétés des matériaux à haute température reste un défi. Les mesures directes de ces propriétés sont limitées par les instruments expérimentaux, et les simulations à l'échelle atomique basées sur des champs de force empiriques sont souvent peu fiables en raison d'un manque de précision. Ce problème peut être résolu à l'aide de techniques d'apprentissage statistique, qui ont récemment vu leur utilisation exploser en science des matériaux. Les champs de force construits par apprentissage statistique atteignent le degré de précision des calculs ab initio ; cependant, leur mise en œuvre dans les méthodes d'échantillonnage est limitée par des coûts de calcul élevés, généralement supérieurs de plusieurs ordres de grandeur à ceux des champs de force traditionnels. Pour surmonter cette limitation, deux objectifs sont poursuivis dans cette thèse : (i) développer des champs de force par apprentissage statistique avec un meilleur compromis précision-efficacité et (ii) créer des méthodes accélérées d'échantillonnage de l'énergie libre afin de faciliter l'utilisation de champs de force d'apprentissage statistique coûteux en termes de calcul. Pour le premier objectif, nous améliorons la construction des champs de force d'apprentissage statistique en nous concentrant sur trois facteurs clés : la base de données, le descripteur de l'environnement atomique local et le modèle de régression. Dans le cadre de la régression par processus gaussien, nous proposons et optimisons des descripteurs basés sur des noyaux échantillonnés par la transformée de Fourier ainsi que de nouvelles méthodes de sélection de points épars pour la régression par noyau. Pour le deuxième objectif, nous développons un schéma d'échantillonnage bayésien rapide et robuste pour estimer l'énergie libre anharmonique, qui est cruciale pour comprendre les effets de la température sur les solides cristallins, à l'aide d'une méthode de force de biais adaptative améliorée. Cette méthode effectue une intégration thermodynamique à partir d'un système de référence harmonique, où les instabilités numériques associées aux fréquences nulles sont éliminées. La méthode d'échantillonnage proposée améliore considérablement la vitesse de convergence et la précision globale. Nous démontrons l'efficacité de la méthode améliorée en calculant les dérivées de second ordre de l'énergie libre, telles que les constantes élastiques, avec une rapidité plusieurs centaines de fois supérieure à celle des méthodes standard. Cette approche permet de prédire les propriétés thermodynamiques du tungstène et des alliages à haute entropie Ta-Ti-V-W à des températures qui ne peuvent être étudiées expérimentalement, jusqu'à leur point de fusion, avec une précision ab initio grâce à l'utilisation de champs de force construits par apprentissage statistique. Une extension de cette méthode permet l'échantillonnage d'un état métastable spécifique sans transition entre différents bassins d'énergie, fournissant ainsi l'énergie libre de formation et de liaison d'une configuration défectueuse. Ce développement aide à expliquer le mécanisme derrière l'observation des cavités dans le tungstène, mécanisme qui ne peut pas être expliqué par les calculs ab initio existants. Le profil d'énergie libre des lacunes dans le système Ta-Ti-V-W est également calculé pour la première fois. Enfin, nous validons l'application de cette méthode d'échantillonnage de l'énergie libre aux liquides. La précision et l'efficacité numérique du cadre de calcul proposé, qui combine des champs de force d'apprentissage statistique et des méthodes d'échantillonnage améliorées, ouvrent de nombreuses possibilités pour la prédiction fiable des propriétés des matériaux à température finie
The properties and behaviors of materials under extreme conditions are essential for energy systems such as fission and fusion reactors. However, accurately predicting the properties of materials at high temperatures remains challenging. Direct measurements of these properties are constrained by experimental instrument limitations, and atomic-scale simulations based on empirical force fields are often unreliable due to a lack of accuracy. This problem can be addressed using machine learning techniques, which have recently become widely used in materials research. Machine learning force fields achieve the accuracy of ab initio calculations; however, their implementation in sampling methods is limited by high computational costs, typically several orders of magnitude greater than those of traditional force fields. To overcome this limitation, this thesis has two objectives: (i) developing machine learning force fields with a better accuracy-efficiency trade-off, and (ii) creating accelerated sampling methods to facilitate the use of computationally expensive machine learning force fields and accurately estimate free energy. For the first objective, we enhance the construction of machine learning force fields by focusing on three key factors: the database, the descriptor of local atomic environments, and the regression model. Within the framework of Gaussian process regression, we propose and optimize descriptors based on Fourier-sampled kernels and novel sparse points selection methods for kernel regression. For the second objective, we develop a fast and robust Bayesian sampling scheme for estimating the fully anharmonic free energy, which is crucial for understanding temperature effects in crystalline solids, utilizing an improved adaptive biasing force method. This method performs a thermodynamic integration from a harmonic reference system, where numerical instabilities associated with zero frequencies are screened off. The proposed sampling method significantly improves convergence speed and overall accuracy. We demonstrate the efficiency of the improved method by calculating the second-order derivatives of the free energy, such as the elastic constants, which are computed several hundred times faster than with standard methods. This approach enables the prediction of the thermodynamic properties of tungsten and Ta-Ti-V-W high-entropy alloys at temperatures that cannot be investigated experimentally, up to their melting point, with ab initio accuracy by employing accurate machine learning force fields. An extension of this method allows for the sampling of a specified metastable state without transitions between different energy basins, thereby providing the formation and binding free energies of defective configurations. This development helps to explain the mechanism behind the observation of voids in tungsten, which cannot be explained by existing ab initio calculations. The free energy profile of vacancies in the Ta-Ti-V-W system is also computed for the first time. Finally, we validate the application of this free energy sampling method to liquids. The accuracy and numerical efficiency of the proposed computational framework, which combines machine learning force fields and enhanced sampling methods, opens up numerous possibilities for the reliable prediction of finite-temperature material properties

This thesis concerns some aspects that have influence on the strength of adhesive layers. The strength is determined by the stress deformation-relation of the layer. This relation is also referred to as cohesive law. The aspects having influence on the cohesive laws that are studied in this work are temperature, fatigue, multi-axial fatigue and mixed mode loading. For each aspect, a model is developed that can be used to describe the influence of the aspects on the cohesive laws numerically, e.g. by using the finite element method. These models are shown to give good agreement with the experimental results when performing simulations that aims at reproducing the experiments. For the aspect of temperature, a FE-model is suggested that can be used to simulate the mechanical behaviour in pure mode loadings at any temperature within the evaluated temperature span. Also, a damage law for modelling high cycle fatigue in a bonded structure in multi-axial loading is presented. Lastly, a new experimental set-up is presented for evaluating strength of adhesives during mixed mode loading. The set-up enables loading with a constant mode-mix ratio and by the experimental results, a potential model for describing the mechanical behaviour of the evaluated adhesive is presented.

This research is about modelling and simulation of how the hydrogen diffuses in high strength steels. The hydrogen diffusion in the material was examined by using finite element software with the help of material properties and some existing data. For modelling and simulating the diffusion analysis in finite element software, a cylindrical type dog-bone shaped specimen was chosen. To determine the diffusion at the centre of specimen, a cross-sectional area of the material was selected to proceed for the analysis. Abaqus software was considered as finite element software to progress the hydrogen diffusion and tensile testing of the specimen. Diffusion analysis was studied under the analogy of heat transfer and also, diffusion analysis with the addition of mechanical load was studied under the analogy of coupled temperature displacement in the Abaqus software. This process has executed for two types of high strength steels 316L and 304L stainless steels. The crack is also considered for analysis to check how it affects the specimen. Further, The 316L and 304L stainless steel results were compared to review that which steel is better to withstand the hydrogen diffusion rate and mechanical load on the material.

Modeling at the structural scale most often requires the use of beam and shell elements. This simplification reduces modeling complexity and computation requirements but sacrifices the accuracy of through-the-thickness information. Several studies have reported various design approaches for analyzing functionally graded material structures. One of these studies proposed a two-node beam element for functionally graded materials (FGMs) based on first order shear deformable (FOSD) theory. The derivation of governing equations included spatial temperature variation. However, only the constant temperature case was carried through in the element formulation. This investigation explore the effects of spatial temperature variation in the axial and through-the-thickness direction of this proposed element and present a new standard three-node beam finite element modified for structure constructed of FGMs. Also, the influence of the temperature dependency of the thermo-elastic material properties on the thermal stresses distribution was studied. In addition, variations in the layer thicknesses within multilayer beam models were studied to determine the effect on stresses and factor of safety. Finally, based on the specific factor of safety, which combines together the strength and mass of the beam, the best layer thicknesses for the beam models were established. The key contributions expected from this research are: 1. development and implementation of a three-node beam element as a finite element code into the commercial computational tool MATLAB® to analyze thermo-mechanical stresses in structures constructed of functionally graded materials; 2. a strategy to simulate different load cases in structures constructed of functionally graded materials; 3. an analysis of the influence of the FGM interlayer thickness on the factor of safety/specific gravity ratio in structures constructed of functionally graded materials under thermo-mechanical loads; 4. and an analysis/comparison of the advantages/benefits of using structures constructed of functionally graded materials with respect to those constructed with homogenous materials.

Gypsum board assemblies are now widely used in buildings, as fire resistant walls or ceilings, to provide passive fire protection. The fire resistance of such systems is fundamentally due to the desirable thermal properties of gypsum. Yet there is wide variability in reported values of thermal properties of gypsum at high temperatures and a lack of understanding of its integrity in fire. To evaluate the fire protection performance of gypsum board assemblies, it is essential to quantify its thermal properties and obtain information on its mechanical properties at high temperatures. Gypsum boards shrink and crack at high temperatures, and this leads to collapse of parts of the gypsum boards in fire. Fall-off of gypsum in fire affects the fire resistance of the assembly considerably, and cannot be overlooked when evaluating the fire resistance of gypsum board assemblies. The current research proposes a model to define the temperature-dependent thermal properties of gypsum boards at high temperatures. Thermal conductivity of gypsum is considered as the most influential parameter in conduction of heat through gypsum, and a hybrid numerical-experimental method is presented for extracting thermal conductivity of various gypsum board products at elevated temperatures. This method incorporates a validated one-dimensional Finite Difference heat conduction program and high temperature test results on small samples of gypsum boards. Moreover, high temperature mechanical tests have been performed on different gypsum board products; thermal shrinkage, strength and stress-strain relationships of gypsum products at elevated temperatures are extracted for use in numerical mechanical analysis. To simulate the structural performance of gypsum boards in fire, a two-dimensional Finite Element model has been developed in ABAQUS. This model successfully predicts the complete opening of a through-thickness crack in gypsum, and is validated against medium-scale fire tests designed and conducted as part of this research. Gypsum fall-off in fire is a complex phenomenon; however, it is believed that delaying the formation of through-thickness cracking will delay falling off of gypsum in fire, and hence improve the fire resistance of gypsum board assemblies. Finally, a study has been performed on the effects of various detailing parameters in gypsum board wall assemblies, and recommendations are offered for improving the fire resistance of such systems.

Structural Health Monitoring (SHM) is a promising tool for condition assessment of bridge structures. SHM of bridges can be performed for different purposes in long or short-term. A few aspects of short- and long-term monitoring of highway bridges are addressed in this research. Without quantifying environmental effects, applying vibration-based damage detection techniques may result in false damage identification. As part of a long-term monitoring project, the effect of temperature on vibrational characteristics of two continuously monitored bridges are studied. Natural frequencies of the structures are identified from ambient vibration data using the Natural Excitation Technique (NExT) along with the Eigen System Realization (ERA) algorithm. Variability of identified natural frequencies is investigated based on statistical properties of identified frequencies. Different statistical models are tested and the most accurate model is selected to remove the effect of temperature from the identified frequencies. After removing temperature effects, different damage cases are simulated on calibrated finite-element models. Comparing the effect of simulated damages on natural frequencies showed what levels of damage could be detected with this method. Evaluating traffic loads can be helpful to different areas including bridge design and assessment, pavement design and maintenance, fatigue analysis, economic studies and enforcement of legal weight limits. In this study, feasibility of using a single-span bridge as a weigh-in-motion tool to quantify the gross vehicle weights (GVW) of trucks is studied. As part of a short-term monitoring project, this bridge was subjected to four sets of high speed, live-load tests. Measured strain data are used to implement bridge weigh-in-motion (B-WIM) algorithms and calculate the corresponding velocities and GVWs. A comparison is made between calculated and static weights, and furthermore, between supposed speeds and estimated speeds of the trucks. Vibration-based techniques that use finite-element (FE) model updating for SHM of bridges are common for infrastructure applications. This study presents the application of both static and dynamic-based FE model updating of a full scale bridge. Both dynamic and live-load testing were conducted on this bridge and vibration, strain, and deflections were measured at different locations. A FE model is calibrated using different error functions. This model could capture both global and local response of the structure and the performance of the updated model is validated with part of the collected measurements that were not included in the calibration process.

La majorité des adhésifs utilisés dans l’industrie marine sont des polymères avec un comportement mécanique qui est fortement influencé par les conditions environnementales (vieillissement hydrique ou température). Par conséquent, il est très important pour les ingénieurs travaillant dans des bureaux d’études d’être capable de prendre en compte ces effets lors des différentes étapes de développement et conception des assemblages collés.Le présent travail propose une méthode d’analyse de l’influence du vieillissement hydrique sur le comportement mécanique d’un adhésif structural époxy dans un assemblage collé. Tout d’abord, un modèle viscoélastique-viscoplastique a été développé pour caractériser la réponse mécanique de l’adhésif dans un joint de colle. Pour cela, le dispositif expérimentalArcan a été utilisé. Le modèle est identifié en utilisant la méthode d’identification inverse et les échantillons sont testés à l’état non-vieilli (pas de vieillissement hydrique). Les résultats obtenus après la démarche d’identification sont utilisés pour prédire le comportement mécanique d’éprouvettes massiques.Dans un deuxième temps, afin de diminuer les temps de saturation d’échantillons, l’évolution des propriétés mécaniques de l’adhésif est analysée sous différentes conditions de vieillissement hydrique (immersion dans l’eau de mer et en humidité relative contrôlée) grâce à des essais sur éprouvettes massiques.Les résultats obtenus seront utilisés pour identifier l’évolution de chaque paramètre du modèle proposé, en fonction de la quantité d’eau absorbée. En parallèle, un modèle de diffusion a été développé pour caractériser le gradient de teneur en eau des joints de colle. Les deux approches sont ensuite combinées pour modéliser les profils d’eau pour différents temps de vieillissement et prédire l’évolution des propriétés mécaniques du joint de colle après le vieillissement. Finalement, pour valider la méthode proposée, la prédiction du modèle est comparée avec des essais réalisés sur assemblages collés vieillis
Most of the adhesives used in the marine industry are polymers with a mechanical behaviour which is strongly influenced by environmental conditions (water activity or temperature). Therefore, it is important for engineers and designers to be able to consider these effects during the different stages of development and manufacturing of a bonded structure.The present work presents a method for analyzing the influence of water ageing on the behaviour of an epoxy adhesive in an adhesively bonded assembly.First, a viscoelastic-viscoplastic model is developed to characterise the mechanical response of the adhesive at initial state in a bonded joint using the modified Arcan device. The model is identified using the inverse identification method and the considered samples are tested at an unaged stage (no water activity). The results obtained after the identification process are used to predict the bulk behaviour of the adhesive. A comparison between numerical results and experimental tests realised on bulk specimens is then made in order to validate this first approach.In a second phase, in order to decrease the times for samples saturation, the evolution of the mechanical properties of the adhesive in bulk form is tested under different water ageing conditions (immersion in seawater and different relative humidity). The obtained results allowed to identify the evolution of the model parameters as a function of water content. In parallel, a diffusion model was developed to characterise the water ingress in the bonded joint. These two approaches are then combined to model the water profiles and to consider the evolution of mechanical properties of a water aged adhesively bonded assembly, for different immersion times. Finally, to validate the framework, the prediction is compared with experimental tests performed on aged specimens

Load bearing Light Gauge Steel Frame (LSF) wall system is a cold-formed steel structure made of cold-formed steel studs and lined on both sides by gypsum plasterboards. In recent times its use and demand in the building industry has significantly increased due to their advantages such as light weight, acoustic performance, aesthetic quality of finished wall, ease of fabrication and rapid constructability. Fire Resistant Rating (FRR) of these walls is given more attention due to the increasing number and severity of fire related accidents in residential buildings that have caused significant loss of lives and properties. LSF walls are commonly made of conventional lipped channel section studs lined with fire resistant gypsum plasterboards on both sides. Recently, greater attention has been given to innovative cold-formed steel sections such as hollow flange sections due to their improved structural efficiency. The reliance on expensive and time consuming full scale fire tests, and the complexity involved in predicting the fire performance of LSF wall studs due to their thin-walled nature and their exposure to non-uniform temperature distributions in fire on one side, have been the main barriers in using different cold-formed steel stud sections in LSF wall systems. This research overcomes this and proposes the new hollow flange section studs as vertical load bearing elements in LSF wall systems based on a thorough investigation into their fire (structural and thermal) performance using full scale fire tests and extensive numerical studies. Test wall frames made of hollow flange section studs were lined with fire resistant gypsum plasterboards on both sides, and were subjected to increasing temperatures as given by the standard fire curve in AS 1530.4 (SA, 2005) on one side. Both uninsulated and cavity insulated walls were tested with varying load ratios from 0.2 to 0.6. LiteSteel Beam (LSB), a welded hollow flange section, which was available in the industry was used to fabricate the test wall panels. Axial deformations and lateral displacements along with the time-temperature profiles of the steel stud and plasterboard surfaces were measured. Five full scale tests were performed, and the test results were compared with those of LSF walls made of lipped channel section studs, which proved the superior fire performance of LSF walls made of hollow flange section studs. The reasons for the superior fire performance are presented in this thesis. The effects of load ratio and plasterboard joint on the fire performance of LSF walls and temperature distribution across the stud cross-sections were identified. Improved plasterboard joints were also proposed. The elevated temperature mechanical properties of cold-formed steels appear to vary significantly as shown by past research. LSBs were manufactured using a combined cold-forming and electric resistance welding process. Elevated temperature mechanical properties of LSB plate elements are unknown. Therefore an experimental study was undertaken to determine the elevated temperature mechanical properties of LSB plate elements. Based on the test results and previous researchers' proposed values, suitable predictive equations were proposed for the elastic modulus and yield strength reduction factors and stress-strain models of LSB web and flange elements. Uninsulated and insulated 2D finite element models of LSF walls were developed in SAFIR using GiD as a pre- and post processor to predict the thermal performance under fire conditions. A new set of apparent thermal conductivity values was proposed for gypsum plasterboards for this purpose. These models were then validated by comparing the time-temperature profiles of stud and plasterboard surfaces with corresponding experimental results. The developed models were then used to conduct an extensive parametric study. Uninsulated and insulated LSF walls with superior fire performances were also proposed. Finite element models of tested walls were also developed and analysed under both transient and steady state conditions to predict the structural performance under fire conditions using ABAQUS. In these analyses, the measured elevated temperature properties of LSB plate elements were used to improve their accuracy. Finite element analysis results were compared with fire test results to validate the developed models. Following this, a detailed finite element analysis based study was conducted to investigate the effects of stud dimensions such as web depths and thicknesses, elevated temperature mechanical properties, types of wall configurations, stud section profiles, plasterboards to stud connections and realistic design fire curves on the fire performance of LSF walls. It was also shown that the commonly used critical temperature method is not appropriate in determining the FRR of LSF walls. Gunalan and Mahendran's (2013) design rules based on AS/NZS 4600 (SA, 2005), and Eurocode 3 Part 1.3 (ECS, 2006) were further improved to predict the structural capacity of hollow flange section studs subjected to non-uniform temperature distributions caused by fire on one side. Two improved methods were proposed and they predicted the FRRs with a reasonable accuracy. Direct Strength Method (DSM) based design rules were then established and they also predicted the FRR of LSF walls made of hollow flange section studs accurately. Finally, spread sheet based design tools were developed based on the proposed design rules. Overall, this research has developed comprehensive fire performance data of LSF walls made of hollow flange section studs, accurate design rules to predict their fire rating and associated design tools. Thus it has enabled the use of innovative hollow flange sections as studs in LSF wall systems. Structural and fire engineers can now use these tools to undertake complex calculations of determining the structural capacities and fire rating of hollow flange section studs subjected to non-uniform temperature distributions, and successfully design them for safe and efficient use in LSF walls of residential and office buildings.

Les préoccupations environnementales actuelles visent à réduire les consommations énergétiques. Dans une démarche d’amélioration des bâtiments existants, l'étude du comportement thermique d’une paroi n'est pas aisée du fait de la méconnaissance de ses propriétés thermophysiques réelles. Ces paramètres sont pourtant prépondérants pour la phase d'optimisation économique des opérations de réhabilitation ou pour vérifier ses performances in situ. Il apparaît donc important de pouvoir caractériser les parois de bâtiment en place. Notre travail vise à développer une méthode de caractérisation thermique d’une paroi adaptée aux applications in situ basée sur une approche active. Le principe d'identification consiste à solliciter thermiquement une face d’accès en imposant un flux de chaleur sous forme d’un créneau et à étudier la réponse en température enregistrée par thermographie infrarouge sur l’autre face. A partir de signaux de flux et de températures mesurés aux limites de la paroi, les propriétés thermophysiques de la paroi seront estimées par méthode inverse. Nous nous sommes dans un premier temps intéressés aux parois homogènes. Le schéma d’inversion est construit autour d’un modèle numérique décrivant la réponse de la paroi suivant la méthode des différences finies en 1D. L’identification de la conductivité thermique et de la chaleur volumique de la paroi est réalisée en optimisant le groupement de paramètres qui permet de minimiser l’écart entre la température normalisée mesurée et la température normalisée simulée. Le coefficient d’échange surfacique global est également identifié à partir du même essai. Dans ce travail, la méthode a été appliquée à une paroi homogène en carreaux de plâtre mise en place au laboratoire. Elle a une épaisseur de 6.5 cm. Cette technique a été utilisée pour les parois multicouches de bâtiments. Les résultats issus de cette procédure d’inversion ont été comparés à des valeurs de référence obtenues à partir d’une procédure classique (NF EN 12664-méthode fluxmétrique). Une bonne concordance des résultats est obtenue. Une autre partie représente les essais in situ
Current environmental concerns are intended to reduce energy consumption. In a process of improving existing buildings, the study of the thermal behavior of a wall is not easy because of the ignorance of its real thermophysical properties. These parameters are yet to dominate the economic optimization phase of the rehabilitation or to check its performance in situ. It therefore appears important to characterize the walls of existing building. Our work aims to develop a method of thermal characterization of a wall suitable for in situ applications based on an active approach. The principle of identification is to apply a heat-face access by imposing a heat flux in the form of a pulse and to study the temperature response recorded by infrared thermography on the other side. From signal flow and temperature measured at the limits of the wall, the thermophysical properties of the wall will be estimated by inverse method. We are at present interested in homogeneous walls. The inversion scheme is built around a digital model describing the response of the wall following the finite difference method in 1D. The identification of the thermal conductivity and heat volume of the wall is achieved by optimizing the group of parameters which minimizes the normalized difference between the temperature measured and the temperature standard simulated. The overall Global exchange coefficient is also identified from the same test. In this work, the method was applied to a homogeneous wall tile plaster introduction to the laboratory. It has a thickness of 6.5 cm. This technique was used for multilayer walls of buildings. The results of this inversion procedure were compared with reference values obtained from a standard procedure (DIN EN 12664-flow meter methods). A good agreement is obtained. Another part is the in situ tests

For microelectronics used in the low-temperature applications, the understanding of their reliability and performance has become an important research subject characterised as electronics to serve under the severe or extreme service conditions. Along with the impact from the increased miniaturization of devices, the various properties and the relevant thermo-mechanical response of the interconnection materials to temperature excursion at micro-scale become a critical factor which can affect the reliable performance of microelectronics in various applications. Pure indium as an excellent interconnection material has been used in pixellated detector systems, which are required to be functional at cryogenic temperatures. This thesis presents an extensive investigation into the thermo-mechanical properties of indium joints as a function of microstructure, strain (loading histories-dependent) and temperature (service condition-sensitive), specifically in the areas as follows: (i) the interfacial reactions and evolution between indium and substrate during the reflow process (liquid-solid) and thermal aging (solid-solid) stages by taking low-temperature cycling into account; (ii) determination of the effects of joint thickness and the types of substrate (e.g. Cu or Ni) on the mechanical properties of indium joints, and the stress- and temperature-dependent creep behaviour of indium joints; (iii) the establishment of a constitutive relationship for indium interconnects under a wide range of homologous temperature changes that was subsequently implemented into an FE model to allow the analysis of the evolution of thermally-induced stresses and strains associated with a hybrid pixel detector.

The cold-formed steel utilization in buildings has increased globally due to its higher strength to weight ratio, ease of transportation and rapid erection and dismantlement. However, cold-formed steel buildings must be designed with adequate Fire Resistance Ratings (FRR). Hence cold-formed Light gauge Steel Frames (LSF) are assembled using channel sections and lined with fire resistive plasterboards to provide load-bearing wall and floor systems. There is an industry need to develop LSF floor systems with improved FRR. Adding multiple layers of plasterboard to increase the FRR of LSF floor systems is not an efficient method. Past research has focused on investigating the behaviour of LSF floor systems made of Lipped Channel Section (LCS) joists. No attempt has been made to use an improved joist section in LSF floor systems. The Hollow Flange Sections (HFS) with torsionally rigid hollow flanges and no free edges have higher local and lateral distortional buckling capacities than the conventional LCSs. This research focuses on investigating the structural and fire performance of LSF floor systems made of HFS joists with a goal to improve their FRRs. Four full scale standard fire tests were undertaken on non-insulated dual and single plasterboard lined LSF floor panels and cavity insulated dual plasterboard lined floor panel made of welded HFS joists known as LiteSteel beams (LSB). Fire tests of these panels undertaken for varying load ratios provided valuable results, which included failure times, joist temperatures and modes, and deflection versus time curves. The floor panels failed due to the section failures of joists. Both non-insulated and cavity insulated LSF floors made of LSB joists showed a significant improvement in the FRRs in comparison to Baleshan's (2012) results for LSF floors made of LCS joists. Another experimental study was undertaken to determine the elevated temperature mechanical properties of the steel used in LSB web and flange elements. The mechanical property reduction variation of LSB steel elements was found to be quite different to that of normal cold-formed steels and was even dissimilar amongst them. The yield strength reduction factors of Eurocode 3 Part 1.2 (ECS, 2005) were proposed for the web elements since they closely followed them whereas a new yield strength reduction factor model was proposed for the flange elements. An identical variation was proposed for the elastic modulus reduction factors of both web and flange elements. Suitable modifications were made to Dolamune Kankanamge and Mahendran's (2011) stress-strain model for improved predictions of LSB web and flange elements' stress-strain curves. A Finite Element (FE) model of an individual simply supported LSB joist was developed and validated using the cold-formed steel design standards and Anapayan et al.'s (2011b) section moment capacity test results. By using the accurate mechanical property reduction factors of LSB steel elements, the FE model was then extended to simulate the full scale fire tests. Finite element analyses (FEA) showed reasonably good agreements in terms of failure times, temperatures and modes, and the mid-span deflection versus time curves. Such good agreements verified the accuracy of the developed FE model to simulate the LSF floor panels made of HFS joists under fire conditions. Thermal FE models of LSF floor systems made of HFS joists were then developed and the time-temperature profiles were compared with the fire test results. They showed better agreements for Tests 1 and 4 whereas there were some discrepancies for Tests 2 and 3. Thermal FEA results obtained using appropriate thermal properties of plywood showed a reasonably good agreement with Baleshan's (2012) fire test results. Parametric studies using the validated model showed that joist section depth and profile had no significant impact on the thermal performance of LSF floor systems whereas steel joist thickness had a significant influence. An extensive FEA based parametric study was then undertaken to investigate the effects of joist thickness, depth, section profile, steel grade and mechanical property reduction factors, and web openings on the structural and fire performances (FRR) of LSF floor systems. Steel joist thickness significantly influenced the FRR of LSF floor systems due to different temperature developments in the steels for varying thicknesses. Joist section depth, section profile and web openings had no significant impact on the FRRs of LSF floor systems. Steel type affected the FRRs of LSF floor systems significantly due to different mechanical property reduction factors, especially different yield strength reduction factors. It was shown that Baleshan's (2012) critical average joist temperature method can be used to determine the FRR of non-insulated dual and single plasterboard lined floor panels made of HFS joists. However, it can be used for cavity insulated floor panels when the load ratio is less than 0.3. Fire test and FEA results showed that LSF floor panels made of LSB joists gave higher FRRs due to improved elevated temperature mechanical properties of LSB plate elements and lower temperature development due to thicker joists. Fire design rules were developed to predict the FRRs of LSF floor systems made of HFS joists based on Eurocode 3 Part 1.3 (ECS, 2006), AS/NZS 4600 (SA, 2005) and Direct Strength Method (DSM). For this purpose, Baleshan's (2012) three fire design rules of LCS joists were used and suitable modifications were made in order to use them for HFS joists. A good agreement was observed between the FRR predictions using two design methods and FEA, and thus they were recommended. In addition, the FRR predictions of HFS joists using the fire design method developed based on DSM were modestly conservative and therefore they were also recommended. Finally, the spread sheet based design tool was developed to undertake the complex calculations in predicting the FRR of LSF floors made of HFS joists with varying sizes and steel types, and subjected to varying load ratios. In summary, this research has significantly improved the knowledge and understanding of the fire performance of LSF floor systems made of hollow flange section joists and developed accurate fire design rules. Structural and fire design engineers can use the developed spread sheet based design tool to predict the fire performance of LSF floor systems made of HFS joists with varying sizes and steel types for a range of applications in commercial and residential buildings.

碩士
東海大學
物理學系
96
The field of quantum spin systems offers a wonderful playground for both theorists and experimentalists to investigate a variety of exotic phases. This system is related to the phenomenon such as superconductivity, superfluidity and supersolid. In this work we will study the thermodynamic properties of the one-dimensional quantum spin systems with the exact diagonalization method. We will verify some of the predictions by Bethe ansatz and other theoretical analyses. We will try to fit the experimental data including specific heat and magnetic susceptibility to get the physical parameters for our quantum spin models. We will take isotropic Heisenberg anti-ferromagnetic chain as the starting model. Then we extend it to include anisotropic spin interaction, alternating anti-ferromagnetic interaction, next-nearest neighbor interaction. Finally we will include the spin-lattice interaction in order to simulate the spin-Peierls transition observed in CuGeO3 material. In addition, we will present some attempts of applying the finite-temperature Lanczos method in the quantum spin systems.

We developed and implemented a first-principles based theory of the Landauer ballistic conductance, to determine the transport properties of nanostructures and molecular-electronics devices. Our approach starts from a quantum-mechanical description of the electronic structure of the system under consideration, performed at the density-functional theory level and using finite-temperature molecular dynamics simulations to obtain an ensemble of the most likely microscopic configurations. The extended Bloch states are then converted into maximally-localized Wannier functions to allow us to construct the Green's function of the conductor, from which we obtain the density of states (confirming the reliability of our microscopic calculations) and the Landauer conductance. A first application is presented to the case of pristine and functionalized carbon nanotubes.
Singapore-MIT Alliance (SMA)

Indiana University-Purdue University Indianapolis (IUPUI)
This study seeks to improve the understanding of inlet conditions of a large rotor-stator cavity in a turbofan engine, often referred to as the drive cone cavity (DCC). The inlet flow is better understood through a higher fidelity computational fluid dynamics (CFD) modeling of the inlet to the cavity, and a coupled finite element (FE) thermal to CFD fluid analysis of the cavity in order to accurately predict engine component temperatures. Accurately predicting temperature distribution in the cavity is important because temperatures directly affect the material properties including Young's modulus, yield strength, fatigue strength, creep properties. All of these properties directly affect the life of critical engine components. In addition, temperatures cause thermal expansion which changes clearances and in turn affects engine efficiency. The DCC is fed from the last stage of the high pressure compressor. One of its primary functions is to purge the air over the rotor wall to prevent it from overheating. Aero-thermal conditions within the DCC cavity are particularly challenging to predict due to the complex air flow and high heat transfer in the rotating component. Thus, in order to accurately predict metal temperatures a two-way coupled CFD-FE analysis is needed. Historically, when the cavity airflow is modeled for engine design purposes, the inlet condition has been over-simplified for the CFD analysis which impacts the results, particularly in the region around the compressor disc rim. The inlet is typically simplified by circumferentially averaging the velocity field at the inlet to the cavity which removes the effect of pressure wakes from the upstream rotor blades. The way in which these non-axisymmetric flow characteristics affect metal temperatures is not well understood. In addition, a constant air temperature scaled from a previous analysis is used as the simplified cavity inlet air temperature. Therefore, the objectives of this study are: (a) model the DCC cavity with a more physically representative inlet condition while coupling the solid thermal analysis and compressible air flow analysis that includes the fluid velocity, pressure, and temperature fields; (b) run a coupled analysis whose boundary conditions come from computational models, rather than thermocouple data; (c) validate the model using available experimental data; and (d) based on the validation, determine if the model can be used to predict air inlet and metal temperatures for new engine geometries. Verification with experimental results showed that the coupled analysis with the 3D no-bolt CFD model with predictive boundary conditions, over-predicted the HP6 offtake temperature by 16k. The maximum error was an over-prediction of 50k while the average error was 17k. The predictive model with 3D bolts also predicted cavity temperatures with an average error of 17k. For the two CFD models with predicted boundary conditions, the case without bolts performed better than the case with bolts. This is due to the flow errors caused by placing stationary bolts in a rotating reference frame. Therefore it is recommended that this type of analysis only be attempted for drive cone cavities with no bolts or shielded bolts.

The current dissertation, titled “Fire Response of Composite aerostructures” deals with a crucial subject of the aeronautics industry that is the fire response of composite aerostructures, more specifically the issue of interest in this work is the fuselage fire burnthrough from an external liquid jet-fuel pool fire. Other fire issues that “bother” the aeronautics industry are the fire spread inside the cabin, smoke generation and toxicity of the fumes, but these are not handled in the current dissertation. Aircraft structures are designed to withstand various loading scenarios during their operational life. These loading scenarios are associated to a great extent with normal aircraft operation (flight manoeuvres, take-off and landing). However there are situations where the aircraft structures are required to assure the safety of the passengers and crew. In the case of an emergency crash landing, the threat of an external jet-fuel fire always exists. Considering that the aircraft structure survives the impact, the survivability of the passengers and crew onboard the aircraft depends solely on the fire resistance of the aircraft structure. A measure of the fire resistance of an aircraft structure is the time needed for the flames to penetrate the fuselage and spread inside the cabin, the so-called, burn-through time. So far, the aircraft fire resistance has been extensively studied by conducting lab, medium and full scale tests. The early lab scale tests were performed by the Federal Aviation Administration (FAA) and involved the Bunsen-burner flammability test of coupons for developing fire safe interior materials. As the application of polymer materials on aircrafts kept increasing, the problem of fire burn-through due to external fire emerged. Marker was one of the first to perform full-scale fuselage burn-through tests to access the insulating performance of materials. Also a statistical analysis was performed by Cherry and Warren that accessed and analyzed data from past accidents and their work resulted in proving the importance of fuselage fire hardening and the passengers’ lives that could be saved using low-cost solutions. These works led the FAA to proposed new fire testing procedures for aircraft materials. The scope of this dissertation was to assess the performance of various structural materials in a pool-fire scenario. A simplified approach is made, approximating the pool-fire conditions with a flat panel burn-through test in accordance to the ISO2685:1998(E) Standard. The originality of the present work comes from the fact that it incorporates a multistage approach in order to investigate the behaviour and response of composite aircraft structures in the possibility of a fire event. The current approach goes down on material level in order to investigate and model the deterioration (decomposition) of the polymer composite. Thus, it investigates and proposes a methodology of how the thermophysical properties of the composite are deteriorated due to the fire event. It proceeds into developing a progressive-damage material model (material properties varying with the deterioration degree) and finally implementing this custom material model into a commercial FE package and solving the loading scenarios. Being more specific the current work begins with a quick review of the literature where incidents and work done on the burnthrough event for the past 20-30 years are summarized. It progresses then to presenting the various types of polymers used in the aircraft industry and their basic decomposition mechanisms, from the unsaturated polyesters to the epoxies and phenolics and in the end reference to the thermoplastics is made. Every organic material, hence, polymers used in aerospace applications, present a set of response characteristics when subjected to fire, specifically the heat release rate, thermal stability index, limiting oxygen index, flammability index, time-to-ignition, surface flame spread, mass loss, smoke density and smoke toxicity. Following is the backbone of this dissertation, the kinetics modelling. Two approaches are made, one simplified using single stage kinetics where the decomposition degree a is calculated based on the Arrhenius reaction theory and using the kinetic triplets (kinetic parameters) extracted from thermogravimetry, TGA, data using the Friedman multi-curve method. The second approach is more complicated and considers multi-stage decomposition of the polymer composite. Specifically a 3-stage reaction network is considered for every material, the LY-Ref, and the two modified batches, one with ammonium polyphosphate AP423 and the other both with AP423 and multi-wall carbon nanotubes MWCNT. Again the kinetic parameters, activation energy EA, frequency factor A, and reaction order n, are extracted for every step using the van Krevelen methodology. In the end using the reaction rates equations the reconstruction of the TGA curves is achieved with an error of less than 5% from the test data. Correlations that consider the material deterioration and affect the thermophysical properties of the materials are proposed. Those expressions are being developed for both of the two kinetic approaches, the single and multi stage. Another crucial part of this work is the measurement and calibration of the applied fire load. Again two fire load approaches are used, one according to the ISO2685 Standard where a propane burner was manufactured and calibrated according to the Standard for medium scale samples testing and a lab scale butane burner for small samples. The ISO2685 burner was also CFD simulated and the models calibrated against analytical expressions, ISO requirements and real measurements. The CFD simulations were performed so the heat flux or heat transfer coefficient to be extracted and used as input for the later thermal FE burnthrough models. The heat flux distribution of the lab-scale AML burner on the specimen surface was measured via a water cooled Schmit-Boelter SBG01 heat flux sensor manufactured by Hukseflux. Manufacturing and material details are presented concerning the samples used for every test campaign. Metallic (AL2024-T3) samples, CFRP neat and modified, and hybrid GLARE ones where manufactured. Also the experimental work performed is described. Cone calorimetry testing data are available, results from thermogravimetry tests, differential scanning calorimetry, and finally the burnthrough tests with both the testing apparatuses, the ISO2685 one and the AML lab-scale burner. The modelling work in this dissertation involved thermal models that were developed into a commercial FE package. It was not part of this work to develop a thermal solver so a commercial one was selected and all the developed methodology was adapted to its requirements and specifications. The boundary conditions on the models are presented both for the ‘hot’ front surface and the rear ‘cooling’ one. For the ‘hot’ one the heat flux distribution is used and for the ‘cooling’ one an equivalent convection is applied that accounts for both convective and radiative cooling. The decomposing material model is implemented into to FE solver via user defined subroutines for the single stage kinetics and the multi-stage approach. Finally the simulations were run and the results and models were compared against the available experimental results. Since so far the burnthrough response of aerostructures was limited to coupon, samples and medium size flat panels. A more realistic approach was performed by developing a mathematical model of a real size test. The certification tests conducted by the FAA are for full size fuselage sectors under the fire load of a burning jet-fuel pan pool-fire. A burning jet-fuel pool fire is a complex phenomenon on its own, combining it with a decomposing fuselage structure make the modeling approach even more difficult to simulate if not impossible. Required data for the pool-sizes under investigation were not available, so data for large external hydrocarbon pool fires from literature were used. Also, because the main characteristic of a jet-fuel (kerosene) pool fire is that the flames are not clear, on the contrary, great amount of shoot is produced making combustion modeling and radiative heat transfer to the fuselage even more of a challenge to model, it was decided to try and tackle this full-scale approach by a simplified the modeling approach. Instead of liquid fuel combustion, an equal hot air stream with mass flow, velocity and temperature properties extracted from literature correlation data was performed. Conclusively, in terms of completeness the benefit analysis performed by Cherry and Warren is presented in brief. The objective of their analysis was to assess the potential benefits, in terms of reduction of fatalities and injuries, resulting from improvements in fuselage burnthrough resistance to ground pool fires. Fire hardening of fuselages will provide benefits in terms of enhanced occupant survival and may be found to be cost beneficial if low-cost solutions can be found. The maximum number of lives saved per year in worldwide transport aircraft accidents, over the period covered by the data, if hardening measures were applied, was assessed to be 12.5 for the aircraft in its actual configuration (when the accidents occurred) and 10.5 for the aircraft configured to later airworthiness requirements. These figures are completely significant and give an extra confirmation that this work on investigating the fire response of composite aerostructures is on the right track. As the work of Cherry and Warren concluded, the fire hardening measures in order to be applicable need to be cost efficient. The concept under which this whole dissertation stepped on was to investigate the fire response of composite aerostructures and the possibility of hardening the structure itself without the use of extra protective layers that add cost and weight to the overall aircraft and its maintenance. In the end it was concluded that there is the possibility of hardening the fuselage structure by design and by material. Incorporating composites into the structure it is possible to prolong the burnthrough time at least for 4-5 minutes before auto ignition occurs on the inner side of the fuselage. Auto ignition of the inner side fuselage cabin materials is mentioned since in NONE of the burnthrough tests of the CFRP composites and the GLARE samples flame penetration was observed.
Στην παρούσα διατριβή με τίτλο «Ανάλυση της απόκρισης σύνθετων πολυμερών υλικών υπό συνθήκες φωτιάς. Εφαρμογή σε αεροπορικές κατασκευές» πραγματοποιείται εργασία στην αριθμητική προσομοίωση και πειραματική διερεύνηση της συμπεριφοράς αεροπορικών κατασκευών σε συνθήκες φωτιάς. Στην μέχρι τώρα βιβλιογραφία οι διάφοροι έλεγχοι για πιστοποίηση των αεροπορικών υλικών αλλά και των αεροσκαφών στο σύνολό τους αποτελούνταν από εκτενείς πειραματικές δοκιμές σε μεσαία κλίμακα καθώς και σε πλήρους κλίμακας κατασκευές. Οι προδιαγραφές των ελέγχων ορίζονται από την Ομοσπονδιακή Διεύθυνση Αεροπλοΐας των Ηνωμένων Πολιτειών της Αμερικής, Federal Aviation Administration FAA. Όπως γίνεται αντιληπτό πλήρους κλίμακας δοκιμές είναι χρονοβόρες αλλά και οικονομικά ασύμφορες, για τον λόγο αυτό τα τελευταία χρόνια πραγματοποιούνται προσπάθειες από την FAA για καθιέρωση Προτύπων ελέγχου μικρής κλίμακας τα οποία σε συνδυασμό με αριθμητικά μοντέλα θα είναι σε θέση να προβλέπουν την συμπεριφορά των αεροπορικών κατασκευών σε συνθήκες φωτιάς από την φάση του σχεδιασμού τους. Θα εξασφαλίζεται έτσι καλύτερη διαχείριση οικονομικών και υλικών πόρων. Στην βιβλιογραφία ο μεγαλύτερος όγκος αριθμητικής μοντελοποίησης έχει πραγματοποιηθεί στους τομείς της ναυπηγικής και των θαλάσσιων κατασκευών καθώς επίσης και τα τελευταία χρόνια στον τομέα της αστικής δόμησης. Αριθμητική δουλεία πάνω στην συμπεριφορά των αεροπορικών κατασκευών είναι υπερβολικά περιορισμένη και εκεί στοχεύει να συμβάλει η παρούσα διατριβή. Οι αεροπορικές κατασκευές εκτός των περιορισμών και προδιαγραφών που θέτουν οι άλλες εφαρμογές απαιτούν την ελαχιστοποίηση του προστιθέμενου βάρους στην κατασκευή. Διάφοροι τύποι πολυμερών συνθέτων υλικών χρησιμοποιούνται στην βιομηχανία, διακρινόμενα σε θερμοσκληρυνόμενα και θερμοπλαστικά. Αρχικά παρουσιάζονται τα θερμοσκληρυνόμενα ξεκινώντας από τους ευρέως χρησιμοποιούμενους πολυεστέρες και βινυλεστέρες, στις φαινολικές και εποξικές ρητίνες καταλήγοντας στους υψηλής θερμοκρασίας κυανεστέρες. Εν συνεχεία γίνεται αναφορά στα συνήθη χρησιμοποιούμενα θερμοπλαστικά, πολυπροπυλένιο PP, Poly-ether ether-ketone PEEK και polyphenylene Sulphide PPS. Φυσικά δεν παραλείπεται να γίνει σύντομη αναφορά και στις τυπικές διεργασίες θερμικής αποσύνθεσης των προαναφερθέντων πολυμερών. Η συμπεριφορά των σύνθετων πολυμερών υλικών σε συνθήκες φωτιάς περιγράφεται από κάποια χαρακτηριστικά μεγέθη τα οποία χρησιμοποιούνται για την ποιοτική και ποσοτική σύγκριση των διαφόρων υποψήφιων αεροπορικών υλικών. Συγκεκριμένα τα μεγέθη αυτά είναι: Heat Release Rate HRR, Thermal Stability Index TSI, Limited Oxygen Index LOI, Extinction Flammability Index ESI, Time-to-Ignition, Surface Flame Spread, Mass Loss, Smoke Density, Smoke Toxicity. Οι διαδικασίες ελέγχου και τα υπολογιζόμενα μεγέθη γίνονται βάσει διεθνών Προτύπων που κυρίως για τον τομέα της αεροναυπηγικής ορίζονται από την Ομοσπονδιακή Διεύθυνση Αεροπλοΐας FAA. Η αριθμητική προσομοίωση προυποθέτει γνώση της συμπεριφοράς των πολυμερών υλικών σε συνθήκες υψηλής θερμοκρασίας, για τον σκοπό αυτό πραγματοποιήθηκαν πειράματα απώλειας μάζας με χρήση θερμογραβιμετρίας TGA κατά την διάρκεια της οποίας η απώλεια μάζας καθώς και ο ρυθμός αυτής παρακολουθούνται και καταγράφονται σαν συνάρτηση του ρυθμού θέρμανσης. Μέσα από αυτά τα δεδομένα μπορεί να πραγματοποιηθεί εκτίμηση του τρόπου αποσύνθεσης του πολυμερούς. Αρχικά πραγματοποιήθηκε η θεώρηση της μονοβάθμιας αντίδρασης (single-stage reaction) που αποτελεί και την πλέον απλουστευμένη προσέγγιση. Στην θεώρηση αυτή θεωρείται πως η πολυμερής μήτρα περνάει από την «παρθένα» κατάσταση στην απανθρακομένη μέσα σε ένα βήμα. Η περιγραφή της αντίδρασης αυτής γίνεται με μια μονοβάθμια αντίδραση τύπου Arrhenius. Σε δεύτερο βήμα χρησιμοποιήθηκε κινητική θεωρία πολλαπλών σταδίων (multi-stage kinetics) σύμφωνα με την οποία πραγματοποιήθηκε ακριβέστερη προσέγγιση της απόσύνθεσης της πολυμερούς μήτρας των συνθέτων υλικών με απόκλιση μικρότερη του 5% από τα πειραματικά δεδομένα της θερμογραβιμετρείας (thermogravimetry). Και στις δύο προσεγγίσεις της αποσύνθεσης υπολογίσθηκαν οι κινηματικές παράμετροι: συντελεστής συχνότητας A (frequency factor), ενέργεια ενεργοποίησης ΕΑ (activation energy), τάξη αντίδρασης n (reaction order) για κάθε στάδιο. Με την ολοκλήρωση αυτού του σταδίου υπήρχε μια αξιόπιστη δυνατότητα αναπαράστασης της διαδικασίας αποσύνθεσης στο πείραμα της θερμογραβιμετρίας. Είναι γνωστό ότι οι διακυμάνσεις της θερμοκρασίας επηρεάζουν της τιμές των θερμοφυσικών ιδιοτήτων των υλικών. Αναλογιζόμενοι ότι στην διαρκεία της επιβολής της φλόγας στα σύνθετα υλικά όχι μόνο η θερμοκρασία αλλά και η σύσταση μεταβάλλεται συνεχώς λόγω της αποσύνθεσης κρίθηκε αναγκαία η ανάπτυξη μιας μεθοδολογίας που θα συμπεριλαμβάνει την επίδραση της αποσύνθεσης στην μεταβολή των θερμοφυσικικών ιδιοτήτων (θερμική αγωγιμότητα, ειδική θερμοχωρητικότητα και πυκνότητα) της πολυμερούς μήτρας και κατά συνέπεια του συνθέτου υλικού. Οι εξαγόμενες μαθηματικές σχέσεις χρησιμοποιήθηκαν στην αριθμητική προσομοίωση που ακολούθησε. Με σκοπό την ορθή αριθμητική μοντελοποίηση κρίνεται αναγκαία η μέτρηση και βαθμονόμηση του θερμικού φορτίου τον πειραματικών δοκιμών. Το μετρούμενο θερμικό φορτίου χρησιμοποιήθηκε εν συνεχεία ως φόρτιση στα αναπτυχθέντα μοντέλα. Χρησιμοποιήθηκαν δύο πειραματικές διατάξεις εφαρμογής φλόγας, μία μεσαίας κλίμακας σύμφωνα με τις διατάξεις του FAA Standard, που περιγράφεται στο ISO2685:1998(E) “Aircraft – Environmental test procedure for airborne equipment – Resistance to fire in designated fire zones” και μίας εργαστηριακής κλίμακος. Πραγματοποιήθηκε μέτρηση με θερμοζεύγη και καλορίμετρο νερού καθώς και αριθμητική μοντελοποίηση με χρήση CFD για την πρώτη διάταξη. Ενώ για την εργαστηριακής κλίμακας έγινε μέτρηση με θερμοζεύγη και ενός αισθητήρα θερμικού φορτίου «water-cooled Hukseflux Schmit-Boelter SBG01 sensor». Εν συνεχεία πραγματοποιήθηκε η κατασκευή των δοκιμίων των υποψήφιων υλικών καθώς και οι πειραματικές δοκιμές και έλεγχοι τους. Συγκεκριμένα πραγματοποιήθηκε: Θερμιδομετρία κώνου (cone calorimetry), Θερμογραβιμετρία (thermogravimetry), Θερμιδομετρία Διαφορικής Ανίχνευσης (Differencial Scanning Calorimetry, DSC), Μέτρηση Θερμικής αγωγιμώτητας, Δοκιμή διείσδυσης φλόγας (Fire burnthrough penetration). Καθώς ο χαρακτηρισμός της αποσύνθεσης των πολυμερών υλικών, η μεταβολή των θερμοφυσικών ιδιοτήτων, η μέτρηση και βαθμονόμηση του επιβαλλόμενου θερμικού φορτίου καθώς και οι πειραματικές δοκιμές έχουν ολοκληρωθεί ακολουθεί η αριθμητική προσομοίωση. Οι συνοριακές συνθήκες θερμικού φορτίου και ψύξης επιλέχθησαν ως εξής. Ως φόρτιση θεωρήθηκε η κατανομή του θερμικού φορτίου (σε kW/m2) στην εμπρός επιφάνεια του πάνελ. Στην ψύξη της πίσω επιφάνειας λήφθηκε υπόψη τόσο η ελεύθερη μεταφορά θερμότητας με επαφή όσο και η ακτινοβολία. Το μοντέλο της συμπεριφοράς του υλικού διαμορφώθηκε κατάλληλα ώστε να γίνει κατανοητό από τις απαιτήσεις ενός εμπορικού κώδικα Πεπερασμένων Στοιχείων επίλυσης θερμικών προβλημάτων και προσομοιώθηκαν οι πειραματικές δοκιμές διείσδυσης φλόγας των δύο πειραματικών διατάξεων, μεσαίας και εργαστηριακής κλίμακος. Πλέον της αριθμητικής προσομοίωσης της συμπεριφοράς σε φωτιά επίπεδων δοκιμίων αεροπορικών κατασκευών, πραγματοποιήθηκε προσπάθεια απλουστευμένης μοντελοποίησης των συνθηκών φλόγας ενός λιμνάζοντος όγκου καυσίμου αεροσκαφών στο εξωτερικό μιας ατράκτου. Δημιουργήθηκε ένα τρισδιάστατο ρευστομηχανικό μοντέλο πρόβλεψης του θερμικού φορτίου στην επιφάνεια μιας τυπικής ατράκτου σύμφωνα με τις προδιαγραφές γεωμετρίας του Προτύπου “Full-scale test evaluation of Aircraft fuel fire burnthrough resistance improvements” DOT/FAA/AR-98/52,1999. Τα ρευστομηχανικά αποτελέσματα συγκρίθηκαν με δεδομένα βιβλιογραφίας για μεγάλες φλεγόμενες δεξαμενές λιμνάζοντος καυσίμου. Εκτός από την μελέτη της απόκρισης των αεροπορικών κατασκευών σε συνθήκες φλόγας σκοπός της παρούσας εργασίας είναι και η παρουσίαση λύσεων οι οποίες θα έχουν την δυνατότητα της βελτίωσης της συμπεριφοράς των υπαρχουσών δομών καθώς και των μελλοντικών σύνθετων δομών. Ενδεικτικά αναφέρεται η δυνατότητα χρήσης νανοεγκλεισμάτων, και βελτιωμένων μονωτικών υλικών, π.χ. aerogels. Όπως έχει ήδη αναφερθεί οι αεροπορικές κατασκευές θέτουν τον περιορισμό της ελαχιστοποίησης του προστιθέμενου βάρους, για τον λόγο αυτό η ενίσχυση των συνθέτων υλικών θα πρέπει να πραγματοποιηθεί σε επίπεδο υλικού και σχεδιασμού. Πρέπει δηλαδή η ίδια η κατασκευή που είναι ικανή να φέρει τα μηχανικά φορτία να εξασφαλίζει και την πιστοποίηση της FAA για συνθήκες φωτιάς. Συνοψίζοντας, η παρούσα διατριβή πραγματοποιεί μια καινοτόμο, γρήγορη και αρκετά ακριβή προσέγγιση του σημαντικότατου ζητήματος της συμπεριφοράς των πολυμερικών σύνθετων αεροπορικών δομών σε συνθήκες φωτιάς Η πολυπλοκότητα του όλου φαινομένου επέβαλε την πραγματοποίηση παραδοχών και απλουστεύσεων. Καθώς όμως με την αυξανόμενη χρήση των συνθέτων υλικών στις αεροπορικές κατασκευές, ο τομέας της ασφάλειας σε συνθήκες φλόγας είναι συνεχώς αυξανόμενος και απαιτητικός. Για αυτό οι παραδοχές και θεωρήσεις της παρούσας διατριβής μπορούν να βελτιωθούν με χρήση νέων υπολογιστικών μεθόδων και πειραματικών δεδομένων με στόχο την ακόμα ακριβέστερη πρόβλεψη της συμπεριφοράς τον αεροπορικών δομών σε συνθήκες φλόγας.