Rozprawy doktorskie na temat „ENERGY DISSIPATORS”

Esta Tesis trata de la simulación numérica y experimental de estructuras de edificios equipadas con disipadores de energía de fricción, sujetos a cargas de sismo. La Tesis pretende contribuir a la evaluación de la eficacia de los disipadores de fricción para reducir la respuesta lateral de los edificios sujetos a sismos. Así, el objetivo global es la evaluación de la utilidad de los disipadores de fricción frente a las cargas sísmicas.

La Tesis está organizada de tal manera que sea fácil su seguimiento. Por esta razón, los capítulos se han escrito siguiendo una secuencia lógica. Una breve descripción de los capítulos se presenta a continuación.

El primer capítulo trata de los aspectos generales del Control Estructural con un especial énfasis en los disipadores de fricción, los cuales se incluyen en el llamado enfoque de 'Control Pasivo'.
El Capítulo 2 proporciona un repaso general al estado del conocimiento relativo a los principios de la fricción seca y a la investigación y aplicación de los dispositivos disipadores de fricción, así como a sus simulaciones numéricas.

Un procedimiento numérico ha sido desarrollado (programa ALMA) en los Capítulos 3 (edificios de una sola planta) y 4 (edificios de varias plantas) para llevar a cabo simulaciones numéricas de edificios equipados con disipadores de fricción y los resultados obtenidos fueron comparados a los que se obtuvieron con el programa comercial ADINA. La similitud entre los resultados de ambos programas es aceptable.

El Capítulo 5 describe los ensayos sobre una mesa vibratoria que se hicieron en dos modelos de edificios: uno de una sola planta y otro de dos plantas; ambos equipados con disipadores de fricción. Los resultados obtenidos se compararon a los resultados conseguidos con ALMA. Los resultados coinciden aceptablemente.

Luego de probar la validez del programa ALMA, éste se usó para hacer un estudio numérico preliminar para obtener la llamada 'carga óptima de deslizamiento' de los disipadores de fricción en el Capítulo 5. Las curvas que se pueden dibujar a partir de los resultados obtenidos con este estudio, pueden ser aplicadas para determinar las cargas óptimas de deslizamiento de edificios de una y varias plantas.

Finalmente, el Capítulo 7 contiene las conclusiones de esta Tesis y algunas propuestas para las futuras líneas de investigación.

Además, se han incluido cinco apéndices. El Apéndice A contiene los principios básicos del Análisis del Contacto. El Apéndice B trata de la solución numérica de las ecuaciones de movimiento para sistemas de uno y varios grados de libertada. Este apéndice está muy relacionado con el contenido de los Capítulos 3 y 4 de esta Tesis. El Apéndice C describe la solución de las ecuaciones de movimiento usando multiplicadores de Lagrange. El Apéndice D describe el equipo de laboratorio y la instrumentación usada en los ensayos. Los resultados de éstos, y la comparación entre éstos y los resultados numéricos (programa ALMA), son presentados en el Apéndice E.

Le développement de turbomachines à faible vitesse spécifique suscite actuellement une forte demande dans de nombreux domaines industriels. De nos jours, les ventilateurs centrifuges constituées d'une roue, d'un diffuseur et d'un canal de retour sont largement utilisés dans de nombreuses applications telles que l'automobile, l'aéronautique, appareils domestiques, etc. En raison de leur grande adaptabilité, ils peuvent fonctionner sur une large plage de débit en particulier avec une vitesse de rotation importante. Cependant, leur utilisation nécessite une meilleure compréhension de leur fonctionnement et notamment de l'interaction rotor-stator pour mieux les concevoir. L'objectif de cette étude est d'analyser et de quantifier l'influence de la solidité des aubes du diffuseur (ou serrage des profils) sur les performances aérauliques du ventilateur centrifuge. Une étude numérique et expérimentale a été menée pour analyser les performances d'un ventilateur centrifuge avec différents niveaux d'interaction rotor-stator. Différentes géométries de diffuseur ont été proposées dans le but de réduire les dissipations énergétiques liées aux phénomènes de recirculation et aux forces de viscosité. L'étude approfondie de la norme ISO 5801 et ISO 5167 a permis de dimensionner et réaliser le banc d'essai et les organes de mesures. Ces normes proposent des calculs et des spécifications qui nous ont permis de fixer des critères quant au choix des éléments du banc d’essai AERO2VACCUM
Nowadays, centrifugal fans are widely used in many industrial applications such as automotive, aeronautic, vacuum cleaner, etc. Due to their high adaptability, they can operate over a wide flow range especially with high speed rotation. In order to reduce the impact on environment, centrifugal fan as other energy conversion system has to display a high efficiency. However, a better understanding of their working and of the rotor-stator interaction is required to enhance their design and to make them widely integrated in current applications. The purpose of this study is to analyze and to quantify the influence of the solidity vaned diffusers on the performance of a centrifugal fan. A computational study has been conducted to analyze the performance of a centrifugal fan under various levels of impeller-diffuser interactions. The objective was to understand the pressure recovery phenomena in each diffuser. Different geometries were proposed to reduce both separation flow phenomenon and viscous losses. The geometries are all composed with the same impeller and return channel, only the vaned diffusers differ; the trailing edges are maintained at the same position while the leading edges have different radial positions. The characterization of the fan has been realized by an experimental setup designed and built in agreement to the standards ISO 5801 and 516

Nesta dissertação estão apresentados os principais conceitos teóricos envolvidos na ação dos jatos formados em dissipadores tipo salto esqui sobre fossa de erosão pré-escavada, desde o trajeto do jato em sua fase aérea e em sua fase submersa, dando especial ênfase às pressões hidrodinâmicas e sua caraterização junto ao fundo da bacia. Para estudar experimentalmente a ação dos jatos nas bacias de dissipação, utilizou-se de um modelo físico de escala 1:100 construído no Laboratório de Obras Hidráulicas do IPH/UFRGS com ensaios de diferentes vazões e diferentes profundidades de fossas pré-escavadas. Os dados foram extraídos dos ensaios no modelo físico através de vasta instrumentação, utilizando-se comportas, pontas linimétricas, piezômetros, sensores de pressão, imagens e outros equipamentos. Foi realizada uma análise criteriosa dos dados gerados no modelo físico a partir das imagens do jato na sua trajetória aérea e no escoamento do jato em sua fase submersa, comparando os dados do modelo aos resultados obtidos por métodos tradicionais de cálculo. Também foram analisados os dados de pressão gerados durante os ensaios, sendo apresentados os resultados relativos às pressões médias e às pressões dinâmicas. As pressões dinâmicas observadas no modelo físico foram comparadas com as calculadas por meio de metodologia teórica abordada na bibliografia com a finalidade de avaliar a qualidade dessa metodologia para aplicação em projetos de engenharia, onde se observou grandes diferenças de magnitude da pressão quando em colchões rasos, e diferenças de distribuição de pressão ao longo do fundo da bacia de dissipação, principalmente quando em colchões profundos. No sentido de ajustar a metodologia aplicada, foi proposta a variação de parâmetros que regulam a forma do bulbo de pressões e a aplicação de um coeficiente de amortecimento de pressões, sendo que assim foi possível melhorar significativamente a distribuição e a magnitude das pressões calculadas devido à ação dos jatos no fundo da bacia de dissipação. Os resultados se mostram satisfatórios até determinada distância após o pico de pressão provocado pelo jato, sendo que após este ponto os efeitos de ressalto hidráulico prevalecem e devem ser pesquisados em estudos futuros.
In this dissertation are presented the main theorical concepts involved in the action of the jets formed in ski jump spillways on stilling basins type pre-excavated scour hole, from the path of the jet in its air stage and in his submerged phase, with particular emphasis on hydrodynamic pressures and its characterization by the bottom of the basin. To experimentally study the action of the jets in stilling basins, was used a physical model in scale 1:100 built in Hydraulic Works Laboratory of IPH/UFRGS with experiments of different flow rates and different depths of pre-excavated scour hole. The data were extracted from experiments on the physical model through extensive instrumentation, using gates, piezometers, pressure sensors, images and other equipment. A careful analysis of the data generated in the physical model from the jet images in your path through the air was performed and the flow of the jet in its submerged phase, comparing the model data to the results obtained by traditional methods of calculation. The pressure data generated during the tests and presented the results for the mean pressures and dynamic pressures were also analyzed. Dynamic pressures observed in the physical model were compared with those calculated by means of theoretical methods for the purpose of evaluating the quality of this methodology for use in engineering projects, where we observed large differences in magnitude of pressure when in shallow water beds, and diferences in pressure distribution over the stilling basin bottom, especially when in deep water beds. In order to adjust the applied methodology, it was proposed to change parameters governing the shape of the pressure bulb and the application of a pressure damping coefficient, and thus was possible to significantly improved the distribution and magnitude of the calculated pressures due to action of the jets in the stilling basin bottom. The results have been satisfactory up to a certain distance after the peak pressure caused by the jet, and after this point the hydraulic jump effects prevail and should be investigated in future studies.

Este artículo consiste en analizar una nueva alternativa de control antisísmico de alta tecnología para la construcción de hospitales en el Perú, que en su normativa exige el uso de aislamiento de base para edificaciones esenciales. Para ello, se opta por la evaluación estructural de una edificación construida con aisladores sísmicos y la misma con implementación del sistema de disipación de energía SLB (Shear Link Bozzo). Los resultados analizados, para cada técnica estudiada de control antisísmico, son: derivas de entrepiso, aceleraciones de piso, balance energético y análisis de rotulas plásticas, en base a la filosofía de funcionalidad continua.
This article consists of analyzing a new high-tech anti-seismic control alternative for the construction of hospitals in Peru, which in its regulations requires the use of base insulation for essential buildings. For this, the structural evaluation of a building constructed with seismic isolators is chosen and the same with the implementation of the SLB (Shear Link Bozzo) energy dissipation system. The results analyzed for each anti-seismic control technique studied are: mezzanine drifts, floor accelerations, energy balance and analysis of plastic hinges, based on the philosophy of continuous functionality.
Trabajo de investigación

In this thesis two all-steel BRB have been designed, manufactured and tested, and both satisfy the testing protocols required by EU and US codes. They are composed of a steel slotted restraining unit which stabilizes the steel core. The first one, the Modular Buckling Restrained Brace (MBRB), is composed by several seriated modules which contain several dissipation units connected in parallel, which yield under shear forces. Although it has good hysteretic response and a high ductility, its core is heavy and expensive to manufacture. The second one,the Slotted Buckling Restraining Brace (SBRB), solves these two shortcomings. It yields under axial forces, like the conventional BRBs, but the usual solid core has been substituted by a perforated plate. The core is a one-piece element composed of two lateral bands, of a nearly uniform section and designed to yield, connected by stabilizing bridges, which behave elastic. The buckling prevention of the lateral bands is done by the restraining unit and the stabilizing bridges. Design expressions have been proposed to design both devices, and a numerical material model have been formulated and implemented in commercial finite element method software to numerically simulate the behavior of the braces, which will reduce the need of full scale tests for its design
En la present tesi, dos BVR totalment metàl·lics s’han dissenyat, fabricat i assajat, satisfent tots dos els requeriments dels protocols definits per les normes europees i americanes. Estan formats per un element de travat ranurat que estabilitza el nucli metàl·lic. El primer d’ells, el Braç de Vinclament Restringit Modular (BVRM), està format per diversos mòduls seriats que a la vegada contenen diverses unitats de dissipació, que plastifiquen sota esforços tallants, connectades en paral·lel. Tot i que té un bon comportament histerètic i una gran ductilitat, el nucli és pesat i difícil de fabricar. El segon braç, anomenat Braç de Vinclament Restringit Ranurat (BVRR), soluciona aquests inconvenients. Plastifica sota esforços axials, de la mateixa manera que els BVR convencionals, però el nucli massís és substituït per una platina perforada. Aquest nucli consisteix en un únic element composat per dos bandes laterals, dissenyades per a plastificar i amb una secció quasi constant, connectades per diversos ponts estabilitzadors que es mantenen sempre en el seu rang elàstic. Aquests ponts, juntament amb l’element de travat, impedeixen el vinclament de les bandes laterals. S’han proposat diverses expressions de disseny pels dos braços. S’ha formulat i implementat, en un programa d’elements finits comercial, un model de material per a simular numèricament el comportament dels braços, reduint així la dependència dels assajos a escala real durant el seu procés de disseny

This thesis demonstrates how new materials and damping technologies were developed and applied to mitigate the adverse effects of vehicular crashes into masonry buildings. Many masonry buildings are vulnerable to vehicular crashes which result in loss of property, loss of habitat or functionality of the building and harm to the occupants of the building and vehicle. Auxetic materials were used in this research with negative Poisson ratio and innovative damping technologies at the wall edges to reduce the accident severity and to save the lives in both the building and the vehicle.

En région montagneuse, les infrastructures et les voies de communications sont soumises à de nombreux risques naturels dont les phénomènes d'origine gravitaire. Au-delà du danger pour les usagers, les conséquences des interruptions du trafic deviennent problématiques d'un point de vue économique et il devient indispensable de sécuriser les itinéraires. La mise en place d'écrans de filets pare-pierres est une des solutions possibles pour la protection contre les éboulements rocheux. Cette thèse porte sur l'étude des écrans souples ou filets métalliques de protection contre les chutes de blocs et plus précisément sur l'écran développé par l'entreprise GTS. Le filet constitutif de ces écrans se différencie par rapport aux systèmes conventionnels par un comportement orthotrope, dû à un maillage spécifique. Dans cette étude nous caractérisons le comportement de ces écrans de filets sous des chargements statiques et dynamiques de type impact par une approche couplant l'expérimentation et la modélisation numérique. L'étude procède pas à pas, les divers constituants sont évalués de façon quasi-statique, en laboratoire, et également in-situ pour reproduire les conditions réelles d'utilisation, en particulier l'aspect dynamique. Une attention particulière concernant les dissipateurs d'énergie, qui représentent l'élément centrale de ce type de structure, nous a permis de mettre au point un élément fusible robuste et fiable. Une campagne d'essais en grandeur réel sur les écrans de filets étudiés a permis de valider deux classes énergétiques (3000 kJ et 5000 kJ) selon les recommandations européennes. Les données recueillis lors des expérimentations ont permis de calibrer et valider différentes modélisations numériques de type éléments finis et éléments discrets. La pertinence de la modélisation a été évaluée au niveau des différentes échelles étudiées, échelle d'une maille, échelle d'une nappe, échelle du dissipateur d'énergie et échelle de la structure entière. Les performances et les limites des deux approches, MEF (méthode des éléments finis) et MED (méthodes des éléments discrets) ont été évaluées pour ce qui est de nos modélisations.

碩士
國立中興大學
土木工程學系
89
According to the related research about the scouring effects of free over-fall will be used in this study. The situation will be discussed in steady flow by using different energy dissipators — apron, water cushion and secondary dam. Facilities, the protected effect and the comparative characteristics of scouring hole will be found (such as the max. depth, max. length, shape factor and volume) in downstream of the energy dissipators. The result would be determined by energy dissipating theory, which is the relations between the scale of dissipators and energy variation. In view of energy disspators, the water cushion had the result of protection better than apron, because of the current hits mutually at the water pond. And the height factorβ=0.13 is the best. The situation of the secondary dam is the moving bed, therefore analysis of the scouring characteristic to make use energy variety of system. In both the rigid and soft energy disspators, the energy loss of scouring increased with slop. The degree of scouring depth and volume could be lowered effectively in rigid energy disspators. In water cushion, the average decreased rate of the scouring length about is 40 percent, and the scouring volume quantity decreases rate reaches 40 percent at least. Although, scouring length can’t be decreased at apron disspators. The scour phenomenon be mollified to a downstream region for protective effects. Scouring is more serious between major and secondary dam if the distance is too short from both of them. Character factorα>1.56 is the suggested application. In this experiment condition, a simple formula is proposed for a relevant establishment. It is expected to be helpful in designs of energy disspators.

Dams and barrages play a very vital role in the economy of a country by providing essential benefits like irrigation, hydropower, flood control, drinking water, recreation, etc. However, when these fail in rare conditions, these may cause catastrophic flooding in the downstream area resulting in huge loss to human life and property. The spillway is among the most important structures of a dam project. It provides the project with the ability to release excess or flood water in a controlled or uncontrolled manner to downstream to ensure the safety of the project. The surface of the spillway should also be such that it is able to withstand erosion or scouring due to the very high velocities generated during the passage of a flood through the spillway. So the safety of spillway against the scouring at downstream is also important. To prevent the scouring of the riverbed and failure of the hydraulic structure, energy dissipators are commonly used for the purpose of dissipating the excessive kinetic energy of flowing water downstream of hydraulic structures. An experimental study was conducted to understand the scouring pattern below downstream of the spillway. A rectangular flume of dimensions 6.0m(length)X0.30m(width)X0.60m(depth), four spillway models(ogee spillway, ski jump bucket, ogee spillway with subsidiary dam, ogee spillway with baffle wall) and locally available material to prepare bed were used in this study. A pitot tube was used to measure the flow velocity and discharge. To measure the pre-jump and post-jump for the calculation of energy loss point gauge was used. In this experimental study total 40 tests were conducted using same run time. The scour pattern (depth and length) were measured after each test run.

Laboratory experiments have shown that screens or porous baffles with a porosity of about 50-60% can be used as effective energy dissipators below small hydraulic structures, either in a single wall, a double wall with the different angles at 45º, 90º and 135º. The experiments were carried out for a range of unstable flow Froude numbers (F1) from about 1.5 to 6, and the relative energy dissipation is comparatively larger than that produced by the hydraulic jumps. The iron sheet screens or porous baffles creates free hydraulic jumps, imposed hydraulic jumps, and in some cases submerged jumps. The downstream flow was found to be supercritical flow and tail water depth after leaving the screen is equal to the .79 times to y2.

碩士
國立中央大學
物理研究所
92
Several kinds of flows in the soap film is created by different electromagnetic forcing and measured by particle tracking. In circle flow the velocity of air is described well by Navier-Stokes equation with only azimuthal velocity component. Using the equation and double films experiment, the velocity in soap film and air are measured. Besides, film's thickness is measured by resonance method. The ratio of energy dissipation rate in air and soap film is found. The important factors of the effect rate are "injection scale" and "frequency response". Comparison of these rates demonstrates that the air friction is a significant energy dissipation mechanism in the system.

碩士
中興大學
土木工程學系所
95
ABSTRACT A vertical drop pool is frequently used in an open channel to dissipate the energy of the approach flow. As the combination of pool length (L), end sill height (h), and discharge (q) meets neither the napped flow nor the skimming flow conditions, the flow oscillates periodically in the vertical drop pool. The purpose of this study is to investigate the characteristics of the periodic oscillatory flow over a vertical drop pool with a subcritical approach flow experimentally. Free surface oscillations of dropping flows were measured using a wave gauge located in the pool. The primary frequency (f0) of the periodic oscillation was then determined by applying spectral analysis to the time series of the gauge measurements. Two flow visualization techniques were employed to reveal the structures of dropping flows qualitatively. In order to understand the relationship between the periodic oscillatory flow pattern with the discharge of the approach flow and the influence of end sill height, series of experiments were conducted by varying discharge (q) and end sill height ratios (h/H, where H is the dropping depth). The flow behaves as the periodic oscillatory flow for h/H ranging from 0.121 to 0.857 with different discharges. When the h/H was equal to 0.429, the largest range of discharge (q) was obtained from 154.6 ~ 223.5 cm2/s for the periodic oscillatory flow patterns in the vertical drop pool. With analyzing the flow visualization results and the time series measurements of wave gauge, flow patterns of the periodic oscillation could be classified by three different modes as follows: (1) Mode1 shows the trend that the f0 decreases with the increase of q for h/H varying from 0.121 to 0.429. A large amount of air bubbles was entrained to the vortex upon the end sill corner. (2) Mode2 presents the trend that the f0 increases with the increase of q for h/H changing from 0.329 to 0.571. By comparing with Mode1, there is no air bubbles entrained into the vortex upon the end sill corner. (3) Mode3 shows that the f0 decreases with the increase of q as h/H is equal to 0.714. The flow features a falling jet impinging on the corner and vertical wall of the end sill without entraining air bubbles upon the end sill corner.

碩士
中興大學
土木工程學系所
94
The measurement of velocity fields of periodic oscillatory flow over a two-dimensional vertical drop energy-dissipator was investigated experimentally. Since a large amount of air bubbles are entrained in the oscillation flow. The highly aerated bubbles caused the traditional particle image velocimetry (PIV) technique to fail due to the uncontrollable scattering of laser light. A modified PIV method, called bubble image velocimetry (BIV), was employed by directly using bubbles as the tracer and measuring the bubble velocity by correlating the “texture” of the bubble images. No laser light sheet was needed while the depth of field was limited to minimize the error. While the fluid velocity in the region where no or few bubbles can be successfully obtained using PIV, the velocity in the high void fraction region can be measured using BIV. To further investigate the velocity characteristics of the periodic oscillatory flow over a two-dimensional vertical drop, PIV and BIV were used in the non-aerated region and the aerated region for quantitative velocity measurement, respectively. The oscillation of water surface was measured using a wave gauge for reference signal. The oscillation phases of particle image velocimetry (PIV) and bubble image velocimetry (BIV) were analyzed using wavelet analysis method. Through combining velocity fields of the non-aerated region and aerated region in the same phase, the cyliclic velocity fields of the oscillatory flow could be presented in detail.

Terrorism pose a serious threat nowadays and many countries have the concern of protecting his people and most important buildings. This concern is an opportunity to strengthen the research of the behaviour of buildings under blast in order to reduce the magnitude of the effects of these catastrophic events. The present thesis aims to create a functional system to improve the security of critical buildings, either new or adapting old buildings to receive this new system. This system also tries to be cost efficient, so this can be used in most buildings. This thesis was centred in one system with two variations. The systems had the same lower reinforced concrete slab with 2,60 x 2,00 m and 0,12 m of thickness. The first system consisted in using 32 steel tubes of 76,1 mm outer diameter arranged uniformly and eight concrete panels, each one with 1,00 x 0,65 m and 0,07 m of thickness, on top of these tubes. The second system was the same concrete panels on top of 32 steel tubes with 48,3 mm outer diameter. To test these systems four blast trials were prepared: the first is the reference specimen and the other two using the two mentioned variations of the system. These slabs were tested simply supported in two parallel edges, with a span of 2,30 m. In all tests 6,00 Kg of the explosive Eurodyn 2000 were used at a distance of 1,85 m from the top of the slab to the centre of the explosive. The results show an improvement in the residual deformation and on the opening of the visible cracks of the base reinforced concrete slab.

La termografia è una tecnica a campo intero, senza contatto e non distruttiva che fornisce la mappa della temperatura da un corpo a temperatura superiore a 0 K, rilevandone la radiazione infrarossa. La caratteristica più importante che rende la termografia un'utile tecnica non distruttiva è la possibilità di ispezionare qualsiasi tipo di corpo senza alcuna interazione con esso, consentendo rilievi ad alta temperatura o ispezioni in ambienti con condizioni critiche senza contaminazioni di materiali e/o sensori. A partire dalla prime osservazione ad opera di Sir William Herschel, nel 1800, la tecnica ha subito una notevole quantità di miglioramenti, infatti i recenti sensori IR beneficiano di un'elevata precisione (la risoluzione è all'incirca dell'ordine del centigrado) e di un'ampia gamma di temperature operative (da -20 Da ° C a 1000 ° C). Tuttavia, anche se le tecniche basate sulla termografia sono ampiamente diffuse in campo accademico, negli ultimi 20 anni si è osservato un ritardo significativo a causa dei maggiori costi delle apparecchiature a infrarossi e soprattutto a causa dell'assenza di standard che disciplinano tale tecnica. In questo senso, le difficoltà derivano dalla difficile collocazione della termografia in uno specifico campo di applicazione a causa della sua grande versatilità. In medicina, il grande interesse della comunità scientifica per la prevenzione del cancro al seno e delle malattie neurologiche comporta la ricerca di metodi più accurati per la diagnosi precoce. In questo senso, la termografia è utile per misurare le temperature superficiali del corpo in relazione al calore prodotto dall'organismo che passa attraverso la pelle e i tessuti. Negli ultimi anni, i ricercatori hanno sviluppato algoritmi accurati per l'analisi di immagini bidimensionali per rilevare la presenza di tumori, nonché per valutare i cambiamenti nella circolazione sanguigna al fine di localizzare la presenza di qualsiasi lesione ischemica. D'altra parte, nel contesto industriale, le parole chiave che portano il ricercatore a trovare nuove tecniche introdotte nel processo di produzione sono il Time-to-Market e la Qualità. Oggigiorno gli imprenditori sono interessati a ridurre il tempo tra il prodotto è fabbricato e venduto, in quanto determina i loro profitti. La riduzione dei tempi di commercializzazione è accompagnata dall'aumento della qualità dei prodotti. La continua richiesta della qualità nei prodotti industriali consente di integrare il controllo di qualità nel processo di produzione. Per fare questo, è necessaria una tecnica che fornisca test in-situ, non distruttivi, in linea durante del processo o a valle di esso, con prestazioni e elaborazione dati veloci. Nell’ambito industriale, si trae vantaggio dalla possibilità di utilizzare una tecnica che non richiede tempi di fermo della produzione, che consente di eseguire manutenzione preventiva nonché azioni rapide e mirate sulle macchine e infine di ridurre il rischio di incidenti e catastrofi. Ad esempio, la termografia per quanto riguarda la tecnica degli ultrasuoni non ha bisogno di un accoppiamento acustico, e non è importante se i rilevatori sono posizionati lontano o vicino al campione da indagare, poiché il contatto non è richiesto. Un altro aspetto interessante legato all'uso di tale tecnica è la possibilità di eseguire anche la caratterizzazione meccanica di materiali e componenti. Infatti, la termografia può essere adottata per valutare il limite di fatica dei campioni da laboratorio e componenti sottoposti a carico dinamico. Il metodo di prova standard per valutare le proprietà di fatica dei materiali richiede una procedura di prova estremamente duratura, sia per il numero di provini da testare sia per i tempi operativi delle macchine. Monitorando con una termocamera le prove a carico ciclico, ed seguendo una specifica procedura di test è possibile studiare la fatica e il danno del materiale, qualunque sia il materiale in un’ottica di riduzione del time-to-market del prodotto sia esso materiale o componente e di risparmio economico. In questa tesi, verrà mostrata la capacità della Termografia alla caratterizzazione della fatica dei materiali (acciai inossidabili, compositi). In particolare, la prima parte (capitoli 1 e 2) si concentrerà sul fornire i concetti di base sulla fisica dei fenomeni di scambio termico e sulla comprensione della teoria dell'approccio TSA (Thermoelastic Stress Analysis). Quest'ultima, come verrà mostrato, è empirica e fornisce due metodi (fase termoelastica e ampiezza del segnale termoelastico) per studiare il comportamento del materiale sottoposto a carico ciclico. Nel capitolo 2, insieme all'approccio TSA, saranno presentati approcci energetici in grado di fornire maggiori informazioni sulle fonti di calore dissipative che producono danneggiamenti nei materiali. Questi approcci sono focalizzati sulla valutazione dei fenomeni irreversibili dai materiali misurando le variazioni di temperatura o valutando l'energia dissipata durante ciclo di isteresi. Nel capitolo 3, il tentativo di unificare gli approcci sarà fornito presentando il modello adottato di variazioni di temperatura. Tali variazioni di temperatura saranno rappresentate secondo un modello che consente di separare tutte le componenti del segnale termico (reversibili ed irreversibili). In letteratura manca di una teoria unificata che valuti e utilizzi nello stesso tempo sia fonti di calore irreversibili termoelastiche reversibili sia dissipative. Inoltre, verrà fornita una spiegazione fisica per ogni componente del segnale termico al fine di migliorare la comprensione dei fenomeni che producono variazioni energetiche durante sollecitazioni cicliche. In effetti, la comprensione dei processi di fatica (fenomeni dissipativi viscosi e plastici) rappresenta la base per spiegare le variazioni di temperatura. Il vantaggio di tale approccio è quello di ottenere con un singolo test / acquisizione / analisi termografica informazioni complementari sul comportamento del materiale in esame. Dopo i capitoli iniziali sulla teoria e la fisica degli scambi di calore, verrà mostrato l'approccio 'energetico' adottato e verranno presentati accuratamente i metodi e gli algoritmi. Tale parte comprende tutti i dettagli delle diverse campagne sperimentali. I risultati interesseranno tutti gli aspetti della fatica: dalla nucleazione della cricca alla meccanica della frattura. Nel capitolo 4, tutti gli aspetti della fatica dei campioni non intagliati saranno discussi con particolare riferimento alla correlazione tra variazioni di temperatura e comportamento viscoso e plastico del materiale. Verrà discusso in dettaglio l’effetto del rapporto di sollecitazione o della microstruttura sulle variazioni di segnale termico. Verrà presentata una procedura per la valutazione del limite di fatica che potrebbe fungere da base di sviluppo per una nuova generazione di prove di fatica mediante metodi termici. Il capitolo 5 è incentrato sulla valutazione del comportamento a fatica dei materiali compositi Uno dei risultati più importanti si riferisce alla possibilità di separare il comportamento della fibra e della matrice al fine di studiare i fenomeni di danno on line in situ. Nel capitolo 6 si troverà l'applicazione dell'approccio presentato alla meccanica della frattura. In questo caso, quando la cricca si è appena sviluppata, è interessante osservare che la termografia offrire supporto nel rilievo della crack driving force e dei fenomeni di crack opening. Inoltre, il modello di rappresentazione delle variazioni di temperature consente anche di ottenere dei parametri importanti per determinare la posizione della cricca e l'area di plastica. L'intera dissertazione di questa tesi, è indirizzata a raccogliere tutte le teorie sullo studio della fatica utilizzando metodi termici, al fine di ottenere più informazioni sul danneggiamento da un singolo test. Infatti, unificando la teoria relativa alle differenti componenti del segnale termico, che in letteratura sono trattati separatamente, è possibile: • ridurre le tempistiche e i costi della campagna prove • ottenere una maggiore comprensione del processo di danneggiamento, e dei legami tra l’energia dissipata e le variazioni di temperatura • consentire il rilievo tempestivo ed il monitoraggio online / in-situ dei processi di danneggiamento mediante un’analisi rapida e non eccessivamente laboriosa. Dal punto di vista del mercato, i metodi e le tecniche presentate, non solo ridurranno il time-to-market dei prodotti, ma contribuiranno ad aumentare la del processo di produzione.
Thermography is a full-field, non-contact, non-destructive technique which provides the temperature map from a body at temperature higher than 0K, by detecting its infrared radiation. The most important characteristic making thermography a useful Non Destructive technique is the possibility to inspect any kind of body without any contact with it; this allows high temperature measurements or critical environment inspections without any material/sensor contaminations. Since the age of its discovery by Sir William Herschel, 1800, the technique undergone a great deal of improvements, in fact recent IR detectors benefit of high accuracy (resolution is roughly in the order of centigrade) and wide operating temperature range (from -20°C to 1000 °C). However, even if the techniques based on thermography are widely diffused in academic field, in the last 20 years a significant delay is observed due to the higher costs of IR equipment and mostly due to the absence of Standards which discipline such the technique. In this sense, difficulties arises from the difficult collocation of thermography in a specific field of application due to its great versatility. In fact, the feasibility of the Thermography extends from medical to many industrial applications. In the field of Medicine, the great interest of scientific community for the prevention of breast cancer and neurological diseases involves the research of more accurate methods for early diagnosis. In this sense, thermography is useful for measuring the surface temperatures from the body as related to the heat produced by the organism passing through the skin and tissues. In recent years, medical researchers developed accurate algorithms for 2-dimension imaging analysis to detect the presence of tumours, as well to evaluate changes in blood circulation in order to localise the presence of any ischemic lesion. On the other hand, in the industrial framework, the keywords which lead the researcher to find new techniques being introduced in the manufacturing process were Time-to-Market and Quality. Nowadays entrepreneurs are interested in reducing the time between the product is manufactured and sold, as it determines their profits. The reduction of the time to market is coupled with increasing the quality of products. The continuous demand of the quality in the industrial products, allows integrating the quality control in the manufacturing process. To do this, a technique providing in-situ, non-destructive tests during or at the end of the process with fast performing and data processing, is required. In particular, by introducing Thermography in the manufacturing process, Industry takes advantage from the possibility of using a technique which does not require any production downtime, allows predicting maintenance as well as fast and focused actions on machines and finally to reduce the risk of accident and catastrophes. For example, Thermography with respect Ultrasound technique does not need of acoustic coupling, and it is not important if the detectors are located far away or near the sample being investigate, since the contact is not required. Another interesting aspect related to the use of Infrared radiation by materials is the possibility to set up a technique that, somehow allows mechanical characterisation, as well. In fact, a part from the process controlling and non-destructive tests for assessing the quality of the products, thermography can be adopted for evaluating the fatigue limit, the yield strength of samples undergoing dynamic loading. The standard test method to evaluate fatigue properties of materials requires extremely long lasting test procedure, great number of material testes, and wide number of operating time of machines while by using an IR detector, by following a specific test procedure it is possible to study fatigue and the damage of the material, whatever the material is. This novel way to perform fatigue tests is consistent with the spirit of ensuring a little time to market of products together with an economic saving. In this thesis, it will be showed the capabilities of the Thermography to the fatigue characterisation of materials (stainless steels, composites). In particular, the first part (chapter 1 and 2) will be focused on providing the basic concepts on the physics of heat exchange phenomena as well as an understanding on the theory of Thermoelastic Stress Analysis (TSA) approach. This latter, as will be showed, is empirical and provides two methods (temperature variation of first order and thermoelastic phase shift) for studying the behaviour of material undergoing dynamic loading. In chapter 2, together with TSA approach, energetic approaches will be presented capable to provide more information on dissipative heat sources producing damage in the materials. These approaches are focused on the assessment of irreversible phenomena from materials by measuring temperature variations or by assessing the energy dissipated during the hysteresis loop. In chapter 3, the attempt to unify the approaches is provided by presenting the adopted model of temperature variations. The model presents the measured temperature evolution, as separeted in its components of first and second order and relatively phase shifts. In effect, literature lacks of a unified theory that assesses and uses in the same time both thermoelastic reversible and dissipative irreversible heat sources. Due to the adopted model, a single temperature variation can be associated with relative physically explanation, with the advantage that by a single test/thermographic acquisition/analysis it is possible to assess complementary information on behaviour of the material being tested. An appropriate study of energy dissipation determining the macroscopic physical phenomena related to the fatigue loading, has been setup before building all the models. In fact, the understanding of the fatigue processes (dissipative viscous and plastic phenomena) represents the basis for explaining the temperature variations. After initial chapters on the theory and physics of heat exchanges, the adopted ‘energetic’ approach will be shown and the methods and algorithms will be accurately presented. Such the part incorporates all the details of experimental campaigns. Results, will interest both the aspect of Fatigue: fatigue of smooth samples and fracture mechanics. In chapter 4, all the aspect of fatigue of smoothed samples will be discussed in sight of viscous and plastic behaviour of material. It will showed the effect of loading ratio or the microstructure on the temperature variation by the sample undergoing cyclic loadings. The result will focus on different types of materials: stainless steels and composites. The procedure to estimate the fatigue limit will be presented together with the damage phenomena localisation by observing the parameter provided by the adopted model. This feature is very important in case of random loading of operating components for detecting the onset of damage. Moreover, the procedure setup for evaluating the fatigue limit involves non-destructive fatigue tests, as it is possible to stop the test when a threshold value of temperature variation has been achieved. This could be used for a new generation of fatigue testing by means of thermal methods. Chapter 5 will be focused on the assessment of fatigue behaviour of composites materials by using all the parameters presented for metals analysis. One of the most important achievements refers to the possibility of separating the behaviour of fiber and matrix in order to study damage phenomena o nline in situ. Chapther 6, will present the application of presented approach on fracture mechanics. In this case, when the crack is just developed it is interesting to observe that the thermography can support in assessing otherwise the Paris’s law. So that, for real operating components, it is possible to make prediction live on the crack growth and prevent catastrophic failures. Moreover, it will shown that by using the presented model, it becomes simple to determine the crack tip position and the plastic area, and distinguish also between static plastic area and the area of crack closure effect. Apart from to present the strong impact of Thermography-based approaches in the field of mechanical characterisation, the whole dissertations of this Thesis, is addressed to gather all the theories on the study of fatigue by using thermal methods, in order to make more information on damage by a single test. The results that will be presented, demonstrate the multistage characteristic of the adopted approach, from the data processing to the data analysis. The purpose of presenting such the temperature variation model and the related analysis, is to unify all the components of temperature variations, which have been treated separately, in literature, in order to achieve: • Experimental campaign testing time and costs reductions • More understanding on specific damage process by assessing the temperature signals • Detection and monitoring online/in-situ of damage processes by means of relatively simple setup. On the point of view of the market, the presented methods and techniques, will not only reduce time-to-market of products, but also include and increase the quality in the manufacturing process.