Hypoid gears have been used extensively in automobiles, aerospace, marine and other applications for decades. The special advantages of hypoid gears come with inherent contact complexities of varying curvature and sliding in both profile and lengthwise direction. Spalling failure is catastrophic and needs to be addressed with deeper roots in gear design. Analytical methods present several limitations. Iterative development from experimentation is expensive and time consuming with different nonlinear parameters difficult to interpret. This thesis aims to develop a method to calculate contact fatigue life for initiation of spalling using finite element methods. Experiments have played a major role in understanding the causal factors for failure, determining the fatigue life and to study the major system design parameters. A failure analysis of the fractured flank is performed. It clarified the design causal factors for failure and the mechanism of failure. Pinion being the vulnerable part is the focus of this thesis, a finite element model was developed on ANSOL HFM and the residual stresses were superposed on MSC Marc. A finite element fatigue analysis is performed on FEMFAT and the component fatigue life is determined. The calculated fatigue life is compared with physical testing results using Weibull statistical analysis in combination with probabilistic bearing life models to formulate emphatical correlation methods. The goal of this thesis is to establish a method to estimate fatigue life by taking up example of computing subsurface fatigue life of a hypoid pinion. The influence factors like the method of contact analysis, different types of residual stresses due to case hardening and shot peening, fatigue criteria, friction, material properties are studied in this thesis to develop a conscience for the methodology to computing contact fatigue life. The bulk material properties based on hardness represented fatigue properties more accurately. Scaled normal stress in critical plane fatigue criteria was found more suitable for contact analysis with pre-stresses and multi-axial non-proportional contact stress state on FEMFAT. Finite element based contact analysis method was found to be more suitable for subsurface fatigue life estimation despite the inherent advantages of the hybrid surface integral method and its accurate representation of friction. It was found that inclusion of friction in model did not change the fatigue life significantly, showing that the influence of hardness, surface topographies lubrication and contact temperature on shear stresses are too large to be neglected. Contact fatigue life increased by a factor of 4.4 times due to shot peening of gears in comparison with case hardening indicating the influence of residual stresses. For the estimation of fatigue life at the initiation of failure, a complete correlation with the fatigue test results could not be achieved and reasons for deviations were clearly identified. The area of damage indicated by this computation method correlated with the damage observed during tests. The observations and calculations indicated premature failure of pinion with explanation of mechanism of failure of pinion flank using contact conditions. / Hypoidväxlar har använts i stor utsträckning i bilar, flyg-, marin- och andra applikationer under årtionden. De speciella fördelarna med hypoidväxlar kommer med inneboende kontaktkomplexitet med varierande krökning och glidning i både profil och längdriktning. Spallingfel är katastrofala och måste hanteras med grundlig redskapsdesign. Analytiska metoder har flera begränsningar. Iterativ utveckling baserad på experiment är dyrt och tidskrävande med olika icke-linjära parametrar som är svåra att tolka. Denna avhandling syftar till att utveckla en metod för att beräkna kontaktutmattningslivslängden för initiering av spalling med finitaelementmetoden. Experiment har spelat en viktig roll för att förstå orsaksfaktorerna för fel, bestämma utmattningslivslängden och för att studera de viktigaste systemdesignparametrarna. En felanalys av den skadade kuggflanken utfördes, vilket förtydligade felens orsaksfaktorer och den underliggande felmekanismerna. Driften är fokus för denna avhandling, en finitaelementmodell utvecklades med ANSOL-HFM och restspänningarna överlagrades med FEM-verktyget MSC-Marc. En slutlig elementutmattningsanalys utfördes med FEMFAT och komponentens utmattningstid bestämdes. Den beräknade utmattningslivslängden korrelerades med fysiska provningsresultat genom att tillämpa statistisk Weibullanalys i kombination med probabilistiska livslängdsmodeller. Målet med denna avhandling är att utveckla en metod för att uppskatta utmattningslivslängden och att tillämpa metoden för att beräkna bulkmaterialets utmattningslivslängd för ett hypoidrev. Påverkningsfaktorer som metoden för kontaktanalys, olika typer av restspänningar på grund av ythärdning och kulblästring, utmattningskriterier, friktion, materialegenskaper studeras i denna avhandling för att utveckla denna metod för att prediktera kontaktutmattningslivslängden. Bulkmaterialegenskaperna för hårdhet representerade utmattningsegenskaperna mera exakt. Skalad normalspänning i kritiska planutmattningskriterier befanns vara mer lämplig för kontaktanalys med förspänningar och multi-axiell icke-proportionell kontaktspänningsstatu än andra egenskaper med FEMFAT. Metodbaserad kontaktanalysmetod visade sig vara mer lämplig för uppskattning av ytutmattningslivslängd trots de inneboende fördelarna med hybridytaintegralmetoden och dess mera exakta friktionsrepresentation. Det visade sig att inkludering av friktion i modellen inte markant förändrade livslängden, vilket visade att påverkan av hårdhet, smörjning av ytorna och kontakttemperatur på skjuvspänningarna är för stor för att försummas. Kontaktutmattningslivslängden ökade med en faktor 4,4 gånger på grund av kugghjulning jämfört med ythärdning, vilket indikerar restspänningspåverkan. För att uppskatta utmattningslivslängden vid inledningen av ytfel kunde en fullständig korrelation med utmattningstestresultaten inte uppnås och orsakerna till avvikelser identifierades tydligt. Det skadade område som indikeras av denna beräkningsmetod korrelerade väl med den skada som observerades under testerna. Observationerna och beräkningarna indikerade tidigt fel i kuggen med förklaring av mekanismen för fel hos kuggflanken med hjälp av aktuellt kontaktförhållande.

<p>This master’s thesis work is a study of oxidation and degradation of nickel-base alloys at high temperatures. The materials studied are designed for use in critical gas turbine components such as turbine blades and vanes. Some of the alloys are used today, whereas others have not yet entered commercial application. In order to maximize the efficiency of gas turbines, there is an ambition to maximize the operating temperatures. There is therefore a demand for materials which can withstand the damage mechanisms active at high temperatures. Among these damage mechanisms are oxidation and microstructural degradation.</p><p>To investigate the oxidation resistance of 7 different monocrystalline and polycrystalline alloys, samples have been exposed isothermally in still air at temperatures between 850 and 1000°C, for exposure times of up to 20000h. Two of the alloys were also exposed cyclically at 950°C. Oxidation during the heat treatment resulted in significant weight changes, which were measured after each cycle for cyclically exposed samples and after completed heat treatment for isothermally exposed samples. The weight change data was used to evaluate the relative oxidation resistance of the alloys. The ranking of the alloys with respect to oxidation resistance was generally in agreement with the oxidation resistance predicted by a simple consideration of the Cr and Al contents of the alloys. However, the single-crystal alloy PWA1483 displayed better oxidation resistance than predicted from its chemical composition.</p><p>Metallographic analysis of the samples indicated that the oxide scales formed consisted of several different types of oxides. The oxide scales were mainly composed of Cr2O3 and Al2O3. Fragments of the oxide scales spalled off, primarily during cooling but also in some cases during the long-term heat treatments. Spalling of the oxide scale accelerated the oxidation process, since the ability of the oxide scale to impede diffusion decreased with its decrease in thickness. Oxidation caused depletion of Al and thereby local dissolution of the aluminum-rich γ′ particles, which are of vital importance to the mechanical properties of the material. A γ′ depleted zone thereby formed underneath the oxide scale. In this zone nitrides and needle-like particles, believed to be topologically close packed μ phase, precipitated during heat treatment. Recrystallization in the depletion zone was observed in some of the monocrystalline materials. MC carbides (M=metal) present in the virgin material decomposed during heat treatment and M23C6 carbides were formed. The γ′ particles coarsened during heat treatment, which resulted in decreased hardness. The hardness decreased with exposure temperature up to 950°C, as expected due to the increased coarsening rate. At 1000°C an unexpected increase in hardness was observed for all sample materials except one. A possible explanation for this hardness increase is redistribution of γ′, by dissolution of γ′ during heat treatment and reprecipitation during cooling as much finer particles. A fine dispersion of γ′ is expected to contribute more to the hardness than a corresponding volume of γ′ in the form of larger particles. For some of the sample series, clear correlations between hardness and γ′ particle size or exposition time were found. These relationships could potentially be used to estimate the exposure temperature of service-exposed material.</p><p>A numerical model was implemented in Matlab to describe the process of oxide growth and spalling, cycle by cycle. The model was successfully adapted to experimental data from the cyclic oxidation measurements. The general applicability of the model to cyclic oxidation data at different temperatures and cycle frequencies was not investigated. At long times of cyclic exposure, the net weight loss of the samples could be well approximated as a linear function of the number of cycles. However, during the last few cycles the amount of oxide spalled in each cycle suddenly decreased. This change in spallation behavior was mainly observed for the samples cooled in air between every cycle and to a much smaller extent for the samples cooled in water. The proposed explanation is that spalling occurred preferentially at a weak subscale interface and that the spalling propensity decreased with decreasing area of this weak interface. The deviating results of the last few cycles were not included in the modeling of the cyclic oxidation process.</p>

Esta dissertação parte da tese que o avanço da Metrópole de São Paulo em direção ao antigo bairro de Santana, em especial, a partir dos anos de 1950, gerou ao espaço geográfico local, uma considerável diversidade espacial, implicando hoje, um mosaico de ealidades, tanto no aspecto de ocupação e uso do solo, como em relação às práticas sócio-espaciais. Essas realidades instauradas em Santana, na perspectiva do presente estudo são refletidas, sobretudo, a partir de duas questões: as intervenções urbanas, uma parcialmente efetivada nos anos de 1970, denominada Plano Cura Piloto de Santana, e outra em curso, intitulada Operação Urbana Vila Maria - Campo de Marte; e o fenômeno da deterioração urbana em Santana, fenômeno este, entendido como uma das contradições geradas a partir da reprodução do espaço urbano sob égide do sistema capitalista. / This report based on the thesis that the growth of São Paulo city towards the old neighborhood Santana, mostly from the fifties on brought to the geographical locus a great deal of landscape diversity. Such diversity led the city to look like a huge mosaic composed of so many different realities both regarding land occupation and usage and social visual specially lifestyles. As for the research herein presented, the differences that are established from two issuer: the urban interventions; one partially effective in the seventies called Plano CURA Piloto Santana and another one named Operação Urbana Vila Maria - Campo de Marte, and the phenomenon is know as one of the conceived contradictions from the reproductions of the urban lifestyle the capitalism system.

Esta dissertação parte da tese que o avanço da Metrópole de São Paulo em direção ao antigo bairro de Santana, em especial, a partir dos anos de 1950, gerou ao espaço geográfico local, uma considerável diversidade espacial, implicando hoje, um mosaico de ealidades, tanto no aspecto de ocupação e uso do solo, como em relação às práticas sócio-espaciais. Essas realidades instauradas em Santana, na perspectiva do presente estudo são refletidas, sobretudo, a partir de duas questões: as intervenções urbanas, uma parcialmente efetivada nos anos de 1970, denominada Plano Cura Piloto de Santana, e outra em curso, intitulada Operação Urbana Vila Maria - Campo de Marte; e o fenômeno da deterioração urbana em Santana, fenômeno este, entendido como uma das contradições geradas a partir da reprodução do espaço urbano sob égide do sistema capitalista. / This report based on the thesis that the growth of São Paulo city towards the old neighborhood Santana, mostly from the fifties on brought to the geographical locus a great deal of landscape diversity. Such diversity led the city to look like a huge mosaic composed of so many different realities both regarding land occupation and usage and social visual specially lifestyles. As for the research herein presented, the differences that are established from two issuer: the urban interventions; one partially effective in the seventies called Plano CURA Piloto Santana and another one named Operação Urbana Vila Maria - Campo de Marte, and the phenomenon is know as one of the conceived contradictions from the reproductions of the urban lifestyle the capitalism system.

Surfaces subjected to rolling and sliding contacts maysuffer from contact fatigue. This thesisdeals with solidmechanic aspects of contact fatigue including the descriptionand verificationof explaining mechanisms. The new mechanism forsurface initiated contact fatigue is basedon tensile surfacestresses from local asperity contacts. It is also realised thatsub-surfaceinitiated contact fatigue is the result of tensileresidual stresses that emanate from plasticdeformation belowthe surface. These mechanisms clearly show that contact fatiguecracksfollow the same rules as ordinary fatigue cracks inhardened steel. The thesis contains four papers that treat a new testprocedure named Standing ContactFatigue (SCF). The results ofthe test procedure have played an important role inthedevelopment and verification of the mechanisms for surfaceand sub-surface contact fatigue. The first part of the research work was experimental. Inthis part the SCF test properties wasdecided, crack resultsconfirmed and crack detection methods developed. Herecomparativestudies were performed using some differentmaterials and mechanical properties. It wasverified that SCFcould detect differences in contact fatigue resistance. Next a finite element model of the SCF test was evaluatedthrough the general-purposeprogram MARC. The model includedgraded material properties that originate from heattreatment.The residual surface deformation and surface compliance wereverified againstexperimental results. Crack initiation wasinvestigated in two ways. Firstly, the principalstresses atcritical locations were computed and plotted in a Haighdiagram. The diagramshowed that the cracks initiate in adirection perpendicular to the principal stress with thelargeststress range provided that the principal stress is tensilesometime during the load cycle. Secondly, some high cycle multiaxial fatigue criteria,including the Haigh principal stresscriterion, was evaluatedagainst the SCF crack initiation results. The surface cracklocation waspredicted by including statistical effects using aweakest-link criterion and a three-parameterWeibulldistribution. The SCF crack propagation was investigated by numericalevaluation ofJ1 andJ2integrals. The crack initiation and propagationphases were separated with a threshold criterionand a directioncriterion. It was found that during crack propagation bothsurface andsub-surface contact fatigue cracks follow thedirection with minimum mode II loading. Key words: contact fatigue mechanism; spall; spalling;surface crack; sub-surface crack;elasto-plastic indentation;contact compliance measurement; mixed-mode fatigue;fatiguecrack growth;J-integral; multiaxial fatigue; weakest-link. / QC 20100407

In the crystalline Si solar cell industry, there is a push to reduce module cost through a combination of thinner substrates and increased cell efficiency. Achieving solar cells with sub-100 µm substrates cost-effectively is a formidable task because such thin substrates impose stringent handling requirements and thermal budget due to their flexibility, ease of breakage, and low yield. Moreover, as the substrate thickness decreases the surface passivation quality dictates the performance of the cells. Crystalline Si heterojunction (HJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) have attracted significant interest in recent years due to their excellent surface passivation properties, potential for high efficiency, low thermal budget and low cost. HJ cells with ultra-passivated surfaces showing > 700 mV open-circuit voltages (Voc) and > 20% conversion efficiency have been demonstrated. In these cells, it has been identified that high-quality a-Si:H films deposited by a low-damage plasma process is key to achieving such high cell performance. However, the options for low-damage plasma deposition process are limited. The main objectives of this work are to develop a low-plasma damage a-Si:H thin film deposition process based on remote plasma chemical vapor deposition (RPCVD) and to demonstrate high efficiency HJ solar cells on bulk substrates as well as on ultra-thin silicon and germanium substrates obtained by a novel, low-cost semiconductor-on-metal (SOM) technology. This manuscript presents a detailed description of the RPCVD system and the process leading to the realization of high quality a-Si:H thin films and high efficiency HJ solar cells. First, p-type a-Si:H thin films are developed and optimized, then HJ solar cells are subsequently fabricated on bulk and ultra-thin Si and Ge SOM substrates without intrinsic a-Si:H passivation. Single HJ cells on ~ 500 µm bulk Si and ~25 µm ultra-thin substrates exhibited conversion efficiencies of η = 16% (Voc = 615 mV, Jsc = 34 mA/cm2, and FF = 77%) and η = 11.2% (Voc = 605 mV, Jsc = 29.6 mA/cm2, and FF = 62.8%), respectively. The performance of the ~25 µm cell was further improved to η = 13.4% (Voc = 645 mV, Jsc = 31.4 mA/cm2, and FF = 66.2%) by implementing the dual HJ architecture without front side i-layer passivation. For single HJ cells based on Ge substrates, the results were η = 1.78 % (Voc = 148 mV, Jsc = 35.1 mA/cm2, and FF = 1.78%) on ~500 µm bulk Ge, compared to η =5.3% (Voc = 203 mV, Jsc = 44.7 mA/cm2, and FF = 5.28%) on ~ 50 µm Ge SOM substrates. Respectively, the results obtained on ultra-thin SOM substrates are among the highest reported in literature for based on comparable architecture and substrate thickness. In order to achieve improved cell performance, dual HJ cells with i-layer passivation of both surfaces were fabricated. First, optimized RPCVD-based i-layer films were developed by varying the deposition temperature and H2 dilution ratio (R). It was found that excellent surface passivation on planar substrates with as-deposited minority carrier lifetimes > 1 ms is achievable by using deposition temperature of 200 ºC and moderate dilution ratio 0.5 ≤ R ≤ 1, even without the more rigorous RCA pre-cleaning process typically used in literature for achieving comparable results. Subsequently, dual HJ solar cells with i-layer films were demonstrated on planar and textured bulk Si substrates showing improved conversion efficiencies of η = 17.3% (Voc = 664 mV, Jsc = 34.34 mA/cm2 and FF = 76%) and η = 19.4% (Voc = 643 mV, Jsc = 38.99 mA/cm2, and FF = 77.5%), respectively. / text

Die Verwendung von Endlosfilamenten aus Carbon als Bewehrungsmaterial für Beton, sogenannter Textilbeton, bietet die Möglichkeit der Sanierung und der Verstärkung bestehender Stahlbetonkonstruktionen. Dabei muss die Frage nach dem Feuerwiderstand von derart verstärkten Tragwerken beantwortet werden. Aufschluss darüber liefern Brandversuche. Mit Textilbeton verstärkte Stahlbetonplatten haben in Brandversuchen nach der Einheits- Temperaturzeitkurve bei 33 % der Traglast mehr als 60 Minuten standgehalten. Bei 50 % der Traglast kam es nach einer Branddauer von 55 Minuten, bei 65 % nach 30 Minuten, zu einem Zugversagen der textilen Verstärkungsschicht. Bei während des Brandes unbelasteten bzw. gering belasteten Platten lagen die im Anschluss ermittelten Resttragfähigkeiten bei 65 % der Bruchlast nach 30 Minuten Branddauer bzw. bei 50 % der Bruchlast nach 60 Minuten Branddauer. Während und nach der Beflammung waren zunehmende Durchbiegungen und Rissbreiten erkennbar. Es traten aber keine Abplatzungen auf, weshalb die Verstärkungsschicht aus Textilbeton als zusätzliche Betondeckung für die Stahlbewehrung angerechnet werden kann. Diese außerordentlich positiven Ergebnisse zeigen, dass für verstärkte Konstruktionen Feuerwiderstandsklassen von F60 bzw. R60 ohne zusätzliche Maßnahmen erreicht werden können. Dies ist von hoher Relevanz für die wirtschaftliche Anwendung dieser Verstärkungsmethode. Für ein umfassendes Verständnis der Interaktion zwischen den Bewehrungen Textil und Stahl sowie der Versagensmechanismen während des Brandes, sind weitere Erkenntnisse über die mechanischen Eigenschaften von Textilbeton im Hochtemperaturbereich nötig. Auch der Einfluss der Oxidation des Carbons konnte nicht abschließend beurteilt werden. / Using endless carbon filaments for concrete reinforcement, so called textile reinforced concrete, the possibility of reconstruction and strengthening of existing concrete structures arises. The question concerning fire resistance of structures strengthened like this has to be answered. Fire tests provide answers. Steel reinforced concrete slabs strengthened with textile reinforced concrete loaded with 33 % of ultimate load survived an ISO-fire for 60 minutes. Loaded with 50 % and 65 % of ultimate load the slabs failed after 55 minutes and 30 minutes of fire exposure due to tension failure of the textile reinforcement layer. Slabs not loaded or with a low load level during fire exposure showed remaining bearing resistances of 65 % of ultimate load after 30 minutes and 50 % of ultimate load after 60 minutes of fire exposure. During and after fire exposure rising deflections and growing crack widths were observed. However no spalling occurred. Therefore the textile reinforced concrete layer can be taken into account as concrete covering for the steel reinforcement. Those extraordinary positive results document, that reinforced concrete structures with additional fibre reinforced concrete can achieve fire resistance classes of R60 without additional provisions. To achieve comprehensive understanding of interaction between steel and fibre reinforcement and failure mechanisms in case of fire more knowledge concerning the mechanic properties of fibre reinforced concrete at high temperatures is essential. The influence of oxidation of the carbon fibres could not fully be answered.

Os concretos de alta compacidade, com resistências elevadas (acima de 40 MPa), estão começando a ser cada vez mais usados em obras de infraestrutura e em edificações de grande porte, devido às importantes vantagens que os mesmos oferecem em termos de capacidade de carga e durabilidade. Um dos pontos acerca de seu desempenho que levanta dúvidas envolve o seu comportamento frente a elevadas temperaturas. Evidências coletadas em incêndios ocorridos em estruturas reais, como o do Channel Tunnel e do Mont Blanc Tunnel, indicaram que, sob certas circunstâncias, existe a possibilidade de ocorrência de um fenômeno, conhecido como spalling explosivo, que causa sérios danos por lascamento ao material, incrementando de forma significativa a degradação térmica. Isso ocorre porque a reduzida porosidade e a baixa permeabilidade favorecem o aprisionamento do vapor de água nos poros, gerando um acréscimo da pressão interna que ocasiona tensões que podem superar a resistência à tração do concreto. O processo de spalling, bem como a sua probabilidade e severidade de ocorrência, dependem de uma série de fatores interdependentes que influenciam o comportamento do material. Contudo, está se formando um consenso de que o teor de umidade é o parâmetro mais influente para o desencadeamento do processo. Estudos efetuados após o reconhecimento da existência do spalling explosivo demonstraram que o mesmo tende a ocorrer somente quando o concreto apresenta teores muito elevados de umidade em sua estrutura de poros. Isso restringe a preocupação a certos tipos de elementos onde o concreto pode estar muito úmido por ocasião do sinistro. Embora estes sejam casos raros, a gravidade dos danos devido ao spalling explosivo justifica que se investiguem as condições nas quais o mesmo pode acontecer e se implementem medidas mitigatórias. Dado o caráter recente das investigações sobre o tema, ainda existe uma carência de estudos específicos que apontem quais as combinações críticas de porosidade e teores de umidade que desencadeiam o fenômeno. Este trabalho foi concebido para ajudar a superar essa lacuna de conhecimento, coletando dados sobre as condições que favorecem o spalling explosivo, bem como averiguando quais são as propriedades mecânicas residuais e as alterações na microestrutura sofridas por concretos com vários patamares de resistência submetidos a altas temperaturas. O trabalho utiliza os dados coletados para fundamentar a proposição de um modelo simplificado de spalling, elaborado com o objetivo de prover uma forma de considerar os efeitos do fenômeno durante uma análise termomecânica. Essa é uma necessidade fundamental para complementar os softwares atualmente utilizados para analisar os efeitos de incêndios sobre estruturas. O módulo de spalling foi implementado computacionalmente de forma que pudesse ser utilizado em consórcio com a ferramenta computacional VULCAN, o que permitiu simular a ocorrência do fenômeno na deterioração de uma viga durante um incêndio. Os resultados indicaram que a perda de seção transversal causada pela ocorrência do spalling explosivo reduz substancialmente o tempo de resistência ao fogo (TRRF), devido à aceleração dos danos e à redução da capacidade portante dos elementos estruturais de concreto. Conclui-se, portanto, que é fundamental considerar a possibilidade de lascamentos explosivos em estruturas suscetíveis ao fenômeno. De acordo com o presente estudo, estas incluem aquelas fabricadas com concretos de resistência maior que 40 MPa, que apresentem grau de saturação em torno ou superiores a 90%. / In the last years, the utilization of high-strength concrete (HSC) in the construction industry, with compressive strength grades greater than 40 MPa, has become wide spread due to remarkable characteristics of durability and load bearing capacity. However, the increased use of HSC has raised concerns regarding the behaviour of such concretes in fire. Evidences which were collected from structures exposed to the effects of fires, like Channel Tunnel and Mont Blanc Tunnel, have indicated that there is a possibility of the occurrence of explosive spalling in this condition. The phenomenon causes serious damages to the material and raises its thermal degradation in a significant way. This occurs because of its low porosity and permeability that contribute for the vapour pressure build-up in the pores, generating stresses which can exceed tensile strength of the material. The probability and severity of spalling process depend on a large number of inter-dependent factors that influence the behaviour of HSC in an unexpected manner. There is a consensus that the moisture content is one of the principal factors of the occurrence of explosive spalling. Recent investigations have demonstrated that spalling tends to occur only when the moisture content in the pores of concrete is elevated. Although fire is considered an exceptional event, the seriousness of spalling in concrete structures justifies the investigation of conditions in that the phenomenon can be happen and the implementation of preventive measures. In spite of the recent advances about the spalling process in HSC at high temperatures, there is still a lack of information in relation to the critical combinations of factors which affect the spalling occurrence. This work intends to help overcoming this lack of knowledge from the experimental data analysis with the purpose of establishing some critical conditions which collaborate in the spalling process as well as investigating the residual mechanical properties and changes in the microstructure of different concrete grades submitted to high temperatures. In addition, the experimental data are used to validate the proposition of a simplified model of spalling which was developed with the objective to take into account the effects of spalling in the thermo-mechanical analysis. This is an important need to complement the fire analysis of concrete structures. The spalling model was incorporated in the VULCAN software to simulate the phenomenon occurrence in a concrete beam exposed to fire. The results shown that the loss of mass in the beam cross-section, caused by spalling, reduces substantially its required time of fire resistance whereas it accelerates the damage in the concrete and aids the reduction of load bearing capacity of structural element. In conclusion, it is essential to consider the possibility of concrete spalling mainly in concrete grades greater than 40 MPa that present a saturation degree higher than 90%.

Os concretos de alta compacidade, com resistências elevadas (acima de 40 MPa), estão começando a ser cada vez mais usados em obras de infraestrutura e em edificações de grande porte, devido às importantes vantagens que os mesmos oferecem em termos de capacidade de carga e durabilidade. Um dos pontos acerca de seu desempenho que levanta dúvidas envolve o seu comportamento frente a elevadas temperaturas. Evidências coletadas em incêndios ocorridos em estruturas reais, como o do Channel Tunnel e do Mont Blanc Tunnel, indicaram que, sob certas circunstâncias, existe a possibilidade de ocorrência de um fenômeno, conhecido como spalling explosivo, que causa sérios danos por lascamento ao material, incrementando de forma significativa a degradação térmica. Isso ocorre porque a reduzida porosidade e a baixa permeabilidade favorecem o aprisionamento do vapor de água nos poros, gerando um acréscimo da pressão interna que ocasiona tensões que podem superar a resistência à tração do concreto. O processo de spalling, bem como a sua probabilidade e severidade de ocorrência, dependem de uma série de fatores interdependentes que influenciam o comportamento do material. Contudo, está se formando um consenso de que o teor de umidade é o parâmetro mais influente para o desencadeamento do processo. Estudos efetuados após o reconhecimento da existência do spalling explosivo demonstraram que o mesmo tende a ocorrer somente quando o concreto apresenta teores muito elevados de umidade em sua estrutura de poros. Isso restringe a preocupação a certos tipos de elementos onde o concreto pode estar muito úmido por ocasião do sinistro. Embora estes sejam casos raros, a gravidade dos danos devido ao spalling explosivo justifica que se investiguem as condições nas quais o mesmo pode acontecer e se implementem medidas mitigatórias. Dado o caráter recente das investigações sobre o tema, ainda existe uma carência de estudos específicos que apontem quais as combinações críticas de porosidade e teores de umidade que desencadeiam o fenômeno. Este trabalho foi concebido para ajudar a superar essa lacuna de conhecimento, coletando dados sobre as condições que favorecem o spalling explosivo, bem como averiguando quais são as propriedades mecânicas residuais e as alterações na microestrutura sofridas por concretos com vários patamares de resistência submetidos a altas temperaturas. O trabalho utiliza os dados coletados para fundamentar a proposição de um modelo simplificado de spalling, elaborado com o objetivo de prover uma forma de considerar os efeitos do fenômeno durante uma análise termomecânica. Essa é uma necessidade fundamental para complementar os softwares atualmente utilizados para analisar os efeitos de incêndios sobre estruturas. O módulo de spalling foi implementado computacionalmente de forma que pudesse ser utilizado em consórcio com a ferramenta computacional VULCAN, o que permitiu simular a ocorrência do fenômeno na deterioração de uma viga durante um incêndio. Os resultados indicaram que a perda de seção transversal causada pela ocorrência do spalling explosivo reduz substancialmente o tempo de resistência ao fogo (TRRF), devido à aceleração dos danos e à redução da capacidade portante dos elementos estruturais de concreto. Conclui-se, portanto, que é fundamental considerar a possibilidade de lascamentos explosivos em estruturas suscetíveis ao fenômeno. De acordo com o presente estudo, estas incluem aquelas fabricadas com concretos de resistência maior que 40 MPa, que apresentem grau de saturação em torno ou superiores a 90%. / In the last years, the utilization of high-strength concrete (HSC) in the construction industry, with compressive strength grades greater than 40 MPa, has become wide spread due to remarkable characteristics of durability and load bearing capacity. However, the increased use of HSC has raised concerns regarding the behaviour of such concretes in fire. Evidences which were collected from structures exposed to the effects of fires, like Channel Tunnel and Mont Blanc Tunnel, have indicated that there is a possibility of the occurrence of explosive spalling in this condition. The phenomenon causes serious damages to the material and raises its thermal degradation in a significant way. This occurs because of its low porosity and permeability that contribute for the vapour pressure build-up in the pores, generating stresses which can exceed tensile strength of the material. The probability and severity of spalling process depend on a large number of inter-dependent factors that influence the behaviour of HSC in an unexpected manner. There is a consensus that the moisture content is one of the principal factors of the occurrence of explosive spalling. Recent investigations have demonstrated that spalling tends to occur only when the moisture content in the pores of concrete is elevated. Although fire is considered an exceptional event, the seriousness of spalling in concrete structures justifies the investigation of conditions in that the phenomenon can be happen and the implementation of preventive measures. In spite of the recent advances about the spalling process in HSC at high temperatures, there is still a lack of information in relation to the critical combinations of factors which affect the spalling occurrence. This work intends to help overcoming this lack of knowledge from the experimental data analysis with the purpose of establishing some critical conditions which collaborate in the spalling process as well as investigating the residual mechanical properties and changes in the microstructure of different concrete grades submitted to high temperatures. In addition, the experimental data are used to validate the proposition of a simplified model of spalling which was developed with the objective to take into account the effects of spalling in the thermo-mechanical analysis. This is an important need to complement the fire analysis of concrete structures. The spalling model was incorporated in the VULCAN software to simulate the phenomenon occurrence in a concrete beam exposed to fire. The results shown that the loss of mass in the beam cross-section, caused by spalling, reduces substantially its required time of fire resistance whereas it accelerates the damage in the concrete and aids the reduction of load bearing capacity of structural element. In conclusion, it is essential to consider the possibility of concrete spalling mainly in concrete grades greater than 40 MPa that present a saturation degree higher than 90%.

Os concretos de alta compacidade, com resistências elevadas (acima de 40 MPa), estão começando a ser cada vez mais usados em obras de infraestrutura e em edificações de grande porte, devido às importantes vantagens que os mesmos oferecem em termos de capacidade de carga e durabilidade. Um dos pontos acerca de seu desempenho que levanta dúvidas envolve o seu comportamento frente a elevadas temperaturas. Evidências coletadas em incêndios ocorridos em estruturas reais, como o do Channel Tunnel e do Mont Blanc Tunnel, indicaram que, sob certas circunstâncias, existe a possibilidade de ocorrência de um fenômeno, conhecido como spalling explosivo, que causa sérios danos por lascamento ao material, incrementando de forma significativa a degradação térmica. Isso ocorre porque a reduzida porosidade e a baixa permeabilidade favorecem o aprisionamento do vapor de água nos poros, gerando um acréscimo da pressão interna que ocasiona tensões que podem superar a resistência à tração do concreto. O processo de spalling, bem como a sua probabilidade e severidade de ocorrência, dependem de uma série de fatores interdependentes que influenciam o comportamento do material. Contudo, está se formando um consenso de que o teor de umidade é o parâmetro mais influente para o desencadeamento do processo. Estudos efetuados após o reconhecimento da existência do spalling explosivo demonstraram que o mesmo tende a ocorrer somente quando o concreto apresenta teores muito elevados de umidade em sua estrutura de poros. Isso restringe a preocupação a certos tipos de elementos onde o concreto pode estar muito úmido por ocasião do sinistro. Embora estes sejam casos raros, a gravidade dos danos devido ao spalling explosivo justifica que se investiguem as condições nas quais o mesmo pode acontecer e se implementem medidas mitigatórias. Dado o caráter recente das investigações sobre o tema, ainda existe uma carência de estudos específicos que apontem quais as combinações críticas de porosidade e teores de umidade que desencadeiam o fenômeno. Este trabalho foi concebido para ajudar a superar essa lacuna de conhecimento, coletando dados sobre as condições que favorecem o spalling explosivo, bem como averiguando quais são as propriedades mecânicas residuais e as alterações na microestrutura sofridas por concretos com vários patamares de resistência submetidos a altas temperaturas. O trabalho utiliza os dados coletados para fundamentar a proposição de um modelo simplificado de spalling, elaborado com o objetivo de prover uma forma de considerar os efeitos do fenômeno durante uma análise termomecânica. Essa é uma necessidade fundamental para complementar os softwares atualmente utilizados para analisar os efeitos de incêndios sobre estruturas. O módulo de spalling foi implementado computacionalmente de forma que pudesse ser utilizado em consórcio com a ferramenta computacional VULCAN, o que permitiu simular a ocorrência do fenômeno na deterioração de uma viga durante um incêndio. Os resultados indicaram que a perda de seção transversal causada pela ocorrência do spalling explosivo reduz substancialmente o tempo de resistência ao fogo (TRRF), devido à aceleração dos danos e à redução da capacidade portante dos elementos estruturais de concreto. Conclui-se, portanto, que é fundamental considerar a possibilidade de lascamentos explosivos em estruturas suscetíveis ao fenômeno. De acordo com o presente estudo, estas incluem aquelas fabricadas com concretos de resistência maior que 40 MPa, que apresentem grau de saturação em torno ou superiores a 90%. / In the last years, the utilization of high-strength concrete (HSC) in the construction industry, with compressive strength grades greater than 40 MPa, has become wide spread due to remarkable characteristics of durability and load bearing capacity. However, the increased use of HSC has raised concerns regarding the behaviour of such concretes in fire. Evidences which were collected from structures exposed to the effects of fires, like Channel Tunnel and Mont Blanc Tunnel, have indicated that there is a possibility of the occurrence of explosive spalling in this condition. The phenomenon causes serious damages to the material and raises its thermal degradation in a significant way. This occurs because of its low porosity and permeability that contribute for the vapour pressure build-up in the pores, generating stresses which can exceed tensile strength of the material. The probability and severity of spalling process depend on a large number of inter-dependent factors that influence the behaviour of HSC in an unexpected manner. There is a consensus that the moisture content is one of the principal factors of the occurrence of explosive spalling. Recent investigations have demonstrated that spalling tends to occur only when the moisture content in the pores of concrete is elevated. Although fire is considered an exceptional event, the seriousness of spalling in concrete structures justifies the investigation of conditions in that the phenomenon can be happen and the implementation of preventive measures. In spite of the recent advances about the spalling process in HSC at high temperatures, there is still a lack of information in relation to the critical combinations of factors which affect the spalling occurrence. This work intends to help overcoming this lack of knowledge from the experimental data analysis with the purpose of establishing some critical conditions which collaborate in the spalling process as well as investigating the residual mechanical properties and changes in the microstructure of different concrete grades submitted to high temperatures. In addition, the experimental data are used to validate the proposition of a simplified model of spalling which was developed with the objective to take into account the effects of spalling in the thermo-mechanical analysis. This is an important need to complement the fire analysis of concrete structures. The spalling model was incorporated in the VULCAN software to simulate the phenomenon occurrence in a concrete beam exposed to fire. The results shown that the loss of mass in the beam cross-section, caused by spalling, reduces substantially its required time of fire resistance whereas it accelerates the damage in the concrete and aids the reduction of load bearing capacity of structural element. In conclusion, it is essential to consider the possibility of concrete spalling mainly in concrete grades greater than 40 MPa that present a saturation degree higher than 90%.