Dissertations / Theses on the topic 'Elevated temperature fatigue'

The work described in this thesis details the design and characteristics of low-Ni maraging steels suitable for use at elevated temperatures. In addition to having a stable microstructure, they require strength, ductility, fatigue- and creep-resistance with low impurity levels suitable for use in aeroengine shafts. Two chemical compositions have been designed based on thermodynamic calculations which independently investigate the suitability of the matrix and the strengthening phases for use at elevated temperatures. It is believed that substituting Ni with Cr will increase the austenite-start temperature, thus retarding the formation of austenite during manufacture and simulated service at 450°C. Avoiding the formation austenite concomitantly with precipitate coarsening is important when designing a suitable shaft material. The designed alloys have exhibited an excellent combination of strength and ductility in the hardened condition achieved through a non-scale distribution of Ni-rich particles. These particles resist growth at 450°C while largely maintaining the mechanical properties. The austenite observed in one alloy transformed to martensite at room temperature during plastic deformation increasing the elongation. However, the increased thermodynamic stability of austenite at 450°C does not permit the martensitic transformation, thus it must be avoided in shaft alloys. The alloy which resisted the formation of austenite displayed excellent fatigue- and creep-resistance in the hardened condition. These properties are attributed to the size and dispersion of the strengthening phases and the level of inclusions within the microstructures. Neither of the designed alloys meets all the mechanical property requirements of the shaft. However, by combining the desirable features of both alloys it is believed that these properties can be achieved though a low-Ni maraging steel.

Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2006.
McDowell, David, Committee Member ; Gokhale, Arun, Committee Member ; Saxena, Ashok, Committee Chair ; Johnson, Steven, Committee Member ; Sanders, Thomas, Committee Member.

Tooling reliability is critical to welding success in friction stir welding, but tooling fatigue is not well understood because it occurs in conditions that are often unique to friction stir welding. A fatigue study was conducted on a commonly used tooling material, H13 tool steel, using constant stress loading at temperatures between 300°C and 600°C, and the results are presented. A model is proposed accounting for temperature and stress effects on fatigue life, utilizing a two-region Arrhenius temperature model. A transition in temperature effect on fatigue life is identified. Implications of the temperature effect for friction stir welding suggest that tooling fatigue life dramatically decreases above 500°C and accelerated testing should be conducted below 500°C.

Polymer matrix composites (PMCâ s) are now being used more and more extensively and over wider ranges of service conditions. Large changes in pressure, chemical environment or temperature influence the mechanical response of such composites. In the present effort, we focus on temperature, a parameter of primary interest in almost all engineering applications. In order to design composite structures without having to perform extensive experiments (virtual design), the necessity of establishing theoretical models that relate the macroscopic response of the structure to the microscopic properties of the constituents arises. In the first part of the present work, a new stiffness versus temperature model is established. The model is validated using data from the literature. The influence of the different polymerâ s properties (Molecular weight, crystallinity, and filler content) on the model are studied by performing experiments on different grades of four polymers PMMA, PEEK, PPS, and PB. This statistical model is proven to be applicable to very different polymers (elastomers, thermoplastics, crystalline, amorphous, cross-linked, linear, filled, unfilledâ ¦) over wide temperature ranges (from the glassy state to the flow region). The most attractive feature of the proposed model is the capability to enable a description of the polymerâ s mechanical behavior within and across the property transition regions. In order to validate the feasibility of using the model to predict the mechanical response of polymer matrix composites, the stiffness-temperature model is used in various micromechanical models (rule of mixtures, compression models for the life prediction of unidirectional PMCâ s in end-loaded bendingâ ¦). The model is also inserted in the MRLife prediction code to predict the remaining strength and life of unidirectional PMCâ s in fatigue bending. End-loaded fatigue experiments were performed. A good correlation between theoretical and experimental results is observed. Finally, the model is used in the Classical Lamination Theory; some laminates were found to exhibit stress reversals with temperature and behaved like thermally activated mechanical switches.
Ph. D.

Metal fatigue is a recurring problem for metallurgists and materials engineers, especially in structural applications. It has been responsible for many disastrous accidents and tragedies in history. Understanding the micro-mechanisms during cyclic deformation and combating fatigue failure has remained a grand challenge. Environmental effects, like temperature or a corrosive medium, further worsen and complicate the problem. Ultimate design against fatigue must come from a materials perspective with a fundamental understanding of the interaction of microstructural features with dislocations, under the influence of stress, temperature, and other factors. This research endeavors to contribute to the current understanding of the fatigue failure mechanisms. Cast aluminum alloys are susceptible to fatigue failure due to the presence of defects in the microstructure like casting porosities, non-metallic inclusions, non-uniform distribution of secondary phases, etc. Friction stir processing (FSP), an emerging solid state processing technique, is an effective tool to refine and homogenize the cast microstructure of an alloy. In this work, the effect of FSP on the microstructure of an A356 cast aluminum alloy, and the resulting effect on its tensile and fatigue behavior have been studied. The main focus is on crack initiation and propagation mechanisms, and how stage I and stage II cracks interact with the different microstructural features. Three unique microstructural conditions have been tested for fatigue performance at room temperature, 150 °C and 200 °C. Detailed fractography has been performed using optical microscopy, scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). These tools have also been utilized to characterize microstructural aspects like grain size, eutectic silicon particle size and distribution. Cyclic deformation at low temperatures is very sensitive to the microstructural distribution in this alloy. The findings from the room temperature fatigue tests highlight the important role played by persistent slip bands (PSBs) in fatigue crack initiation. At room temperature, cracks initiate along PSBs in the absence of other defects/stress risers, and grow transgranularly. Their propagation is retarded when they encounter grain boundaries. Another major finding is the complete transition of the mode of fatigue cracking from transgranular to intergranular, at 200 °C. This occurs when PSBs form in adjacent grains and impinge on grain boundaries, raising the stress concentration at these locations. This initiates cracks along the grain boundaries. At these temperatures, cyclic deformation is no longer microstructure- dependent. Grain boundaries don’t impede the progress of cracks, instead aid in their propagation. This work has extended the current understanding of fatigue cracking mechanisms in A356 Al alloys to elevated temperatures.

Includes abstract.
Includes bibliographical references (leaves 133-141).
The research studies revealed that there was a distinct difference in fatigue performance as a result of a temperature increase from 20°C to 45°C, as characterised by both SN and Paris fatigue tests. There was also, but to a lesser degree, an ageing effect. The temperature performance factor for the SN curves was between 1.6 and 4.6, while for the Paris characterisation the temperature performance factor was between 3 and 3.5.

Includes bibliographical references.
A series of failures in underground water-carrying uPVC piping in a luxury resort in Dubai gave rise to a research opportunity to determine the effect of temperature on fatigue life performance of uPVC piping. Two different modes of testing were used to determine this temperature effect, namely SN and Fracture Mechanics Paris testing. The temperatures tested were 20%C and 45%C. In addition to temperature tests, a potential ageing effect was also investigated by comparing pipes which had been in service in the resort, and previously unused piping. The SN tests consisted of externally and symmetrically stressing, across the diameter, sections of pipe from the luxury Madinat Jumeirah resort in Dubai where the failures had occurred. The Paris equation generating FM tests used Compact Tension specimens and produced an equation relating the crack growth rate to the cyclic stress intensity amplitude. In addition, material properties were measured which could then be used for fatigue lifetime predictions. In addition to the lifetime tests, fracture toughness tests were also completed. These were done with a view to determining the fracture toughness of the material, and also to ascertain if there was an orientation effect for crack growth. The potential ageing effect was also investigated. This was achieved by means of using differently orientated specimens. SENB specimens were used to determine circumferential fracture toughness and C-Shaped specimens for longitudinal cracks (the direction of on-site crack growth). Fracture surfaces were inspected and calculations performed to indicate critical flaw sizes were broadly consistent with linear elastic fracture mechanics (LEFM). Using the material properties garnered from the Paris fatigue and fracture toughness tests, lifetime predictions were made and compared to the SN data for cycles to failure The research studies revealed that there was a distinct difference in fatigue performance as a result of a temperature increase from 20%C to 45%C, as characterised by both SN and Paris fatigue tests. There was also, but to a lesser degree, an ageing effect. The temperature performance factor for the SN curves was between 1.6 and 4.6, while for the Paris characterisation the temperature performance factor was between 3 and 3.5.

This thesis studies dwell fatigue crack growth in an advanced nickel disc alloy RR1000 at elevated temperature. RR1000 with different grain sizes and γ’ precipitate distributions are tested at both 650 and 700°C to investigate the effect of microstructure and testing temperature on dwell fatigue crack growth. Dwell fatigue tests with different dwell times at the peak load are conducted to study the effect of dwell time on crack growth rate and crack propagation mechanism. A transition from cycle-dependent transgranular fatigue crack growth to time-dependent oxide induced intergranular crack growth is found when the dwell time exceeds a critical value. Linear elastic stress intensity factor (K) threshold values for intergranular crack growth under sustained load are also measured to evaluate conditions when intergranular crack growth occurs progressively during the dwell period. Impacts of prior dwell fatigue loading on subsequent fatigue crack growth are also studied.

Increasing use of fiber reinforced ceramic matrix composites (CMC's) materials is needed, especially for hostile environments such as elevated temperatures. However, some fundamental issues regarding how these materials should be made for optimized performance are far from being settled. This study focuses on the modeling of the tensile behavior of unidirectional CMC using statistical methods and micro-mechanical analysis, based on laboratory observations. The model can be used to examine the effect of performance-influencing parameters on the strength of unidirectional CMC, thus shed light on how such material should be put together. The tensile strength model was then modified such that the behavior of unidirectioal CMC under cyclic tensile load can be studied. Results from the tensile strength model suggest that the Weibull modulus, m, of the strength of the reinforcing fibers and the fiber/matrix interfacial shear stress both have significant effect on the strength and toughness of the unidirectional composite: a higher m value and a lower interfacial shear stress result in a lower strength; a lower value of m and a higher interfacial shear stress results in a higher strength but lower toughness. Calculations from the tensile fatigue model suggest that a lower m value results in a longer fatigue life.
Ph. D.

Composite materials are under consideration for the replacement of steel helical tendons in unbonded flexible pipes utilized by the offshore oil industry. Higher strength to weight ratios and increased corrosion resistance are the primary advantages of a composite material for this application. The current study focuses on the life prediction of a PPS/AS-4 carbon fiber composite proposed for the above employment. In order to accomplish this task, the properties of the material were experimentally characterized at varying temperatures, aging times and loadings. An analytic technique was developed to predict tensile rupture behavior from bend-compression rupture data. In comparison to tensile rupture tests, bend-compression rupture data collection are uncomplicated and efficient; thus, this technique effectively simplifies and accelerates the material characterization process. The service life model for the flexible pipe composite armor was constructed with MRLife, a well established performance simulation code for material systems developed by the Materials Response Group at Virginia Tech. In order to validate MRLife for the current material, experimental data are compared to life prediction results produced by the code. MRLife was then applied to predict the life of the flexible pipe composite armor in an ocean environment. This analysis takes into account the flexible pipe structure and the environmental and mechanical loading history of an ocean service location. Several parameter studies of a flexible pipe in a hypothetical environment were conducted. These analyses highlight certain loadings and conditions which are particularly detrimental to the life of the material.
Master of Science

Multiple step tests under cyclic strain control have been performed using cylindrical specimens of cast polycrystalline Inconel 738LC superalloy at 23, 700, 500, 800 and 900 °C in laboratory atmosphere to obtain cyclic stress-strain curves. During cyclic straining of specimen were obtained cyclic hardening-softening curves. Their progress changed with temperature and strain amplitude. Evaluated cyclic stress-strain curves are shifted to lower stresses with increasing temperature. Surface relief was observed in fatigued specimens under SEM and metalography under optic microscopy. Slip markings were studied on specimen surface fatigued at 700 °C .Stress-strain response is compared and discussed in relation to the surface observations - persistent slip markings.

Pure aluminum deformed in torsion (shear) at elevated temperatures reaches a broad "peak" stress and then undergoes about a 17% decrease in flow stress with deformation to roughly 1-2 equivalent uniaxial strain. Beyond this strain the flow stress is approximately constant. The sources for this softening are unclear. The suggested basis includes texture softening, microstructural softening, and enhanced dynamic recovery. Experiments were performed where specimens were deformed in torsion to various strains within the softening regime followed by compression tests at ambient and elevated temperature. Analysis of the compressive yield strengths indicate that the softening is at least substantially explained by a decrease in the average Taylor factor due to the development of texture.
Graduation date: 2002

博士
國立中山大學
機械與機電工程學系研究所
94
This thesis was concerned on the investigation of mechanical properties of centrally notched and unnotched AS-4/PEEK (APC-2) composite laminates due to static tensile and tension-tension (T-T) fatigue tests empirically and systematically. Then, statistical analyses were used to determine and quantify the significant thermomechanical variables that influence the durability/life of the composite laminates. Typical laminates were made from sixteen prepregs of APC-2 and manufactured by a modified curing process. After drilling one hole with various diameters in the center of the samples respectively, the lay-ups were conducted on tension fracture and T-T fatigue test at different temperatures. From the parametric study we achieved the important results as follows. The cross-ply laminate possesses the higher ultimate strength, fatigue strength and longitudinal stiffness than those of the quasi-isotropic at the same temperature. Notch effect decays the laminate strength seriously, but changes the stiffness irregularly. As test temperature rising both strength and stiffness of lay-ups degrade significantly. Combining both effects of notch and temperature under severe environmental condition, it is found the cross-ply laminate possesses more resistance than that of the quasi-isotropic to cyclic loading. However, the quasi-isotropic laminate is more capable of sustaining the original strength than that of the cross-ply. Finally, the multiple regression analysis results showed that the hygrothermal environmental effects and cyclic loading were decoupled for APC-2 composite system. A semi-empirical model, reliably set up after the said programs, predicts conservative values, and should be adequate for use in preliminary designs. That is the main contribution in this study. Also, for the purposes of design and application, the predicted models efficiently treat experimental data instead of conventional curve-fitting methods.

碩士
國立中山大學
機械工程學系
87
The thesis is aimed to investigate the mechanism and mechanical properties of thermoplastic AS-4 / PEEK composite laminates subjected to tension-tension fatigue at elevated temperature. We adopt diaphragm forming method by controlling temperature, pressure, vacuum and time condition according to the obtained best curing process to form composite laminates of low crystallinity, trans- crystallinity and good fiber / matrix interfaces. We use the common type of laminates of cross-ply [0 / 90] and quasi-isotropic [0 / 45 / 90 / -45] . Static tension test is performed to measure the elastic modulus and ultimate strength at four different temperatures, such as 25C, 75C, 125C, 175C. And the tension-tension fatigue test is conducted to yield fatigue strength and the S-N curves. Then, we use the degradation of stiffness to explain the fatigue properties at elevated temperature. At last, we observe the failure cross section of composite laminates and understand the failure initiation and mechanism through Scanning Electron Microscope (SEM). The result of experiments can be concluded as follows. The ultimate strength of cross-ply composite laminates is larger than that of quasi-isotropic at room and elevated temperature. Also the fatigue strength of cross-ply composite laminates is larger than that of quasi-isotropic at room temperature. However, the degradation of the fatigue strength of cross-ply composite laminates is less than that of quasi-isotropic when the temperature is raising smoothly. As for the failure mechanism, we obviously observe that the ductile and brittle failure of matrix occur at room and elevated temperature.

博士
國立中山大學
機械與機電工程學系研究所
100
The innovative Al/APC-2 hybrid nanocomposite fiber metal laminates (FMLs) were successfully fabricated. To overcome the usual problem of delamination, the Al alloy 2024-T3 thin sheets were treated by chromic acid anodic (CAA) method to achieve perfectly bonding with matrix PEEK eventually. It was found much better than the previously surface treatment method of CrO3-based chemical etching. A systematic study of hybrid specimens subjected to both static tensile and fatigue tests was conducted at elevated temperatures to obtain their mechanical properties, fatigue lives and failure mechanisms. From the tensile tests, the mechanical properties of Al/APC-2 hybrid cross-ply and quasi-isotropic nanocomposite FLMs at elevated temperatures were received, such as ultimate tensile strength and longitudinal stiffness. Also, the predicted stress-strain curves was proposed and in good agreement with experimental data. The average values of received notched strength were affected significantly by stress concentration and high temperature. The modified point stress criterion (PSC) was used with the varied characteristic length dependent on nature of material and specimen geometry. The predicted notched strengths by the modified PSC model were not only precisely validated, but extended to the application at elevated temperatures. The received fatigue data were plotted in S-N curves at variously elevated temperatures. The predictions of fatigue life curves were also presented and verified. The predicted S-N curves were compared with experimental data and found quite accurate.

abstract: The use of solar energy to produce power has increased substantially in the past few decades. In an attempt to provide uninterrupted solar power, production plants may find themselves having to operate the systems at temperatures higher than the operational capacity of the materials used in many of their components, which affects the microstructural and mechanical properties of those materials. Failures in components that have been exposed to these excessive temperatures have been observed during operations in the turbine used by AORA Solar Ltd. A particular component of interest was made of a material similar to the Ni-based superalloy Inconel 718 (IN 718), which was observed to have damage that is believed to have been initiated by Foreign Object Damage (FOD) and worsened by the high temperatures in the turbine. The potential links among the observed failure, FOD and the high temperatures of operation are investigated in this study. IN718 is a precipitation hardened nickel superalloy with resistance to oxidation and ability to withstand high stresses over a wide range of temperatures. Several studies have been conducted to understand IN 718 tensile and fatigue properties at elevated temperatures (600- 950°C). However, this study focuses on understanding the behavior of IN718 with FOD induced by a stream of 50 μm Alumina particles at a velocity of 200 m/s. under high cycle fatigue at an elevated temperature of 1050 °C. Tensile tests were conducted for both as-received and heat treated (1050 °C in air for 8hrs) samples at room and high temperature. Fatigue tests were performed at heat treated samples at 1050 °C for samples with and without ablation. The test conditions were as similar as possible to the conditions in the AORA turbine. The results of the study provide an insight into tensile properties, fatigue properties and FOD. The results indicated a reduction in fatigue life for the samples with ablation damage, where crack nucleation occurred either at the edge or inside the ablation region and multisite cracking was observed under far field stresses that were the same than for pristine samples, which showed single cracks. Fracture surfaces indicate intergranular fracture, with the presence of secondary cracks and a lack of typical fatigue features, e.g., beach marks which was attributed to environmental effects and creep.
Dissertation/Thesis
Masters Thesis Materials Science and Engineering 2017

碩士
國立中山大學
機械工程學系研究所
88
ABSTRACT PEEK matrix reinforced by carbon fibers as one thermoplastic composite material is studied. Thermoplastic composites have the advantages of high specific stiffness and strength, longer fatigue life, good resistance to moisture absorption and high temperature condition. The thesis is aimed to investigate the mechanical properties and fracture mechanism of the centrally notched AS-4/PEEK composite laminates subjected to tension-tension fatigue loading at elevated temperature. We use three common types of laminates, such as cross-ply , quasi-isotropic and angle-ply . After centrally notched, we first obtain the base-line data of mechanical properties by tensile tests at five different temperatures, such as 25℃、75℃、100℃、125℃、150℃. Then, the fatigue tests are conducted, we receive the fatigue strength and life and establish the stress-life curves. The fatigue characteristics and fracture mechanism of a centrally notched composite laminate at elevated temperature are also recorded and observed. The empirical results can be concluded as follows. At the same temperature, the laminate of cross-ply possesses the largest ultimate strength and fatigue strength, quasi-isotropic the second angle-ply the smallest. As for the elastic modulus, the laminate of cross-ply is larger than that of quasi-isotropic. However, the large strain of angle-ply is within the plastic range that is out of the limit of the study. Thus, a further investigation is needed for angle-ply laminates alone. After centrally notched, the net area is reduced of the specimen, and then the elastic modulus is raised and the ultimate strength and fatigue strength of composite materials are lower. As the temperature increasing, the ultimate strength, fatigue strength and elastic modulus are all decreasing.

碩士
國立中山大學
機械與機電工程學系研究所
106
The thesis aims to investigate the mechanical properties and fatigue characteristics of inclined single-edge-cracked Titanium/AS-4/PEEK/SiO2 nanocomposite laminates at elevated temperature. The nanocomposite laminates used was a sandwich structure of cross-ply AS-4/PEEK (APC-2) with SiO2 nano-powder between the titanium sheets. Then they were cut into a predetermined dimensions. Each sample was cut a unilateral crack by wire processing at factory. The length of the crack was fixed at 3 mm, and the inclined angles θ were 0°, 45° and 60°. In experiments, the tensile tests were carried out at various ambient temperatures of 100°C , 125°C , and 150°C . The fatigue tests were also performed at three ambient temperatures of 100°C , 125°C , and 150°C . The constant stress amplitude tension-tension cyclic tests were carried out by using load-control mode at sinusoidal loading wave with frequency of 5 Hz and the stress ratio of 0.1. After finishing all tensile and fatigue tests, we obtain the results as the greater the angle of cracks, the greater the ultimate strength of the laminate, and can also withstand more resistance to fatigue load. Second, the higher the ambient temperature is, the lower the ultimate strength of laminates is, and the less the fatigue life is. Third, the effect of crack angle on strength and fatigue life is higher than that of ambient temperature. Fourth, the nano-powder can increase the ultimate strength of laminates, but doesn’t have significant improvement in fatigue resistance.

碩士
國立中山大學
機械與機電工程學系研究所
90
ABSTRACT This thesis is aimed to investigate the fatigue life, strength, damage and fracture process in AS-4/PEEK APC-2 composite laminates subjected to both elevated temperature and loading sequences. Our main work is experiment. All specimens are 16-ply thick with lay-up of quasi-isotropic laminates. We accomplish static tensile test, fatigue test of constant stress amplitude, and two-step loading at elevated temperatures, i. e., 75℃and 125℃. The residual strength and stiffiness are obtained. We also use Miner’s rule to analyze the data acquired from experiment and to discuss the fatigue properties and fracture mechanism subjected to both elevated temperatures and loading sequences of combination. Finally, we perform ultrasonic C-Scan non-destructive test to examine laminates. In comparsion with experimental data, we can further understand the damage process and fracture mechanism in laminates. The experimental results can be concluded as follows. The damage of specimens at 125℃ is more serious than that at 75℃.Furthermore, if the loading sequence is low-high, the cumulative damage value will be smaller than 1,whilst it will be larger than 1 due to reverse loading sequence. The sequence of decreasing residual strength and residual stiffiness of specimen associated with the temperature and stress level in combination is high temperature－low stress, low temperature－low stress, high temperature－high stress and low temperature－high stress.

Doctoral Thesis in Civil Engineering
The rehabilitation of existing reinforced concrete (RC) structures has been recognized as a great challenge in the field of Civil Engineering construction sector. The use of strengthening techniques with fibre reinforced polymer (FRP) materials is considered a promising option. Over the last decade, many research works were dedicated to fully understand the possibilities of the strengthening technique that consists on inserting FRP bars or rods into pre-cut grooves opened in the concrete cover of the element to be strengthened. This technique is named by NSM (Near Surface Mounted technique), being the FRP reinforcing material typically bonded to the concrete substrate with an epoxy adhesive. The research carried out until now has been mainly focused on flexural and shear strengthening, and on the bond behaviour. Nevertheless, critical aspects such as durability and time-dependent behaviour requires important and significant studies in order to fully understand behaviour of this technique. The present PhD thesis intends to contribute for the knowledge on the durability and timedependent behaviour of RC slabs strengthened with CFRP (Carbon Fibre Reinforced Polymer) strips according to NSM technique. Thereby, the main objectives outlined were the following: (i) performing experimental tests to study the durability and the time-dependent behaviour of RC slabs strengthened with NSM CFRP systems, as well as, the intervening materials, under different environmental conditions; and, (ii) applying analytical and numerical models to predict the structural and time-dependent behaviour of RC slabs strengthened with NSM CFRP systems. The experimental programmes included RC slabs strengthened with NSM CFRP systems and samples of materials (concrete, epoxy adhesive and CFRP strip). The RC slabs and the samples were submitted to accelerated ageing tests, to identify eventual mechanisms of degradation provided by chemical, physical and environmental degradation factors. Thus, different environmental conditions (immersion in water, immersion in water with chlorides, wet-dry cycles with water with chlorides, thermal cycles, freeze-thaw cycles, exterior tempered environment and maritime environment) and loading (sustained and cyclic fatigue loading) were studied. The eventual synergies between environmental and viscoelastic effects were also evaluated. During the development of this work, elevated service temperatures (up to 80 °C) were also identified as a critical aspect on the global structural response of the NSM CFRP systems. Therefore, an additional experimental programme was performed in order to analyse this aspect. With the purpose of collecting significant data on the durability and time-dependent tests, the period of exposure to the environmental conditions studied lasted between a four and twenty-four months. At the end, all specimens were tested up to the failure (by means of monotonic quasi-static tests). The mechanical behaviour of concrete and CFRP strips was marginally affected, in contrast to the significant changes observed in the epoxy adhesive. The results obtained in the tests of the strengthened RC slabs have shown a marginal variation on their ultimate response due to the action of the environmental conditions studied, as well as, due to the fatigue action. The slabs submitted to the elevated temperatures were the ones in which major changes in the global response were observed. In the creep tests of slabs, the deformation throughout the time revealed to be mostly dependent on the creep of the concrete. Based on the results obtained in experimental tests, analytical modelling of creep of epoxy adhesive and creep of slabs was carried out using existing models in the literature. Therefore, an analytical-numerical tool was proposed to evaluate the creep of epoxy adhesive since early ages using the Generalized Kelvin Model, showing satisfactory results. The analytical prediction of the time-dependent behaviour of slabs was based on the age-adjusted effective modulus method, revealing good predictive results when applied to this type of structures. With the purpose of better understanding the results observed in time-dependent and failure tests of slabs, several numerical models based on element finite method (FEM) were also developed. In general, the used existing tools allowed to simulate with enough accuracy the experimental responses.
A reabilitação de estruturas existentes em geral, e de betão armado (BA), em particular, tem-se revelado como sendo um grande desafio em várias vertentes relevantes da Engenharia Civil. No contexto específico da reabilitação estrutural, o recurso a técnicas de reforço baseadas no uso de polímeros reforçados com fibras (FRP) tem revelado um caráter bastante promissor. Na última década, foi efetuada uma quantidade significativa de trabalhos de investigação dedicados à exploração das potencialidades da técnica de reforço que recorre à aplicação de laminados ou varões de FRP em ranhuras pré-executadas na camada do betão de recobrimento do elemento a reforçar. Esta técnica de reforço é correntemente designada por NSM (Near Surface Mounted strengthening technique), sendo que os reforços em FRP são normalmente colados ao betão através de um adesivo de origem epoxídica. A investigação até agora desenvolvida neste contexto, tem incidido, fundamentalmente, no reforço à flexão e corte, e no estudo da aderência. Contudo, aspetos críticos como a durabilidade e o comportamento diferido necessitam de importantes e significativos estudos dedicados, de modo a compreender a resposta destes sistemas. A presente tese de Doutoramento pretende contribuir para o conhecimento da durabilidade e do comportamento diferido de lajes de BA reforçadas com laminados de CFRP (Carbon Fibre Reinforced Polymer) de acordo com a técnica NSM (sistemas NSM CFRP). Para o efeito, foram definidos os dois seguintes principais objetivos: (i) realização de ensaios experimentais para estudar a durabilidade e o comportamento diferido de lajes de BA reforçadas com sistemas NSM CFRP, bem como dos materiais intervenientes, sob o efeito de diferentes condições ambientais; (ii) aplicar modelos analíticos e numéricos para prever a resposta estrutural e comportamento diferido de lajes de BA reforçadas com sistemas NSM CFRP. O extenso programa experimental incluiu faixas de lajes de BA reforçadas com sistemas NSM CFRP e provetes dos materiais constituintes (betão, adesivo epoxídico e CFRP). As faixas de lajes e provetes foram submetidos a ensaios de envelhecimento acelerado, de modo a identificar os eventuais mecanismos de degradação proporcionados por fatores de degradação de origem química, física e ambiental. Para o efeito foram consideradas diferentes condições ambientais (imersão em água, imersão em água com cloretos, ciclos de molhagem e secagem com água com cloretos, ciclos térmicos, ciclos gelo-degelo, ambiente exterior temperado/quente, ambiente marítimo) e de carregamento (carregamento prolongado e carregamento cíclico de fadiga). A eventual sinergia entre os efeitos ambientais e efeitos viscoelásticos (fluência) foi também avaliada. Durante o desenvolvimento deste trabalho, verificou-se que as temperaturas de serviço elevadas (na ordem dos 80 ºC) podem ser determinantes na resposta de sistemas NSM CFRP. Com vista a analisar esta questão, efetuouse um programa de experimental adicional específico para o efeito. Tendo em vista reunir o máximo de informação e resultados significativos nos ensaios de durabilidade e comportamento diferido, o período de exposição nas condições ambientais estudadas variou entre quatro e vinte e quatro meses. No final de cada período, todos os provetes foram ensaiados até à rotura. O betão e o laminado de CFRP teve o seu comportamento mecânico praticamente inalterado, ao contrário das significativas alterações observadas no adesivo epóxi. Na análise dos resultados obtidos nos ensaios quase estáticos das faixas de lajes reforçadas concluiu-se que a capacidade última destas praticamente não foi afetada pelos ambientes estudados, assim como, no caso dos ensaios quase estáticos pós-fadiga. As faixas de lajes ensaiadas sob temperaturas elevadas, foram aquelas em que se detetaram mais alterações do seu comportamento global. Nos ensaios de comportamento diferido das faixas de laje BA reforçadas com o sistema NSM CFRP, a deformação ao longo do tempo revelou ser maioritariamente dependente da fluência do betão. Com base nos resultados obtidos nos ensaios experimentais, foram desenvolvidas modelações analíticas do comportamento diferido do adesivo epoxídico e das lajes recorrendo a modelos existentes. No estudo da fluência do adesivo, foi proposta uma ferramenta analíticonumérica para avaliar esta desde as primeiras idades através do modelo generalizado de Kelvin, revelando resultados bastante satisfatórios. A previsão analítica do comportamento diferido das lajes reforçadas foi realizada com base no método do módulo efetivo ajustado, mostrando bons resultados quando aplicado a este tipo de estruturas. Foram também desenvolvidas simulações numéricas com recurso a ferramentas existentes baseadas no método de elementos finitos com o intuito de ajudar a interpretar os resultados observados nos ensaios de carregamento diferido e de rotura das lajes. De uma forma geral, as ferramentas existentes permitiram similar as respostas experimentais com rigor considerado adequado.
The research has been carried out at the Department of Civil Engineering of University of Minho, under the supervision of Professor José Manuel de Sena Cruz, and co-supervision of Professor Miguel Ângelo Dias Azenha. This work was financially supported by the Portuguese Foundation for the Science and Technology (Fundação para a Ciência e Tecnologia, FCT) under the grant number SFRH/BD/89768/2012, which is gratefully acknowledge. The work developed would not have existed without the financial support of this institution. This work was also incorporated in a research project CutInDur: Long-term structural and durability performance of concrete elements strengthened with the NSM technique, with the reference number PTDC/ECM/112396/2009, supported by FEDER funds through the Operating Program for Competiviness Factors – COMPETE and FCT.