The degree of cure and temperature are consistent variables used in models to describe the state of material behaviour development for a thermoset during cure. Therefore, the validity of a cure kinetics model is an underlying concern when combining several material models to describe a part forming process, as is the case for process modeling. The goals of this work are to identify sources of uncertainty in the decision-making process from cure measurement by differential scanning calorimeter (DSC) to cure kinetics modeling, and to recommend practices for reducing uncertainty. Variability of cure kinetics model predictions based on DSC measurements are investigated in this work by a study on the carbon-fiber-reinforced-plastic (CFRP) T800H/3900-2, an interlaboratory Round Robin comparison of cure studies on T800H/3900-2, and a literature review of cure models for Hexcel 8552. It is shown that variability between model predictions can be as large as 50% for some process conditions when uncertainty goes unchecked for decisions of instrument quality, material consistency, measurement quality, data reduction and modeling practices. The variability decreases to 10% when all of the above decisions are identical except for the data reduction and modeling practices. In this work, recommendations are offered for the following practices: baseline selection, balancing heats of reaction, comparing data over an extensive temperature range (300 K), choosing appropriate models to describe a wide range of behaviour, testing model reliability, and visualization techniques for cure cycle selection. Specific insight is offered to the data reduction and analysis of thermoplastic-toughened systems which undergo phase separation during cure, as is the case for T800H/3900-2. The evidence of phase separation is a history-dependent Tg-α relationship. In the absence of a concise outline of best practices for cure measurement by DSC and modeling of complex materials, a list of guidelines based on the literature and the studies herein is proposed. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A limited number of studies have been conducted in the literature to examine the effect of stacking sequence and fiber orientation on the compressive behavior of fiber reinforced polymer (FRP) confined concrete. This thesis presents the results of an experimental investigation examining the effect of parameters such as fiber orientation, amount of confinement, and specimen size on the behavior of FRP-confined concrete. As part of the experimental study, a large set of concrete cylinders having two different sizes (100 mm x 200 mm and 150 mm x 300 mm) were jacketed with carbon fiber reinforced polymer (CFRP) sheets having various orientations and tested under pure axial compressive loading. The specimens were confined using various CFRP stacking sequences, with fibers oriented at 0⁰, 90⁰, and ±45⁰ (both unidirectional and woven). Furthermore, within each stacking sequence, the numbers of layers was varied between 4, 6, and 8 to examine the impact of number of plies on the behavior of the FRP-confined concrete cylinders. In addition, the research program included a large number of CFRP coupons made from CFRP laminates having the same properties as the CFRP jackets used in the strengthening of the cylinder series. The analytical program assesses the accuracy and suitability of using various FRP confinement models in the literature to predict the stress-strain response of the confined cylinders tested in the experimental program. The results indicate that parameters such as fiber orientation, stacking sequence, number of confinement layers and specimen size have a direct impact on the strength, ductility and stress-strain behavior of CFRP confined concrete. However, the level of influence varies from one parameter to the other, with the results demonstrating that fiber orientation has a more noticeable effect when compared to the other parameters. The results of the analytical program demonstrate the need to develop reliable confinement models which can take into account the effects of fiber orientation.

En los últimos años, debido al elevado coste de la reparación de las estructuras de hormigón armado con acero por el efecto de la corrosión, se ha tomado conciencia de la necesidad de utilizar nuevos materiales sustitutivos de las barras de acero corrugado. Estos materiales deben garantizar la seguridad y las condiciones de servicio, estéticas y funcionales de la estructura durante el periodo de vida útil para el cual fue diseñado. En la búsqueda de nuevos materiales que sustituyan a las barras de acero corrugado, aparecen hace pocos años las barras de polímero reforzado con fibras carbono (CFRP). Este tipo de materiales es más ligero, posee una resistencia a tracción superior a las barras de acero y, además, solucionan el problema de la corrosión. Antes de introducir este nuevo tipo de barras es necesaria la comprobación de la adherencia con el hormigón en las distintas condiciones térmicas a las que se encuentran expuestas las estructuras, debido a que éste es uno de los parámetros más importantes en el diseño de la estructura. Además, de ser una de las hipótesis en las que se basan las normativas actuales de diseño de estructuras de hormigón armado. Este trabajo estudia el comportamiento adherente entre las barras de CFRP y el hormigón en diferentes condiciones térmicas. Para ello, se lleva a cabo una campaña de 175 ensayos experimentales de pullout. Estos ensayos se realizan en condiciones térmicas de Hielo/Deshielo, 5 ºC, 20 ºC, 40 ºC y 80 ºC, tanto en barras de CFRP con tratamientos superficiales de arenado, texturizado y corrugado, como en barras de acero corrugado. Previamente a la realización de los ensayos experimentales de pullout, se realizan 21 ensayos de diámetro equivalente y 21 ensayos de tracción longitudinal en ambos tipos de barras. Además, en las barras de CFRP se realizan ensayos complementarios de análisis termogravimétrico (TGA), de calorimetría diferencial de barrido (DSC) y microdureza. Al mismo tiempo, se obtienen imágenes con microscopia óptica, microscopia electrónica de barrido (SEM) y microscopia de fuerza atómica (AFM) de las muestras ensayadas a pullout. Los resultados obtenidos muestran que los diferentes tratamientos superficiales de las barras de CFRP, que a diferencia del acero no se encuentran estandarizados, movilizan distintos mecanismos adherentes y modifican el comportamiento adherente de las barras de CFRP, siendo el tratamiento de corrugado el único que produce una adherencia similar a las barras de acero corrugado. Además, se comprueba que las condiciones térmicas afectan de forma diferente a los distintos tipos de barras de CFRP. / Calvet Rodríguez, V. (2014). Influencia de los distintos tratamientos superficiales de las barras de fibra de carbono en su comportamiento adherente con el hormigón [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/38100 / TESIS

The need to mitigate the effects of corrosion on the reinforced concrete structure, and to do so permanently, has led to the proposal of replacing steel bars with other materials that respond adequately to the presence of water without developing corrosive processes. These reinforcements shall ensure the safety and service conditions of the structure over a lifetime and in very adverse climatic conditions. One of the materials being proposed, and de facto already replacing in some places for years, are the rods of polymer reinforced with carbon fibers. The present work studies the adherent behavior of this type of bars at high temperatures and for long periods. If it has planned a experimental plan consisting of a series of envelopes of different moments and conditional spatial environments, so much so that the CFRP with the surface of the surface of the arena and the corrugado, as in the barras of steel. In order to determine the influence of the high temperatures in the concrete, tests of rupture to compression of each kneading performed and at the same ages and temperatures as those experienced in the pullout tests have been carried out. The study was completed with complementary tests on the CFRP bars of optical microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM) of the samples tested at pullout. The results obtained for each age and temperature confirm that the high temperatures affect Corrugated CFRP bars causing a decrease of their adherent tension, without affecting in a decisive way in the case of CFRP bars. The reasons for this unequal behavior as well as the combined influence of high temperatures at the age of 180 days have been justified because of the impact of increased concrete strength. / La necesidad de mitigar los efectos de la corrosión en las estructuras de hormigón armado, y hacerlo de forma permanente, ha llevado a la propuesta de sustitución de las barras de acero por otros materiales que respondan adecuadamente ante la presencia de agua sin desarrollar procesos corrosivos. Estas armaduras deberán garantizar la la seguridad y las condiciones de servicio de la estructura durante un periodo de vida útil y en condiciones climáticas muy adversas. Uno de los materiales que están proponiéndose, y de facto ya sustituyendo en algunos lugares desde hace años, son las barras de polímero reforzado con fibras de carbono. El presente trabajo estudia el comportamiento adherente de este tipo de barras ante elevadas temperaturas y durante largos períodos de tiempo. Se ha planteado un plan experimental consistente en una serie de ensayos de pullout a diferentes edades días y condiciones térmicas, tanto en barras de CFRP con tratamientos superficiales de arenado y corrugado, como en barras de acero corrugado. Para determinar la influencia de las altas temperaturas en el hormigón, se han realizado ensayos de rotura a compresión de cada amasada realizada y a las mismas edades y temperaturas que las experimentadas en los ensayos de pullout. Se ha completado el estudio con ensayos complementarios sobre las barras de CFRP de microscopía óptica, microscopía electrónica de barrido (SEM) y microscopía de fuerza atómica (AFM) de las muestras ensayadas a pullout. Los resultados que se han obtenido para cada edad y temperatura pueden deberse a que las altas temperaturas afectan a las barras de CFRP Corrugadas provocando una disminución de su tensión adherente, sin que afecten de manera determinante en el caso de las barras de CFRP Arenadas. Se ha justificado el por qué de este comportamiento desigual así como la influencia combinada de las altas temperaturas en las edades de 180 días, debido al impacto del aumento de resistencia del hormigón. / La necessitat de mitigar els efectes de la corrosió en les estructures d'hormigó armat, i fer-ho de forma permanent, ha portat a la proposta de substitució de les barres d'acer per altres materials que responguin adequadament davant la presència d'aigua sense desenvolupar corrosius processos. Aquestes armadures hauran de garantir la seguretat i les condicions de servei de la estructura durant un període de vida útil i en condicions climàtiques molt adverses. Un dels materials que estan proposant, i de fet ja s'han substituït en alguns llocs des de fa anys, són les barres de polímer reforçat amb fibres de carboni. El present treball estudia el comportament adherent d'aquest tipus de barres davant elevades temperatures i durant llargs períodes. S'ha plantejat un pla experimental consistent en una sèrie d'assajos de pullout a diferents edats i condicions tèrmiques, tant en barres de CFRP com amb tractaments superficials d'arenado i corrugat, com en barres d'acer Corrugat. Per determinar la influència de les altes temperatures en el formigó, s'han realitzat assaigs de ruptura a compressió de cada amassada realitzada i a les mateixes edats i temperatures que les experimentades en els assaigs de retirada. S'ha completat l'estudi amb assaigs complementaris sobre les barres de CFRP de microscopía òptica, microscopía electrònica de barrejat (SEM) i microscòpia de força atòmica (AFM) de les mostres assajades a retirada. Els resultats que s'han obtingut per a cada edat i temperatura confirmen que les altes temperatures afecten les barres de CFRP Corrugadas provocant una disminució de la seva tensió adherent, sense que afectin de manera determinant en el cas de les barres de CFRP Arenadas. S'ha justificat el perquè d'aquest desigual comporta així com la influència combinada de les altes temperatures en les edats de 180 dies, a causa de l'impacte de l'augment de resistència del formigó. / Cos-Gayón López, FJ. (2017). Influencia de elevadas temperaturas en la adherencia de barras de fibra de carbono con el hormigón [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/91482 / TESIS

Carbon fiber based composites are an essential material in weight-critical applications such as in the aerospace industry. However, these materials are susceptible to damage such as matrix microcracking and fiber/matrix debonding (diffuse damage), which occurs at stresses much lower than the failure stress. A T700/M21 [0/90]s laminate was tensile loaded to introduce diffuse damage and prepared for a study on the initiation of transverse microcracks. The material was tensile loaded in a [+45/-45]s orientation to induce diffuse damage. A diffuse damage indicator was developed by measuring the decrease in shear stiffness. Samples with diffuse damage levels of 0, 0.05, 0.10, 0.15, 0.20, and 0.25 were prepared to be tensile tested in a [0/90]s orientation to induce microcracks. A successful development of the microcracking test procedure was performed. The edge of the material was studied with optical microscopy and x-ray to establish the structure of the fiber bundle geometry when undamaged. A sample containing microcracks was treated with diiodomethane dye penetrant, which successfully highlighted microcracks during x-ray imaging. The application time was not sufficient to produce consistent x-ray images over time, so a 45 minute soak time was recommended instead. The same damaged sample was subjected to a tomographic scan without a dye penetrant and while unloaded. Transverse microcracks were successfully identified from the data, although the results were not clean enough and likely omitted some smaller microcracks. Results are expected to be cleaner if performed during tensile testing. Future tensile testing will quantify the induced crack density of samples containing various degrees of initial diffuse damage, either using x-rays with a dye penetrant or using x-ray microtomography.

In Florida a number of highway bridges were retrofitted on their reinforced concrete (RC) girders with carbon-fiber reinforced polymers (CFRP) during the 1990's. Their conditions, after being in service for approximately 10 years, are of significant interest to the State's highway authority, as well as researchers in the region. This paper will evaluate if a load test on one of such bridges, which was retrofitted with CFRP at the girders in the splash-zone and thus was subjected to severe environmental conditions, is a feasible technique to evaluate the actual condition of the CFRP. A 3-dimensional Finite Element Model (FEM) was utilized to assess the load-deflection behavior of the bridge. An analytical study was used to evaluate the effective moment of inertia of the strengthened beams modeled on the FEM. The results indicate that the deflection change due to the amount of CFRP sheets assumed to be effective on the beam is insignificant. The paper also shows that it would not be feasible to estimate changes in the properties in the CFRP based only on deflection and strain measurements.

A cheaper access to space is needed in current times and new technologies need to be developed to reduce the cost of space access to increase productivity. This thesis presents a study on carbon fiber reinforced polymer (CFRP) composites which is an enabling technology for cost reduction in space vehicles. A literature review of the behavior of CFRP composite has been conducted and it was found that the currently used IM7/977 carbon fiber reinforced epoxy composites do not microcrack at a lower number of thermal cycles. Nano-composites and Thermoplastic matrix composites have been found as two promising alternatives for cryogenic applications. With the use of nano sized inclusions in currently used epoxy resins, coefficient of thermal expansion can be reduced while increase in strength and fracture toughness can be achieved. Some thermoplastics were found to have non-linear stress-strain relationships with signs of ductility even at 4.2K. Both of these resin systems show promise in reducing microcracking at cryogenic temperatures.

No description available.

As the existing steel infrastructure inevitably continues to age and deteriorate, engineers are increasingly looking for innovative and effective methods for repairing and maintaining existing structures. Structural steel components can degrade due to the surrounding environmental conditions, and are susceptible to corrosion damage when exposed to aggressive environments and deicing salts. The conventional methods for repairing steel structures can be labor-intensive and time-consuming, and add considerable weight to the existing structure. One alternative is utilizing carbon fiber reinforced polymers (CFRP). Many studies have documented the ability of CFRPs to enhance the strength of existing structures. Furthermore, CFRP offers the benefits of being non-corrosive and having a high strength-to-weight ratio. Most studies on steel strengthening have focused on the bond behavior of CFRP to steels having a smooth surface condition, which are not representative of deteriorated structures in greater need of retrofitting. Further research has examined the durability of CFRP-steel bonds relative to environmental conditions that do not reflect the service life conditions for typical applications. In this work, a comprehensive study is conducted on the effects of the surface condition and environmental exposure on the bond between CFRP and steel. The influence of corrosion and simulated corrosion pitting is evaluated to determine whether structures with non-uniform surfaces are adequate for CFRP retrofits. In addition, the durability of CFRP-steel bonded systems is investigated through laboratory hygrothermal aging and in-situ environmental conditioning to multiple environments in Virginia. The research can be useful in the development of guidelines that will assist engineers determine if a CFRP retrofit solution is applicable in a given environmental setting and appropriate for the level of deterioration of the structure. / Doctor of Philosophy / As steel structures continue to age, engineers are looking for innovative and effective methods for repairing and maintaining the existing steel infrastructure. Steel components in structures can degrade due to the surrounding environmental conditions. The conventional methods for repairing steel structures can be labor-intensive and time-consuming, and add considerable weight to the existing structure. One alternative is utilizing carbon fiber reinforced polymers (CFRP). The ability of CFRPs to enhance the strength of existing structures has been widely documented. Furthermore, CFRP offers the benefits of being non-corrosive and avoids adding considerable weight to the structure. Most studies on steel strengthening have focused on the bond behavior of CFRP to steels having a smooth surface condition, which are not representative of deteriorated structures in greater need of retrofitting. Further research has examined the durability of CFRP-steel bonds relative to environmental conditions that do not reflect the service life conditions for typical applications. In this work, a comprehensive study is conducted on the effects of the surface condition and environmental exposure on the bond between CFRP and steel. The influence of corrosion and simulated corrosion pitting is evaluated to determine whether structures with non-uniform surfaces are adequate for CFRP retrofits. In addition, the durability of CFRP-steel bonds exposed to laboratory and field conditions is investigated. The research can be useful in the development of guidelines that will assist engineers determine if the application of CFRPs retrofit solution is suitable in a given environment and appropriate for the level of deterioration of the structure.

Fiber reinforced polymers (FRP) are an efficient and inexpensive method of repairing deteriorating infrastructure. FRP sheets can be applied to spalling bridge sections and columns to prevent further deterioration and increase stiffness. However, the effect of the environment on the long-term durability of FRP and how the various damage mechanisms initiate and develop are not known. Systems for structural health monitoring are being sought as a means of managing important components in transportation systems as assets in light of modern life cycle cost concepts. This study characterizes a fiber optic sensor for use in detecting acoustic emissions (AE) in FRP. The results of AE analysis (signal amplitude, frequency spectra, MARSE, and in-plane displacement) caused by simulated fiber fracture experiments and other types of mechanical loading in FRP test coupons are reported. The applications to the development of FRP structural health monitoring systems are also discussed. / Master of Science