In many reinforced concrete structures built in the 1970's and earlier, bottom beam reinforcement is not continuous and if a column support is lost due to terrorist attack or other unexpected action, the structure could be vulnerable to progressive collapse. The use of CFRP material may provide a solution for rehabilitating such structures. CFRP materials cannot develop full tensile capacity unless they are properly anchored to the reinforced concrete structure. The intent of this study is to find an effective method of anchoring CFRP material to a reinforced concrete beam so that the ultimate tensile strength of the CFRP is realized. In this study, ten reinforced concrete beams rehabilitated using different configurations of anchors were tested to assess the effectiveness of the anchors. Both CFRP anchors and CFRP U-wraps were investigated. The rehabilitated beams were loaded until failure of the CFRP material or anchor occurred. Different failure modes, strengths and deformation capacities of the rehabilitated beams were observed depending on the configurations of anchors. The maximum capacity was obtained using a combination of CFRP anchors and U-wraps. / text

The objective of this research is the evaluation of shear behavior of full-scale reinforced concrete T-beams strengthened with carbon fiber reinforced polymer (CFRP) sheets and CFRP anchors. Although the CRFP material has high tensile strength, premature failure due to debonding CFRP sheets prevents utilizing that strength. The use of CFRP anchors prevents this failure, so the CFRP sheets are able to reach ultimate strain. The current shear design is based on plasticity, which assumes that all steel (ductile material) stirrups, across the critical section yield at ultimate. However the strain in the CFRP (brittle material), is essential to estimate the shear contribution of CFRP. To evaluate the validity of CFRP strengthening for shear, 24 tests were conducted with several parameters including shear-span-to-depth ratio, depth of beams, different transverse reinforcement ratios, and the layout of CFRP strips. In addition, a simple shear behavior model was developed to explain the differences between ductile and brittle material. From test observation, the use of CFRP anchors resulted in U-wrap application to perform like continuous wrapping which implies that a CFRP strip reached rupture strain because the anchors prevented debonding failure. However, all FRP strips did not rupture simultaneously because the strain distribution across a critical crack was not uniform. The average strain across the critical crack was about 0.005. Therefore a conservative value of effective strain (0.004) was selected for design purposes. In addition, when a beam is strengthened with CFRP, interactions between the contributions of the CFRP, steel or concrete must be taken into account. Factors ka, ks, and kf were introduced in the proposed shear design equations. Factor ka reflects the change in the material contributions as the shear span to depth ratio (a/d ratio) changes in deep beams. Factors ks and kf account for the change in steel or CFRP shear contribution due to the change in the critical crack angle as well as the interactions between the steel and FRP transverse reinforcement. As the amount of either steel or FRP material increase, the efficiency of the other material decreases. / text

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.

Steel bridges and structures often need strengthening due to increased live loads, or repair due to corrosion or fatigue cracking. This thesis explores the use of adhesively bonded prestressed carbon fibre reinforced polymers (CFRP) strips in retrofitting intact steel girders, through experimental and analytical investigations. The first part of the research program investigates the behaviour of CFRP-strengthened steel beams comprised of W Structural Sections (W ) with cover plates welded to the tension flange. Six beams, 2000 mm long, were tested under cyclic loads to examine the effects of CFRP strip strengthening on the fatigue life. The CFRP strip prestressing process, type of CFRP strip, level of prestressing, and the location of the CFRP strips were the main parameters examined in this study. Debonding at the end of strip was a significant problem that can be controlled by applying a proper end clamp. The maximum increase in fatigue life observed in the experiments was 125 percent, for a specimen strengthened using high modulus CFRP strips bonded onto the cover plates with the highest level of prestressing. An analytical model and a finite element model were developed for analyzing the strengthened beams. A fracture mechanic analysis was performed to investigate the effects of prestressing on the crack growth rates at the critical weld toe. The models were verified using experimental results, and then used to perform parametric studies. It is shown that the effectiveness of reinforcement is greatest for beams with strips on the cover plate, higher CFRP elastic modulus, and higher prestressing level. In general, this study demonstrates that steel beams can indeed be successfully strengthened or repaired using prestressed CFRP materials.

No description available.

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.

L'étude présentée dans ce mémoire traite du perçage orbital de l'alliage de titane Ti6Al4V et du composite à fibres de carbone CFRP sur des structures épaisses. Ce cas d'étude est extrait d'une problématique industrielle, provenant de la société Airbus qui souhaite intégrer à la structure primaire du mât de réacteur des pièces en CFRP afin de diminuer sa masse. Mais l'association de ces deux matériaux aux propriétés antagonistes pose de nombreux problèmes en matière de perçage. C'est la raison pour laquelle de nouvelles alternatives au perçage classique ont été recherchées. Parmi ces alternatives possibles, le perçage orbital avec micro-lubrification a montré des perspectives intéressantes. C'est pourquoi il a été choisi dans cette application industrielle. Mais ce procédé reste encore assez peu étudié et il existe donc peu de retours d'expériences et beaucoup de développements à réaliser. Le procédé de perçage orbital est très différent du perçage axial conventionnel. L'alésage est réalisé avec un outil de diamètre inférieur au trou, qui décrit une trajectoire hélicoïdale dans le matériau. L'ensemble des travaux présentés se focalisent sur la caractérisation en vue de l'optimisation du procédé de perçage orbital. Pour réaliser ceci, plusieurs aspects ont été abordés. Tout d'abord, une modélisation géométrique et cinématique de l'opération a été développée. La prise en compte de la géométrie exacte de l'outil et des conditions de coupe a permis de définir la géométrie du copeau à chaque instant. Cette connaissance est très importante pour la compréhension du mécanisme d'enlèvement de matière réalisé, elle permet d'estimer le chargement de l'outil et les conditions dans lesquelles s'effectue l'usinage. À partir de cette première modélisation géométrique, une modélisation des efforts de coupe a été mise en place. Pour cela, un modèle d'effort de type mécanistique a été utilisé. Son application a été adaptée au perçage orbital, afin de représenter au mieux l'opération. Les efforts ainsi modélisés ont été comparés à ceux observés expérimentalement afin de valider la modélisation proposée. Cela a permis d'envisager l'utilisation de cette modélisation pour une meilleure compréhension du processus d'enlèvement de matière présent. L'influence des entrées du modèle, à savoir les conditions de coupe et la géométrie de l'outil a été étudiée. L'autre apport de ces travaux réside dans la caractérisation du perçage orbital d'empilages CFRP/Ti6Al4V. En effet, de nombreux essais ont été mis en place pour caractériser le procédé de perçage orbital. Des procédures expérimentales ont donc été mises en place. Tout d'abord, le moyen d'essai instrumenté a dû être caractérisé afin qu'il corresponde au mieux aux moyens utilisés par l'industriel et surtout qu'il permette de réaliser des essais fiables et répétitifs. Les plans d'expériences mis en place par la suite ont permis de définir l'influence des paramètres de coupe sur les efforts et sur les diamètres réalisés. Dans cette phase de caractérisation, les défauts présents en perçage ont également été étudiés. Ainsi, des tendances ont pu être observées. Les résultats obtenus dans ces travaux dans ces travaux ont permis enfin d'envisager les voies d'optimisation du procédé, à travers le pilotage des avances, la stratégie de perçage, mais aussi la géométrie de l'outil. Des pistes ont été proposées et doivent faire l'objet d'études complémentaires. la modélisation mise en place et l'identification des phénomènes intervenant pendant l'opération ont d'ailleurs permis de poser les bases d'une surveillance du process. Celle-ci peut s'envisager de façon passive, pour contrôler le bon déroulement de l'opération, mais également de façon active pour agir en temps réel sur le pilotage du procédé, en fonction des phénomènes identifiés, afin de garantir la qualité souhaitée. / The study presented in this thesis deals with the orbital drilling of Ti6Al4V titanium alloy and CFRP carbon fiber composite. This case study is taken from an industrial problem, from the Airbus company wishing to incorporate parts CFRP to reduce its mass. But the combination of these two materials with antagonistic properties poses many problems for drilling. This is why new alternatives to conventional drilling have been sought. Among these alternatives, orbital drilling with micro-lubrication showed interesting prospects. That is why it was chosen in this industrial application. But this process is still relatively unexplored and there is little feedback and many developments to achieve. Orbital drilling process is very different from the conventional axial bore. The bore is machined with a smaller diameter tool than the hole, which describes a helical path in the material. All work presented focus on the characterization for the optimization of the orbital drilling process. To achieve this, several aspects were discussed. First, a geometric modeling and kinematics of operation has been developed. The inclusion of the exact geometry of the tool and cutting conditions helped to define the geometry of the chip at every moment. This knowledge is important for understanding the achieved material removal mechanism, it allows to estimate the loading of the tool and the conditions in which machining is performed. From this first geometric modeling, modeling of cutting forces was established. For this, a model of mechanistic type of effort was used. Its application was adapted to orbital drilling in order to best represent the operation. The thus modeled efforts were compared to those observed experimentally in order to validate the proposed model. This allowed to consider the use of this model for a better understanding of this material removal process. The influence of model inputs, namely the cutting conditions and tool geometry was studied. Another contribution of this work is the characterization of the orbital drilling of CFRP stacks / Ti6Al4V. Indeed, many tests were developed to characterize the orbital drilling process. Experimental procedures have therefore been put in place. First of all, the instrumented test means had to be characterized so that it better corresponds to the means used by the manufacturer and above all it allows to carry out reliable and repeatable testing. The experimental design implemented subsequently helped to define the influence of cutting parameters on the efforts and realized diameters. In this phase of characterization, the bore in errors have also been studied. Thus, the trends have been observed. The results obtained in this work in the meeting helped to consider the process optimization of routes, through the control of advances, the drilling strategy, but also the geometry of the tool. Tracks have been proposed and are subject to further study. modeling implementation and the identification of phenomena occurring during the operation have also laid the foundation for process monitoring. This can be considered passively, to monitor the smooth running of the operation, but also actively to act in real time to the control of the process, based on identified phenomena, to ensure the desired quality.

Les propriétés finales des structures composites sont gouvernées par les paramètres de cuisson. En effet, ces derniers influencent non seulement les taux volumiques de chaque composant et les changements physico-chimiques de la matrice lors de la polymérisation mais également la variation de l'ensemble des paramètres géométriques à l'échelle de la structure considérée (épaisseurs des plis et inter-plis, paramètres de percolation, etc.). Nous proposons ici de suivre in situ et en temps réel le processus de cuisson de composites fabriqués à partir de préimprégnés unidirectionnels T700/M21 en utilisant des mesures d'impédance électrique. L'instrumentation utilisée (électrodes flexibles : flex et toile) est suffisamment robuste pour proposer des mesures fiables et reproductibles sur la caractérisation électrique du matériau composite pendant sa cuisson. Afin de pouvoir caractériser le changement du comportement électrique anisotrope en cours de cuisson et de proposer une étude des paramètres nécessaires à une future modélisation (pouvant mener à l'asservissement des moyens de cuisson), une analyse microstructurale multi-échelle a été réalisée. Cette dernière a permis de suivre l'évolution des différents paramètres géométriques retenus en fonction du temps de cuisson et à différentes échelles (micro-, méso- et macroscopiques): fractions surfaciques ou volumiques des constituants (fibres, résine et porosité), paramètres de percolation (nombre de points de contacts surfaciques et distances de percolation) et épaisseurs (stratifiés, plis et inter-plis). Les mesures électriques obtenus en temps réel de cuisson sont corrélées non seulement à la variation des paramètres rhéologiques obtenus lors d'essais DSC et DMA (à cycle de cuisson identique) mais également à la variation des paramètres géométriques en cours de cuisson. L'étude de faisabilité a été réalisée avec plusieurs types de cuisson : cycles sans défauts (à l'étuve et à l'autoclave) et cycles perturbés (présence d'une anomalie avant ou en cours de cuisson). Les résultats obtenus montrent la possibilité de rendre compte de l'évolution de l'ensemble des paramètres étudiés et ainsi le fort potentiel des mesures d'impédances électriques dans le monitoring de la cuisson de ce type de matériau. En ce qui concerne l'instrumentation utilisée (flex ou toile), la présence des électrodes à cœur ne doit pas perturber l'intégrité de la pièce composite réalisée. Pour cela, une étude d'intrusivité a été menée à travers des analyses microstructurales comparatives (sur des échantillons cuits en étuve et autoclave) et des essais de caractérisation mécanique (flexion 3 points et traction). Il en ressort que les électrodes de type toile semblent être un très bon compromis mesures électriques/non intrusivité. Le matériau est utilisé ici comme capteur mais pourra également servir d'actionneur (fonctionnalisation du matériau : thermique et mécanique) et l'instrumentation utilisée restera par la suite en son sein pour suivre l'état du matériau en service (SHM). Dans cette perspective, les travaux de thèse se sont également intéressés à l'étude de la possibilité de suivre le comportement mécanique d'échantillons unidirectionnels soumis à une flexion trois points via les mesures de la résistance électrique. / The final properties of composite structures are governed by the curing parameters. Indeed, they not only affect the volume rate of each component and the physicochemical changes of the matrix during the polymerization but also the variations of the set of geometric parameters at the structure scale (plies and inter- plies thicknesses, percolation parameters, etc.). We propose to follow in situ and in real time the curing process of composites made from unidirectional T700 / M21 prepregs using electrical impedance measurements. The instrumentation used (flexible electrodes: flex and weave) is robust enough to offer reliable and reproducible measurements on electrical characterization of the composite material during curing. In order to characterize the change of the anisotropic electrical behaviour during curing and propose a study of the parameters needed for future modelling (which can lead to the complete control of the curing process), a multi-scale microstructural analysis was performed. This latter has tracked the evolution of different geometric parameters depending on curing time and at different scales (micro, meso and macroscopic): surface or volume fractions (fibre, matrix and porosity), percolation parameters (contact points and percolation distances) and thicknesses (laminate, plies and inter-plies). Electrical measurements obtained during curing are correlated not only to the change of rheological parameters obtained during DSC and DMA tests (identical cure cycle) but also to the variation of the geometrical parameters during curing. The feasibility study was carried out with several types of cures: cycles without defects (in oven and autoclave) and disturbed cycles (defect before or during curing). The results show the possibility to account for the evolution of all studied parameters and thus the potential of the electrical impedance measurement to monitor the curing of this kind of composite material. Regarding the instrumentation used for this study (flex or weave), it must not disturb the integrity of the produced composite part. For the purpose, an intrusiveness study was conducted through comparative microstructural analyses (on samples cured in oven and autoclave) and mechanical characterization tests (3-point bending and tensile tests). It shows that the weave electrodes seem to be a very good compromise electrical measurements / non intrusiveness. The material is used here as a sensor but can also be used as actuator (thermal and mechanical material functionalization) and the instrumentation will remain thereafter within it to monitor its structural health (SHM). In this perspective, the thesis work was also interested in monitoring the mechanical behaviour of unidirectional samples subjected to three-point bending via electrical resistance measurements.

Master of Science / Department of Architectural Engineering / Donald J. Phillippi / For this thesis, research was performed on CFRP bonded composite lap-joints with one and two continuous laminas through the lap. Composite wraps used to retrofit existing structures use lap joints to maintain their integrity. The use of composites for retrofitting structures has many advantages over traditional methods, such as steel jacketing, and is becoming more widely accepted in the structural engineering industry. While much literature exists documenting the performance of composite wraps as a whole when applied to concrete columns, less information is available on the behavior of the lap-joint of the wrap. Developing a better understanding of how the lap-joint behaves will help researchers further understand composite column wraps. This research sought to determine what affect continuous middle laminas may have on the stiffness of lap joints and whether or not stress concentrations exist in the lap-joint due to a change in stiffness.

Master of Science / Department of Civil Engineering / Hayder A. Rasheed / There is a high need to repair or replace many bridges in the state of Kansas. 23% of the bridges in Kansas are labeled structurally deficient or functionally obsolete. A majority of these bridges serve rural areas and are damaged due to overloading during harvest season. A state-of-the-art method of performing structural health monitoring on these bridges followed by an effective method of strengthening and repair was researched and presented in this thesis. The first phase of this research involved researching multiple devices to be used for state-of-the-art health monitoring. After deciding on an appropriate system, multiple tests were performed to determine the systems performance compared against conventional systems. The system was tested on a laboratory scale pre-stressed concrete T-beam. The system was tested on its ability to effectively record and transmit acceleration data. If this system were to be implemented on an actual bridge, KDOT could make a decision to repair or strengthen the bridge based on the results. The next phase of the research was to determine an effective strengthening procedure using carbon fiber reinforced polymer (CFRP). Reinforced concrete beam specimens were cast and tested in the lab. The specimens consisted of rectangular and T-shaped cross-sections to create different failure modes when tested in bending. The primary issue when strengthening with CFRP is the issue of early separation failure when using CFRP in the longitudinal direction only. In an effort to prove this, the specimens were strengthened with five layers of CFRP and tested in four-point bending until failure. In an effort to prevent early separation failure, CFRP “U-wraps” were applied to provide shear resistance and additional anchorage for the flexural CFRP. The beams were then tested in flexure until failure by FRP rupture or concrete crushing followed by FRP rupture. The test results indicate that the U-wraps allowed the FRP to reach full capacity and fail in FRP rupture. The use of CFRP provided a strength increase of about 220% over the control beam specimens while significantly reducing the ultimate deflection.