Carbon Fiber Reinforced Polymer (CFRP) composites exhibit exceptional specific stiffness and strength properties. However, their use in structural applications is often constrained with high safety margins out of concern for their brittle and sudden failures. This study proposes manipulating the tensile failure mechanism by utilizing a discontinuous overlapped architecture, which has been demonstrated in the literature to non-linearize the tensile stress-strain response of CFRP laminates. Continuous Carbon fiber 3D-printing provides freedom in building complex morphologies and adjusting the resin content, enabling intricate discontinuous patterns for further tuning the stress-strain response. This study characterizes the constituents and tensile properties of 3D-printed continuous UD laminates. Then, an investigation is conducted on the mechanical tensile response of a 3D-printed discontinuous laminates design and the effect of discontinuity pattern length, and post-processing.

Power harvesting, energy harvesting, power scavenging, and energy scavenging are four terms commonly used to describe the process of extracting useful electrical energy from other ambient energy sources using special materials called transducers that have the ability to convert one form of energy into another. While the words power and energy have vastly different definitions, the terms "power harvesting" and "energy harvesting" are used interchangeably throughout much of the literature to describe the same process of extracting electrical energy from ambient sources. Even though most of the energy coupling materials currently available have been around for decades, their use for the specific purpose of power harvesting has not been thoroughly examined until recently, when the power requirements of many electronic devices has reduced drastically. The overall objective of this research is to typify the power source characteristics of various transducer devices in order to find some basic way to compare the relative energy densities of each type of device and, where possible, the comparative energy densities within subcategories of harvesting techniques. Included in this research is also a comparison of power storage techniques, which is often neglected in other literature sources. An initial analysis of power storage devices explores the background of secondary (rechargeable) batteries and supercapacitors, the advantages and disadvantages of each, as well as the promising characteristics of recent supercapacitor technology developments. Also explored is research into the effectiveness of piezoelectric energy harvesting for the purpose of battery charging, with particular focus on the current output of piezoelectric harvesters. The first objective involved presenting and verifying a model for a cantilever piezoelectric bimorph. Next, an investigation into new active fiber composite materials and macro fiber composite devices utilizing the d31 coefficient is performed in comparison to a monolithic piezoelectric bimorph. The information gathered here was used to design a two bimorph device termed the mobile energy harvester (MEH). Worn by a human being at the waste level, the MEH harvests energy from each footfall during walking or running. The next objective involved characterizing small temperature gradient (less than 200 oC) thermoelectric generators (TEGs). Four TEGs were linked in series and joined with a specially made aluminum base and fin heat sink. This device was then mounted to the exhaust system of an automobile and proved capable of recharging both an 80 and a 300 milliamp-hour battery. A switching circuit concept to step up the output voltage is also presented. However, the circuit proves somewhat difficult to implement, so an alternative DC/DC device is proposed as a possible solution. With the advent of highly efficient, low voltage DC to DC converters, it is shown that their high current, low voltage output can be converted to a higher voltage source that is suitable for many electronic and recharging applications. As extensive literature exists on the capabilities of photovoltaic and electromagnetic energy harvesting, no original experimentation is presented. Instead, only a brief overview of the pertinent technological advances is provided in this document for the purpose of comparison to piezoelectric and thermoelectric energy harvesting. The main research focus, as described above, is dedicated to designing and performing original experiments to characterize cutting edge piezoelectric and thermoelectric transducer materials. To conclude and unify the document, the final section compares the power harvesting techniques with one another and introduces methods of combining them to produce a hybrid, multiple energy domain harvesting device. A piezoelectric-electromagnetic harvesting combination device is presented and scrutinized, revealing that such a device could improve the amount of energy extracted from a single harvesting unit. The research presented here not only expands on the present understanding of these materials, but also proposes a new method of creating a hybrid power harvesting device utilizing two of the energy coupling domains, electromechanical and piezoelectric. The goal is to maximize the harvested energy by tapping into as many ambient sources as are available and practical. / Master of Science

This research is focused on investigating the effect of carbon nanotubes on macroscale composite laminate properties, such as, interlaminar shear strength, interlaminar fracture toughness and electrical conductivity along with studying the micro and nano-scale interactions of carbon nanotubes with epoxy matrix via thermomechanical and electrical characterization of nanocomposites. First an introduction to the typical advanced composite laminates and multifunctional nanocomposites is provided followed by a literature review and a summary of recent status on the processing and the characterization work on nanocomposites and composite laminates. Experimental approach is presented for the development of processing techniques and appropriate characterization methods for carbon nanotubes reinforced epoxy resin based multi-functional nanocomposites and carbon fiber reinforced polymer composite laminates modified with carbon nanotubes. The proposed work section is divided into three sub-sections to describe the processing and the characterization of carbon nanotube reinforced epoxy matrix nanocomposites, woven-carbon fabric epoxy matrix composite laminates modified with selective placement of nanotubes and unidirectional carbon fiber epoxy matrix composite laminates modified with carbon nanotubes. Efforts are focused on comparing the effects of functionalized and unfunctionalized carbon nanotubes on the advanced composite laminates. Covalently functionalized carbon nanotubes are used for improved dispersion and fiber-matrix bonding characteristics and compared with unfunctionalized or pristine carbon nanotubes. The processing of woven carbon fabric reinforced epoxy matrix composite laminates is performed using a vacuum assisted resin transfer molding process with selective placement of carbon nanotubes using a spraying method. The uni-directional carbon fiber epoxy matrix pre-preg composites are processed using a hot press technique along with the spraying method for placement of nanotubes. These macroscale laminates are tested using short beam shear and double cantilever beam experiments for investigating the effect of nanotubes on the interlaminar shear stress and the interlaminar fracture toughness. Fractography is performed using optical microscopy and scanning electron microscopy to investigate the structure-property relationship. The micro and nano-scale interactions of carbon nanotubes and epoxy matrix are studied through the processing of unfunctionalized and functionalized single wall carbon nanotube reinforced epoxy matrix nanocomposites. The multifunctional nature of such nanocomposites is investigated through thermo-mechanical and electrical characterizations.

The objective of this investigation was to characterize two cyanate ester resins and a eight harness satin (8HS) graphite fabric for use in resin film infusion (RFI) processing. Two cyanate ester resin systems were characterized to determine their cure-kinetics, and viscosities during cure. A 8HS graphite fabric was tested in compaction and through the thickness permeability. A one-dimensional, through the thickness, flow and cure computer simulation was run. The resin cure-kinetics models predicted the curing behavior of the resins as functions of time, temperature, and degree of cure. The proposed viscosity models determined the resin viscosity as a function of temperature and degree of cure. The 8HS graphite fabric was tested in compaction and through the thickness permeability to determine the effect of compaction pressure on fiber volume fraction and in turn on through the thickness permeability. The one-dimensional RFI flow and cure simulation combined the cure-kinetics and viscosity models of the resins with the characteristics of the graphite fabric and determined resin infiltration and cure times. The proposed cure-kinetics and viscosity models were more than adequate in modeling the cure and flow behavior of the cyanate ester resin systems. Power law curve fits accurately represented the compaction and through the thickness permeability of the 8HS graphite fabric. Finally, the one-dimensional RFI flow and cure simulation showed that resin viscosity was the major influence on the infiltration times. / Master of Science

The design of fiber-reinforced thermoplastic (FRT) parts is hindered by the determination of the various empirical parameters associated with the fiber orientation models. A method for obtaining these parameters independent of processing doesn't exist. The work presented here continues efforts to develop a rheological test that can obtain robust orientation model parameters, either by fitting directly to orientation data or by fitting to stress-growth data. First, orientation evolution in a 10 wt% long-glass-fiber-reinforced polypropylene during two homogeneous flows (startup of shear and planar extension) was compared. This comparison had not been performed in the literature previously, and revealed that fiber orientation is significantly faster during planar extension. This contradicts a long-held assumption in the field that orientation dynamics were independent of the type of flow. In other words, shear and extension were assumed to have equal influence on the orientation dynamics. A non-lubricated squeeze flow test was subsequently implemented on 30 wt% short-glass-fiber-reinforced polypropylene. An analytical solution was developed for the Newtonian case along the lateral centerline of the sample to demonstrate that the flow is indeed a superposition of shear and extension. Furthermore, an existing fiber orientation model was fit to the gap-wise orientation profile, demonstrating that NLSF can, in principle, be used to obtain fiber orientation model parameters. Finally, model parameters obtained for the same FRT by fitting to orientation data from startup of steady shear are shown to be inadequate in predicting the gap-wise orientation profile from NLSF. This work is rounded out with a comparison of the fiber orientation dynamics during startup of shear and non-lubricated squeeze flow using a long-fiber-reinforced polypropylene. Three fiber concentrations (30, 40, and 50 wt%) were used to gauge the influence of fiber concentration on the orientation dynamics. The results suggest that the initial fiber orientation state (initially perpendicular to the flow direction and in the plane parallel to the sample thickness) and the fiber concentration interact to slow down the fiber orientation dynamics during startup of shear when compared to the dynamics starting from a planar random initial state, particularly for the 40 and 50 wt% samples. However, the orientation dynamics during non-lubricated squeeze flow for the same material and initial orientation state were not influenced by fiber concentration. Existing orientation models do not account for the initial-state-dependence and concentration-dependence in a rigorous way. Instead, different fitting parameters must be used for different initial states and concentrations, which suggests that the orientation models do not accurately capture the underlying physics of fiber orientation in FRTs. / Ph. D. / In order to keep pace with government fuel economy legislation, the automotive and aerospace industries have adopted a strategy they call “lightweighting”. This refers to decreasing the overall weight of a car, truck, or plane by replacing dense materials with less-dense substitutes. For example, a steel engine bracket in a car could be replaced with a high-temperature plastic reinforced with carbon fiber. This composite material will be lighter in weight than the comparable steel component, but maintains its structural integrity. Thermoplastics reinforced with some kind of fiber, typically carbon or glass, have proven to be extremely useful in meeting the demands of lightweighting. Thermoplastics are materials that can be melted from a feedstock (typically pellets), reshaped in the melted state through use of a mold, and then cooled to a solid state, and some common commodity-grade thermoplastics include polypropylene (used for Ziploc bags) and polyamides (commonly called Nylon and used in clothing). Although these commodity applications are not known for their strength, the fiber reinforcement in the automotive applications significantly improves the structural integrity of the thermoplastics. The ability to melt and reshape thermoplastics make them incredibly useful for highthroughput processes such as injection molding. Injection molding takes the pellets and conveys them through a heated barrel using a rotating screw. The melted thermoplastic gathers at the tip of the barrel, and when a set volume is gathered, the screw is rammed forward to inject the thermoplastic into a closed mold of the desired shape. This process typically takes between 30-60 seconds per injection. This rate of production is crucial for the automotive industry, as manufacturers need to put out thousands of parts in a short period of time. The improvement to mechanical properties of the thermoplastics is strongly influenced by the orientation of the reinforcing fibers. Although design equations connecting the part’s mechanical properties to the orientation of the fibers do exist, they require knowledge of the orientation of the fibers throughout the part. Fibers in injection-molded parts have an extremely complicated orientation v state. Measuring the orientation state at each point would be too laborious, so empirical models tying the flow of the thermoplastic through the mold to the evolving orientation state of the fibers have been developed to predict the orientation state in the final part. These predictions can be used in lieu of direct measurements in the part design equations. However, the orientation models rely on empirical fitting parameters which must be obtained before injection molding simulations are performed. There is currently no standard test for obtaining these parameters, nor is there a standardized look-up table. The work presented in this dissertation continues efforts to establish such a test using simple flows in a laboratory setting, independent of injection molding. Previous work focused exclusively on using shearing flow (e.g. pressure-driven flow found in injection molding) to obtain these parameters. However, when these parameters were used in simulations of injection molding, the agreement between measured and predicted fiber orientation was mediocre. The work here demonstrates that another type of flow, namely extensional flow, must also be considered, as it has a non-negligible influence on fiber orientation. this is crucial to injection molding, as injection molding flows have elements of both shearing and extensional flow. The first major contribution from this dissertation demonstrates that extensional flow (e.g. stretching a film) has a much stronger influence than shearing flow, even at the same overall rate of deformation. The second major contribution used a combination shear/extensional flow to demonstrate that the empirical model parameters, thought to be characteristic of the composite, are actually strongly influenced by the type of flow experienced by the sample, and that no single set of model parameters can fit the full orientation state. The final major contribution extends the previous case to long-fiber reinforcement at multiple fiber concentrations which are of industrial interest. This finds the same results, that the model parameters are dependent on the type of flow experienced by the sample. The flow-dependence of the parameters is a crucial point to address in future work, as the flows found in injection molding contain both shearing and extensional flow. By further developing this flow-type dependence, future injection molding simulations should become more accurate, and this will make computer-aided injection-molded part design much more efficient.

An enhanced demand for lightweight materials in automotive applications has resulted in the growth of the use of injection molded discontinuous fiber-reinforced thermoplastics. During the intensive injection molding process, severe fiber breakage arises in the plasticating stage leading to a broad fiber length distribution. Fiber orientation distribution (FOD) is another highly anisotropic feature of the final injection molded parts induced by the mold filling process. The mechanical and other properties can be highly dependent on the fiber length distribution and fiber orientation distribution. The residual fiber length in the final part is of great significance determining the mechanical performances of injection molded discontinuous fiber reinforced thermoplastic composites. One goal of this research is to develop a fiber length characterization method with reproducible sampling procedure in a timely manner is described. In this work is also proposed an automatic fiber length measurement algorithm supported by Matlab®. The accuracy of this automatic algorithm is evaluated by comparing the measured results using this in-house developed tool with the manual measurement and good agreement between the two methods is observed. Accurate predictions of fiber orientation are also important for the improvement of mold design and processing parameters to optimize mechanical performances of fiber-reinforced thermoplastics. In various fiber orientation models, a strain reduction factor is usually applied to match the slower fiber orientation evolution observed experimentally. In this research, a variable strain reduction factor is determined locally by the corresponding local flow-type and used in fiber orientation simulation. The application of the variable strain reduction factor in fiber orientation simulations for both non-lubricated squeeze flow and injection molded center-gated disk, allows the simulated fiber re-orient rate to be dependent on the local flow-type. This empirical variable strain reduction factor might help to improve the fiber orientation predictions especially in complex flow, because it can reflect the different rates at which fibers orient during different flow conditions. Finally, the stiffness of injection-molded long-fiber thermoplastics is investigated by micro-mechanical methods: the Halpin-Tsai (HT) model and the Mori-Tanaka model based on Eshelby's equivalent inclusion (EMT). We proposed an empirical model to evaluate the effective fibers aspect ratio in the computation for the fiber bundles under high fiber content in the as-formed fiber composites. After the correction, the analytical predictions had good agreement with the experimental stiffness values from tensile tests on the composites. Our analysis shows that it is essential to incorporate the effect of the presence of fiber bundles to accurately predict the composite properties. / PHD / An enhanced demand for lightweight materials in automotive applications has resulted in the growth of the use of injection molded discontinuous fiber-reinforced thermoplastics. The injection molding process results in fiber length and fiber orientation distributions in the final parts. The mechanical and other properties can be highly dependent on the fiber length distribution and fiber orientation distribution. This work focuses on the process-structure-property relationship of fiber-thermoplastic composites. A novel fiber length measurement procedure and an automatic fiber length measurement tool were developed to improve the accuracy of fiber length measurement. The existing fiber orientation models have been improved by integration of the flow-type dependent fiber orientation kinetics. To improve the stiffness predictions, an empirical model has been developed to include the effects of fiber clumping on the elastic properties of injection molded fiber composites.

Strengthening of existing structures with Carbon Fibre Reinforced Polymers (CFRP) is a method that has been more common in the building sector during the last decades. The materials strength in relation to its weight is a huge advantage but the lack of knowledge in the building sector results that professionals uses more proven materials such as steel to strength structures. In this report five minor projects in which steel was the strengthening material has been analysed to see if CFRP could be a competitive strengthening material considering mainly practical and economical aspects. The main purpose of this report was to evaluate when CFRP is the most suitable option for strengthening of concrete structures. The results showed that CFRP was applicable in every project but the total cost were higher comparing to the steel solution in four out of five projects. The results indicate that it is difficult to motivate CFRP regarding the economical aspect in relation to minor project that were evaluated in this report. Nevertheless, the tendency is that the advantages with CFRP is more useful when there are more comprehensive projects such as advanced steel works and when it is necessary to save room volumes.

Ce travail de thèse a pour objectif d’analyser le comportement en vibration des composites non-hybrides et hybrides lin-verre. Dans une première partie, une démarche d’analyse modale a été mise en place pour étudier le comportement mécanique et dynamique de ces matériaux. Ceci a permis d’une part, d’identifier les propriétés élastiques et les coefficients d’amortissement de ces composites à partir de leurs fréquences propres, et d’autre part, d’effectuer une comparaison avec les composites traditionnels. La deuxième partie de ce travail est consacrée à une modélisation de l’amortissement des composites non-hybrides et hybrides. Cette modélisation, basée sur la théorie des stratifiés avec cisaillement transverse, a été développée en utilisant la méthode des éléments finis. Plusieurs aspects ont été étudiés comme l’orientation des fibres, la séquence d’empilement, l’architecture des renforts, le choix des séquences d’empilement pour l’hybridation. Cette analyse a conduit enfin à optimiser les performances mécaniques et dissipatives des composites non hybrides et hybrides lin-verre.La dernière partie de ce travail est consacrée à l’étude d’un vieillissement caractérisé par une immersion des matériaux dans l’eau. Dans un premier temps, des essais de vibration ont été réalisés à différentes périodes d’immersion pour identifier l’impact de ce vieillissement sur les propriétés mécaniques et dissipatives des composites non hybrides et hybrides, ainsi que leur évolution en fonction de la durée d’immersion. Enfin, la réversibilité de ces propriétés a été également analysée en effectuant un cycle de vieillissement jusqu’à la saturation puis une opération de séchage. / This PhD research work aimed at analysing the free vibration behaviour of non-hybrid and hybrid flax-glass composites. First, a modal analysis approach was developed to study the mechanical and dynamic behaviour of these materials. Their elastic and damping properties were identified from their natural frequencies and a comparison with the traditional composites was carried out. In the second part, a finite element modelling of the damping of non-hybrid and hybrid composites was implemented by considering the classical laminate theory, taking into account the transverse shear effects. Different topics were studied such as the fibres orientation, the stacking sequence, the reinforcement architecture, the choice of the stacking sequence layers for the hybridisation. This analysis resulted in optimising both mechanical and damping performances of non-hybrid composites and hybrid flax-glass composites. In the last part of this work, the effect of water ageing on the dynamical and mechanical properties of non-hybrid and hybrid glass-flax composites was studied. To this end, these composites were subjected to free vibrations at different ageing durations in order to identify the effect of water ageing on their mechanical and damping properties and their evolution with ageing time. Finally, a cycle of ageing until saturation was reached followed by a drying operation, which was carried out to analyse the reversibility of their properties.

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Composite materials are widely used in temperature fluctuating environments, which make these materials highly prone to cracking. The cracking phenomenon is a result of high thermal stresses that are generated by the mismatch in properties of the composite constituents, particularly the mismatch in the thermal expansion coefficient. The main objective of this study is to understand the micromechanics of such a phenomenon. The problem has been investigated using the finite element method (FEM). The analyses were performed utilizing 3-D prism and axisymmetric models. Hexagonal fiber packing of unidirectional composites was considered. The dimensions of the models were assumed such that the models could provide sufficient information on the behavior near the free surface as well as the interior of fiber composites. Properties of the constituents were considered to be temperature dependent. The elasto-plastic and visco-elastic characteristics of the materials were also included. The transient thermal analysis of the models showed that, for most practical applications, the temperature gradient in the composite constituents has minor effects on the stresses generated. Therefore, several stress analyses were performed assuming a uniformly changing temperature throughout the composite. The elastic analysis of thermal stresses and deformations showed high radial and hoop stress concentrations occurring at the fiber end on the free surface. This is contrary to the shear-lag theorem, which assumes that these stress components are negligible. An overlapping hypothesis, based on the deformation of the fiber and matrix, is proposed to explain such high radial and hoop stresses. Using regular FEM elements, it was concluded that the stresses are singular in nature. The stress singularity was numerically investigated and found to be of the type r -á with á being dependent on the material properties but having a value close to 1/3. The elasto-visco-plastic behavior of composites was also analyzed. Large plastic strains were localized at the fiber end even for a small temperature change. Creep effects that were significant at elevated temperatures brought about some stress relaxation during the manufacturing process. Thermally induced stress concentration in composites can be controlled, to some extent, by changing the geometry of the free surface. The analysis of such effects indicated that reduction of the contact angle between the fiber and the matrix on the fire surface reduced the high radial and hoop stress magnitudes. Also, the influence of covering the free surface of the composite with a thin layer of matrix-like material was studied. The magnitudes of the radial and hoop stress components were substantially reduced. The case when the cover and the composite are made in separate stages (two-stage covering), was also studied. Based on the analysis, effective and practical ways of applying the cover are recommended. To verify the effects of the covering process, experiments were conducted on large-scale laboratory-made composite samples. The samples with the free surface covered with a thin layer of matrix-like material showed no trace of cracking or fiber/matrix debonding even after 1000 thermal cycles. On the other hand, in the samples without cover, exposed to identical thermal cycling, numerous matrix cracks and extensive fiber/matrix debonding were observed.