Abstract Organically modified montmorillonite by a long chain alkylammonium surfactant was used to prepare polyimide/clay nanocomposites in this study. Several attempts were made in an effort to achieve fully exfoliated nanocomposites. These included the one-step method, two-step method and in-situ polymerization method. At the same time, the effects of polyimide structures and clay contents were studied. Two dianhydrides and two diamines were used to prepare polyimide/clay nanocomposites via the two-step method. The polyimide/clay nanocomposites with various clay contents from 1.5 ~ 10 wt % were prepared via the two-step method too. The structure of polyimides and the dispersion level of clay were identified by Fourier transform infrared spectroscopy (FTIR) and X-ray diffractometry (XRD). Thermogravimetric analysis (TGA) was performed to demonstrate the thermal stability of the nanocomposites. TGA and XRD results indicate the surfactants are intercalated into the layers of clay. FTIR results indicate the all polyimides in the nanocomposites are formed successfully. XRD results indicate the BPDA-ODA/clay nanocomposite within 3 % by weight of clay via the two-step method is shown to have the best dispersion level of clay. These results are consistent with observations from TGA. The temperature at 10 % by weight loss of the nanocomposite is 31 ¢J greater than that of pure BPDA-ODA. The formation mechanism of polyimide/clay nanocomposites via the two-step method can be described by three distinct steps. A polyamic acid/clay mixture with an exfoliated morphology is first formed. A portion of solvents and intercalated surfactants are then either degraded or expelled from the clay gallery under thermal imidization, resulting in a reduced gallery height of 1.32 nm. On the other hand, portion of the clay layers show an exfoliated morphology due to the effective surfactant and polyimide molecules. As a result, a partially exfoliated polyimide/clay nanocomposite is obtained.

This study introduces a time-dependent micromechanical model for a viscoelastic-viscoplastic analysis of particle-reinforced composite and hybrid composite. The studied particle-reinforced composite consists of solid spherical particle and polymer matrix as constituents. Polymer constituent exhibits time-dependent or inelastic responses, while particle constituent is linear elastic. Schapery's viscoelastic integral model is additively combined with a viscoplastic constitutive model. Two viscoplastic models are considered: Perzyna's model and Valanis's endochronic model. A unit-cell model with four particle and polymer sub-cells is generated to obtain homogenized responses of the particle-reinforced composites. A time-integration algorithm is formulated for solving the time-dependent and inelastic constitutive model for the isotropic polymers and nested to the unit-cell model of the particle composites. Available micromechanical models and experimental data in the literature are used to verify the proposed micromechanical model in predicting effective viscoelasticviscoplastic responses of particle-reinforced composites. Filler particles are added to enhance properties of the matrix in the fiber reinforced polymer (FRP) composites. The combined fiber and particle reinforced matrix forms a hybrid composite. The proposed micromechanical model of particle-reinforced composites is used to provide homogenized properties of the matrix systems, having filler particles, in the hybrid composites. Three-dimensional (3D) finite element (FE) models of composite's microstructures are generated for two hybrid systems having unidirectional long fiber and short fiber embedded in cubic matrix. The micromechanical model is implemented at the material (Gaussian) points of the matrix elements in the 3D FE models. The integrated micromechanical-FE framework is used to examine time-dependent and inelastic behaviors of the hybrid composites.

A series of PMMA/silica hybrid composites were prepared by a sol-gel process in different catalytic and drying. Their thermal properties were analysized by DSC and TGA, the micro-structures by SEM, and the chemical reactions by FTIR. The highest decomposed temperature of these hybrid composites were found for samples prepared at low pH due to the hydrogen bonding. DSC data indicated the samples cured at 25 and 140 oC existed a higher Tg due to unhydrolyzed TEOS. The heat-treated hybrid composites exhibited more compact structures. The size of SiO2 particles from SEM increases with increasing drying temperature and pH value. The hybrid composites prepared in acid condition showed more Si-O-Si bonding than Si-O-C bonding in FTIR. In addition, in FTIR spectra the shifts have been observed from a non-hydrogen-bonded C=O at 1733cm-1 to a hydrogen-bonded carbonyl at 1725cm-1.

This thesis details a study of the structural behavior of Hybrid-Composite Beams (HCB) consisting of a fiber reinforced polymer (FRP) shell with a concrete arch tied with steel prestressing strands.  The HCB offers advantages in life cycle costs through reduced transportation weight and increased corrosion resistance. By better understanding the system behavior, the proportion of load in each component can be determined, and each component can be designed for the appropriate forces. A long term outcome of this research will be a general structural analysis framework that can be used by DOTs to design HCBs as rapidly constructible bridge components. This study focuses on identifying the load paths and load sharing between the arch and FRP shell. Testing was performed by applying point loads on simple span beams (before placing the bridge deck) and a three beam skewed composite bridge system.  Curvature from strain data is used to find internal bending forces, and the proportion of load within the arch is found. Additionally, a stress integration method is used to confirm the internal force contributions.  The tied arch carries about 80% of the total load for the non-composite case without a bridge deck.  When composite with a bridge deck, the arch has a minimal contribution to the HCB stiffness and strength as it is below the neutral axis. For this composite case the FRP shell and prestressing strands resist about 85% of the applied load while the bridge deck carries the remaining 15% to the end diaphragms and bearings. / Master of Science

A micromechanical analysis of the representative volume element (RVE) of a unidirectional flax/jute fiber reinforced epoxy composite is performed using finite element analysis (FEA). To do so, first effective mechanical properties of flax fiber and jute fiber are evaluated numerically and then used in evaluating the effective properties of ax/jute/epoxy hybrid composite. Mechanics of Structure Genome (MSG), a new homogenization tool developed in Purdue University, is used to calculate the homogenized effective properties. Numerical results are compared with analytical solution based on rule of mixture, Halpin-Tsai as well as Tsai-Hahn equations. The effect of the volume fraction of the two different fibers is studied. Mechanical performance of hybrid composite is compared with the mechanical performance of single fiber composites. Synergistic effect due to hybridization is studied using analytical method given in literature, finite element method based MSG and Classical Lamination Theory (CLT). It is found that, when Poisson ratio is taken into consideration, elastic modulus shows synergy due to hybridization. Finally, impact properties of ax/jute/epoxy hybrid composite material are studied using Charpy impact testing.

The objective of the research presented in this thesis was to study and optimize the transverse deck reinforcement for a skewed concrete bridge deck supported by Hybrid Composite Beams (HCB's).  An HCB consists of a Glass Fiber Reinforced Polymer outer shell, a concrete arch, and high strength seven wire steel strands running along the bottom to tie the ends of the concrete arch together.  The remaining space within the shell is filled with foam.  The concrete arch does not need to be cast until the beam is in place, making the HCB very light during shipping.  This lowers construction costs and time since more beams can be transported per truck and smaller cranes can be used.  HCB's are quite flexible, so AASHTO LRFD's design model for bridge decks, as a one-way slab continuous over rigid supports, might not apply well to the HCB's deck design. A skewed three HCB girder bridge with a reinforced concrete deck and end diaphragms was built in the laboratory at Virginia Tech.  Concentrated loads were applied at locations chosen to maximize the negative and positive moments in the deck in the transverse direction.  The tests revealed that the transverse reinforcement was more than adequate under service loads. An Abaqus model was created to further study the behavior of the bridge and to help create future design recommendations.  The model revealed that the HCB bridge was behaving more like a stiffened plate at the middle section of the bridge, indicating that the flexibility of the girders needed to be considered. / Master of Science

A test program for the Hybrid-Composite Beam (HCB) was conducted prior to its use for the replacement of a skewed, simply-supported bridge (Tide Mill Bridge). The HCB is an innovative combination of conventional materials and ideas in a structural beam. The beam consists of a concrete arch tied with prestressing strand that is placed within a Fiber-Reinforced Polymer (FRP) box. Behavior in individual HCB's and a three HCB-system was examined to determine the appropriateness of the current design methodology developed by John Hillman and the simplifying assumptions made within it. Such assumptions include strain compatibility and linear-elastic behavior. Three HCB's were tested at the structures laboratory at Virginia Tech. During individual beam tests, the predicted behavior of the FRP box and prestressing strand agreed with experimental results. The tests revealed the arch was susceptible to local bending and behaved far differently from predicted. Overall, the beams were shown to behave linearly. A final test was performed to apply the design live load to the system. Slight non-linear behavior was observed in the beams. Distribution factors for the system were also investigated and compared to AASHTO and Hillman's model. AASHTO factors were conservative for exterior girders but unconservative for interior girders. Hillman's factors were often conservative but were in agreement for the shear in the exterior girder. The current design procedure appeared to predict FRP and strand behavior well, but the behavior of the arch appeared to differ greatly from the other components of the HCB. / Master of Science

Flexible risers that carry hydrocarbon fuels from the subsea facilities to the floatation units above the sea surface are composed of multiple metallic and polymeric layers (in their wall). Among these layers, the tensile armour layer consists of several helically wound metallic wires; these tensile armour layers carry the weight of the riser, provide tensile stiffness & strength and maintain the structural integrity of the riser structure during harsh underwater currents. However, as the oil & gas fields in shallow waters are receding, the oil & gas industry is being forced to move towards deeper offshore waters, where the metallic tensile armour wires pose limitations (fatigue, corrosion, weight, etc.). In this thesis an alternative to metallic tensile armour wires will be presented in form of a flexible hybrid composite formed by stacking seven pultruded composite (carbon and vinyl-ester) circular rods in form of hexagonal pack, held together by an over-braid (Dyneema fibres) sleeve. The manufacturing process for hybrid composite tensile armour wires will be studied and their mechanical properties will be presented. A multi-scale finite element model developed for hybrid composite wires will be presented in this thesis to help further understand the mechanical properties of hybrid composite wires.

Renewable energy is a serious alternative to deliver the energy needs of an increasing world population and improve economic activity. Wind energy provides better environmental and economic benefits in comparison with the other renewable energy sources. Wind energy is capable of providing 72 TW (TW = 10^12 W) of electric power, which is approximately four and half times the world energy consumption of 15.8 TW as reported in 2006. Since power output extracted from wind turbines is proportional to the square of the blade length and the cube of the wind speed, wind turbine size has grown rapidly in the last two decades to match the increase in power output. As the blade length increases, so does its weight opening up design possibilities to introduce hybrid glass and carbon fiber composite materials as lightweight structural load bearing alternatives. Herein, we investigate the feasibility of introducing modular composite tubulars as well as hybrid sandwich composite skins in the next generation blades. After selecting a target energy output, 8 MW with 80 m blade, airfoil geometry and the layup for the skin as well as internal reinforcements are proposed. They are incorporated into the computational blade via linear shell elements for the skin, and linear beam elements for the composite tubulars to assess the relationship between weight reduction and structural performance. Computational simulations are undertaken to understand the static and dynamic regimes; specifically, displacements, stresses, and vibration modes. The results showed that the composite layers did not exhibit any damage. However, in the balsa core of the sandwich skin, the von Mises stress exceeded its allowable at wind speeds ranging from 11.0 m/sec to 12.6 m/sec. In the blades with composite tubular reinforcement, two different types of damage are observed: a. Stress concentrations at the tubular-skin attachments, and b. Highest von Mises stress caused by the flapping bending moment. The vibration studies revealed a strong coupling mode, bending and twist, at the higher natural frequencies of the blade with tubular truss configuration. The weight saving measures in developing lighter blades in this study did not detract from the blades structural response for the selected load cases.

ABSTRACT DETERMINATION OF MECHANICAL PROPERTIES OF HYBRID FIBER REINFORCED CONCRETE Yurtseven, Alp Eren M.Sc. Department of Civil Engineering Supervisor: Prof. Dr. Mustafa Tokyay Co-Supervisor: Asst. Prof. Dr. . &Ouml / zg&uuml / r Yaman August 2004, 82 pages Fiber reinforcement is commonly used to provide toughness and ductility to brittle cementitious matrices. Reinforcement of concrete with a single type of fiber may improve the desired properties to a limited level. A composite is termed as hybrid, if two or more types of fibers are rationally combined to produce a composite that derives benefits from each of the individual fibers and exhibits a synergetic response. This study aims to characterize and quantify the mechanical properties of hybrid fiber reinforced concrete. For this purpose nine mixes, one plain control mix and eight fiber reinforced mixes were prepared. Six of the mixes were reinforced in a hybrid form. Four different types of fibers were used in combination, two of which were macro steel fibers, and the other two were micro fibers. Volume percentage of fiber inclusion was kept constant at 1.5%. In hybrid reinforced mixes volume percentage of macro fibers was 1.0% whereas the remaining fiber inclusion was v composed of micro fibers. Slump test was carried out for each mix in the fresh state. 28-day compressive strength, flexural tensile strength, flexural toughness, and impact resistance tests were performed in the hardened state. Various numerical analyses were carried out to quantify the determined mechanical properties and to describe the effects of fiber inclusion on these mechanical properties. Keywords: Fiber Reinforcement, Hybrid Composite, Toughness, Impact Resistance