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.

Understanding the origins of the multiaxial material properties of soft tissues is crucial for quantifying the tissue material properties that govern the material behavior for acurate predictive modeling of the biological systems. Unlike many engineering structural materials, biological materials exhibit complex material behavior due to structural anisotropy and various interactions between each of the microstructrual units of the tissue. Therefore, this study aims to quantify the shear material behavior of rabbit patellar tendon under simple shear loading along the fiber direction and perpendicular to the fiber direction and to understand the role of tissue shearing and its contribution to the overall mechanical behavior of the tendon. It was hypothesized that tendon demonstrates anisotropic material response under simple shear. Results suggests that tendon exhibits direction-dependent viscoelastic shear properties, reflecting structural anisotropy. The data obtained from our present study could be used for development of constituent models using internal state variable theory.

Abstract Electromagnetic shielding of nylon-66 composites applied to laser modules was studied experimentally and theoretically. The effects of conductive carbon fiber length and weight percentage upon the shielding effectiveness (SE) of nylon composites were investigated. The result showed that the SE of long carbon fiber filled nylon-66 composites was found to be higher SE than short carbon fiber filled nylon-66 composites under the same weight percentage of carbon fibers. In addition, higher electromagnetic shielding was obtained for the composite with higher contents of carbon fibers at the same length. The SE of conductive carbon fiber filled nylon-66 composites was measured to be 41 dB at low frequency of 30 MHz and 59 dB at high frequency of 1.5 GHz. The results of SE predicted by the proposed theoretical model and the results measured by experiments were in good agreement with each other for carbon fibers filled nylon-66 composites of different lengths. The effects of fiber orientation on SE of nylon and LCP composites were also investigated. The result showed that the SE of LCP composites was found to be higher than nylon composites under the same weight percentage of carbon fiber. This is due to that the fiber orientation in LCP composites attempts to keep the same direction.

Natural fibers, such as hemp and flax, are emerging as cheaper reinforcing fibers for polymer composites. Renew-ability, comparable specific properties, and biodegradability make natural fibers more attractive than glass fibers. Vacuum Assisted Resin Transfer Molding (VARTM) is widely used to manufacture medium-to-large sized composites. The non-woven mats used in VARTM must meet manufacturing (permeability) and structural (volume fraction (Vf), thickness, fiber orientation, properties) requirements. Unlike glass mats, natural fiber mats are not available commercially. Design and development of natural fiber mats require knowledge on the relationship among manufacturing, structure and properties of these mats and their composites. Developing this knowledge is the objective of this thesis. Effect of needle punch density on hemp fiber mat structure (areal density, Vf, fiber orientation distribution (FOD), thickness, permeability) was systematically studied. The FOD was characterized non-destructively using X-ray tomography. The Effect of consolidation pressure during composite manufacturing on its structure (Vf, thickness, FOD) was studied. The modulus and strength of needle-punched hemp mat – thermoset polyester composites, manufactured using VARTM and compression molding, were measured. A predictive model for these properties and a modeling approach for the evolution of FOD and thickness during mat manufacturing were developed and validated. The results of these studies were used to understand the relationship. The modulus and the strength of the composites were significantly influenced by the Vf and the FOD, the evolution of which during composite manufacturing depended on the consolidation pressure and the mat structure. The latter depended on mat manufacturing parameters, namely the punch density used to bind the fibers together and the areal density of the web of fibers formed during air laying, and the FOD in the web. The permeability of the mat decreased with increasing the punch density and was found to be a function of both the Vf and the FOD. Despite this, the manufacturing of composite was not adversely affected, and the tensile modulus increased with punch density. The mat composite was modeled as an equivalent laminate, whose lay-up was determined using its FOD. The properties of equivalent laminate that was predicted using lamination theory compared well with the experimental results.

Natural fibers, such as hemp and flax, are emerging as cheaper reinforcing fibers for polymer composites. Renew-ability, comparable specific properties, and biodegradability make natural fibers more attractive than glass fibers. Vacuum Assisted Resin Transfer Molding (VARTM) is widely used to manufacture medium-to-large sized composites. The non-woven mats used in VARTM must meet manufacturing (permeability) and structural (volume fraction (Vf), thickness, fiber orientation, properties) requirements. Unlike glass mats, natural fiber mats are not available commercially. Design and development of natural fiber mats require knowledge on the relationship among manufacturing, structure and properties of these mats and their composites. Developing this knowledge is the objective of this thesis. Effect of needle punch density on hemp fiber mat structure (areal density, Vf, fiber orientation distribution (FOD), thickness, permeability) was systematically studied. The FOD was characterized non-destructively using X-ray tomography. The Effect of consolidation pressure during composite manufacturing on its structure (Vf, thickness, FOD) was studied. The modulus and strength of needle-punched hemp mat – thermoset polyester composites, manufactured using VARTM and compression molding, were measured. A predictive model for these properties and a modeling approach for the evolution of FOD and thickness during mat manufacturing were developed and validated. The results of these studies were used to understand the relationship. The modulus and the strength of the composites were significantly influenced by the Vf and the FOD, the evolution of which during composite manufacturing depended on the consolidation pressure and the mat structure. The latter depended on mat manufacturing parameters, namely the punch density used to bind the fibers together and the areal density of the web of fibers formed during air laying, and the FOD in the web. The permeability of the mat decreased with increasing the punch density and was found to be a function of both the Vf and the FOD. Despite this, the manufacturing of composite was not adversely affected, and the tensile modulus increased with punch density. The mat composite was modeled as an equivalent laminate, whose lay-up was determined using its FOD. The properties of equivalent laminate that was predicted using lamination theory compared well with the experimental results.

The use of carbon-carbon (C/C) composites for clutch application requires a basic understanding of the structural characteristics of the composites that control their frictional and engineering properties. These are related to the microstructure of the matrix and fiber architecture, with the character of fiber/matrix interface and type of defects, porosity and microcracks being the most relevant. The purpose of this study is to examine and characterize the relation between the fiber architecture of selected C/C composites and its relation to their frictional properties when subjected to different normal forces and relative humidity. Friction tests is conducted using a Brüker Universal Friction Tester (UFT). This study also seeks to characterize and analyze the microstructure and fiber architecture through Polarized Light Microscopy, X-Ray Diffraction and Ultrasound Scans. This study shows that the Coefficient of Friction (COF) at constant normal force and RPM are always slightly lower for the samples with surface fibers orientated at 45° relative to the direction of rotation compared to samples with surface fibers orientated 0/90° at 50% relative humidity. The percent difference ranges from 1.62% to 15.30%. However, at 85% relative humidity, the average COF at the constant normal force and RPM are always slightly higher for the 45° compared to 0/90° samples for Rotor samples, while in contrast the average COF are always lower for the 45° samples compared to 0/90° samples for Stator samples. The percent difference ranges from 3.14% to 35.46%. This study found significant differences between the 0/90° samples and the 45° samples. There is indication that the fiber orientation can cause differences between frictional properties even if the clutches are made from the same material. The change in humidity also significantly changes the resulting COF.

No description available.

<p> An electrically conductive thermoplastic composite (ECTPC) consists of electrically conductive filler(s) at a concentration above percolation threshold distributed in an insulating polymer matrix. The high concentration of the filler required to achieve high electrical conductivity for ECTPC is usually accompanied with the deterioration of mechanical properties and a large increase in the viscosity which prevents feasible processing of these materials in common polymer processing equipments such as injection molding machinery. The initial focus of this work was to control these drawbacks by using combinations of conductive fillers namely carbon fiber (CF) and carbon black (CB) to create a hybrid-filler composite. Cyclic olefin copolymer (COC), an amorphous polyolefin, was used as the matrix material. It was found that carbon black and carbon fiber synergistically contribute to the transport of electrons through the matrix. The synergism exists at various filler concentrations including when one of the fillers was present below its percolation threshold, but not at high carbon fiber content. Results showed that where the concentration of CF was several fold higher than carbon black a good trade-off between viscosity and conductivity can be achieved so that the obtained composites can be reasonably processed tn common processing equipment e.g. in an injection molding machine </p> <p> Carbon fiber is preferred to carbon black as it leads to ECTPC with higher electrical conductivity and lower viscosity. However, the high aspect ratio fibers preferentially align in the flow direction leading to ECTPCs which have electrical conductivity several orders of magnitude greater in the in-plane rather than through-plane. We focused on foaming as a strategy to reorient the fibers toward the through-plane direction in foam injection molding. Through a fractional factorial experimental design, the effect of injection rate, melt temperature and mold temperature on electrical conductivity was screened at two levels for foam and nonfoam COC/CF(lO vol%)-CB(2 vol%) injection molded composites. It was found that foaming significantly enhanced the through-plane fiber orientation and through-plane conductivity of the hybrid composite at low injection rate and high melt temperature. The concurrence of the melt flow and bubble growth was considered to be the key mechanism for fiber reorientation while the cell size and shape should not disrupt the conductive path spanning the bulk of the material. </p> <p> The importance of the relative length scale of the fillers on cell size and subsequently, electrical conductivity was investigated by injection molding. Results showed that where the length scale of the filler was comparable to the cell size, as for foamed COC/CF composites, the conductivity considerably decreases with foaming. The drop was greater in the through plane direction and smaller in the in-plane direction for the composites with larger average fiber length. Also smaller cells led to a larger drop in the composite conductivity. It was observed that where the length scale of the filler was much smaller than the cell size as such for COC/CB composites, foaming enhanced the electrical conductivity particularly in the through-plane directions and its effects became more pronounced at lower carbon black concentrations. It was proposed that induced carbon black coagulation by foaming was the main reason for the observed improvement in conductivity. For COC/CF-CB hybrid composites, enhancement in through-plane conductivity, particularly at CB concentration below percolation, via foaming inferred that CB aggregates significantly contributed in improving fiber-fiber contacts. </p> <p> Reorientation of the fibers by foaming was found to be very dependent on processing conditions. High viscosity and fiber- fiber interactions can hinder fiber rotation. The general understanding of the investigation was that fiber reorientation may occur where the cells are much larger than the fibers. In comparison, a series of nonfoam injection molded composites containing CF, CB and CF-CB were foamed in a batch process to avoid flow effects. The insignificant change in fiber orientation with foaming proved that fibers can not rotate by the growth of an adjacent cell in the absence of shear. Also, a large drop in electrical conductivity with foaming as compared to the foam injection molded composites suggested that particle relocalization can not occur in batch foaming. </p> / Thesis / Doctor of Philosophy (PhD)

No description available.

Within this research, factors affecting the orientation of injection molded long fiber composites in an end-gated plaque were investigated. Matrix viscosity was found to have a small effect on fiber orientation. The impact matrix viscosity had on orientation was dependent on fiber loading. At lower fiber loadings, the higher viscosity material had a more asymmetric orientation profile throughout the samples and less of a shell-core-shell orientation. At higher fiber loadings, there were few differences in orientation due to matrix viscosity. Fiber concentration was found to have a larger influence on fiber orientation than matrix viscosity. Increased fiber concentration led to a lower degree of flow alignment and a broader core region at all locations examined, following the trend previously reported for short fiber composites. The orientations of three different fiber length distributions of glass fiber (GF) were compared. The longer fibers in the fiber length distribution were shown to have a disproportionate effect on orientation, with weight average aspect ratio being better than number average aspect ratio at indicating if the GF and CF samples orientated comparably. To improve properties transverse to the main flow direction, the super critical carbon dioxide aided deagglomeration of multi-walled carbon nanotubes (CNTs) was used to create injection molded multiscale composites with CNT, CF, and polypropylene. The addition of CNTs greatly improved the tensile and electrical properties of the composites compared to those without CNTs. The degree of improvement from adding CNTs was found to be dependent on CF concentration, indicating that the CNTs were most likely interacting with the CF and not the polymer. A CNT concentration of 1 wt% with a tenfold degree of expansion at 40 wt% CF proved to be optimum. A large improvement in the tensile properties transverse to the flow direction was found implying that the CNTs were not highly flow aligned. Tensile and electrical properties began to fall off at higher CNT loadings and degrees of expansion indicating the importance of obtaining a good dispersion of CNTs in the part. / Ph. D.