Fibrous composites

Nanoscale silica particles are functionalized and ultrasonically dispersed into a
mixture of polyethylene glycol and ethanol, and then reinforced with Kevlar. The stab or
puncture resistance of the flexible nanophased materials system supersedes recent
advances made in this area. Through SEM scans, thermal and chemical analysis, it is
evident that the functionalized nanoparticles offer multiple facets of resistance to
penetration of a sharp impactor. The improvement in protection is traced to the
formation of siloxane bonds during functionalization. The framework for a theoretical
model is established to estimate penetration depth under low velocity impact of a sharp
object through the flexible composite. For comparison ofthese novel fabric composites,
a method is also introduced to evaluate penetration resistance quantitatively. The method
is capable of showing subtle changes that would otherwise be missed.

In fabrication of nanoparticle-reinforced polymers, two critical factors need to be
taken into account to control properties of the final product; nanoparticle
dispersion/distribution in the matrix; and interfacial interactions between nanoparticles and
their surrounding matrix. The focus of this thesis was to examine the role of these two
factors through experimental methodologies and molecular-level simulations. Carbon
nanotubes (CNTs) and vinyl ester (VE) resin were used as nanoparticles and matrix,
respectively.
In a parametric study, a series of CNT/VE nanocomposites with different CNT
dispersion conditions were fabricated using the ultrasonication mixing method. Thermomechanical
properties of nanocomposites and quality of CNT dispersion were evaluated.
By correlation between nanocomposite behavior and CNT dispersion, a thermomechanical
model was suggested; at a certain threshold level of sonication energy, CNT dispersion would be optimal and result in maximum enhancement in properties. This
threshold energy level is also related to particle concentration. Sonication above this
threshold level, leads to destruction of nanotubes and renders a negative effect on the
properties of nanocomposites.
In an attempt to examine the interface condition, a novel process was developed to
modify CNT surface with polyhedral oligomeric silsesquioxane (POSS). In this process, a
chemical reaction was allowed to occur between CNTs and POSS in the presence of an
effective catalyst. The functionalized CNTs were characterized using TEM, SEM-EDS,
AFM, TGA, FTIR and Raman spectroscopy techniques. Formation of amide bonds
between POSS and nanotubes was established and verified. Surface modification of CNTs
with POSS resulted in significant improvement in nanotube dispersion. In-depth SEM
analysis revealed formation of a 3D network of well-dispersed CNTs with POSS
connections to the polymer. In parallel, molecular dynamics simulation of CNT-POSS/VE
system showed an effective load transfer from polymer chains to the CNT due to POSS
linkages at the interface. The rigid and flexible network of CNTs is found to be responsible
for enhancement in elastic modulus, strength, fracture toughness and glass transition
temperature (Tg) of the final nanocomposites.

This study focused on the fracture and fatigue crack growth behavior in polyvinylchloride (PVC) and polyethersulfone (PES) foams. A new sandwich double cantilever beam (DCB) test specimen was implemented. Elastic foundation and finite element analysis and experimental testing confirmed that the DCB specimen is appropriate for static and cyclic crack propagation testing of soft polymer foams. A comprehensive experimental mechanical analysis was conducted on PVC foams of densities ranging from 45 to 100 kg/m3 and PES foams of densities ranging from 60 to 130 kg/m3. An in-situ scanning electron microscope study on miniature foam fracture specimens showed that crack propagation in the PVC foam was inter-cellular and in the PES foam, failure occurred predominately by extensional failure of vertical cell edges. Sandwich DCB specimens were loaded cyclically as well. For the PVC foams, the crack growth rates were substantially influenced by the density. For the PES foams, there was no clear indication about the influence of foam density on the crack growth rate.

Enhancement of mechanical, thermal and hygrothermal properties of carbon fiber/vinyl ester (CFVE) composites through nanoparticle reinforcement has been investigated. CFVE composites are becoming more and more attractive for marine applications due to two reasons : high specific strength and modulus of carbon fiber and low vulnerability of vinyl ester resin to sea water. However, the problem with this composite system is that the fiber matrix (F/M) interface is inherently weak. This leads to poor mechanical properties and fast ingress of water at the interface further deteriorating the properties. This investigation attempts to address these deficiencies by inclusion of nanoparticles in CFVE composites. Three routes of nanoparticle reinforcement have been considered : nanoparticle coating of the carbon fiber, dispersion of nanoparticles in the vinyl ester matrix, and nanoparticle modification of both the fiber and the matrix. Flexural, short beam shear and tensile testing was conducted after exposure to dry and wet environments. Differential scanning calorimetry and dynamic mechanical analysis were conducted as well. Mechanical and thermal tests show that single inclusion of nanoparticles on the fiber or in the matrix increases carbon/vinyl ester composite properties by 11-35%. However, when both fiber and matrix were modified with nanoparticles, there was a loss of properties.

An experimental investigation was undertaken to determine the effects of marine environmental exposure on the mechanical properties of vinylester resins (VE510A and VE8084) and carbon fiber/VE510A vinylester composites. The effect of carbon fiber sizing on the composite strengths was also examined. Neat resins were exposed to marine environments until moisture content reached a point of saturation after which they were tested in tension, compression and shear. Compared to the baseline dry specimens, specimens subjected to moisture showed overall increased ductility and a reduction in strength. Dry and moisture saturated composite specimens were tested in tension and compression in different orientations. Longitudinal specimens were tested in in-plane shear and interlaminar shear. Composites with F-sized carbon fibers displayed overall higher strength than those with G-sized fibers at both dry and moisture saturated conditions. An analysis of moisture absorption of the composites was performed which vii shows that the moisture up-take is dominated by the fiber/matrix region which absorbs up to 90% of the moisture. The composites experienced reduced strength after moisture absorption. The results revealed that the fiber sizing has stronger effect on the fiber/matrix interface dominated strengths than moisture up-take.

An experimental study was conducted on the strength and toughness characteristics of concrete made from recycled aggregate, cement and fly ash reinforced with reclaimed high density polyethylene plastic (HDPE) fibers. The objectives of the investigation were: (1) to evaluate the performance of a sustainable concrete containing up to 90% recycled materials; (2) to determine the variation of strength and toughness with a Fiber Factor incorporating length, width and amount of HDPE fibers; (3) to identify the best performing mix design based on tensile strength and toughness and (4) to provide some guidelines for the use of this sustainable composite in Civil Engineering construction. The results showed that the HDPE fiber reinforcements did not improve the compressive strength of the mixture. However, HDPE fibers improved the ductility and toughness which may be beneficial for structural and pavement applications.

Degradation of the Carbon Fiber/Vinylester (CF/VE) polymer matrix composites due to different electrochemical interactions when exposed to seawater or at high temperature had been experimentally investigated. Water uptake behavior of composite specimen was examined based on weight gain measurement. Three point bending test was performed to quantify the mechanical degradation of composite immersed in seawater with different environmental and electrochemical interactions. Finally, Electrochemical Impedance Spectroscopy (EIS) was used to better understanding of the degradation process in CF/VE composite produced by interactions between electrochemical and different environmental conditions. A detailed equivalent circuit analysis by using EIS spectra is also presented in an attempt to elucidate the degradation phenomenon in composites.

Durability of the composite materials in marine environments has been investigated experimentally and with analytical and numerical methods. The main focus of this study is on the integrity of the fiber/matrix interface under seawater exposure. A single-fiber compression test specimen called the Outwater-Murphy (OM) test has been analyzed using mechanics of materials principles and linear elastic fracture mechanics. Sizing of the OM specimen was conducted so that debonding of the fiber from the interface should be achieved prior to yielding of the matrix and global instability failure. Stress analysis of the OM specimen has been conducted from theory of elasticity and finite element analysis. A superelement technique was developed for detailed analysis of the stress state at the fiber/matrix interface. The interface stress state at the debond site in the OM specimen, i.e. at the hole edge, was identified as biaxial tension at the fiber/matrix interface. Characterization of cure and post-cure of 8084 and 510A vinlyester resins has been performed using cure shrinkage tests based on dynamic mechanical analysis and coated beam experiments. In addition, moisture absorption, swelling and the influence of moisture on the mechanical properties of the resins were determined. Testing of OM specimens consisting of a single carbon or glass fiber embedded in vinylester resin at dry conditions and after seawater exposure revealed that the debond toughness was substantially reduced after exposure of the OM specimen to seawater. C(F) did not debond. Macroscopic carbon/vinylester woven composites where the fibers were sized with F sizing were tested in shear at dry conditions and after four weeks of seawater exposure. The shear strength was very little affected after the short immersion time.

In this research, the degradation of carbon fiber/vinylester composites in marine environments was experimentally investigated. Additionally, two types of carbon fiber surface treatments, namely Polyhedral Oligomeric Silsesquioxane (POSS) and the industrial surface treatment F0E, were evaluated to determine their effectiveness in creating a fiber/matrix (F/M) interface for use in the marine environment. Electrochemical Impedance Spectroscopy (EIS) was explored as a new application of an existing technique for use in measuring the amount of water at the F/M interface in carbon fiber/vinylester composites. EIS spectra were used to determine equivalent electric circuit models that allow for the prediction of water at the interface. The location of water within the composite was determined through Positron Annihilation Lifetime Spectroscopy (PALS). Interlaminar shear strength and transverse tensile tests were carried out for dry conditions and after hygrothermal exposure of the composites to study the influence of the integrity of the F/M interface on the macroscopic response of the composite.