The mechanical characterization of the interfacial shear strength (IFSS) of carbon nanotube (CNT) coated carbon fibers and the interlaminar fracture toughness of woven fabric carbon fiber/epoxy composites toughened with CNT/epoxy interleave films is presented. The deposition of multiwalled carbon nanotubes (MWCNT) onto the surface of carbon fibers through thermal chemical vapor deposition (CVD) was used in an effort to produce a graded, multifunctional interphase region used to improve the interfacial strength between the matrix and the reinforcing fiber. Characterization of the IFSS was performed using the single-fiber fragmentation test. It is shown that the application of a MWCNT coating improves the interfacial shear strength between the coated fiber and matrix when compared with uncoated fibers. The effect of CNT/epoxy thin interleave films on the Mode I interlaminar fracture toughness of woven fabric carbon/epoxy composites is examined using the double-cantilever beam (DCB) test. Initiation fracture toughness, represented by critical strain energy release rate (GIC), is shown to improve over standard un-toughened composites using amine-functionalized CNT/epoxy thin films. Propagation fracture toughness is shown to remain unaffected using amine-functionalized CNT/epoxy thin films with respect to standard un-toughened composites.

Research is presented which encompasses a study of epoxies based on diglycidyl ether of bisphenol A (DGEBA) cured with 2-ethyl-4-methylimidazole (EMI-24) in the presence of the nonionic surfactant Triton X-100. Interest in this epoxy system is due partially to the potential application as a waterborne replacement for solvent cast epoxies in E-glass laminated printed circuit boards. This research has revealed that the viscoelastic behavior of the cured epoxy is altered when serving as the matrix in a glass composite. The additional constraining and coupling of the E-glass fibers to the segmental motion of the epoxy matrix results in an increased level of viscoelastic cooperativity. Current research has determined that the cooperativity of an epoxy/E-glass composite is also sensitive to the surface chemistry of the glass fibers. Model single-ply epoxy/E-glass laminates were constructed in which the glass was pretreated with either 3-aminopropyltriethoxysilane (APS) or 3-glycidoxypropyltrimethoxysilane (GPS) coupling agents. Dynamic mechanical analysis (DMA) was then used to create master curves of the storage modulus (E') in the frequency domain. The frequency range of the master curves and resulting cooperativity plots clearly varied depending on the surface treatment of the glass fibers. It was determined that the surfactant has surprisingly little effect in the observed trends in cooperativity of the composites. However, the changes in cooperativity due to the surface pretreatment of the glass were lessened by the aqueous phase of the waterborne resin. Moisture uptake experiments were also performed on epoxy samples that were filled with spherical glass beads as well as multi-ply laminated composites. No increases in the diffusion constant could be attributed to the surfactant. However, the surfactant did enhance the final equilibrium moisture uptake levels. These equilibrium moisture uptake levels were also sensitive to the surface pretreatment of the E-glass. / Ph. D.

Master of Science / Department of Mechanical and Nuclear Engineering / Kevin Lease / Carbon fiber reinforced composites (CFRP’s) are a mainstay in many industries, including the aerospace industry. When composite components are damaged on an aircraft, they are typically repaired with a composite patch that is placed over the damaged material and cured into the existing composite material. This curing process involves knowledge of the curing time necessary to sufficiently cure the patch. The inexact nature of curing composites on aircraft causes a significant waste of time and material when patches are unnecessarily redone. Knowing how differences in cure cycle affect the strength of the final material could reduce this waste. That is the focus of this research. In this research, the interfacial shear strength (IFSS) of carbon fiber/epoxy composites was investigated to determine how changes in cure cycle affect the overall material strength. IFSS is a measure of the strength of the bond between the two materials. To measure this, the microbond method was used. In this method, a drop of epoxy is applied to a single carbon fiber. The specimen is cured and the droplet is sheared from the fiber. The force required to debond the droplet is recorded and the data is analyzed. The IFSS of AS4/Epon828, T650/Epon828, and T650/Cycom 5320-1 composites were evaluated. For the former two material systems, a cure cycle with two steps was chosen based on research from others and then was systematically varied. The final cure time was changed to determine how that parameter affected the IFSS. It was found that as the final cure time increased, so did the IFSS and level of cure achieved by the composite to a point. Once the composite reached its fully cured state, increasing the final cure time did not noticeably increase the IFSS. For the latter material system (T650/Cycom 5320-1), the two cure cycles recommended by the manufacturer were tested. These had different initial cure steps and identical final cure steps. Although both cure cycles caused high IFSS, the cycle with the higher initial temperature, but shorter initial cure time achieved a higher level of cure than that with a longer time, but shorter temperature.

Fiber sizings are used to improve the performance of fiber-reinforced polymer composites made from low-cost fiber and matrix materials. Evaluation of three sizings, poly(vinylpyrrolidone) (PVP), a carboxyl modified polyhydroxyether (PHE), and a standard industrial sizing (G'), have revealed tremendous improvements in static mechanical and enviro-mechanical properties. The focus of this work is to determine if these improvements in performance can be ascertained from a micromechanical test for interfacial shear strength (IFSS) on as-processed materials. The accomplishment of this goal would create more information with fewer experiments and a need for less experimental materials. In this study, a nanoindenter uniquely outfitted with a blunt tip is effectively used to obtain microindentation results where the debond load is extracted directly from the experimental load-deflection curve. Shear lag and finite element analyses are used to evaluate the mechanics of the system, but both methods show limitations with regard to determining interfacial stresses in an experimental system. In the results obtained, the PHE and Gâ materials outperform the PVP in IFSS, but the bulk properties for PVP and PHE outperform the Gâ material, suggesting the presence of another dominant mechanism. Despite better retention of bulk properties after hygrothermal exposure, PHE experiences degradation in IFSS that PVP does not. The PHE loses 10% of its original IFSS after 576 hours of 65ºC moisture exposure, while PVP improves by 25%. The tensile strengths for PHE and PVP decrease 7% and 10% respectively at 576 hours exposure. Finite element modeling shows that matrix swelling due to moisture absorption increases interfacial shear stresses, a finding supported by a comparison of wet and dry specimens subjected to equivalent aging times. Matrix swelling is not, however, responsible for the increase in IFSS of the PVP material. The relationship between tensile strength and IFSS proves to be small as predicted by a tensile strength model, but processing defects and other failure processes that are not included in the tensile strength model appear to have strong influences over the experimental results. IFSS is important in composite materials, but in the case of the G', PHE and PVP materials, other factors dominate fiber direction tensile performance. Therefore, this one simple micromechanical test provides significant insight into the composite material behavior, but it does not provide the same magnitude of information as from bulk composite experiments. / Master of Science

Master of Science / Department of Mechanical and Nuclear Engineering / Kevin B. Lease / This research was intended to measure the interfacial shear strength between fiber/ matrix systems and to investigate the relation between structure-mechanical properties and performance of fiber/matrix systems. This work conducted a systematic study on model fiber/matrix systems to enhance the fundamental understanding on how variation of polymeric compositions (and hence, different structures), different curing conditions, and fiber surface treatments influence the interactions between the fiber and matrix. In order to measure the interfacial shear strength of fiber/matrix systems, the microdroplet technique was used. In this technique a polymer droplet was deposited on a fiber in the liquid state. Once the droplet was cured a shear force was applied to the droplet in order to detach the droplet from the fiber. The amount of the force needed to de-bond the droplet was directly related to the strength of the bonds formed between the fiber and matrix during the curing process. In addition, the micro-droplet technique was used to evaluate effects of different crosslinker ratio of fiber/ matrix system and also to see if different curing conditions affect the interfacial shear strength of fiber/ matrix system. Surface treatment was also conducted to evaluate its effects on the interfacial shear strength of the fiber/ matrix system using microdroplet technique. The interfacial shear strength of fiber/ matrix system increased along with the increase of crosslinker ratio to a limiting value, and it decreased as long as the crosslinker ratio increased. Curing condition also caused the interfacial shear strength of fiber/ matrix system to increase when it was cured at higher temperature. Fiber surface treatment exhibited a significant effect to the interfacial shear strength as well as the fiber/ matrix contact angle measurement.

The non-reactive Dreiding and the reactive ReaxFF atomic potentials were applied within a family of atom molecular dynamics (MD) simulations to investigate and understand interfacial adhesion in graphene/vinyl ester composites. First, a liquid vinyl ester (VE) resin was equilibrated in the presence of graphene surfaces and then cured, resulting in a gradient in the monomer distribution as a function of distance from the surfaces. Then the chemically realistic relative reactivity volume (RRV) curing algorithm was applied that mimics the known radical addition regiochemistry and monomer reactivity ratios of the VE monomers during three-dimensional chain-growth polymerization. Surface adhesion between the cured VE resin and the graphene reinforcement surfaces was obtained at a series of VE resin “crosslink densities.” Both pristine and oxidized graphite sheets were employed separately in these simulations using a Dreiding potential. The pristine sheets serve as a surrogate for pure carbon fibers while oxidizing the outer graphene sheets serve as a model for oxidized carbon fibers. Hence, the effects of local monomer distribution and temperature on the interphase region formation and surface adhesion can be investigated. Surface adhesion was studied at various curing conversions and as a function of temperature. Uniaxial loading simulations were performed at different curing conversions for both models to predict the composites’ modulus of elasticity, Poisson’s ratio, and yield strength. The same analysis was performed for the neat cured matrix. The glass transition temperature (Tg) for the homogenized composite and neat VE matrix was determined at different degrees of curing. Subsequent MD simulations were performed to predict structural damage evolution and fracture in the neat VE matrix. The ReaxFF potential was used to quantify irreversible damage due to bond breakage in the neat VE matrix for different degrees of cure, stress states, temperatures, and strain rates. The predicted damage mechanisms in the bulk VE thermosetting polymer were directly compared to those for an amorphous polyethylene (PE) thermoplastic polymer.

A functional nanometer-scale hybrid coating layer with multi-walled carbon nanotubes (MWCNTs) and/or nanoclays, as mechanical enhancement to ‘heal’ surface microcracks and environmental barrier layer is applied to alkaliresistant glass (ARG) fibres. The nanostructured and functionalised traditional glass fibres show both significantly improved mechanical properties and environmental corrosion resistance. Early warning material damage can be achieved by carbon nanotubes concentrated interphases in the composites. / Eine funktionale nanometerskalige Hybridbeschichtung mit multi-walled carbon nanotubes (MWCNTs) und/oder Nanoclay wurde als mechanische Verbesserung des „Ausheilens“ von Oberflächen-Mikrorissen und Barriereschicht gegenüber Umwelteinflüssen auf alkaliresistente Glasfasern (ARG) appliziert. Die nanostrukturierten und funktionalisierten traditionellen Glasfasern zeigen signifikant verbesserte mechanische Eigenschaften und Korrosionsbeständigkeit. Die Frühwarnung des Materialversagens kann durch Carbon Nanotubes, konzentriert in der Grenzschicht der Composites, erreicht werden.

The doctoral thesis is aimed at preparation of glass fiber reinforced polymer composites with controlled interphase formed by plasma-polymerized tetravinylsilane and tetravinylsilane/oxygen thin films. The thin polymer films of specific physico-chemical properties and thickness were deposited to improve interfacial adhesion of glass fiber/polyester composites. The fiber surface modification was performed by using plasma enhanced chemical vapor deposition in low-temperature RF plasma operating in an various effective power range and different treatment time. Test results were examined in relation to the interlayer thickness and different treatment conditions. The prepared interlayers were analyzed to evaluate physico-chemical composition and properties (XPS, RBS, ERDA, FTIR and spectroscopic elipsometry). Selected mechanical properties were evaluated by AFM. Mechanical response of plasma interlayers was evaluated by short beam shear test and direct method of testing the interfacial shear strength using microindentation. The interphase shear failure was controlled by the shear strength at the interlayer/fiber interface as follows from experimental and model data.

A functional nanometer-scale hybrid coating layer with multi-walled carbon nanotubes (MWCNTs) and/or nanoclays, as mechanical enhancement to ‘heal’ surface microcracks and environmental barrier layer is applied to alkaliresistant glass (ARG) fibres. The nanostructured and functionalised traditional glass fibres show both significantly improved mechanical properties and environmental corrosion resistance. Early warning material damage can be achieved by carbon nanotubes concentrated interphases in the composites. / Eine funktionale nanometerskalige Hybridbeschichtung mit multi-walled carbon nanotubes (MWCNTs) und/oder Nanoclay wurde als mechanische Verbesserung des „Ausheilens“ von Oberflächen-Mikrorissen und Barriereschicht gegenüber Umwelteinflüssen auf alkaliresistente Glasfasern (ARG) appliziert. Die nanostrukturierten und funktionalisierten traditionellen Glasfasern zeigen signifikant verbesserte mechanische Eigenschaften und Korrosionsbeständigkeit. Die Frühwarnung des Materialversagens kann durch Carbon Nanotubes, konzentriert in der Grenzschicht der Composites, erreicht werden.

In recent years, due to growing environmental awareness, considerable attention has been given to the development and production of natural fibre reinforced polymer (both thermoset and thermoplastic) composites. The main objective of this study was to reinforce epoxy and polylactic acid (PLA) with hemp fibre to produce improved composites by optimising the fibre treatment methods, composite processing methods, and fibre/matrix interfacial bonding. An investigation was conducted to obtain a suitable fibre alkali treatment method to: (i) remove non-cellulosic fibre components such as lignin (sensitive to ultra violet (UV) radiation) and hemicelluloses (sensitive to moisture) to improve long term composites stability (ii) roughen fibre surface to obtain mechanical interlocking with matrices (iii)expose cellulose hydroxyl groups to obtain hydrogen and covalent bonding with matrices (iv) separate the fibres from their fibre bundles to make the fibre surface available for bonding with matrices (v) retain tensile strength by keeping fibre damage to a minimum level and (vi) increase crystalline cellulose by better packing of cellulose chains to enhance the thermal stability of the fibres. An empirical model was developed for fibre tensile strength (TS) obtained with different treatment conditions (different sodium hydroxide (NaOH) and sodium sulphite (Na2SO3) concentrations, treatment temperatures, and digestion times) by a partial factorial design. Upon analysis of the alkali fibre treatments by single fibre tensile testing (SFTT), scanning electron microscopy (SEM), zeta potential measurements, differential thermal analysis/thermogravimetric analysis (DTA/TGA), wide angle X-ray diffraction (WAXRD), lignin analysis and Fourier transform infrared (FTIR) spectroscopy, a treatment consisting of 5 wt% NaOH and 2 wt% Na2SO3 concentrations, with a treatment temperature of 120oC and a digestion time of 60 minutes, was found to give the best combination of the required properties. This alkali treatment produced fibres with an average TS and Young's modulus (YM) of 463 MPa and 33 GPa respectively. The fibres obtained with the optimised alkali treatment were further treated with acetic anhydride and phenyltrimethoxy silane. However, acetylated and silane treated fibres were not found to give overall performance improvement. Cure kinetics of the neat epoxy (NE) and 40 wt% untreated fibre/epoxy (UTFE) composites were studied and it was found that the addition of fibres into epoxy resin increased the reaction rate and decreased the curing time. An increase in the nucleophilic activity of the amine groups in the presence of fibres is believed to have increased the reaction rate of the fibre/epoxy resin system and hence reduced the activation energies compared to NE. The highest interfacial shear strength (IFSS) value for alkali treated fibre/epoxy (ATFE) samples was 5.2 MPa which was larger than the highest value of 2.7 MPa for UTFE samples supporting that there was a stronger interface between alkali treated fibre and epoxy resin. The best fibre/epoxy bonding was found for an epoxy to curing agent ratio of 1:1 (E1C1) followed by epoxy to curing agent ratios of 1:1.2 (E1C1.2), 1: 0.8 (E1C0.8), and finally for 1:0.6 (E1C0.6). Long and short fibre reinforced epoxy composites were produced with various processing conditions using vacuum bag and compression moulding. A 65 wt% untreated long fibre/epoxy (UTLFE) composite produced by compression moulding at 70oC with a TS of 165 MPa, YM of 17 GPa, flexural strength of 180 MPa, flexural modulus of 10.1 GPa, impact energy (IE) of 14.5 kJ/m2, and fracture toughness (KIc) of 5 MPa.m1/2 was found to be the best in contrast to the trend of increased IFSS for ATFE samples. This is considered to be due to stress concentration as a result of increased fibre/fibre contact with the increased fibre content in the ATFE composites compared to the UTFE composites. Hygrothermal ageing of 65 wt% untreated and alkali treated long and short fibre/epoxy composites (produced by curing at 70oC) showed that long fibre/epoxy composites were more resistant than short fibre/epoxy composites and ATFE composites were more resistant than UTFE composites towards hygrothermal ageing environments as revealed from diffusion coefficients and tensile, flexural, impact, fracture toughness, SEM, TGA, and WAXRD test results. Accelerated ageing of 65 wt% UTLFE and alkali treated long fibre/epoxy (ATLFE) composites (produced by curing at 70oC) showed that ATLFE composites were more resistant than UTLFE composites towards hygrothermal ageing environments as revealed from tensile, flexural, impact, KIc, SEM, TGA, WAXRD, FTIR test results. IFSS obtained with untreated fibre/PLA (UFPLA) and alkali treated fibre/PLA (ATPLA) samples showed that ATPLA samples had greater IFSS than that of UFPLA samples. The increase in the formation of hydrogen bonding and mechanical interlocking of the alkali treated fibres with PLA could be responsible for the increased IFSS for ATPLA system compared to UFPLA system. Long and short fibre reinforced PLA composites were also produced with various processing conditions using compression moulding. A 32 wt% alkali treated long fibre PLA composite produced by film stacking with a TS of 83 MPa, YM of 11 GPa, flexural strength of 143 MPa, flexural modulus of 6.5 GPa, IE of 9 kJ/m2, and KIc of 3 MPa.m1/2 was found to be the best. This could be due to the better bonding of the alkali treated fibres with PLA. The mechanical properties of this composite have been found to be the best compared to the available literature. Hygrothermal and accelerated ageing of 32 wt% untreated and alkali treated long fibre/PLA composites ATPLA composites were more resistant than UFPLA composites towards hygrothermal and accelerated ageing environments as revealed from diffusion coefficients and tensile, flexural, impact, KIc, SEM, differential scanning calorimetry (DSC), WAXRD, and FTIR results. Increased potential hydrogen bond formation and mechanical interlocking of the alkali treated fibres with PLA could be responsible for the increased resistance of the ATPLA composites. Based on the present study, it can be said that the performance of natural fibre composites largely depend on fibre properties (e.g. length and orientation), matrix properties (e.g. cure kinetics and crystallinity), fibre treatment and processing methods, and composite processing methods.