M.Tech. (Chemistry) / Please refer to full text to view abstract

Submitted in fulfillment of the requirements for the degree of Master in Engineering: Mechanical Engineering, Durban University of Technology, Durban, South Africa, 2017. / Despite the huge improvements made in the development of vehicle brake pad materials, problems such long stopping distances, noise pollution, and heat dissipation still continue to persist. In this regard, a novel polymer-based hybrid nanocomposite brake pad (HC) has been developed. Here, a combination of carbon-based materials, including those at a nanoscale, was used to produce the brake pad. The coefficient of friction, wear rate, noise level, and interfacial temperature was investigated and compared with that of a commercial brake pad material (CR). It was found that the brake pad performance varied with the formulation of each pad. Hybrid nanocomposite brake pads material exhibited superior performance in most tests when compared to the commercial brake pad. They exhibited a 65% lower wear rate, 55% lower noise level, 90% shorter stopping distance, and 71 % lower interfacial temperature than the commercial brake pad (CR). Furthermore, mechanical properties such as hardness, compressive strength, shear strength, and impact resistance were also evaluated. The material exhibited a 376% higher shear strength, 100% improved compressive strength, 77% greater modulus and 100% higher impact strength than the commercial brake pad. The hardness of both brake pads material was statistically comparable. Additionally, the thermal stability, degradation, water and oil absorption behaviour were measured. It was found that HC brake pad material exhibited a 100% lower water absorption and 80% oil absorption rate. The brake pads also exhibited a thermal stability within the brake pad standard maximum working temperature of 300 -400 0C. The superior performance of hybrid nanocomposite brake pad material observed was due to synergism between the carbon-carbon additives and uniform dispersion of carbon fiber as shown in Figure 4.16. Scanning electron microscopy study was subsequently performed on fracture and worn surfaces of the brake pads. The micrographs show changes in the structural formation after the incorporation of carbon based fillers. It also shows the smooth structure and uniform dispersion of the carbon fiber. The smooth surface of the worn brake pad is an indicative of a harder structure. No ploughing or score marks were evident. Hence, it was deduced that the reinforced had superior mechanical and tribological properties. These improved properties are suggestive of materials that may be successfully used for brake pad application. / M

An investigation of the effect of R-ratio on the mode II fatigue delamination of AS4/3501-6 carbon/epoxy composites has been undertaken. Experiments have been performed on end notched cantilever beam specimens over a wide range of R-ratios (-l ≤R ≤0.50). The measured delamination growth rate data have been correlated with the mode II values of strain energy release rate range ∆G[formula omitted]), maximum strain energy release rate (G[formula omitted]) and stress intensity factor range (∆K[formula omitted]). The growth rate is dependent on the R-ratio over the range tested. For a constant level of ∆G[formula omitted], the crack growth rate decreases with increasing R-ratio. A similar trend is observed when the data is plotted as a function of G[formula omitted]. The effect of plotting the growth rate as a function of ∆K[formula omitted] is to produce an R-ratio dependence opposite to that obtained by either the ∆G[formula omitted] or G[formula omitted] approach. For a constant level of ∆K[formula omitted], the crack growth rate increases with increasing R-ratio. Master equations which completely characterize the fatigue behaviour as a function of ∆G[formula omitted] and ∆K[formula omitted] have been derived, based on the observation that the growth rate law exponent, n and constant, A are unique functions of R-ratio. Values for n are surprisingly large and increase with increasing R-ratio whereas values for A decrease with increasing R-ratio. The effect of time-at-load has been considered in an attempt to explain the existence of the R-ratio dependence of the growth rate. The correct trend can be established for the exponent, n but not for the constant, A. Friction between the crack faces, particularly at higher R-ratios, is proposed as a possible explanation for the observed anomaly. Further evidence of a frictional mechanism operating at higher R-ratios has been discovered through a postmortem fracture surface examination. Additional fractographic observations are presented over the entire range of R-ratios tested. In regions subjected to negative R-ratio cycling, there is no evidence of the characteristic mode II hackle features. Instead, loose rounded particles of matrix material are found. An extensive amount of hackling is observed in regions subjected to low positive R-ratio cycles. The extent of hackle damage visibly decreases in areas where higher levels of R-ratio are imposed. A correlation between the general fracture surface morphology and the fatigue data provides support for the hypothesis that energy for delamination is always available in sufficient quantity, and that growth is dependent on the stresses ahead of the crack tip being sufficiently high. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Ph.D. (Chemistry) / In this study, carbon based electrode materials such as glassy carbon, graphene, diamond and exfoliated graphite were explored as suitable electrode materials for electrochemical detection, electrochemical and photoelectrochemical degradation of organic water pollutants. Graphene modified glassy carbon electrode sensor was developed for bisphenol A. Cyclic voltammetry was used to study the electrochemical properties of the prepared graphene- modified glassy carbon electrode using potassium ferricyanide as a redox probe. The prepared graphene- modified glassy carbon electrode exhibited more facile electron kinetics and enhanced current of about 75% when compared to the unmodified glassy carbon electrode...

Among current technologies used for the treatment of polycyclic aromatic hydrocarbons (PAHs) -contaminated media, adsorption has been reported to offer relatively high PAH removal efficiency while being rapid and cost-effective. Therefore, the main goal of this research was to assess and optimize the adsorption process for the removal of PAHs from contaminated water and soil using recoverable magnetic activated carbon-based composites. In the first phase, 6 different composites, 3 magnetic powder activated carbon (MPAC) composites and 3 magnetic granular activated carbon (MGAC) composites, were synthesized, and then, characterized using XRD, FE-SEM, EDS, and FTIR methods. The adsorption experiments revealed that all the recoverable MPACs and MGACs were capable of removing the PAHs from water, with removal percentages ranging from 87.2 to 99.3%. The PAH-loaded MPAC and MGAC with the highest PAH removal efficiency were also subjected to a series of desorption studies. The results indicated that the PAHs desorption was in the range 38.1-60.1% for low molecular weight (LMW) PAHs and 23.4 to 57.2% for the high molecular weight (HMW) PAHs. In the second phase, the adsorption kinetics and isotherms studies were performed on MPAC synthesized by a precipitation (MPAC-Prec.) method, which showed the highest PAH removal efficiency among the prepared magnetic activated carbons (MACs). The PAHs adsorption by MPAC-Prec. was rapid, reaching equilibrium in 6 h with the removal efficiency ranging from 95.6 to 100.0% under the conditions of this study. Among the studied kinetics models, pseudo-second order fitted the experimental data very well, implying that all the MPAC adsorption sites had an equal affinity for PAHs. The results of the kinetic studies also indicated that the greater molecular weight PAHs had a slower adsorption rate due to the slower transfer of their molecules to the MPAC adsorption sites. With an R2 in the range 0.73-0.96, the Langmuir model described the isotherms adsorption of LMW and HMW PAHs better than the other isotherms models. Furthermore, according to the Langmuir model, the maximum adsorption capacity of MPAC-Prec. was determined to be between 8.7 and 11.4 µg/mg for the LMW PAHs, and 8.4 and 20.2 µg/mg for the HMW PAHs. In the third phase, a series of soil washing tests using MGAC synthesized by co-precipitation (MGAC-CoP) method, were carried out to explore the effect of MAC on the PAHs removal from soil. The employed MGAC was the second most efficient MAC in the PAHs adsorption experiments (first phase of research), and it showed greater recovery from soil washing mixture compared to the MPAC-Prec. in the preliminary tests. The MGAC-CoP composite had a surface area and total pore volume of 837.9 m2/g and 0.5 cm3/g, respectively, which were approximately 10% lower than the bare GAC, according to BET test results. Soil washing parameters were optimized for the treatment of a real contaminated soil, which were MGAC-CoP dose of 2% (w/w), washing time of 24 h, liquid to soil ratio of 15:1, stirring speed of 100 rpm, pH of 8.3, and temperature of 25 ˚C. Under these optimized conditions, an average PAHs removal of 47.4% was obtained. Among the LMW and HMW PAHs, anthracene (ANT), and fluoranthene (FLUO) showed the highest affinity to MGAC during the treatment process, with 57.7% and 67.1% removal from soil, respectively. The thermodynamic studies revealed that the adsorption of the LMW and HMW PAHs onto MGAC in soil washing was non-spontaneous and endothermic as the values of Gibbs free energy (∆G˚>0) and Enthalpy change (∆H˚>0) were positive. In the fourth phase, the efficiency of MGAC-CoP in surfactant-enhanced soil washing for the PAH removal and the recovery of the surfactant solution was studied. The effective parameters of soil washing with the surfactant (Tween 80) were assessed using a real contaminated soil sample, and the results showed that 5% Tween 80, a liquid to soil ratio of 10:1, and a 72-hour washing time at 20°C were optimum operating conditions. Under these conditions, the average PAHs removal efficiency was 67.6%, which was higher than the 47.4% obtained for the same soil with no surfactant addition in phase 3. The possibility of recycling and reusing the Tween 80 solution was investigated by adding MGAC-CoP to the soil and surfactant solution mixture during the soil washing process. For this purpose, 5% Tween 80 and 2% (w/w) MGAC were used in 7 successive washing cycles, with no regeneration process for the MGAC composite. The results revealed that the combination of surfactant and MGAC was capable of removing 68.6, 70.7, 70.3, 61.6, 55.5, 50.2, and 39.4% of the PAHs from soil in the 7 washing cycles, respectively. Furthermore, the recycled Tween 80 and non-regenerated MGAC did not produce any waste or effluent after 6 times reuse in the treatment process, while successfully recovered and reused. This implies that soil washing with Tween 80 and MGAC is a very affordable, efficient, and practical method for remediation of PAH-contaminated soils.

No description available.

An infrared microscope system was used to measure the temperatures at the interfaces of graphite-epoxy and carbon-PEEK composites in unidirectional sliding contact with sapphire. Effects of fiber orientation and velocity on tribological parameters were examined. Oscillating contact conditions with graphite-epoxy were also examined. Surface temperatures on the order of 100-160°C were measured at relatively low rates of frictional heat generation. The corresponding coefficients of friction were on the order of 0.45-0.65. In graphite-epoxy, fiber orientation was seen to affect coefficient of friction and wear significantly; but surface temperature was very little affected by fiber orientation. In carbon-PEEK, fiber orientation affected the coefficient of friction, wear and surface temperatures significantly. Surface temperatures in both materials initially increased with velocity, but stayed constant as the glass transition temperature of the matrix material was reached. The total wear is believed to be due to a combination of adhesive wear and fatigue wear. Comparison of the measured surface temperatures with theoretical predictions is done. A “two-velocity-regime” tribological model is proposed to explain the tribological behavior of polymer composites. / Master of Science

The influence of crenulated noncircular fibers on the micromechanical stress states due to a transverse strain and to a temperature change in carbon/carbon composites is examined using the finite element method. Stresses at the interface of both fully bonded and fully disbonded fibers having two crenulation amplitudes and with two fiber volume fractions are presented. In each case, these interface stresses are compared to stresses at the interface of circular fibers which have the same degree of disbond and fiber volume fraction and are under the same loading conditions. For the disbonded cases, deformed meshes showing locations of fiber/matrix contact are also included. In addition to the interface stress states, selected composite properties are also computed and compared in each case examined. Interest in studying noncircular fibers stems from a desire to increase the transverse properties of carbon/carbon by introducing a mechanical interlocking between the fiber and the matrix. Results presented here indicate that this interlocking does in fact occur. Evidence from the interface stress data suggests, however, that any possible advantage of this interlocking may be outweighed by the disadvantage of stress concentrations which arise at the interface due to the crenulated geometry of the fibers / Master of Science

The objective of this study was to observe and characterize the out-of-plane fracture of a 2-D carbon-carbon composite and to gain an understanding of the factors influencing the stress distribution in such a laminate. The experimental portion of this study consisted of performing an out-of-plane tensile test in a scanning electron microscope and determining the modes of failure. Failure was found to be interlaminar, with cracks propagating along the fiber-matrix interface. Finite element analyses of a two-ply carbon-carbon composite under in-plane, out-of-plane, and thermal loading were performed. Stress distributions were studied as a function of stacking sequence, undulation aspect ratio, and undulation offset ratio. The results indicated that under out-of-plane loading σ_x and τ_xz were strongly dependent on the geometric parameters studied, but σ_z and σ_y were relatively independent of geometry. Under in-plane loading all components of stress were strong functions of the geometry, and large interlaminar stresses were predicted in regions of undulation. The thermal analysis predicted the presence of large in-plane normal stresses throughout the laminate and large interlaminar stresses in regions of undulation. An elasticity solution was utilized to analyze an orthotropic fiber in an isotropic matrix under uniform thermal load. The analysis reveals that the stress distributions in the fiber are singular when the radial stiffness C_rr is greater than the hoop stiffness C₀₀. Conversely, if C_rr < C₀₀ the maximum stress in the composite is finite and occurs at the fiber-matrix interface. In both cases the stress distributions are radically different than those predicted assuming the fiber to be transversely isotropic (C_rr = C₀₀). / Ph. D.

Identifying material parameters in composite plates is a necessary first step in a variety of structural applications. For example, understanding the material parameters of carbon fiber composite is important in investigating sensor and actuator placement on micro-air-vehicle wings for control and wing morphing purposes. Knowing the material parameters can also help examine the health of composite structures and detect wear or defects. Traditional testing methods for finding material parameters such as stiffness and damping require multiple types of experiments such as tensile tests and shaker tests. These tests are not without complications. Methods such as tensile testing can be destructive to the test specimens while use of strain gages and accelerometers can be inappropriate due to the lightweight nature of the structures. The proposed inverse problem testing methods using digital image correlation via high speed cameras can potentially eliminate the disadvantages of traditional methods as well as determine the required material parameters including stiffness and damping by conducting only one type of experiment. These material parameters include stiffness and damping for both isotropic and orthotropic materials, and ply angle layup specifically for carbon fiber materials. A finite element model based on the Kirchoff-Love thin plate theory is used to produce theoretical data for comparison with experimental data collected using digital image correlation. Shaker experiments are also carried out using digital image correlation to investigate the modal frequencies as validation of the results of the inverse problem. We apply these techniques first to an aluminum plate for which material parameters are known to test the performance and efficiency of the method. We then apply the method to a composite plates to determine not only these parameters, but also the layup angle. The inverse problem successfully estimates the Young's modulus and damping for the aluminum material. In addition, the vibration analysis produces consistent resonance frequencies for the first two modes for both theoretical and experimental data. However, carbon fiber plates present challenges due to limitations of the Kirchoff-Love plate theory used as the underlining theoretical model for the finite element approximation used in the inverse problem, resulting in a persistent mismatch of resonance frequencies in experimental data. / Graduation date: 2012