Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.

The fabrication and mechanical properties of Ti2AlC composites reinforced with the alumina oxide fibers, such as NextelTM 720 and ALBF1, were described in this thesis. Alumina fibers and Ti2AlC powders were dispersed in the water and slip cast in the molds to form green bodies. Sedimentation test were carried out to optimize pH of the slurry. It was found that suspensions prepared with PAA as a dispersant and has an excellent stability in the pH range of 4 ~ 5. Composite green bodies were densified by pressureless sintering or hot isotatic pressing (HIP) at different temperatures. The microstructure of fabricated samples was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and porosimetry. It was found out that HIPing at 1300 oC for 4 hrs at 100 MPa results in almost fully dense composites with majority phases being alumina fibers and Ti2AlC. However, fully dense Ti2AlC composites could not be obtained by the pressurless sintering, even at temperature as high as 1400 oC at which reaction between Ti2AlC and NextelTM 720 was observed. The double torsion (DT) tests were carried out at room temperature to measure the fracture toughness of the HIPed pure and 5vol% alumina fiber reinforced Ti2AlC. DT results showed increase in the fracture toughness of Ti2AlC reinforcing with NextelTM 720 alumina fibers. However, fracture toughness of the samples reinforced with ALBF1 was lower than that of pure Ti2AlC because of the low relative densities of those composites. SEM study of the fracture surfaces after DT tests showed that toughening mechanisms by crack bridging and fiber pull outs at the crack tip are operative in all reinforced samples. In addition, elastic moduli of HIPed Ti2AlC measured by Resonant Ultrasound Spectroscopy (RUS) do not show significant change due to reinforcement with alumina fibers, while the Vickers hardness of composites was found to be larger for Ti2AlC reinforced with NextelTM 720 and lower for the samples reinforced with ALBF1.

Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.

Metal carbides have attracted much attention over the past several years due to their unique qualities. The purpose of this research is to develop a cermet that demonstrates desired properties of nano-size titanium carbide (TiC) and copper-nickel (Cu-Ni) metals. In this study stoichiometric, nano-size TiC was synthesized through a patented carbothermal synthesis process (U.S. Patent No.: 5,417,952). The resulting TiC was sintered with varying copper (12.5-37.5wt %) and nickel (12.5-25wt %) contents. Hardness, fracture toughness, and microscopy studies were performed. Average hardness ranging between 325-1292 HV were found, with copper content showing a strongly negative correlation with hardness. Fracture toughness values were found to be between 4.85-14.36 Mpa*m^.5; TiC content had a nearly linear, negative correlation with fracture toughness. Samples with high copper to nickel ratios showed poor homogeneity and wetting.

Made available in DSpace on 2014-06-11T19:34:59Z (GMT). No. of bitstreams: 0 Previous issue date: 2002-10Bitstream added on 2014-06-13T18:45:52Z : No. of bitstreams: 1 graca_mla_dr_guara.pdf: 5050152 bytes, checksum: dce8c01ce72d2ebc0e9caea4d6df5d6b (MD5) / Neste trabalho foi feita uma análise detalhada dos micromecanismos de iniciação da fratura em amostras entalhadas para cinco aços e duas ligas de alumínio. Com esse objetivo foram obtidas curvas de transição frágil-dúctil e de tenacidade à fratura em função do raio da raiz do entalhe, e realizados ensaios interrompidos antes da fratura da amostra. Análises fractográficas e micrográficas das regiões de iniciação das fraturas foram realizadas por microscopia eletrônica de varredura. A variação dos micromecanismos de iniciação em função da variação da capacidade plástica local na raiz dos entalhes, como induzida pela variação da temperatura de ensaio e pela variação do raio da raiz, foi analisada. De um modo geral, três tipos de micromecanismos de iniciação foram observados. Um frágil, em que a iniciação envolve a nucleação de uma microtrinca à frente do entalhe e sua subsequente propagação instável. Dois dúcteis, um pela ruptura por cisalhamento localizado ao longo de linhas de cisalhamento máximo formadas na raiz do entalhe, e outro pela formação de microcavidades cuja ligação entre si e a ponta do entalhe envolve um processo misto de cisalhamento localizado e de coalescência de microcavidades. Aspectos de modelos que relacionam tenacidade com a microestrutura foram discutidos com base nos micromecanismos observados. / In this study a detailed analysis of the micromechanisms of the fracture initiation in notched specimens was made, for five steels and two aluminum alloys. With that purpose brittle/ductile transition and fracture toughness x r1/2 curves were obtained, and interrupted tests before the fracture of the sample were used. Fractographic and micrographic analysis of the fracture initiation areas were accomplished by scanning electron microscopy. The variation of the initiation micromechanisms in function of the variation of the local plastic capacity in the notch root, as induced by the variation of the test temperature and by the variation of the notch root radius, was analyzed. In a general way, three types of initiation micromechanisms were observed. A brittle one, where the initiation involves the microcrack nucleation ahead of the notch and its subsequent unstable propagation. Two ductile, one by localized shear rupture along the maximum shear lines formed in the notch root, and other by microvoids nucleation whose link to each other and the notch tip involves a mixed process of localized shear and microcavoid coalescence. Aspects of relating models of fracture toughness with microstructure were discussed, based in the observed micromechanisms.

No description available.

Cordierite was adhered to molybdenum using various metallic interlayers of copper, nickel, and chromium. The development of a coating adhesion test methodology was required to choose between interface designs. An indentation method was chosen because of ease in testing and availability of fracture mechanics interpretations of test data. The interfacial fracture toughness was determined from indentation load vs. crack length data by examining the residual stress and critical buckling load of the ceramic coatings. The interfacial fracture toughness values obtained using a slightly different indentation analysis agree with those in the literature. Quantitative chemical analysis of the interface microstructure was used to explain differences in interfacial fracture toughness values for samples with different metallic interlayer designs. The best interface design for adhering cordierite glass-ceramic coatings to molybdenum was found to be molybdenum / 2 μm copper / 4 μm chromium / cordierite. / Master of Science

The primary objective of this study is to test the applicability to plastics of a fracture toughness testing tool developed for metals. The intent is to study pre-test conditioning of several plastic materials and the effect of the depth of the razor notch cut in the chevron notched fracture toughness test specimens. The study includes the careful preparation of samples followed by conditioning in various environments. Samples were subjected to laboratory air for a specific duration or to a controlled temperature-humidity condition as per the ASTM D1870. Some of the samples were subjected to vacuum conditioning under standard test specifications. Testing was conducted using the conventional three-point bend test as per ASTM D5045-95. ASTM E1304, which sets a standard for short rod and bar testing of metals and ceramics provides some basis for conducting chevron notched four-point bend tests to duplicate the toughness tool. Correlation of these results with the ASTM test samples is determined. The four-point bend test involves less specimen machining as well as time to perform the fracture toughness tests. This study of fracture toughness testing has potential for quality control as well as the fracture property determination.

The present study investigates microcracking and damage progression in IM7/977-2, IM7/5555, and IM7/5276-1 [0/90/90/0] laminates. For each material system, seven to eight small coupons were axially loaded in a tensile substage. At increments of around 50 MPa the surfaces of the specimens were inspected via optical microscopy so that a history of microcracking damage as a function of applied loading could be charted. In the IM7/977-2 laminates microcracks were found to initiate on average at around 1050MPa; microcracking initiation for the other two systems was around 850 to 900 MPa. Also, the IM7/977-2 system displayed a steeper increase in crack density as a function of applied loading than the other two systems. The IM7/5555 system was the only system that achieved a microcracking saturation density; the saturation density was found to be around 17 cracks per centimeter. While the IM7/977-2 and IM7/5276-1 systems typically broke into two pieces at failure, the IM7/5555 specimens shattered into pieces. In addition, delaminations were observed in a majority of the IM7/5555 specimens at loadings 250MPa under the failure loads.

This study reports a novel and simple technique for successfully coating multi-walled carbon nanotubes (MWCNTs) on to the surface of carbon fibre (CF) fabric for the production of multi-scale CF-epoxy composites. Initially, epoxy composites with multi-scale reinforcement were produced by resin infusion (RI) using woven CF fabric coated with a dispersion of 1 wt. % MWCNTs in an epoxy binder of low molar mass. The effects of this reinforcement on the CF-epoxy interface with MWCNTs was studied in mode I and mode II interlaminar fracture toughness (ILFT) using double-cantilever beam (DCB) and 4 point end-notch flexure (4ENF) tests, respectively. Relative to an equivalent composite reinforced with non-coated CF reinforcement, the binder/MWCNTs coating increased significantly the ILFT of the CF-epoxy composite; in mode I by 105% and in mode II by 50%. This increase in ILFT was attributed to two main effects: Firstly, the binder alone (without MWCNTs), which has a much lower glass transition temperature (Tg) than that the matrix (45 vs. 140 °C), hindered crack propagation and increased the ILFT of the epoxy matrix by 25% for mode I and 15% for mode II; Secondly, the energy absorbing mechanisms of MWCNTs during fracture particularly pull-out and crack bridging. However the Tg of the matrix epoxy of the multi-scale composites was reduced to 118 °C compared to 140 °C, for the unmodified composite, due to phase mixing with the low Tg binder. For RI processing, the CF volume fraction of the composites prepared using coated CF was ≈50% compare to at ≈55% for the composite with non-coated CF. Curing agents were added to the binder, which not only increased the Tg from ≈50 °C to ≈100 °C, but also increased the Tg of the matrix epoxy of the multi-scale composites to 154 °C. Relative to an equivalent composite reinforced with non-coated CF reinforcement, the curable-binder/MWCNTs coating increased the ILFT of the CF-epoxy composite; in mode I by 120% and in mode II by 90%. A hybrid RI-hot press (HP) process was used to prepare CF-epoxy composites from coated fabrics with CF volume fractions of ≈55%. The damping curves for the HP-composites consisted of a β-peak, due to the formation of a third mixed phase, in addition to a γ-peak (assigned to the Tg of the binder) and an α-peak (assigned to the Tg of matrix epoxy). The β-peak, and the uniformly distributed nodular particles observed on the fracture surface of the matrix, by SEM, for HP-composites, are indicative of the formation of mixed-phase particles due to reaction induced phase separation (RIPS). Relative to an equivalent RI-composite, the curable-binder/MWCNTs treatment increased the ILFT of the CF-epoxy multi-scale composite; in mode I by 134% and in mode II by 15% for HP-composites. Impact test results showed that HP-composites absorbed more energy, due to CF fracture, compared to equivalent RI composites, which showed larger delamination areas after 5 J and 10 J impact. The out-of-plane electrical conductivity and thermal conductivity of the HP-composite with CF coated with curable-binder/MWCNTs was increased by ≈38% and ≈50%, respectively, compared to the composite with non-coated CF, indicating formation of MWCNTs networks in the matrix rich areas of the multi-scale composite.