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This thesis describes the processing and characterization of uniform, interpenetrating, nickel-cubic stabilized zirconia composites. This work was performed in support of future development in the study and modelling of Metal-Ceramic (M/C) Functionally Graded Materials. Since it is of fundamental interest to understand the relationship between microstructure and behaviour in these materials, the characterization has focused on uniform composites. To minimize experimentation while maximizing productivity, the material grades selected for this study include M:C ratios by volume of 25:75, 50:50, and 75:25, along with the elemental materials (i.e. 100% nickel and 100% cubic zirconia). Solid state powder processing was developed including the steps: colloidal processing (by tape casting), lamination, organic burnout, hydrogen reduction of metal oxides, and hot press consolidation. These techniques support the even distribution of phases and yield high density composites (>98% of theoretical). Unfortunately the nickel powder used in this study produced atypical properties which complicated the analysis of mechanical behaviour. Composite thermal dilation, elastic, and mechanical properties were determined. Through analysis of residual stresses and fracture behaviour ideas regarding the modelling of M/C composites and FGMs are addressed. Continuum modelling approaches seem appropriate. / Thesis / Master of Engineering (ME)

This dissertation presents a new method for obtaining fully anisotropic stiffness tensor for materials containing discontinuous curverd fibers. It is demonstrated that the definition of fiber configuration and configuration averaging allow us to obtain better match with experimental results when compared to theory relying on the assumption of straight fibers. The experimental results are obtained using novel X-ray micro-tomography setup allowing observation of material microstructure under load. / Ph. D.

In this work, the thermo-mechanical response and failure of loaded, fire-exposed composite structures was studied. Unique experimental equipment and procedures were developed and experiments were performed to assess the effects of mechanical loading and fire exposure on the service life of composite beams. A series of analytical models was assembled to describe the fire growth and structural response processes for the system used in the experiments. This series of models consists of a fire model (to predict the heat flux to the fire-exposed beam), a thermal response model (to calculate the temperature distribution within the beam due to this heat flux), a stiffness-temperature model (to calculate the loss in stiffness at elevated temperatures), a mechanical response model (to compute the strain distribution within the loaded beam), and a material failure model (to calculate the strain at which the beam is expected to fail). Each of these models is independently validated by comparing predictions with experimental results. The models are then used to predict the times-to-failure for beams over a range of fire and loading conditions. The predicted failure times agree fairly well with experimental results, but it is expected that the agreement could be improved with improvements to the first model in the series - the fire model. / Master of Science

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Recent work in the area of composite laminates has focused on the characterization of the strength of laminates constructed from very thin plies. Interlaminar shear and normal stress components have been shown to be concentrated on the edges, the so-called edge effect, of unidirectional laminates at the interface between plies of different fiber orientation. Research has shown that decreasing ply thickness can reduce these interlaminar stress edge effects, and delay delamination in quasi-isotropic laminate specimen for laminates of equal total thickness. First ply failure stress has also been shown to increase with decreasing ply thickness. For these reasons, there has been a great deal of interest in laminated composites constructed from very thin plies. This work studies the impact of manufacturing tolerances on ply orientation on the mechanical properties of the constructed laminate. Direct Monte Carlo simulation is used to model the variance introduced in the manufacturing process. First-order variance-based sensitivity analysis using a local analysis of variance technique is used to study the contribution of each individual ply to the variation in as built mechanical properties. Variation in mechanical properties of thick-ply and thin-ply laminate designs are compared to study if thin-ply laminate designs show more or less variation than their thick-ply counterparts. This work has found potential impacts of ply angle variation on variance of as-built stiffness in laminates of different ply thicknesses. These differences are attributable to the total ply count in a laminate. For a fixed height laminate, the ply count is inversely proportional to thickness, yielding the apparent benefit of thin plies. Using thinner plies in a sub-laminar stacking arrangement, repeating a sublaminate instead of repeating plies, reduces sensitivity to manufacturing errors and would suppress tranverse failure modes. / Master of Science / Carbon fiber reinforced polymer composites, a material consisting of carbon fiber filaments bound within a polymer matrix, are commonly used in aerospace applications for their excellent strength to weight ratio. This class of materials is highly tailorable, with strength and stiffness controlled by the number of fiber layers, their thickness, and each layer's respective orientation. Variability in these characteristics arising from manufacturing processes can result in changes in the laminate's engineering properties. This work shows that characterizing the impacts to the engineering properties through Monte-Carlo simulation of variability in the orientation is possible. A Monte-Carlo simulation is a type of statistical simulation where a sample population is generated using an assumed mean and standard deviation. Engineering and statistical analyses can then be performed on this sample population to determine the variability in the engineering properties of the population. In addition, the variability in the population can be studied as a function of each individual fiber layer to understand individual impacts based on orientation and position within the larger composite. Using these analysis techniques presented in this work allows for the study of laminate variability prior to manufacturing, allowing engineers to better understand the material during the design of complex aerospace structures.

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In this thesis, a novel in-situ method for incorporating nanoscale ceramic particles into metal has been developed. The ceramic phase is introduced as an organic-polymer precursor that pyrolyzes in-situ to produce a ceramic phase within the metal melt. The environment used to shield the melt from burning also protects the organic precursor from oxidation. The evolution of volatiles (predominantly hydrogen) as well as the mechanical stirring causes the polymer particles to fragment into nanoscale dispersions of a ceramic phase. These “Polymer-based In-situ Process-Metal Matrix Composites” (PIP-MMCs) are likely to have great generality, because many different kinds of organic precursors are commercially available, for producing oxides, carbides, nitrides, and borides. Also, the process would permit the addition of large volume fractions of a ceramic phase, enabling nanostructural design, and production of MMCs with a wide range of mechanical properties, meant especially for high temperature applications. An important and noteworthy feature of the present process, which distinguishes it from other methods, is that all the constituents of the ceramic phase are built into the organic molecules of the precursor (e.g., polysilazanes contain silicon, carbon, and nitrogen); therefore, a reaction between the polymer and the host metal is not required to produce the dispersion of the refractory phase. The polymer precursor powder, with a mean particle size of 31.5 µm, was added equivalent to 5 and 10 weight % of the melt (pure magnesium) by a liquid metal stir-casting technique. SEM and OM microstructural observations show that in the cast structure the pyrolysis products are present in the dendrite boundary region in the form of rod/platelets having a thickness of 100 to 200 nm. After extrusion the particles are broken down into fine particles, having a size that is comparable to the thickness of the platelets, in the 100 to 200 nm range, and are distributed more uniformly. In addition, limited TEM studies revealed the formation of even finer particles of 10-50 nm. X-ray diffraction analysis shows the presence of a small quantity of an intermetallic phase (Mg2Si) in the matrix, which is unintended in this process. There was a significant improvement in mechanical properties of the PIP-MMCs compared to the pure Mg. These composites showed higher macro-and micro-hardness. The composite exhibited better compressive strength at both room temperature and at elevated temperatures. The increase in the density of PIP-composites is less than 1% of Mg. Five weight percent of the precursor produced a two-fold increase in the room-temperature yield strength and reduced the steady state creep rate at 723 K by one to two orders of magnitude. PIP-MMCs showed higher damping capacity and modulus compared to pure Mg, with the damping capacity increasing by about 1.6 times and the dynamic modulus by 11%-16%. PIP-composites showed an increase in the sliding wear resistance by more than 25% compared to pure Mg.

In recent times, composite materials have gained mainstay acceptance as a structural material of choice due to their tailorability and improved thermal, specific strength/stiffness and durability performance. Carbon-Carbon (C/C) composites are used for high temperature applications such as exit nozzles for rockets, leading edges for missiles, nose cones, brake pads etc. Mechanical property estimation of C/C composites is challenging due to their highly heterogeneous microstructure. Computed Tomography (CT) images (volumetric imaging) coupled with Scanning Electron Microscopy (SEM) reveal a highly heterogeneous microstructure comprised of woven C-fibers, amorphous C-matrix, irregularly shaped voids, cracks and other inclusions. The images also disclose structural hierarchy of the C/C composite at different length scales. Predicting the mechanical behavior of such complex hierarchical materials like C/C composites forms the motivation for the present work. A systematic study to predict the effective mechanical properties of C/C Composite using numerical homogenization has been undertaken in this work. The Micro-Meso-Macro (MMM) principle of ensemble averages for estimating the effective properties of the composite has been adopted. The hierarchical length scales in C/C composites has been identified as micro (single fiber with matrix), meso (fabric) and macro (laminate). Numerical homogenization along with periodic boundary conditions (PBCs) have been used to estimate the effective engineering properties of the material at different length scales. Concurrently, mechanical testing has also been carried out at macro (compression tests) and micro scale (using nano-indentation studies) to characterize the mechanical behavior of C/C composites.