Cheat Sheets - Summary Engineering Sciences PDF

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CERAMICS Kaolinite - consists of layered pairs i.e. one layer of (Si2O5)2- and one layer of Al2(OH)4 2+, strongly bonded to each other. However - the bonds between layers is poor - so interlayer cleavage (shear between layers) is easy and hence slippage between layers is easily achieved. This slippage means that it can be used as a solid lubricant. Porosity = There are equations that show an intense dependency of strength and stiffness on the volume fraction of porosity present. Porosity acts as mechanical holes in the material, thus effectively reducing its cross-sectional area. This porosity can be controlled by allowing longer for the intergranular diffusion process (sintering) to occur, or by raising the firing (sintering) temperature and pressure. E=E0(1 - 1.9P + 0.9P2) - now when P=0.05, E=310MPa so E0 can be calculated as 341.7MPa therefore - the stiffness at zero porosity is 341.7MPa and the stiffness at 10% porosity (P=0.1) can be calculated as 280MPa. COMPOSITES Why can most engineering alloys be described as composite materials? Composites by definition, are materials that contain 2 or more constituents, and the properties of these constituents contribute to the property portfolio of the material as a whole. Alloys by definition, are materials that contain 2 or more constituents, and the properties of these constituents contribute to the property portfolio of the material as a whole! What is the main difference between a particle toughened composite and a dispersion strengthened composite? Particles tend to be on a micro/macro level (sizewise), such as carbon black, alumina and stone aggregate. Dispersions tend to be sub-micron in size, such as precipitation zones in metal alloys. This manifests itself in the essential strengthening mechanisms, that have to be treated on a molecular level with dispersion-strengthened composites, and must therefore include effects of atomic species, lattice parameters etc. Also – dispersion composites gain their strength from a subsequent heat treatment, and this strength drops off at elevated usage temperatures. The strengthening mechanisms in particulate composites are permanent and require no specialised initiation process. There are 3 types of fibrous reinforcement; what are they? Give two examples of each. Fibre carbon, glass, KEVLAR, alumina etc. Whisker - SiC, SiN, alumina, graphite etc. Wires - steel, molybdenum, tungsten etc. ZrO2 based composites are the best example of toughened ceramic composites. Why are they so good at preventing crack growth? The local change in crystal structure that characterizes PSZ. If PSZ particles are dispersed in a ceramic matrix, along with a stabilizer such as MgO or CaO, transformation toughening can occur. This is where the particles act as crack arresters when contacted by a crack, absorbing the associated crack tip energy. This renders the material tougher. For the following constituents, calculate the longitudinal and transverse stiffness of a continuous fibre composite of fibre volume fraction 0.6: Carbon fibre - stiffness = 130GPa, Epoxy resin - stiffness = 3GPa. Ef = 130GPa Em = 3GPa Vf = 0.6 longitudinal modulus = EmVm + EfVf =3(0.4)+130(0.6)=79.2GPa Transverse modulus = EmEf / (VmEf + VfEm) = 3(130) / (0.4(130)+0.6(3)) = 14.61GPa OPTICAL PROPERTIES Aluminium oxide can be totally transparent, translucent or totally opaque. What factors determine which optical form this ceramic will take? Porosity, and crystallinity mainly. The more holes or grain boundaries there are, the more potential scattering sites and the less likelihood there is of the light getting through. What gives ruby, emerald and sapphire their distinctive colours? Their purity! e.g. ruby consists of a small amount of chromium ions that optically are red, similarly, copper etc. in emerald, and cobalt etc. in sapphire, manganese in amethyst etc. Zinc selenide has a band gap of 2.58eV. Over what range of wavelengths of visible light is it transparent? Zinc selenide has a band gap of 2.58eV. This means that only photons having energies of 2.58eV or greater are absorbed by valence-band-toconduction-band electron transitions. Thus, photons having energies less than 2.58eV are not absorbed. The minimum photon energy for visible light is 1.8eV (equation 21.16) which corresponds to a wavelength of 0.7um. The wavelength of a photon having an energy of 2.58eV is merely...