KOMA Omtenta - förklaringar

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• Crystal structure : the manner in which atoms, ions, or molecules are arranged in a material. Most common metals exist in at least one of three relatively simple crystal structures: Face-centred cubic (FCC), Body-centred cubic (BCC), or Hexagonal close-packed (HCP). In ceramics, crystal structure is determined by the charge magnitude on each ion and the radius of each type of ion
• Polymer molecules Most polymeric materials are composed of very large molecular chains with side groups of various atoms (O, Cl, etc.) or organic groups such as methyl, ethyl, or phenyl groups.Differences in spatial arrangements of these side atoms or groups of atoms lead to isotactic, syndiotactic, and atactic stereoisomers. When a repeat unit contains a double chain bond, both cis and trans geometric isomers are possible
• Point defects Point defects are those associated with one or two atomic positions; these include vacancies (or vacant lattice sites) and self-interstitials (host atoms that occupy interstitial sites)
• Diffusion mechanisms . Vacancy diffusion occurs via the exchange of an atom residing on a normal lattice site with an adjacent vacancy. For interstitial diffusion, an atom migrates from one interstitial position to an empty adjacent one.
• Strain hardening Strain hardening is the enhancement in strength (and decrease of ductility) of a metal as it is deformed plastically. Degree of plastic deformation may be expressed as percent cold work. Yield strength, tensile strength, and hardness of a metal increase with increasing percent cold work; ductility decreases
• For thermoplastic polymers, cite five factors that can contribute to the scatter of fatigue life data Five factors that lead to scatter in fatigue life data are (1) specimen fabrication and surface preparation, (2) microstructure characteristics, (3) specimen alignment in the test apparatus, (4) variation in mean stress, and (5) variation in test cycle frequency
• Eutectoid reaction solid-state transformation when one solid phase transforms isothermally into two other solid phases upon cooling (e.g. α→β+γ).
• Continuous cooling transformation (CCT) diagram valid only for constant-temperature heat treatments and permit determination of times at which a phase transformation begins and ends. CCT diagrams make possible the prediction of microstructural products for specified heat treatments.
• Glass transition temperature a liquid into a rubbery material and finally into a rigid solid
• Glass-ceramics Glass-ceramics are initially fabricated as glasses and then, by heat treatment, crystallized to form fine-grained polycrystalline materials. Two properties of glass-ceramics that make them superior to glass are improved mechanical strengths and lower coefficients of thermal expansion (which improves thermal shock resistance).
• Hardenability parameter used to determine the influence of composition to form martensite structure for special heat treatment
• Structural composites : laminar composites and sandwich panels. Laminar composites are composed of a set of two-dimensional sheets that are bonded to one another. Sandwich panels consist of two strong and stiff sheet faces that are separated by a core material or structure
• Thermal stress are generated by the rapid heating or cooling of a body of material leading to temperature gradients between the outside and interior portions and Page 7 of 10 accompanying differential dimensional changes
• Compare the dependencies of Elastic Modulus and Tensile Strength on chemical composition, atomic bonding and microstructure in solids Elastic Modulus depends on chemical composition and atomic bonding, is independent of other microstructure characteristics in solids such as grain size. Tensile Strength depends on all three aforementioned characteristics: chemical composition, atomic bonding and microstructure
• Which elements can be used as interstitial atoms in interstitial-type solid solutions? Describe reasons, mechanism of formation and possibilities. Only four elements from the top of the periodic table can be used in interstitial-type solid solutions. These are hydrogen (H), Boron (B), Carbon (C) and Nitrogen (N). The prime reason for that is the atom sizes
• Compare fracture mechanisms between ductile metals and polymers Ductile metals demonstrate significant amount of plastic deformation before fracture. In polymer materials, ductile fracture is more common for thermoplastic polymers
• Compare the possibilities of controlling structural properties in single-phase crystalline materials to those in polymers. metals, structural properties can be controlled through solid-solutioning, controlling grain size during solidification as well as plastic deformation processing and heat treatment for recovery / recrystallisation / grain growth. In polymers, the control of structural properties can be achieved through the control of molecular weight, degree of crystallinity and pre-deformation by drawing. A distinct difference in the control of structural properties in polymers is that heat treatment for annealing may lead to the increase of strength and decrease in ductility. These trends are opposite to metals and are attributed to the increase of percent crystallinity, crystallite size, perfection, and the modifications of spherulite structure.
• What makes carbon unique? Carbon is unique from many perspectives and therefore does not belong to a particular material category. Its ability to form both sp3 and sp2 hybridizations leading to allotropic transformations as well as to exist in amorphous make it unique. Former hybridization leads to the formation of diamond structure, which is similar to the zinc blende structure of ceramic materials. The later hybridization leads to the formation of graphite structure, which is crystalline and has many properties in common with metals. An atomic-level monolayer of the later is called graphene and may also lead to the formation of carbon nano-tubes and fullerenes. Finally, small-diameter high-strength and high-modulus fibres can be made of carbon. Their structure is based on graphene monolayers in a large variety of arrangements. They are used as reinforcements fibres in high-performance polymermatrix composites, similar to ceramic glass and polymer fibre composites.
• Describe the unique advantages of each composite material type. Polymer-matrix composites (PMCs) are designed for high stiffness and yield strength. Metal-Matrix Composites (MMCs) are primarily designed for increased service temperatures. Ceramic-Matrix Composites (CMCs) are design for increased fracture toughness

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