User:Raleway

Raleway
Type	Chemistry
Category	Lab
Event Information
Latest Appearance	2017
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2017 2014 2013
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2017

Materials Science tests knowledge of the properties and characteristics of metals, ceramics, polymers and composite materials, with a focus on material characterization techniques, intermolecular forces, and surface chemistry.

Event Overview

Rules Review

Each pair of competitors from a team will be allowed 5 front and back pages of cheat sheets. There will a lab section unless listed otherwise, and students must bring safety apparel themselves or be unable to do the lab due to safety concerns. Students will be tested on the field of Materials Science and topics are limited to those listed below, though the scope of each topic is nearly unlimited.

Tips and Tricks

Materials Science covers a vast amount of knowledge, hence the 5 cheat sheets allowed for the event. The extent of each topic is nearly unlimited, such that teams will need to know each topic well without relying on cheat sheets. Graphs are especially important, but only so long as they are useful - too many basic graphs will take up valuable space. Also, cheat sheets can contain large tables of information that would otherwise be impossible to memorize, such as atomic packing for different elements and compounds, melting and boiling points, and more. Many teams have found an optimal layout of 0.5" margins, 3 columns, and Times New Roman font. The font size will depend largely on the amount of information contained, but teams should be cautious to still keep their cheat sheets readable. The best competitors will know their cheat sheets extraordinarily well, such that they can find small bits of information quickly. Less competitive teams will either rely on their cheat sheets for basic information or rely on the find function during practice and become lost in actual tests. Lastly, screen protectors are incredibly useful for protecting cheat sheets from any damage or spills in the lab section of the event.

Physical Characteristics

Metals

Mechanical properties: Metals are stiff, strong, ductile, malleable, resistant to fracture, good conductors of electricity and heat, and are lustrous. Many metals, such as iron, cobalt, and nickel, also have good magnetic properties.

Diamagnetism: Diamagnetism is present in all materials. An applied magnetic field induces another magnetic field in the opposite direction, providing a repulsive force. Diamagnetic materials could be considered non-magnetic, as diamagnetism is negligible compared to other, stronger forms of magnetism.

Paramagnetism: Paramagnetism exists due to unpaired electrons in some elements and compounds, which generate a magnetic dipole due to their spin. Paramagnets do not retain magnetization unless they are in the presence of an applied magnetic field.

Ferromagnetism: Ferromagnetism also exists due to unpaired electrons, but works in a different manner. Magnetic moments align themselves parallel to each other and to an applied magnetic field in order to maintain a lowered state. These magnetic moments align even in the absence of an applied magnetic field. Every ferromagnetic material has a Curie temperature, above which it loses any ferromagnetic properties.

Ceramics

Ceramics are relatively stiff and strong, hard, brittle, susceptible to fracture, insulators of heat and electricity, and are very resistant to high temperatures and harsh conditions. Some ceramics (glass ceramics) can be transparent or translucent, while others (clay ceramics) are opaque. Some oxide ceramics also exhibit magnetic behavior.

Polymers

Mechanical properties: Polymers are low density, but not as stiff or strong as metals or ceramics. They are extremely ductile and pliable, chemically inert in a multitude of environments, nonmagnetic (diamagnetic), and usually are not conductive (polymers with a backbone of alternative double and single bonded carbon can conduct electricity through the movements of free electron pairs). Polymers soften or decompose at modest temperatures.

Degree of polymerization: The degree of polymerization, or DP, is defined as the number of monomer units in a macromolecule or polymer molecule. A homopolymer contains only one type of monomer unit, and the number-average degree of polymerization is given by [math]M_n/M_0[/math] where M(n) is the number-average molecular weight and M(0) is the molecular weight of the monomer unit.

Composites

The properties of composites are entirely dependent on their constituent parts and are usually enhanced by fiber lengths. Strength is dependent on fiber length, where smaller fibers are larger. Ductility, meanwhile, can be lowered by using matrices instead of fibers. Ceramic composites are resistant to degradation, but are brittle; metal matrices are used at high temperatures. Construction also matters. Carbon-carbon composites, which reinforce graphite with fibers, have high tensile strength, resistance to creep, fracture toughness, strength, and thermal conductivity; laminar composites, meanwhile, have high strength in the 2D plane only; sandwich panel cores have low elasticity, light weight, and high shear strength, while their sheets resist tension and compression.

Manufacturing Techniques and Natural Occurrences

Metals

Natural Occurrences

Native State:

Metals are said to be in their native state if they are found in their elementary form. Generally, less active metals like gold, silver, copper, and platinum can be found in their native state.

Combined State

Metals are said to be in their combined state if they are found in nature in the form of compounds. Generally, reactive metals occur in their combined state. Combined state metals are found in the crust of the earth as oxides, carbonates, sulfides, silicates, phosphates, etc.; these are known as minerals. Minerals extracted from the crust of the earth are not pure, but instead associated with many earthly, rocky, and siliceous impurities. These impurities associated with minerals are collectively known as the gangue or matrix, which is commercially valueless. Not all minerals can be used, depending on economic factors or the extraction process. Minerals from which metals can be conveniently and economically extracted are known as ore. For example, aluminum can be found naturally in two minerals, both bauxite and China clay. However, extraction of aluminum is much cheaper and easier from bauxite than from China clay, so bauxite is considered an ore of aluminum while China clay is not. Similarly, copper can be found as copper glance, cuprite, malachite, and copper pyrite, but only copper pyrite is considered to be an ore of copper. All ores are minerals, but not all minerals are considered to be ores.

Mining

Metals are often extracted from Earth by means of mining ores that are rich sources of requisite elements, such as bauxite. Ore is located by prospecting techniques, followed by exploration and examination of deposits. Mineral sources are generally divided into surface mines, which are mined by excavation using heavy equipment, and subsurface mines. Once Ore is mined, metals must be extracted, usually by chemical or electrolytic reduction. Pyrometallurgy uses high temperatures to convert ore into raw metals, while hydrometallurgy employs aqueous chemistry for the same purpose.methods used depend on metal and their contaminants. When a metal ore is an ionic compound of that metal and a nonmetal, the ore must usually be smelted — heated with a reducing agent — to extract the pure metal. Many common metals, such as iron, are smelted using carbon as a reducing agent. Some metals, such as aluminum and sodium, have no commercially practical reducing agent and are extracted using electrolysis instead. Sulfide ores are not reduced directly to metal but are roasted in air to convert them to oxides.

Forming Techniques

Hot Working:

Hot working is when a metal is deformed at a temperature higher than its temp of recrystallization Forging: Forging is mechanically working or deforming a single piece of a normally hot metal; this may be accomplished by the application of successive blows or by continuous squeezing. Forgings are classified as either closed or open die. For the closed die, a force is brought to bear on two or more die halves having the finished shape such that the metal is deformed in the cavity between them. For the open die, two dies having simple geometric shapes (e.g., parallel flat, semicircular) are employed, normally on large workpieces. Forged articles have outstanding grain structures and the best combination of mechanical properties. Wrenches, and automotive crankshafts and piston connecting rods are typical articles formed using this technique.

Rolling:

Rolling, the most widely used deformation process, consists of passing a piece of metal between two rolls; a reduction in thickness results from compressive stresses exerted by the rolls. Cold rolling may be used in the production of sheet, strip, and foil with a high-quality surface finish. Circular shapes, as well as I-beams and railroad rails, are fabricated using grooved rolls. Extrusion: For extrusion, a bar of metal is forced through a die orifice by a compressive force that is applied to a ram; the extruded piece that emerges has the desired shape and a reduced cross-sectional area. Extrusion products include rods and tubing that have rather complicated cross-sectional geometries; seamless tubing may also be extruded.

Drawing:

Drawing is the pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side. A reduction in cross section results, with a corresponding increase in length. The total drawing operation may consist of a number of dies in a series sequence. Rod, wire, and tubing products are commonly fabricated in this way.

Casting Techniques

Casting is when a heated metal is poured into a mold in order to achieve a desired shape.

Sand Casting:

Sand Casting With sand casting, probably the most common method, ordinary sand is used as the mold material. A two-piece mold is formed by packing sand around a pattern that has the shape of the intended casting. Furthermore, a gating system is usually incorporated into the mold to expedite the flow of molten metal into the cavity and to minimize internal casting defects. Sand-cast parts include automotive cylinder blocks, fire hydrants, and large pipe fittings.

Die Casting:

In die casting, the liquid metal is forced into a mold under pressure and at a relatively high velocity and allowed to solidify with the pressure maintained. A two piece permanent steel mold or die is employed; when clamped together, the two pieces form the desired shape. When complete solidification has been achieved, the die pieces are opened and the cast piece is ejected. Rapid casting rates are possible, making this an inexpensive method; furthermore, a single set of dies may be used for thousands of castings. However, this technique lends itself only to relatively small pieces and to alloys of zinc, aluminum, and magnesium, which have low melting temperatures.

Investment Casting:

For investment (sometimes called lost wax) casting, the pattern is made from a wax or plastic that has a low melting temperature. Around the pattern is poured a fluid slurry, which sets up to form a solid mold or investment; plaster of Paris is usually used. The mold is then heated, such that the pattern melts and is burned out, leaving behind a mold cavity having the desired shape.This technique is employed when high dimensional accuracy, reproduction of fine detail, and an excellent finish are required—for example, in jewelry and dental crowns and inlays. Also, blades for gas turbines and jet engine impellers are investment cast.

Lost Foam Casting:

A variation of investment casting is lost foam (or expendable pattern) casting. Here the expendable pattern is a foam that can be formed by compressing polystyrene beads into the desired shape and then bonding them together by heating. Alternatively, pattern shapes can be cut from sheets and assembled with glue. Sand is then packed around the pattern to form the mold. As the molten metal is poured into the mold, it replaces the pattern which vaporizes. The compacted sand remains in place, and, upon solidification, the metal assumes the shape of the mold. With lost foam casting, complex geometries and tight tolerances are possible. Furthermore, in comparison to sand casting, the lost foam is a simpler, quicker, and less expensive process, and there are fewer environmental wastes. Metal alloys that most commonly use this technique are cast irons and aluminum alloys; furthermore, applications include automobile engine blocks, cylinder heads, crankshafts, marine engine blocks, and electric motor frames.

Continuous Casting:

At the conclusion of extraction processes, many molten metals are solidified by casting into large ingot molds.The ingots are normally subjected to a primary hot-rolling operation, the product of which is a flat sheet or slab; these are more convenient shapes as starting points for subsequent secondary metal-forming operations (i.e., forging, extrusion, drawing). These casting and rolling steps may be combined by a continuous casting (sometimes also termed “strand casting”) process. Using this technique, the refined and molten metal is cast directly into a continuous strand that may have either a rectangular or circular cross section; solidification occurs in a water-cooled die having the desired cross-sectional geometry. The chemical composition and mechanical properties are more uniform throughout the cross sections for continuous castings than for ingot-cast products. Furthermore, continuous casting is highly automated and more efficient.

Miscellaneous Techniques

Power Metallurgy Yet another fabrication technique involves the compaction of powdered metal, followed by a heat treatment to produce a more dense piece. The process is appropriately called powder metallurgy, frequently designated as P/M. Powder metallurgy makes it possible to produce a virtually nonporous piece having properties almost equivalent to the fully dense parent material. Diffusional processes during the heat treatment are central to the development of these properties. This method is especially suitable for metals having low ductilities since only small plastic deformation of the powder particles need to occur. Metals having high melting temperatures are difficult to melt and cast, and fabrication is expedited using P/M.

Ceramics

Simple Ceramics

Traditional ceramics are made from natural materials such as clay that have been hardened by heating at high temperatures (driving out water and allowing strong chemical bonds to form between flakes of clay). When artists make ceramic works of art, they first mold clay, often mixed with other raw materials, into the desired shape. Special ovens called kilns are used to "fire" (heat)shaped object until it hardens. Clay consists of a large number of very tiny flat plates, stacked together but separated by thin layers of water.water allows plates to cling together, but also acts as a lubricant, allowing plates to slide past one another. As a result, clay is easily molded into shapes. High temperatures drive out water and allow bonds to form between plates, holding them in place and promoting the formation of a hard solid. Binders such as bone ash are sometimes added to clay to promote strong bond formation, which makes ceramic resistant to breakage.common clay used to make flower pots and roof tiles is usually red-orange because of the presence of iron oxides. Porcelain is a hard, tough ceramic that is less brittle than ceramics that preceded it. Its strength allows it to be fashioned into beautiful vessels with walls so thin they can even be translucent. It is made from kaolin mixed with china stone, and the mixture is heated to a very high temperature (1,300°C, or 2,372°F). Bone China has a composition similar to that of porcelain, but at least 50 percent of the material is finely powdered bone ash. Like porcelain, bone china is strong and can be formed into dishes with very thin, translucent walls. Stoneware is a dense, hard, gray or tan ceramic that is less expensive than bone china and porcelain, but it is not as strong. As a result, stoneware dishes are usually thicker and heavier than bone china or porcelain dishes.

Advanced Ceramics

Preparation of an advanced ceramic material usually begins with a finely divided powder that is mixed with an organic binder to help powder consolidate so that it can be molded into the desired shape. Before it is fired, the ceramic body is called "green." green body is first heated to a low temperature in order to decompose or oxidize binder. It is then heated to a high temperature until it is "sintered," or hardened, into a dense, strong ceramic. At this time, individual particles of original powder fuse together as chemical bonds form between them. During sintering ceramic may shrink by as much as 10 to 40 percent. Because shrinkage is not uniform, additional machining of ceramic may be required in order to obtain a precise shape. Sol-gel technology allows better mixing of ceramic components at the molecular level and hence yields more homogeneous ceramics because ions are mixed while in solution. In Sol-gel process, a solution of an organometallic compound is hydrolyzed to produce a "sol," a colloidal suspension of a solid in a liquid. Typically Solution is a metal alkoxide such as tetraethoxysilane in an alcohol solvent.sol forms when individual formula units polymerize (link together to form chains and networks).sol can then be spread into a thin film, precipitated into tiny uniform spheres called microspheres, or further processed to form a gel inside a mold that will yield a final ceramic object in the desired shape.many cross-links between formula units result in a ceramic that is less brittle than typical ceramics. Although Sol-gel process is very expensive, it has many advantages, including low-temperature requirements; Porous ceramics are made by the sol-gel process. These ceramics have sponge-like structures, with much more like lacunae, or openings, that can make up for 25 to 70 percent of volume.pore size can be large, or as small as 50 nanometers (2 × 10 −6 inches) in diameter. Because of a large number of pores, porous ceramics have enormous surface areas (up to 500 square meters, or 5,382 square feet, per gram of ceramic), and so can make excellent catalysts. For example, zirconium oxide is a ceramic oxygen sensor that monitors air-to-fuel ratio exhaust systems of automobiles. Aerogels are solid foams prepared by removing liquid from gel during a sol-gel process at high temperatures and low pressures. Because aerogels are good insulators, have very low densities, and do not melt at high temperatures

Polymers

Addition Polymerization (Chain Reaction Polymerization)

A process by which monomer units are attached one at a time in chainlike fashion to form a linear macromolecule. The composition of the resultant product molecule is an exact multiple for that of the original reactant monomer. Three distinct stages—initiation, propagation, and termination—are involved in addition polymerization. During the initiation step, an active center capable of propagation is formed by a reaction between an initiator (or catalyst) species and the monomer unit.Propagation involves the linear growth of the polymer chain by the sequential addition of monomer units to this active growing chain molecule.Chain growth is relatively rapid; the period required to grow a molecule consisting of, say, 1000 repeat units is on the order of to s. Propagation may end or terminate in different ways. First, the active ends of two propagating chains may link together to form one molecule.The other termination possibility involves two growing molecules that react to form two “dead chains”

Condensation Polymerization (Step Reaction Polymerization)

The formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species. There is usually a small molecular weight by-product such as water that is eliminated (or condensed). No reactant species has the chemical formula of the repeat unit, and the intermolecular reaction occurs every time a repeat unit is formed.This stepwise process is successively repeated, producing a linear molecule. The chemistry of the specific reaction is not important but the condensation polymerization mechanism is. Furthermore, reaction times for condensation are generally longer than for addition polymerization.

Polymer Additives

Fillers

Filler materials are most often added to polymers to improve tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, and other properties. Materials used as particulate fillers include wood flour (finely powdered sawdust), silica flour and sand, glass, clay, talc, limestone, and even some synthetic polymers.

Plasticizers

The flexibility, ductility, and toughness of polymers may be improved with the aid of additives called plasticizers. Their presence also produces reductions in hardness and stiffness. Plasticizers are generally liquids having low vapor pressures and low molecular weights. The small plasticizer molecules occupy positions between the large polymer chains, effectively increasing the interchain distance with a reduction in the secondary intermolecular bonding.

Stablilizers

The flexibility, ductility, and toughness of polymers may be improved with the aid of additives called plasticizers. Their presence also produces reductions in hardness and stiffness. Plasticizers are generally liquids having low vapor pressures and low molecular weights. The small plasticizer molecules occupy positions between the large polymer chains, effectively increasing the interchain distance with a reduction in the secondary intermolecular bonding.

Composites

Manufacturing a fibre-reinforced composite involves several steps in which filler (fiber) is produced and then if required, aligned prior to the introduction of the matrix. Fibers produced from polymers, glass, and metals can be produced from the molten state by drawing them. Most carbon fibers are first drawn from a polymer, for example, poly (propenonitrile) (polyacrylonitrile), which is then oxidized, followed by pyrolysis. If Fibres are unaligned (random orientation) with each other, filler and matrix can be blended together as powders or slurries, shaped or molded, and matrix hardened and bonded to filler by heat or by chemical reaction. shaping may be accomplished by injection molding or casting of the composite. If required, additional machining is done before any further treatment to harden composite. The production of aligned fiber (Figure 1) composites is more complicated. fibers may be used as monofilaments, or they may be twisted into yarns of up to 10 000 filaments. yarns are then woven or knitted into two or three-dimensional fabrics and can be then formed into tapes by weaving or braiding. Filaments can also be processed into non-woven mats of fibers, randomly orientated in two dimensions.mats, tapes and fabrics may be impregnated with a matrix material (or materials from which matrix is made) before final assembly and processing, and are then referred to as a prepreg (pre-impregnated). Thus, there is no need to handle individual chemicals. It is simply rolled out and used as a laminate. Curing is accomplished by heating.fabrics and mats are placed in molds, sometimes by hand, and impregnated with the matrix material and processed. They can be compressed mechanically or by pressure or vacuum forming techniques. Filaments or tapes can also be wound, passing through a bath of the matrix before being processed into a solid shape. A simple example of a prepreg is bandage that is used to set broken limbs (Unit 67). the bandage is made of a polyester and it is impregnated with a linear polyurethane, matrix, bandage, or prepreg, is flexible and is wound around broken limb. On soaking, cross-linkages between polyurethane molecules react with cross-linkages between molecules, producing a strong but light cast.

Carbon Fiber Reinforced Polymer Composites (CRFPs)

Carbon fibers are generally made by heating filaments of poly (propenonitrile) (polyacrylonitrile, PAN) at ca 500 K, under an atmosphere of air to form oxidized PAN. oxidized PAN is then placed into a furnace with an inert atmosphere of a gas such as argon, and heated to ca 2000 K, a process known as pyrolysis, fiber heating of a substance in absence of air. fiber contains chains of carbon atoms which are bonded side-to-side (ladder polymers), forming narrow sheets of carbon atoms, known as graphene.

Uses: One can imagine it as a piece of chicken wire on a very small scale. Graphene is also basic structure of graphite and of carbon nanotubes. In the case of a carbon fiber, fibers sheets merge to form a single circular filament. C fibers are used as a filler in a continuous matrix of a polymer, often an epoxy resin. Layers of carbon fiber cloth are laid down in shape required, usually in a mold which is filled with epoxy resin and heated.

Glass Fiber Reinforced Polymer Composites (GFRCs)

Glass fiber is made from silica, sodium carbonate, and calcium carbonate together with other compounds to give properties required. materials are heated to ca 1700 K in a furnace and then extruded directly from the furnace through metal (a platinum/rhodium alloy) orifices of various diameters (4 - 34 urn) to produce filaments. A high-speed winder revolving faster than existing molten glass draws them under tension into fine filaments. a number of orifices vary from 200 - 8000.filaments are coated with a lubricant to protect them and bundled together on a drum, looking like a spool of a thread. In fiberglass, in which glass fibers are filler(dispersed phase), the matrix is usually polyester resin though epoxy/acrylic polymers also used.

Uses: FRPs are used widely I the manufacture of boats for reasons of cost and maintenance. While large ships are usually constructed of steel, over 80% of marine hulls, less than about 40 m in length, are made of glass fiber reinforced polymer. This is a much cheaper process and the hull is easier to maintain. Additionally, there are certain applications in which magnetic, electrical or thermal properties of CFRPs are advantageous, an example being minesweepers which need to be non-magnetic in order to avoid activation of mines.Glass-phenolic resin prepregs are also used to improve protection for armored vehicles.

Aramid Reinforced Polymer Composites (ARPCs)

An aramid is a polymeric aromatic amide. Kevlar® is a particularly widely used aramid. It is very difficult to handle as the only effective solvent is concentrated sulfuric acid. Kevlar® is made from lightweight atoms but is very strong and flexible, weight for weight five times as strong as steel. Its strength comes from way polymer chains are aligned, fiber flat molecules being held together by hydrogen bonds. These sheets of molecules can stack up fiber axis (Figure 2). These fibers, already strong, are used as filler in APCs with a phenolic resin or epoxy resin as a matrix. They are useful when dissipating and absorbing energy, and they are also able to resist abrasion.

Uses: These composites have been used widely in aviation, for helicopter rotor blades, in sport, to make tennis, badminton and squash racquets, and in boats such as kayaks and dinghies.

Additional Information

These fibers, already strong, are used as the filler in ARPCs with a phenolic resin or epoxy resin as a matrix. They are particularly useful where energy has to be absorbed and dissipated, and they are also able to resist abrasion. Manufacturing a fibre-reinforced composite involves several steps in which the filler (fiber) is produced and then if required, aligned prior to the introduction of the matrix. Fibers produced from polymers (for example, the aramids, see below), glass and metals can be produced from the molten state by drawing them. Most carbon fibers are first drawn from a polymer, for example, poly (propenonitrile) (polyacrylonitrile), which is then oxidized, followed by pyrolysis. If the fibers are unaligned (random orientation) with each other, the filler and matrix can be blended together as powders or slurries, shaped or molded, and the matrix hardened and bonded to the filler by heat or by chemical reaction. The shaping may be accomplished by injection molding or casting of the composite. If required, additional machining is done before any further treatment to harden the composite. The production of aligned fiber composites is more complicated. The fibers may be used as monofilaments, or they may be twisted into yarns of up to 10 000 filaments. The yarns are then woven or knitted into two or three-dimensional fabrics and can be then formed into tapes by weaving or braiding. Filaments can also be processed into non-woven mats of fibers, randomly orientated in two dimensions. The mats, tapes, and fabrics may be impregnated with a matrix material (or materials from which the matrix is made) before final assembly and processing, and are then referred to as a prepreg (pre-impregnated). Thus, there is no need to handle the individual chemicals. It is simply rolled out and used as a laminate. Curing is accomplished by heating. The fabrics and mats are placed in molds, sometimes by hand, and impregnated with the matrix material and processed. They can be compressed mechanically or by pressure or vacuum forming techniques. Filaments or tapes can also be wound, passing through a bath of the matrix before being processed into a solid shape. A simple example of a prepreg is the bandage that is used to set broken limbs (Unit 67). The bandage is made of a polyester and it is impregnated with a linear polyurethane, the matrix. The bandage, the prepreg, is flexible and is wound around the broken limb. On soaking, the polyurethane molecules react with water to form cross-linkages between the molecules, producing a strong but light cast.

Physical Tests

Creep Rate

In materials science, creep (sometimes called cold flow) tendency of a solid material to move slowly or deform permanently under influence of mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below yield strength of material. Creep is more severe in materials that are subjected to heat for long periods, and generally increases as they near their melting point. The rate of deformation is a function of material properties, exposure time, exposure temperature and applied structural load. Depending on magnitude of applied stress and its duration,deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause blade to contact casing, resulting in failure of blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. For example, moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking. Unlike brittle fracture, creep deformation does not occur suddenly upon application of stress. Instead, strain accumulates as a result of long-term stress. Therefore, creep is a "time-dependent" deformation.

Stages of Creep: Initial stage, or primary creep,strain rate is relatively high, but slows with increasing time. This is due to work hardening.strain rate eventually reaches a minimum and becomes near constant. This is due to balance between work hardening and annealing (thermal softening). This stage is known as secondary or steady-state creep. This stage is most understood.characterized "creep strain rate" typically refers torate in this secondary stage. Stress dependence of this rate depends on creep mechanism. In tertiary creep,strain rate exponentially increases with stress because of necking phenomena or internal voiding decreased effective area of specimen. Fracture always occurs at tertiary stage.

Mechanisms of Creep: The mechanism of creep depends on temperature and stress. Various mechanisms are: Bulk diffusion (Nabarro-Herring creep) Climb — here strain is actually accomplished by climb Climb-assisted glide — here climb is an enabling mechanism, allowing dislocations to get around obstacles Grain boundary diffusion (Coble creep) Thermally activated glide — e.g., via cross-slip

General Creep Equation: where is creep strain, C is a constant dependent on material and particular creep mechanism, m and b are exponents dependent on creep mechanism, Q is activation energy of creep mechanism, σ is applied stress, d is grain size of material, k is Boltzmann's constant, and T is absolute temperature.

Fatigue Limit

Fatigue limit, endurance limit, and fatigue strength are all expressions used to describe a property of materials:amplitude (or range) of cyclic stress that can be applied to material without causing fatigue failure. Ferrous alloys and titanium alloys have a distinct limit, an amplitude below which there appears to be no number of cycles that will cause failure. Other structural metals such as aluminium and copper, do not have a distinct limit and will eventually fail even from small stress amplitudes. In these cases, a number of cycles (usually 107) is chosen to represent fatigue life of material.

Definitions

Bulk Modulus

The bulk modulus (K or B) of a substance is a measure of how compressible that substance is. It is defined as the ratio of infinitesimal pressure increase resulting relative decrease of volume. In addition, The bulk modulus K>0 can be formally defined by equation K= -VdPdV where P is pressure, V is volume, and dP/dV denotes derivative of pressure with respect to volume It is also equal to K=pdPdp where ρ is density and dP/dρ denotes derivative of pressure with respect to density (i.e. pressure rate of change with volume).inverse of bulk modulus gives a substance's compressibility. Other moduli describe material's response (strain) to other kinds of stress: shear modulus describes the response to shear, and Young's modulus describes the response to linear stress. For a fluid, only the bulk modulus is meaningful. For an anisotropic solid such as wood or paper, these three moduli do not contain enough information to describe its behavior, and one must useful generalized Hooke's law.

Thermodynamic Relation: Strictly speaking, bulk modulus is a thermodynamic quantity, and in order to specify a bulk modulus it is necessary to specify how temperature varies during compression: constant temperature (isothermal KT) constant-entropy (isentropic KS) and other variations are possible. Such distinctions are especially relevant for gases. For an ideal gas, isentropic bulk modulus KS is given by

and isothermal bulk modulus is given by

where γ is heat capacity ratio p is pressure. When Gas is not ideal, these equations give only an approximation of bulk modulus. In a fluid, bulk modulus K and density ρ determine speed of sound c (pressure waves), according to Newton-Laplace formula

In solids, and have very similar values. Solids can also sustain transverse waves: for these materials one additional elastic modulus, for example shear modulus, is needed to determine wave speeds.

Measurement: It is possible to measure bulk modulus using powder diffraction under applied pressure. It is a property of a fluid which shows its ability to change its volume under its pressure.

Crystal Types

Bond Types

Ionic:

Ionic compounds can be recognized by a combination of positively charged cations and negatively charged anions. They form a three-dimensional ionic lattice.

Ionic compounds often are soluble in water.

In their solid form, ionic compounds do not conduct electricity. However, ionic compounds conduct electricity when dissolved in water or melted as ions are free to move.

Most ionic compounds have a very high melting point and an extremely low volatility.

Metallic:

Metallic compounds are characterized by their "sea" of free electrons. These electrons are free to move and balance out the positively charged cations of the metal atoms.

Metallic compounds are not soluble in water.

Metallic compounds conduct electricity both as solids and as liquids, but not when dissolved in water.

Most metallic compounds have a very high to extremely high melting point and an extremely low volatility due to the strong attractive force between the electrons and cations.

However, the fairly unrestricted motion of electrons allows for properties like ductility and malleability.

Covalent (Network):

Covalent compounds are small molecules composed of covalently bonded atoms, while network compounds have an ordered structure of atoms connected by covalent bonds.

Neither covalent nor covalent network compounds are soluble in water, nor do they conduct electricity.

Covalent compounds (simple covalent bonds) have relatively low melting points and high volatility, while covalent network compounds (giant covalent bonds) have extremely high melting points and extremely low volatility due to intermolecular forces.

Dipole-Dipole/LDF:

These compounds are composed of either covalent nonpolar or barely polar molecules.

They are soluble in both hexane and ethanol due to their lack of charge but are not soluble or conductive in water.

They have very low melting points and high volatility.

Hydrogen:

Hydrogen bonded compounds are characterized by O-H, N-H, or F-H bonds. These bonds cause regions of positive and negative charge, creating intermolecular attractions.

They are soluble but not conductive in water and ethanol.

They have very low melting points and extremely low volatility.

Crystallinity

Crystalline solids are composed of 1 or more crystals containing completely constant, rigid long range order.

Semi-crystalline solids are also heavily bonded but lack the rigidity and constant structure of crystalline solids.

Amorphous solids still possess short-range order but have significantly less chain linkage.

Polymers are usually semi-crystalline to some degree, with some crystalline regions with rigid chain linkage and some amorphous regions.

Metals follow a relatively crystalline structure.

Finally, ceramics vary in crystallinity, from highly crystalline, vitrified fired ceramics to amorphous glasses.

Atomic Packing

FCC Cell

Face Centered Cubic

Also known as cubic close packed.

In the FCC cell, atoms are located at each of the 8 corners as well as in the centers of each of the 6 faces.

FCC follows an ABCABC close packing pattern - there are 3 repeating layers, where the atoms of the third layer are located above holes in the first and second layers.

FCC is the densest of the cubic packing arrangements, with an atomic packing factor of 0.74. Each unit cell contains 4 atoms and has a side length of [math]A = 4R/\sqrt{2}[/math].

Each atom in the FCC matrix has a coordination number of 12.

BCC Cell

Body Centered Cubic

In the BCC cell, atoms are located at each of the 8 corners as well as in the center of the cubic cell.

BCC is less dense than FCC, with an atomic packing factor of 0.68. Each unit cell contains 2 atoms and has a side length of [math]A = 4R/\sqrt{3}[/math].

Each atom in the BCC matrix has a coordination number of 8.

HCP Cell

Hexagonal Close Packing

HCP is another close-packed arrangement.

The HCP cell is composed of two hexagons of 6 atoms each, an additional atom in the center of each hexagon, and a triangle of atoms in between the two hexagons.

HCP differs from FCC in that HCP follows an ABAB packing pattern - there are only 2 repeating layers, where the atoms of the third layer are located above the atoms of the first layer, not above gaps.

HCP also has an atomic packing factor of 0.74, the maximum possible. Each unit cell contains 6 atoms and has two parameters, A (side length) and B (height).

Each atom in the HCP matrix has a coordination number of 12.

SC Cell

Simple Cubic

Simple cubic is a very basic arrangement, only containing atoms at each corner of the unit cell.

SC is the least dense, with an atomic packing factor of 0.52. Each unit cell contains 1 atom and has a side length of [math]A = 2R[/math]

Each atom in the SC matrix has a coordination number of 6.

Atomic Packing Factor (APF)

The atomic packing factor describes the amount of space occupied by atoms.

For example, in the simple cubic packing, each cell has side length 2R and contains 1 atom of radius R.

The volume occupied by the atom is [math](4/3)\pi *R^3[/math], while the total volume is [math](2R)^3 = 8R^2[/math]

The fraction of space occupied by atoms is [math](4/3)\pi *R^3/8R^3[/math] = [pi/6] = 0.524, exactly the amount listed above.

This equation will work for any of the other crystal structures assuming the correct side lengths are used.