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Hooke's Law

Hooke's law

In physics, Hooke's law of elasticity is an approximation which states that if a spring is elongated by some distance, x, the restoring force exherted by the spring, F, is proportional to x by a constant factor, k. That is, :

F=-kx.

When this holds, we say that the spring is a linear spring. For many applications, a prismatic rod, with length L and cross sectional area A, can be treated as a linear spring. Its extension (strain) is linearly proportional to its tensile stress, σ by a constant factor, the modulus of elasticity, E. Hence, :

\sigma = E \cdot \varepsilon

or :

\Delta L = \frac \times F \times \frac   = \frac \times L \times \sigma.

It is named after the 17^th century physicist Robert Hooke, who initially published it as the anagram ceiiinosssttuv, which he later revealed to mean ut tensio sic vis, or as the extension, the force. This approximation holds for only some materials under certain loading conditions. Materials for which Hooke's law is a useful approximation are known as linear-elastic or "Hookean" materials. Steel exhibits linear-elastic behavior in most engineering applications; Hooke's law is valid for it throughout its elastic range (i.e., for stresses below the yield strength). For some other materials, such as Aluminum, Hooke's law is only valid for a portion of the elastic range. For these materials a proportional limit stress is defined, below which the errors associated with the linear approximation are negligible. Materials such as rubber, for which Hooke's law is never valid, are known as "non-hookean". The stiffness of rubber is not only stress dependent, but is also very sensitive to temperature and loading rate. The graph below shows a stress-strain curve for low-carbon steel. Hooke's law is only valid for the portion of the curve between the origin and the yield point. stress-strain curve
3. Rupture
4. Strain hardening region
5. Necking region.]] Applications of the law include spring operated weighing machines. Originally the law applied only to stretched springs, but subject to physical constraints it also applies to compression springs.

Spring equation

The most commonly encountered form of Hooke's law is probably the spring equation, which relates the force exerted by a spring to the distance it is stretched by a spring constant, k, measured in force per length. :

F=-kx

The negative sign indicates that the force exerted by the spring is in direct opposition to the direction of displacement. It is called a "restoring force", as it tends to restore the system to equilibrium. The potential energy stored in a spring is given by :

U=kx^2

which comes from adding up the energy it takes to incrementally compress the spring. That is, the integral of work over distance. This potential can be visualized as a parabola on the U-x plane. As the spring is stretched in the positive x-direction, the potential energy increases (the same thing happens as the spring is compressed). The corresponding point on the potential energy curve is higher than that corresponding to the equilibrium position (x=0). The tendency for the spring is to therefore decrease its potential energy by returning to its equilibrium (unstretched) position, just as a ball rolls downhill to decrease its gravitational potential energy. If a mass is attached to the end of such a spring and the system is bumped, it will oscilate with a natural frequency (or resonant angular (circular) frequency) of :

\omega_n = \sqrt.

Generalized Hooke's law

When working a with three-dimensional stress state, a 4^th order tensor (c_ijkl) containing 81 elastic coefficients must be defined to link the stress tensor (σ_ij) and the strain tensor (or Green tensor) (ε_kl). :

\sigma_ = \sum_ c_ \cdot \varepsilon_

Due to the symmetry of the stress and strain tensor, only 36 elastic coefficients are independent. As stress is measured in units of pressure and strain is dimensionless, the entries of c_ijkl are also in units of pressure. Generalization for the case of large deformations is provided by models of neo-Hookean solid and Mooney-Rivlin solid.

Zero-length springs

Hooke's law does not apply in some special physical conditions. In 1932 Lucien LaCoste invented the zero-length spring. A zero-length spring has a physical length equal to its stretched length. Its force is proportional to its entire length, not just the stretched length, and its force is therefore constant over the range of flexures in which the spring is elastic (that is, it does not follow Hooke's Law). Theoretically, with the correct mass, a pendulum using such a spring as a return can have an infinite natural period. Long-period pendulums enable seismometers to sense the slowest, most penetrating waves of distant earthquakes. Zero-length springs also find use in gravimeters, which need them to have linear sense-pendulums. Some door springs, especially for screen doors, are zero-length springs to reduce the energy of a slammed door. Zero-length springs sometimes smooth auto suspensions. Physically, one common form of a practical zero-length spring is a leaf-spring curled almost in a circle, with the ends mounted to flexible restraints. A convenient form is a helical spring whose wire is twisted while it is being wound (common in screen-door springs). Another common design is a torque-spring or bar. Zero-length springs usually require special compliant mountings, sometimes require precise adjustments to enter zero-length mode, and often have a limited range of motion.

Links

- [http://www.mssu.edu/seg-vm/bio_lucien_lacoste.html A Biography of Lucien LaCoste, inventor of the zero-length spring]
- [http://physics.mercer.edu/earthwaves/zero.html Zero Length Springs in Seismometers] Category:Continuum mechanics Category:Eponymous laws ko:훅의 법칙 ja:振動運動#フックの法則

Spring

Spring has several meanings: As commonly used:
- Spring (season), a season of the year.
- Spring (device), a common mechanical part.
- Spring (water), a natural source of water. As a place or location:
- Spring, Texas, a town in the United States.
- Springs, Gauteng, a city in South Africa.
- Springs, Western Australia, a place in Australia.
- Three Springs, Western Australia, another place in Australia. As a personal last name:
- Howard Spring
- Sherwood C. Spring
- Cecil Spring-Rice As a musical title:
- Spring, a Dutch band.
- Spring, a musical album by Finn Coren inspired by William Blake's poetry.
- Spring is a violin concerto in The Four Seasons by Antonio Vivaldi.
- Spring, a song by Rammstein. As a mathematical surface:
- Spring (math) As computer terminology:
- Spring framework, an enterprise Java framework for web applications.
- Spring operating system, an experimental operating system from Sun Microsystems.
- TA Spring is an open-source RTS game inspired by Total Annihilation.

Force

:For other senses of this word, see force (disambiguation). In physics, a force is an external cause responsible for any change of a physical system. For instance, a person holding a dog by a rope is experiencing the force applied by the rope on their hand, and the cause for its pulling forward is the force exercised by the rope. The kinetic expression of this change is, according to Newton's second law, acceleration, non kinetic expressions such as deformation can also occur. The SI unit for force is the newton.

Elementary concepts

Force in its most primitive definition can be thought of as that which when acting alone causes an object to accelerate. In a practical sense forces can be divided into two groups: contact forces and field forces. Contact forces require the physical contact of one object with other such as a hammer striking a nail or the force exerted by a gas under pressure - gas produced by exploding gunpowder forces a heavy ball out of a cannon. Field forces on the other hand need no physical medium of contact. Gravity and magnetism are examples of such forces. It should be noted however, that fundamentally all forces are in fact field forces. The force of hammer striking the nail in the previous example turns out to be a clash of the electric forces in both hammer and nail. Nevertheless it is appropriate in some cases to maintain these two classifications for ease of understanding.

Quantitative definition

In physics models, the point-like system is used, where objects are represented as one-dimensional points at their centre of mass. The only change the system can experience is a change of its momentum (its speed). Since the rise of the atomic theory, any physical system has been considered in classical physics as composed of point-like systems called atoms or molecules. Therefore, all forces can be defined by their effect; that is, by the change of movement they induce on point-like systems. This change of movement can be quantified by the acceleration (the derivative of velocity). The discovery by Isaac Newton that a given force will induce an acceleration in inverse proportion to a quantity called the mass of inertia or inertial mass which is independent of the speed of the system is Newton's second law. This law allows us to predict the effect of a force on any point-like system whose mass is known. It is usually written as: :F = dp/dt = d(m·v)/dt = m·a (in the case where m does not depend on t) where :F is the force (a vector quantity), :p is the momentum, :t is the time, :v is the velocity, :m is the mass, and :a=d²x/dt² is the acceleration, the second derivative with respect to t of the position vector x. If the mass m is measured in kilograms and the acceleration a is measured in metres per second squared, then the unit of force is kilogram × metre/second squared. This unit is called the newton: 1 N = 1 kg x 1 m/s². This equation is a system of three second-order differential equations with respect to the three-dimensional position vector which is an unknown function of time. This equation can be solved if F is a known function of x and some of its derivatives and if the mass m is known. Morevover the boundary conditions are required; for example, the values of the position vector and x and the velocity v at the starting time, say t=0. Of course, this formula is only useful if one knows the numerical values of F and m. The definition above is an implicit definition, arrived at as follows. One defines a reference system (one litre of water) and a reference force (the gravitational force applied by the Earth on it at the altitude of Paris). One takes Newton's second law for granted (one postulates that it is true) and measures the acceleration induced by the reference force on the reference system. This gives us a mass unit (1 kg) and a force unit (the older unit of 1 kilogram-force = 9.81 N). Once this is done, one can measure any force by the acceleration it induces on the reference system and measure the inertial mass of any system by measuring the acceleration induced on this system by the reference force. Force is often considered a fundamental quantity in physics, but there are more fundamental quantities, such as momentum (p = mass m x velocity v). Energy, measured in joules, is still less fundamental than force and momentum, because it is defined as work, and work is defined in terms of force. The two most fundamental theories of nature - quantum electrodynamics and general relativity - do not contain the concept of force at all. Although not the most fundamental quantity in physics, force is an important basic mathematical concept from which other concepts, such work and pressure (measured in pascals), are derived. Force is sometimes confused with stress.

Types of force

There are four known fundamental forces in nature.
- Nuclear forces acting between subatomic particles
- Electromagnetic forces between electric charges
- Weak forces arising from radioactive decay
- Gravitational forces between masses Quantum field theory accurately models the first three fundamental forces, but does not model quantum gravity. Quantum gravity on a large scale can, however, be described by general relativity. The four fundamental forces describe every observable phenomenon including the many other forces observed such as: Coulomb's force (the force between electrical charges), gravitational force (force between masses), magnetic force, frictional forces, centripetal, centrifugal, impact force, and spring force, to name a few. Forces can also be classified into conservative forces and nonconservative forces. Conservative forces are equivalent to the gradient of a potential, and include gravity, electromagnetic force, and spring force. Nonconservative forces include friction and drag.

Properties of force

Because momentum is a vector, then force, being its time derivative, is also a vector - it has magnitude and direction. Forces can be added together using the parallelogram of force. When two forces act on an object, the resulting force, the resultant, is the vector sum of the original forces. This is called the principle of superposition. The magnitude of the resultant varies from zero to the sum of the magnitudes of the two forces, depending on the angle between their lines of action. If the two forces are equal, but opposite, the resultant is zero. This condition is called static equilibrium, with the result that the object remains at rest or moves with a constant velocity. As well as being added, forces can be can also be broken down (or 'resolved'). For example, a horizontal force pointing northeast can be split into two forces, one pointing north, and one pointing east. Summing these component forces using vector addition yields the original force. Force vectors can also be three-dimensional, with the third (vertical) component at right-angles to the two horizontal components.

Forces in theory

The total (Newtonian) force, in newtons, on an object at any given time is defined as the rate of change of the object's velocity multiplied by the object's mass: :

\mathbf = \lim_ \frac

where :m is the inertial mass of the particle (measured in kilograms) :v_o is its initial velocity (measured in metres per second) :v is its final velocity (measured in metres per second) :T is the time from the initial state to the final state (measured in seconds); :Lim T→0 is the limit as T tends towards zero. Force was so defined to explain the effects of superimposing situations: if in one situation, a force is experienced by a particle, and if in another situation another force is experienced by that particle, then in a third situation, which (according to standard physical practice) is taken to be a combination of the two individual situations, the force experienced by the particle will be the vector sum of the individual forces experienced in the first two situations. This superposition of forces, and the definition of inertial frames and inertial mass, are the empirical content of Newton's laws of motion. There are other ways to look at the above definition of force. First, the mass of a body multiplied by its velocity is called its momentum, p, so the above definition is equivalent to: :

\textbf=

If F is not constant over Δt, then this is the definition of average force over the time interval. To apply it at an instant we apply an idea from calculus. If we graph p as a function of time, the average force will be the slope of the line connecting the momentum at two times. Taking the limit as the two times get closer together gives the slope at an instant, which is called the derivative: :

\textbf=

Many forces are associated with a potential energy field. For instance, the gravitational force acting upon a body can be seen as the action of the gravitational field that is present at the body's location. The potential field is defined as that field whose gradient is equal and opposite to the force produced at every point: :

\textbf=-\nabla U

The derivative of force with respect to time is called yank. Higher order derivatives are sometimes used, but they lack names because of their rarity. In most explanations of mechanics, force is usually defined only implicitly, in terms of the equations that work with it. Some physicists, philosophers and mathematicians, such as Ernst Mach, Clifford Truesdell and Walter Noll, have found this problematic and sought a more explicit definition of force. According to the Special theory of relativity the mass of an object increases as it's velocity and therefore it's energy increases. The law of force must then be modified to the following: :

\textbf=

where :v is the mass's velocity :c is the speed of light. Note that the equation is undefined if the mass's speed is equal to c because one then has to divide by zero. This is one reason most physicists believe an object with nonzero rest mass can not be accelerated to the speed of light, as this would require an infinite force.

Units of measurement

The SI unit used to measure force is the newton (symbol N), which is equivalent to kg·m·s⁻².

Non-SI units of force and mass

The F=m·a relationship can be used with any consistent units (SI or CGS). If these units are not consistent, a more general form, F=k·m·a, can be used, where the constant k is a conversion factor dependent upon the units being used. For example, in imperial engineering units, F is measured in "pounds force" or "lbf", m in "pounds mass" or "lb", and a in feet per second squared. In this particular system, one needs to use the more general form above, usually written F=m·a/g_c with the constant normally used for this purpose g_c = 32.174 lb·ft/(lbf·s²) equal to the reciprocal of the k above. As with the kilogram, the pound is colloquially used as both a unit of mass and a unit of force. 1 lbf is the force required to accelerate 1 lb at 32.174 ft per second squared, since 32.174 ft per second squared is the standard acceleration due to terrestrial gravity. Another imperial unit of mass is the slug, defined as 32.174 lb. It is the mass that accelerates by one foot per second squared when a force of one lbf is exerted on it. When the standard gee (an acceleration of 9.80665 m/s²) is used to define pounds force, the mass in pounds is numerically equal to the weight in pounds force. However, even at sea level on Earth, the actual acceleration of free fall is quite variable, over 0.53% more at the poles than at the equator. Thus, a mass of 1.0000 lb at sea level at the equator exerts a force due to gravity of 0.9973 lbf, whereas a mass of 1.000 lb at sea level at the poles exerts a force due to gravity of 1.0026 lbf. The normal average sea level acceleration on Earth (World Gravity Formula 1980) is 9.79764 m/s², so on average at sea level on Earth, 1.0000 lb will exerts a force of 0.9991 lbf. The equivalence 1 lb = 0.453 592 37 kg is always true, by definition, anywhere in the universe. If you use the standard gee which is official for defining kilograms force to define pounds force as well, then the same relationship will hold between pounds-force and kilograms-force (an old non-SI unit is still used). If a different value is used to define pounds force, then the relationship to kilograms force will be slightly different—but in any case, that relationship is also a constant anywhere in the universe. What is not constant throughout the universe is the amount of force in terms of pounds-force (or any other force units) which 1 lb will exert due to gravity. By analogy with the slug, there is a rarely used unit of mass called the "metric slug". This is the mass that accelerates at one metre per second squared when pushed by a force of one kgf. An item with a mass of 10 kg has a mass of 1.01972661 metric slugs (= 10 kg divided by 9.80665 kg per metric slug). This unit is also known by various other names such as the hyl, TME (from a German acronym), and mug (from metric slug). Another unit of force called the poundal (pdl) is defined as the force that accelerates 1 lbm at 1 foot per second squared. Given that 1 lbf = 32.174 lb times one foot per second squared, we have 1 lbf = 32.174 pdl. The kilogram-force is a unit of force that was used in various fields of science and technology. In 1901, the CGPM improved the definition of the kilogram-force, adopting a standard acceleration of gravity for the purpose, and making the kilogram-force equal to the force exerted by a mass of 1 kg when accelerated by 9.80665 m/s². The kilogram-force is not a part of the modern SI system, but is still used in applications such as:
- Thrust of jet and rocket engines
- Spoke tension of bicycles
- Draw weight of bows
- Torque wrenches in units such as "meter kilograms" or "kilogram centimetres" (the kilograms are rarely identified as units of force)
- Engine torque output (kgf·m expressed in various word orders, spellings, and symbols)
- Pressure gauges in "kg/cm²" or "kgf/cm²" In colloquial, non-scientific usage, the "kilograms" used for "weight" are almost always the proper SI units for this purpose. They are units of mass, not units of force. The symbol "kgm" for kilograms is also sometimes encountered. This might occasionally be an attempt to disintinguish kilograms as units of mass from the "kgf" symbol for the units of force. It might also be used as a symbol for those obsolete torque units (kilogram-force metres) mentioned above, used without properly separating the units for kilogram and metre with either a space or a centered dot.

Conversions

Below are several coversion factors between various mesurements of force:
- 1 kgf (kilopond kp) = 9.80665 newtons
- 1 metric slug = 9.80665 kg
- 1 lbf = 32.174 poundals
- 1 slug = 32.174 lb
- 1 kgf = 2.2046 lbf

Forces in everyday life

Forces are part of everyday life, with examples such as:
- gravity: objects fall, even after being thrown upwards, or slide and roll down
- friction: floors and objects that are not extremely slippery
- spring force, objects resist tensile stress, compressive stress and/or shear stress, objects bounce back.
- electromagnetic force: attraction of magnets
- movement created by force: the movement of objects when force is applied.

Forces in the laboratory

Founding experiments

- Galileo Galilei used rolling balls to disprove the Aristotelian theory of motion (1602 - 1607)
- Henry Cavendish's torsion bar experiment measured the force of gravity between two masses (1798)

Instruments to measure forces

- spring balance
- pivot balance
- forcemeter

History

Force was first described by Archimedes.

References

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External links

- [http://jumk.de/calc/force.shtml Calculation: force F - English and American units to metric units]
- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units]
- [http://www.patbelford.com/gallery/web3d/education/forceworkpower/index.html Interactive demonstration of Force-Work-Power Relationship] Category:Introductory physics ko:힘 ms:Daya (fizik) ja:力 simple:Force (physics)

Strain

Strain refers to several different things:
- Strain (materials science), the deformation of materials caused by stress on a body;
- Strain (biology), a variant of a plant, virus or bacteria;
- Strain (injury), a muscle injury.
- Strain (manga) ja:ひずみ

Tensile stress

Tensile stress (or tension) is the stress state leading to expansion; that is, the length of a material tends to increase in the tensile direction. The volume of the material stays constant; therefore in a uniaxial material the length increases in the tensile stress direction and the other two directions will decrease in size (see Poisson's ratio for detail). In the uniaxial manner of tension, tensile stress is induced by pulling forces across a bar, specimen, etc. Tensile stress is the opposite of compressive stress. Structural members in direct tension are ropes, soil anchors and nails, bolts, etc. Beams subjected to bending moments may include tensile stress as well as compressive stress and/or shear stress. Tensile stress may be increased until the reach of tensile strength, namely the limit state of stress. The formula for tensile stress is: Tensile Stress = Force / Cross-sectional Area

Robert Hooke

Robert Hooke, FRS (July 18, 1635 - March 3, 1703), one of the greatest experimental scientists of the seventeenth century, played an important role in the scientific revolution. Born in Freshwater on the Isle of Wight, Hooke received his early education at Westminster School. In 1653, Hooke won a place at Christ Church, Oxford. There he met Robert Boyle, and gained employment as his assistant. It is possible that Hooke formally stated Boyle's Law, as Boyle was not a mathematician. In 1660, he discovered Hooke's law of elasticity, which describes the linear variation of tension with extension in an elastic spring. In 1662, Hooke gained appointment as Curator of Experiments to the newly founded Royal Society, and took responsibility for experiments performed at its meetings. In 1665 he published a book entitled Micrographia, which contained a number of microscopic and telescopic observations, and some original biology. Indeed, Hooke coined the biological term cell -- so called because his observations of plant cells reminded him of monks' cells. Also in 1665 he gained appointment as Professor of Geometry at Gresham College. Robert Hooke also achieved fame as Surveyor to the City of London and chief assistant of Christopher Wren, helping to rebuild London after the Great Fire in 1666. He worked on designing the Royal Greenwich Observatory and the infamous Bethlem Royal Hospital (which became known as 'Bedlam'). He died in London in 1703. Although he was wealthy from his work in the City, he never married. No authenticated portrait of him survives, although the historian Lisa Jardine claims one portrait of John Ray represents Robert Hooke, and a seal used by Hooke displays a man's head that some have argued portrays Hooke. Both these claims remain in dispute, however.

Achievements

John Ray.]] In addition to the book Micrographia and Hooke's Law, Hooke invented the anchor escapement and may also have invented the balance spring before Christiaan Huygens. Devices known as escapements regulate the rate of a watch or clock, and the anchor escapement represented a major step in the development of accurate watches. The balance spring also regulates the flow of energy from the mainspring of a timepiece. It coils and uncoils with a natural periodicity, allowing for fine adjustment of the period of ticks. Modern spring watches still use balance springs, and derivative designs of Hooke's anchor escapement remain in common use. Historians sometimes credit Hooke with inventing the compound microscope, a design consisting of multiple lenses (usually three - an eyepiece, a field lens and an objective). While he did give much advice on new microscope designs to the instrument-maker Christopher Cock, this attribution appears incorrect, since Zacharias Janssen had already assembled compound microscopes in 1590. However, Hooke's microscopes achieved 30x magnification, which far outstripped the capabilities of any previous instruments. Hooke once called his compound microscopes "offensive to my eye" and "much strained and weakened the sight". Leeuwenhoek found his animalcules and Hooke was asked to confirm his findings. Hooke's other significant achievements include the construction of the first Gregorian reflecting telescope and the discovery of the first binary star. He also receives credit with inventing the first practical universal joint, sometimes called the Hooke joint, although the Italian mathematician Girolamo Cardano had proposed the idea about a century earlier and may or may not have built one. Hooke also experimentally demonstrated the inverse-square law of gravity, but lacked the mathematics to formally prove it.

Hooke and Newton

Robert Hooke and Isaac Newton entertained a considerable mutual dislike for each other. They fell out in 1672 when Hooke criticized Newton's presentation showing that prisms split white light rather than modifying it. Newton expressed fury that Hooke seemed unable to grasp his ground-breaking discovery, and threatened to leave the Royal Society. Relations between the men grew worse as time progressed. In 1679, Hooke wrote to Newton advocating an inverse square law of gravitation, though he lacked the mathematical ability to formally prove it. When Newton published his Principia Mathematica in 1687, including a proof of an inverse square law, he failed to credit Hooke at all. It is possible that this dispute may be overplayed: Gunther suggests that the two men held each other in some regard until quite late, citing as evidence their correspondence over matters such as the inverse-square law of gravitation, which Hooke (an undoubtedly gifted experimenter) had demonstrated. The famous Newton quote, "If I have seen further, it is by standing on the shoulders of giants", appeared originally in a letter to Hooke, and this has been interpreted as a sarcastic remark directed against Hooke. This is somewhat speculative: Hooke and Newton had exchanged many letters in tones of mutual regard, and Hooke was not of particularly short stature, although he was of slight build and had been afflicted from his youth with a severe stoop. Part of the problem was caused by Newton's retreat to Cambridge during the years around the Plague and Great Fire. Hooke remained in London, demonstrating regularly at the Royal Society, while Newton's work often took longer to reach the Society. Hooke made what are apparently barbed references to the benefits of attending the Society in person rather than receiving the reports at a later date. At a time when science was progressing by leaps and bounds it was inevitable that two men with such similar interests would come up with similar ideas. Whether Hooke or Newton first invented the reflecting telescope is a matter of conjecture, but it is the case that Hooke did demonstrate what is now known as the Newtonian telescope some time before Newton is credited with inventing it, as well as documenting "Newton's rings" before Newton did. Newton's animosity towards Hooke extended to the removal of Hooke's portrait in the Royal Society (long believed destroyed but recently rediscovered) and an attempt (prevented) to have Hooke's papers in the Society burned. It is probably thanks to Newton that Hooke's name remained relatively unknown until the latter part of the 20th Century, although Hooke's own unsympathetic character was undoubtedly also a factor: he was known to have had acrimonious exchanges with many other contemporaries, and to bear grudges.

Hooke the architect

standing on the shoulders of giants Robert Hooke was an important architect. He was the official London Surveyor after the Great Fire of 1666. As well as the Bethlem Royal Hospital, other buildings designed by Hooke include: The Royal College of Physicians (1679); Ragley Hall in Warwickshire; and the church at Willen, Buckinghamshire. Hooke's collaboration with Christopher Wren was particularly fruitful and yielded and The Royal Observatory at Greenwich, The Monument (to the Great Fire) and St Paul's Cathedral, whose dome uses a method of construction conceived by Hooke. In the reconstruction after the Great Fire, Hooke proposed redesigning London's streets on a grid pattern with wide boulevards and arteries along the lines of the Champs-Élysées, (this pattern was subsequently used for Liverpool and many American cities), but was prevented by problems over property rights. Many property owners were surreptitiously shifting their boundaries and disputes were rife. So London was rebuilt along the original mediaeval streets. It is interesting to note that the modern-day curse of congestion in London has its origin in petty disputes in the 17th Century.

Mass Media

Robert Hooke is one of many real-life personages featured in the historical adventure novels The Baroque Cycle by American author Neal Stephenson; Hooke's skill in the sciences and surgical arts are used to great (and often darkly comedic) effect throughout the cycle.

Books

- Early Science in Oxford vol vii, Dr. R. T. Gunther, ed., privately printed, 1923-67.
- Robert Hooke, Margaret 'Espinasse. William Heinmann Ltd, 1956.
- The Curious Life of Robert Hooke: The Man who Measured London, Lisa Jardine. Harper Collins Publishers, 2003. ISBN 0007149441.
- London's Leonardo: The Life and Work of Robert Hooke, Jim Bennett, Michael Cooper, Michael Hunter and Lisa Jardine. Oxford University Press, 2003. ISBN 0198525796.
- England's Leonardo: Robert Hooke and the Seventeenth-century Scientific Revolution, Allan Chapman. Institute of Physics Publishing, 2004. ISBN 0750309873.
- Robert Hooke and the English Renaissance, Allan Chapman and Paul Kent (editors). Gracewing, 2005. ISBN 0852445873.

External links

- [http://www.roberthooke.org.uk roberthooke.org.uk]
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- [http://freespace.virgin.net/ric.martin/vectis/hookeweb/roberthooke.htm Hooke Timeline]
- [http://physics.iop.org/IOP/Press/PR1203.html Robert Hooke – the face of England's Leonardo?] from the Institute of Physics
- [http://www.rod.beavon.clara.net/leonardo.htm England's Leonardo] lecture on Robert Hooke
- [http://archive.museophile.org/ox/univ-col/boyle-hooke.html Robert Boyle and Robert Hooke] Hooke, Robert Hooke, Robert Hooke, Robert Hooke, Robert Hooke, Robert Hooke, Robert Hooke, Robert Hooke, Robert Hooke, Robert Hooke, Robert ms:Robert Hooke ja:ロバート・フック

Steel

Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. Carbon acts as a hardening agent, preventing iron atoms, which are naturally arranged in a lattice, from sliding past one another. Varying the amount of carbon and its distribution in the alloy controls qualities such as the hardness, elasticity, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. One classical definition is that steels are iron–carbon alloys with up to 1.5 percent carbon by weight; alloys with higher carbon content than this are known as cast iron. Currently there are several classes of steels in which carbon is replaced with other alloying materials, and carbon, if present, is undesired. A more recent definition is that steels are iron-based alloys that can be plastically formed (pounded, rolled, etc.).

Iron and steel

plastically Iron, like most metals, is not found in the Earth's crust in a native state. Iron can be found in the crust only in combination with oxygen or sulfur. Typically Fe₂O₃—the form of iron oxide found as the mineral hematite, and FeS₂—Pyrite. Iron oxide is a soft sandstone-like material with limited uses on its own. Iron is extracted from ore by removing the oxygen by combining it with a preferred chemical partner such as carbon. This process, known as smelting, was first applied to metals with lower melting points. Copper melts at just over 1000 °C, while tin melts around 250 °C. Both temperatures could be reached with ancient methods that have been used for at least 6000 years (since the Bronze Age). Since the oxidation rate itself increases rapidly beyond 800 °C, it is important that smelting take place in a fairly oxygen-free environment. Unlike copper and tin, liquid iron dissolves carbon quite readily, so that smelting results in an alloy containing too much carbon to be called steel. Bronze Age Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, or allotropes, with very different properties; understanding these is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic structure ferrite or α-iron, a fairly soft metallic material that can dissolve only a small concentration of carbon (no more than 0.021 wt% at 910 °C). Above 910 °C ferrite undergoes a phase transition from body-centered cubic to a face-centered cubic configuration, called austenite or γ-iron, which is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04 wt% carbon at 1146 °C). As carbon-rich austenite cools, the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, and resulting in a cementite-ferrite mixture. Cementite is a stochiometric phase with the chemical formula of Fe₃C. Cementite forms in regions of higher carbon content while other areas revert to ferrite around it. Self-reinforcing patterns often emerge during this process, leading to a patterned layering known as pearlite due to its pearl-like appearance, or the similar but less beautiful bainite. Perhaps the most important allotrope is martensite, a chemically metastable substance with about four to five times the strength of ferrite. Martensite has a very similar unit cell structure to austenite, and identical chemical composition. As such, it requires extremely little thermal activation energy to form. The heat treatment process for most steels involves heating the alloy until austenite forms, then quenching the hot metal in water or oil, cooling it so rapidly that the transformation to ferrite or perlite does not have time to take place. The transformation into martensite, by contrast, occurs almost immediately, due to a lower activation energy. Martensite has a lower density than austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, these internal stresses can cause a part to shatter as it cools; at the very least, they cause internal work hardening and other microscopic imperfections. At this point, if its carbon content is high enough to produce a significant concentration of martensite, the metal resembles spring steel: extremely hard, but very brittle. Often, steel undergoes further heat treatment at a lower temperature to destroy some of the martensite (by allowing enough time for cementite, etc., to form) and help settle the internal stresses and defects. This softens the steel, producing a more ductile and fracture-resistant metal. Because time is so critical to the end result, this process is known as tempering, source of the term tempered steel. Other materials are often added to the iron-carbon mixture to tailor the resulting properties. Nickel and manganese in steel add to its tensile strength and make austenite more chemically stable, chromium increases the hardness and melting temperature, and vanadium also increases the hardness while reducing the effects of metal fatigue. Large amounts of chromium and nickel (often 18 and 8 %, respectively) are added to stainless steel so that a hard oxide forms on the metal surface to inhibit corrosion. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel. On the other hand sulfur, nitrogen, and phosphorus make steel more brittle, so these commonly found elements must be removed from the ore during processing. When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. To become steel, it must be melted and reprocessed to remove the correct amount of carbon, at which point other elements can be added. Once this liquid is cast into ingots, it usually must be "worked" at high temperature to remove any cracks or poorly mixed regions from the solidification process, and to produce shapes such as plate, sheet, wire, etc. It is then heat-treated to produce a desirable crystal structure, and often "cold worked" to produce the final shape. In modern steelmaking these processes are often combined, with ore going in one end of the assembly line and finished steel coming out the other. These can be streamlined by a deft control of the interaction between work hardening and tempering.

History of iron and steelmaking

Iron was in limited use long before it became possible to smelt it. The first signs of iron use come from Ancient Egypt and Sumer, where around 4000 BC small items, such as the tips of spears and ornaments, were being fashioned from iron recovered from meteorites (see Iron: History). About 6% of meteorites are composed of an iron-nickel alloy, and iron recovered from meteorite falls allowed ancient peoples to manufacture small numbers of iron artifacts. Meteoric iron was also fashioned into tools in precontact North America. Beginning around the year 1000, the Thule people of Greenland began making harpoons and other edged tools from pieces of the Cape York meteorite. These artifacts were also used as trade goods with other Arctic peoples: tools made from the Cape York meteorite have been found in archaeological sites more than 1000 miles (1600 km) away. When the American polar explorer Robert Peary shipped the largest piece of the meteorite to the American Museum of Natural History in New York City in 1897, it still weighed over 33 tons. The name for iron in several ancient languages means "sky metal" or something similar. In distant antiquity, iron was regarded as a precious metal, suitable for royal ornaments.

The Iron Age

ton Beginning between 3000 BC to 2000 BC increasing numbers of smelted iron objects (distinguishable from meteoric iron by their lack of nickel) appear in Anatolia, Egypt and Mesopotamia (see Iron: History). The oldest known samples of iron that appear to have been smelted from iron oxides are small lumps found at copper-smelting sites on the Sinai Peninsula, dated to about 3000 BC. Some iron oxides are effective fluxes for copper smelting; it is possible that small amounts of metallic iron were made as a by-product of copper and bronze production throughout the Bronze Age. In Anatolia, smelted iron was occasionally used for ornamental weapons: an iron-bladed dagger with a bronze hilt has been recovered from a Hattic tomb dating from 2500 BC. Also, the Egyptian ruler Tutankhamun died in 1323 BC and was buried with an iron dagger with a golden hilt. An Ancient Egyptian sword bearing the name of pharaoh Merneptah as well as a battle axe with an iron blade and gold-decorated bronze haft were both found in the excavation of Ugarit (see Ugarit). The early Hittites are known to have bartered iron for silver, at a rate of 40 times the iron's weigh, with Assyria. Iron did not, however, replace bronze as the chief metal used for weapons and tools for several centuries, despite some attempts. Working iron required more fuel and significantly more labor than working bronze, and the quality of iron produced by early smiths may have been inferior to bronze as a material for tools. Then, between 1200 and 1000 BC, iron tools and weapons displaced bronze ones throughout the near east. This process appears to have begun in the Hittite Empire around 1300 BC, or in Cyprus and southern Greece, where iron artifacts dominate the archaeological record after 1050 BC. Mesopotamia was fully into the Iron Age by 900 BC, central Europe by 800 BC. The reason for this sudden adoption of iron remains a topic of debate among archaeologists. One prominent theory is that warfare and mass migrations beginning around 1200 BC disrupted the regional tin trade, forcing a switch from bronze to iron. Egypt, on the other hand, did not experience such a rapid transition from the bronze to iron ages: although Egyptian smiths did produce iron artifacts, bronze remained in widespread use there until after Egypt's conquest by Assyria in 663 BC. Iron smelting at this time was based on the bloomery, a furnace where bellows were used to force air through a pile of iron ore and burning charcoal. The carbon monoxide produced by the charcoal reduced the iron oxides to metallic iron, but the bloomery was not hot enough to melt the iron. Instead, the iron collected in the bottom of the furnace as a spongy mass, or bloom, whose pores were filled with ash and slag. The bloom then had to be reheated to soften the iron and melt the slag, and then repeatedly beaten and folded to force the molten slag out of it. The result of this time-consuming and laborious process was wrought iron, a malleable but fairly soft alloy containing little carbon. Wrought iron can be carburized into a mild steel by holding it in a charcoal fire for prolonged periods of time. By the beginning of the Iron Age, smiths had discovered that iron that was repeatedly reforged produced a higher quality of metal. Quench-hardening was also known by this time. The oldest quench-hardened steel artifact is a knife found on Cyprus at a site dated to 1100 BC.

Developments in China

Archaeologists and historians debate whether bloomery-based ironworking ever spread to China from the West. Around 500 BC, however, metalworkers in the southern state of Wu developed an iron smelting technology that would not be practiced in Europe until late medieval times. In Wu, iron smelters achieved a temperature of 1130°C, hot enough to be considered a blast furnace. At this temperature, iron combines with 4.3% carbon and melts. As a liquid, iron can be cast into molds, a method far less laborious than individually forging each piece of iron from a bloom. Cast iron is rather brittle and unsuitable for striking implements. It can, however, be decarburized to steel or wrought iron by heating it in air for several days. In China, these ironworking methods spread northward, and by 300 BC, iron was the material of choice throughout China for most tools and weapons. A mass grave in Hebei province, dated to the early third century BC, contains several soldiers buried with their weapons and other equipment. The artifacts recovered from this grave are variously made of wrought iron, cast iron, malleabilized cast iron, and quench-hardened steel, with only a few, probably ornamental, bronze weapons. During the Han Dynasty (202 BC–AD 220), Chinese ironworking achieved a scale and sophistication not reached in the West until the eighteenth century. In the first century, the Han government established ironworking as a state monopoly and built a series of large blast furnaces in Henan province, each capable of producing several tons of iron per day. By this time, Chinese metallurgists had discovered how to puddle molten pig iron, stirring it in the open air until it lost its carbon and became wrought iron. (In Chinese, the process was called chao, literally, stir-frying.) Also during this time, Chinese metallurgists had found that wrought iron and cast iron could be melted together to yield an alloy of intermediate carbon content, that is, steel. According to legend, the sword of Liu Bang, the first Han emperor, was made in this fashion. Some texts of the era mention "harmonizing the hard and the soft" in the context of ironworking; the phrase may refer to this process.

India

Perhaps as early as 300 BC, although certainly by AD 200, high quality steel was being produced in southern India by what Europeans would later call the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in crucibles and heated until the iron melted and absorbed the carbon. The resulting high-carbon steel, called پولاد (pulâd) in Persian and wootz by later Europeans, was exported throughout much of Asia. A solid pillar of rust-resistant steel forged in 4th century AD and now standing for many centuries next to the Kutab Minar in Delhi is a testimony of the steel manufaturing skills of Indian artisans. The famous Damascus sword was made of steel imported from India.

Middle East

By the 9th century, smiths in the Abbasid caliphate had developed techniques for forging wootz to produce steel blades of unusual flexibility and sharpness (Damascus steel). The secret of forging this kind of steel was lost, even in the Middle East, by around 1600, and only recently have metallurgists found methods for reproducing its properties.

Ironworking in medieval Europe

The middle ages in Europe saw the construction of progressively larger bloomeries. By the 8th century, smiths in northern Spain had developed a style that become known as a Catalan forge, a furnace about 1 meter (3 feet) tall, capable of smelting up to 150 kg (350 lb) of iron in each batch. In succeeding centuries, smiths in the Frankish empire and later the Holy Roman Empire scaled up this basic design, increasing the height of the flue to as tall as 5 meters (16 feet) and smelting as much as 350 kg (750 lb) of iron in each batch. These larger furnaces required more draft than could be provided by human power, and forging the large blooms that resulted was also beyond the capabilities of a single man. To this end, waterwheels were employed to power the bellows and hammers. Eventually, the scaling up of the bloomery reached a point where the furnace was hot enough to produce cast iron. Although the brittle cast iron may initially have been a nuisance to the smith, as it was too brittle to be forged, the spread of cannon to Europe in the 1300s provided an application for iron casting, cast iron cannonballs. The oldest known blast furnace in Europe was constructed at Lapphyttan in Sweden, sometime between 1150 and 1350. Other early European blast furnaces were built throughout the Rhine valley: blast furnaces were in operation near Liège (a city in modern-day Belgium) in the 1340s, and at Massevaux in France by 1409. The first English blast furnace was not built until 1496, when Henry VII commissioned a new ironworks at Newbridge, in a part of Sussex known as the Weald. Despite this late start, the production of English iron foundries rapidly grew, in no small part due to foreign craftsmen hired by Henry to bring the craft of iron casting to England. In 1543, William Levett, a Wealden ironmaster, and Peter Baude, a French craftsman in Henry VIII's employ, cast the Weald's first one-piece iron cannon. English iron cannons gained a reputation for being superior to, and less expensive than, the bronze cannons made elsewhere in Europe, and at least initially, efforts to copy them outside the Weald failed. The superiority of English cannons over Spanish ones has been credited as one factor in England's 1588 defeat of the Spanish Armada. In 1619, Jan Andries Moerbeck, a Dutch ironmaster, began importing Wealden iron ore for comparison to the ore available on the Continent. One difference he observed was that the English ore contained some calcareous material, and soon after, Dutch ironmasters introduced the use of limestone as a flux in the blast furnace. This practice improved the separation of slag from the cast iron and improved the quality of Continental cast iron.

Ironworking in early modern Europe

Dutch Also by the early 1600s, ironworkers in western Europe had found a means (called cementation) to carburize wrought iron without individually forging each piece. Wrought iron bars and charcoal were packed into stone boxes, then held at a red heat for up to a week. During this time, carbon diffused into the iron, producing a product called cement steel or blister steel. For many years the best steels could be produced by buying expensive iron ore from Sweden. Although it was not understood at the time, Swedish ore had very low phosphorus content compared to most ores (notably those in England), which allowed for a finer and stronger crystal structure. Sales of Swedish ore generated considerable trade income, and local development helped the country become the industrialised nation it remains to this day. By the 18th century, deforestation in western Europe was making ironworking and its charcoal-hungry processes increasingly expensive. In 1709 Abraham Darby began smelting iron using coke, a refined coal product, in place of charcoal at his ironworks at Coalbrookdale in England. Although coke could be produced less expensively than charcoal, coke-fired iron was initially of inferior quality compared to charcoal-fired iron. It was not until the 1750s, when Darby's son refined the coking process to reduce the amount of sulfur in the coke that coke-fired furnaces became widespread. Another 18th-century European development was the reinvention of the puddling furnace. In particular, the form of coal-fired puddling furnace developed by the British engineer Henry Cort in 1784 made it possible to convert cast iron into wrought iron in large batches, finally rendering the ancient bloomery obsolete. Wrought iron produced using this method became a major metal in the English midlands' emerging toy industry. The combination of the blast furnace and the puddling furnace allowed iron to be produced at either end of the carbon spectrum, depending on the user's needs. As for alloys of intermediate carbon content (that is, steel), crucible steel was rediscovered in the 1740s by Benjamin Huntsman in Handsworth in England. In his process, wrought iron and cast iron were heated in small ceramic crucibles, melting together to form steel. While producing steel superior to cement steel, the crucible steel process remained relatively expensive in both time and fuel, and could not be used in any sort of modern industrial scale. The strong steels produced were however in high demand for specialty products such as cutlery and weapons. Sheffield's Abbeydale Industrial Hamlet has preserved a waterwheel powered, scythe-making works dating from Huntsman's times. It is still operated for the public, several times per year, using crucible steel made on the Abbeydale site.

Industrial steelmaking

Abbeydale Industrial Hamlet The problem of mass-producing steel was solved in 1855 by Henry Bessemer, with the introduction of the Bessemer converter at his steelworks in Sheffield, England. (An early converter can still be seen at the city's Kelham Island Museum). In the Bessemer process, molten pig iron from the blast furnace was charged into a large crucible, and then air was blown through the molten iron from below, igniting the dissolved carbon from the coke. As the carbon burned off, the melting point of the mixture increased, but the heat from the burning carbon provided the extra energy needed to keep the mixture molten. After the carbon content in the melt had dropped to the desired level, the air draft was cut off: a typical Bessemer converter could convert a 25-ton batch of pig iron to steel in half an hour. In 1867, the German-British engineer Sir William Siemens introduced an improved puddling furnace – the regenerative furnace – that used brick heat exchangers to preheat the incoming air and conserve fuel. The next year Pierre and Émile Martin, French ironmasters who had licensed Siemens' furnace design, developed a method for measuring the carbon content of molten iron. Thus, the decarburization could be stopped at the steel stage rather than proceeding all the way to wrought iron. This open-hearth process coexisted in industrial practice with the Bessemer process for many years, but eventually proved more economical and displaced it. Reasons for this include its ability to recycle scrap metal in addition to fresh pig iron, its greater scalability (up to hundreds of tons per batch, compared to tens of tons for the Bessemer process), and the more precise quality control it permitted. quality control Initially, only ores low in phosphorus and sulfur could be used for quality steelmaking; ores rich in those elements yielded brittle metals little better than cast iron. This problem was solved in 1878 by Percy Carlyle Gilchrist and his cousin Sidney Gilchrist Thomas at the ironworks at Blaenavon in Wales. Their modified Bessemer process used a converter lined with limestone or dolomite, and additional lime was added to the molten metal as a flux. This added basic material removed phosphorus and sulfur from the steel as insoluble calcium or magnesium phosphates and sulfates. This development expanded the range of iron ores that could be used to make steel, especially in France and Germany, where high-phosphorus ores abounded. These developments increased the availability and decreased the price of steel; 22 thousand tonnes were produced in 1867, 500 thousand in 1870, 1 million in 1880 and 28 million by 1900. Today, worldwide annual production is around 1.1 billion tonnes. This widespread availability of inexpensive steel powered the industrial revolution and modern society as we know it. It also led to the introduction of newer "niche" steels (such as stainless steel), all of them dependent on the wide availability of inexpensive iron and steel and the ability to alloy it at will.

Types of steel

Alloy steels were known from antiquity, being nickel-rich iron from meteorites hot-worked into useful products. In a modern sense, alloy steels have been made since the invention of furnaces capable of melting iron, into which other metals could be thrown and mixed.
- Carbon steel
- Damascus steel, which was famous in ancient times for its durability and ability to hold an edge, was created from a number of different materials (some only in traces), essentially a complicated alloy with iron as main component.
- Stainless steels and surgical stainless steels contain a minimum of 10.5% chromium, often combined with nickel, to resist corrosion (rust). Some stainless steels are nonmagnetic.
- Tool steels
- HSLA Steel (High Strength, Low Alloy)
- Advanced High Strength Steels
- Ferrous superalloys Though not an alloy, there exists also galvanized steel, which is steel that has gone through the chemical process of being hot-dipped or electroplated in zinc for protection against rust.

Production methods

- Crucible technique or puddling - the original steel making technique, developed in India as wootz, used in the Middle East as Damascus steel and independently redeveloped in Sheffield by Benjamin Huntsman in 1740, and Pavel Anosov in Russia in 1837.
- Bessemer process, the first commercial-scale steel production process
- Open hearth furnace
- Basic oxygen steelmaking
- Electric arc furnace a form of secondary steelmaking from scrap, though the process can also use direct-reduced iron

References

- [http://www.geology.ucdavis.edu/~cowen/~GEL115/index.html Essays on geology, history, and people]
- [http://www.staff.hum.ku.dk/dbwagner/KoreanFe/KoreanFe.html Early iron in China, Korea, and Japan]
- [http://www.davistownmuseum.org/toolPreBlastFurnace.html Precursors of the blast furnace]
- [http://members.lycos.nl/cvdv/historycastiron.htm Early progress in the melting of iron]
- [http://www.tilthammer.com/bio/index.html Steel City founders]

External links

- [http://www.soas.ac.uk/art/tanavoli.html Exhibition of Persian Steel]
- [http://www.ae.msstate.edu/vlsm/materials/alloys/steel.htm Steel Alloys and Their Classification]
- [http://www.ce.berkeley.edu/~paulmont/CE60/quench/ Quenched & Tempered Steel Alloys]
- [http://www.key-to-steel.com/Articles/Art12.htm Quench hardening of steel]
- [http://cic.nist.gov/vrml/cis2.html CIMsteel Integration Standards] Category:Alloys
- ja:鋼 simple:Steel

Yield strength

Yield strength, or the yield point, is defined in engineering as the amount of strain that a material can undergo before moving from elastic deformation into plastic deformation. Elastic deformation is spring-like deformation, where a material will return to its original shape. The stress felt by a material given a certain strain is defined by linear relationship, with a slope defined by the modulus of elasticity (E). If too much strain is applied, the material will deform permanently, or plastically. The yield point is often defined, due to the lack of a clear border between the elastic and plastic regions, by a 0.2% offset from the linear region. The point where this offset line intersects the stress-strain curve is the yield point. See also yield (engineering), tensile strength

External links

- [http://www.key-to-steel.com/Articles/Art43.htm Engineering Stress-strain Curve] Category:Continuum mechanics

Aluminum

x Aluminium or aluminum (Symbol Al) (see the spelling section below) is a silvery and ductile member of the poor metal group of chemical elements. Its atomic number is 13. Aluminium is found primarily as the ore bauxite and is remarkable for its resistance to oxidation (due to the phenomenon of passivation), its strength, and its light weight. Aluminium is used in many industries to make millions of different products and is very important to the world economy. Structural components made from aluminium are vital to the aerospace industry and very important in other areas of transportation and building in which light weight, durability, and strength are needed.

Properties

transport Aluminium is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. Aluminium is nontoxic (as the metal) nonmagnetic and non-sparking. Pure aluminium has a tensile strength of about 49 megapascals (MPa) and 700 MPa if it is formed into an alloy. Aluminium is about one-third as dense as steel or copper; is malleable, ductile, and easily machined and cast; and has excellent corrosion resistance and durability due to the protective oxide layer. It is also nonmagnetic and nonsparking and is the second most malleable metal (after gold) and the sixth most ductile. ductile

Applications

Whether measured in terms of quantity or value, the use of aluminium exceeds that of any other metal except iron, and it is important in virtually all segments of the world economy. Pure aluminium has a low tensile strength, but readily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon. When combined with thermo-mechanical processing these aluminium alloys display a marked improvement in mechanical properties. Aluminium alloys form vital components of aircraft and rockets as a result of their high strength to weight ratio. When aluminium is evaporated in a vacuum it forms a coating that reflects both visible light and radiant heat. These coatings form a thin layer of protective aluminium oxide that does not deteriorate as silver coatings do. In particular, nearly all modern mirrors are made using a thin reflective coating of aluminium on the back surface of a sheet of float glass. Telescope mirrors are also coated with a thin layer of aluminium, but are front coated to avoid internal reflections even though this makes the surface more susceptible to damage. Telescope Diet Coke.]] Some of the many uses for aluminium are in:
- Transportation (automobiles, airplanes, trucks, railroad cars, marine vessels, etc.)
- Packaging (cans, foil, etc.)
- Water treatment
- Construction (windows, doors, siding, building wire, etc.
- Consumer durable goods (appliances, cooking utensils, etc.)
- Electrical transmission lines (aluminium conductors are half the weight of copper for equal conductivity and lower in price[http://www.metalprices.com])
- Machinery.
- Although non-magnetic itself, aluminium is used in MKM steel and Alnico magnets.
- Super purity aluminium (SPA, 99.980% to 99.999% Al) is used in electronics and CDs.
- Powdered aluminium is commonly used for silvering in paint. Aluminium flakes may also be included in undercoat paints, particularly wood primer — on drying, the flakes overlap to produce a water resistant barrier.
- Anodised aluminium is more stable to further oxidation, and is used in various fields of construction.
- Most modern computer CPU heat sinks are made of aluminium due to its ease of manufacture and good heat conductivity. Copper heat sinks are smaller although more expensive and harder to manufacture. Aluminium oxide, alumina, is found naturally as corundum (rubies and sapphires), emery, and is used in glass making. Synthetic ruby and sapphire are used in lasers for the production of coherent light. Aluminium oxidises very energetically and as a result has found use in solid rocket fuels, thermite, and other pyrotechnic compositions. Aluminium is also a superconductor, with a superconduting critical temperature of 1.2 Kelvin.

Engineering use

Improper use of aluminium can result in problems, particularly in contrast to iron or steel, which appear "better behaved" to the intuitive designer, mechanic, or technician. The reduction by two thirds of the weight of an aluminium part compared to a similarly sized iron or steel part seems enormously attractive, but it should be noted that it is accompanied by a reduction by two thirds in the stiffness of the part. Therefore, although direct replacement of an iron or steel part with a duplicate made from aluminium may still give acceptable strength to withstand peak loads, the increased flexibility will cause three times more deflection in the part. Where failure is not an issue but excessive flex is undesirable due to requirements for precision of location or efficiency of transmission of power, simple replacement of steel tubing with similarly sized aluminium tubing will result in a degree of flex which is undesirable; for instance, the increased flex under operating loads caused by replacing steel bicycle frame tubing with aluminium tubing of identical dimensions will cause misalignment of the power-train as well as absorbing the operating force. To increase the rigidity by increasing the thickness of the walls of the tubing increases the weight proportionately, so that the advantages of lighter weight are lost as the rigidity is restored. Aluminium can best be used by redesigning the part to suit its characteristics; for instance making a bicycle of aluminium tubing which has an oversize diameter rather than thicker walls. In this way, rigidity can be restored or even enhanced without increasing weight. The limit to this process is the increase in susceptibility to what is termed "crippling" failure, where the deviation of the force from any direction other than directly along the axis of the tubing causes folding of the walls of the tubing. For instance, a common aluminium soft drink can should be able to support an enormous weight directly along its axis; in practice, however, the walls of the can buckle, crumple, and/or fold up under even a mild force, due to minute deviations from the precise axial direction, making possible the common pastime of flattening an empty can by slamming it against one's forehead. The latest models of the Corvette automobile, among others, are a good example of redesigning parts to make best use of aluminium's advantages. The aluminium chassis members and suspension parts of these cars have large overall dimensions for stiffness but are lightened by reducing cross-sectional area and removing unneeded metal; as a result, they are not only equally or more durable and stiff as the usual steel parts, but they possess an airy gracefulness which most people find attractive. Similarly, aluminium bicycle frames can be optimally designed so as to provide rigidity where required, yet have flexibility in terms of absorbing the shock of bumps from the road and not transmitting them to the rider. The strength and durability of aluminium varies widely, not only as a result of the components of the specific alloy, but also as a result of the particular manufacturing process; for this reason, it has from time to time gained a bad reputation. For instance, a high frequency of failure in many early aluminium bicycle frames in the 1970s resulted in just such a poor reputation; with a moment's reflection, however, the widespread use of aluminium components in the aerospace and automotive high performance industries, where huge stresses are undergone with vanishingly small failure rates, proves that properly built aluminium bicycle components should not be unusually unreliable, and this has subsequently proved to be the case. Similarly, use of aluminium in automotive applications, particularly in engine parts which must survive in difficult conditions, has benefited from development over time. An Audi engineer commented about the V12 engine, producing over 500 horsepower (370 kW), of an Auto Union race car of the 1930s which was recently restored by the Audi factory, that the aluminium alloy of which the engine was constructed would today be used only for lawn furniture and the like. Even the aluminium cylinder heads and crankcase of the Corvair, built as recently as the 1960s, earned a reputation for failure and stripping of threads in holes, even as large as spark plug holes, which is not seen in current aluminium cylinder heads. Often, aluminium's sensitivity to heat must also be considered. Even a relatively routine procedure such as welding is complicated by the fact that aluminium will melt long before it gets even dully red hot; therefore, unlike steel or iron, where the experienced welder can know from its hue how close the metal is to the melting point, welding aluminium requires a degree of expertise incorporating an almost intuitive sense of the metal's temperature, or else the part suddenly and without warning melts into a puddle. Aluminium also will accumulate internal stresses and strains under conditions of overheating; while not immediately obvious, the tendency of the metal to "creep" under sustained stresses results in delayed distortions, for instance the commonly observed warping or cracking of aluminium automobile cylinder heads after an engine is overheated, sometimes as long as years later, or the tendency of welded aluminium bicycle frames to gradually twist out of alignment from the stresses accumulated during the welding process. For this reason, many uses of aluminium in the aerospace industry avoid heat altogether by joining parts using adhesives; this was also used for some of the early aluminium bicycle frames in the 1970s, with unfortunate results when the aluminium tubing corroded slightly, loosening the bond of the adhesive and leading to failure of the frame. Stresses from overheating aluminium can be relieved by heat-treating the parts in an oven and gradually cooling, in effect annealing the stresses; this can also result, however, in the part becoming distorted as a result of these stresses, so that such heat-treating of welded bicycle frames, for instance, results in a significant fraction becoming misaligned. If the misalignment is not too severe, once cooled they can be bent back into alignment with no negative consequences; of course, if the frame is properly designed for rigidity (see above), this will require enormous force.

Household wiring

Because of its high conductivity and relatively low price compared to copper at the time, aluminium was introduced for household electrical wiring to a large degree in the United States in the 1960s. Unfortunately, many of the wiring fixtures at the time were not designed to accept aluminium wire. More specifically:
- The greater coefficient of thermal expansion of aluminium, causes the wire to expand and contract relative to the dissimilar metal screw connection, eventually loosening the connection.
- Pure aluminium has a tendency to "creep" under steady sustained pressure (to a greater degree as the temperature rises), again producing a degree of looseness in an initially tight connection.
- Galvanic corrosion from the dissimilar metals increases the electrical resistance of the connection. In combination, these properties caused connections between electrical fixtures and aluminium wiring to overheat which resulted in several fires. As a result, aluminium household wiring has become unpopular, and in many jurisdictions is not permitted in very small sizes in new construction. However, aluminium wiring can be safely used with fixtures whose connections are designed to avoid loosening and overheating. Older fixtures of this type are marked "Al/Cu", and newer ones are marked "CO/ALR". Otherwise, aluminium wiring can be terminated by crimping it to a short "pigtail" of copper wire, which can be treated as any other copper wire. A properly done crimp, requiring high pressure produced by the proper tool, is tight enough not only to eliminate any thermal expansion of the aluminium, but also to exclude any atmospheric oxygen and thus prevent corrosion between dissimilar metals. New alloys are used for aluminium building wire today in combination with aluminium terminations. Connections made with these standard industry products are as safe and reliable as copper connections. :See also:Aluminum wire

History

The oldest suspected (although unprovable) reference to aluminium is in Pliny the Elder's Naturalis Historia: One day a goldsmith in Rome was allowed to show the Emperor Tiberius a dinner plate of a new metal. The plate was very light, and almost as bright as silver. The goldsmith told the Emperor that he had produced the metal from ordinary clay. He also assured the Emperor that only he, himself, and the gods knew how to produce this metal from clay. The Emperor became very interested, and, as a financial expert, he was also worried. He feared that all his treasures of gold and silver would fall in value if people started producing this bright metal from clay. Therefore, instead of giving the goldsmith the recognition the latter had anticipated, he ordered him to be beheaded. [http://www.findarticles.com/p/articles/mi_m2843/is_n3_v19/ai_16836663 Notes] - [http://www.world-aluminium.org/history/antiquity.html Source] The ancient Greeks and Romans used salts of this metal as dyeing mordants and as astringents for dressing wounds, and alum is still used as a styptic. Further Joseph Needham suggested finds in 1974 showed the ancient Chinese used aluminium (see the link for "Notes" above). In 1761 Guyton de Morveau suggested calling the base alum 'alumine'. In 1808, Humphry Davy identified the existence of a metal base of alum, which he named (see Spelling below for more information on the name). Friedrich Wöhler is generally credited with isolating aluminium (Latin alumen, alum) in 1827 by mixing anhydrous aluminium chloride with potassium. However, the metal had been produced for the first time two years earlier in an impure form by the Danish physicist and chemist Hans Christian Ørsted. Therefore almanacs and chemistry sites often list Øersted as the discoverer of aluminium.[http://www.chemicalelements.com/elements/al.html#isotopes Source] Still it would further be P. Berthier who discovered aluminium in bauxite ore and successfully extracted it. The Frenchman Henri Saint-Claire Deville improved Wöhler's method in 1846 and described his improvements in a book in 1859, chief among these being the substitution of sodium for the considerably more expensive potassium. The American Charles Martin Hall of Oberlin, OH applied for a patent (400655) in 1886 for an electrolytic process to extract aluminium using the same technique that was independently being developed by the Frenchman Paul Héroult in Europe. The invention of the Hall-Héroult process in 1886 made extracting aluminium from minerals cheaper, and is now the principal method in common use throughout the world. Upon approval of his patent in 1889, Hall, with the financial backing of Alfred E. Hunt of Pittsburgh, PA, started the Pittsburgh Reduction Company, renamed to Aluminum Company of America in 1907, later shortened to Alcoa. Alcoa Aluminium was selected as the material to be used for the apex of the Washington Monument, at a time when one ounce cost twice the daily wages of a common worker in the project. [http://www.tms.org/pubs/journals/JOM/9511/Binczewski-9511.html Source] Germany became the world leader in aluminium production soon after Adolf Hitler seized power. By 1942, however, new hydroelectric power projects such as the Grand Coulee Dam gave the United States something Nazi Germany could not hope to compete with, namely the capability of producing enough aluminium to manufacture sixty thousand warplanes in four years. [http://www.phpsolvent.com/wordpress/?page_id=341]

Natural occurrence

Although aluminium is an abundant element in Earth's crust (believed to be 7.5% to 8.1%), it is very rare in its free form and was once considered a precious metal more valuable than gold. Napoleon III of France had a set of aluminium plates reserved for his finest guests. Others had to make do with gold ones. Aluminium has been produced in commercial quantities for just over 100 years. Aluminium was, when it was first discovered, extremely difficult to separate from its ore. Aluminium is among the most difficult metals on earth to refine, despite the fact that it is one of the planet's most common. The reason is that aluminium is oxidised very rapidly and that its oxide is an extremely stable compound that, unlike rust on iron, does not flake off. The very reason for which aluminium is used in many applications is why it is so hard to produce. Recovery of this metal from scrap (via recycling) has become an important component of the aluminium industry. Recycling involves simply melting the metal, which is far less expensive than creating it from ore. Refining aluminium requires enormous amounts of electricity; recycling it requires only 5% of the energy to produce it. A common practice since the early 1900s, aluminium recycling is not new. It was, however, a low-profile activity until the late 1960s when the exploding popularity of aluminium beverage cans finally placed recycling into the public consciousness. Other sources for recycled aluminium include automobile parts, windows and doors, appliances, containers and other products. Aluminium is a reactive metal and it is hard to extract it from its ore, aluminium oxide (Al₂O₃). Direct reduction, with carbon for example, is not economically viable since aluminium oxide has a melting point of about 2000°C. Therefore, it is extracted by electrolysis — the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. By this process, the actual operational temperature of the reduction cells is around 950 to 980°C. Cryolite was originally found as a mineral on Greenland, but has been replaced by a synthetic cryolite. Cryolite is a mixture of aluminium, sodium, and calcium fluorides: (Na₃AlF₆). The aluminium oxide (a white powder) is obtained by refining bauxite, which is red since it contains 30 to 40% iron oxide. This is done using the so-called Bayer process. Previously, the Deville process was the predominant refining technology. The electolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the ore is in the molten state, its ions are free to move around. The reaction at the negative cathode is :Al³⁺ + 3e^- → Al Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off. At the positive electrode (anode) oxygen gas is formed: :2O^2- → O₂ + 4e^- This carbon anode is then oxidised by the oxygen. The anodes in a reduction must therefore be replaced regularly, since they are consumed in the process: :O₂ + C → CO₂ Contrary to the anodes, the cathodes are not consumed during the operation, since there is no oxygen present at the cathode. The carbon cathode is protected by the liquid aluminium inside the cells. Cathodes do erode, mainly due to electrochemical processes. After 5 to 10 years, depending on the current used in the electrolysis, a cell has to be reconstructed completely, because the cathodes are completely worn. Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The world-wide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters reach approximately 12.8 kW·h/kg (46.1 MJ/kg). Reduction line current for older technologies are typically 100 to 200 kA. State-of-the-art smelters operate with about 350 kA. Trials have been reported with 500 kA cells. Electric power represents about 20 to 40% of the cost of producing aluminium, depending on the location of the aluminium smelter. Smelters tend to be located where electric power is plentiful and inexpensive, such as South Africa, the South Island of New Zealand, Australia, China, Middle-East, Russia, Iceland and Quebec in Canada. China is currently (2004) the top world producer of aluminium. Suriname depends on aluminium exports for 70% of its export earnings.[http://www.cia.gov/cia/publications/factbook/geos/ns.html#Econ]

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