In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform. The concept is similar to the rolling of dough. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature the process is known as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is known as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process, cold rolling processes the most tonnage out of all cold working processes. Roll stands holding pairs of rolls are grouped together into rolling mills that can process metal steel, into products such as structural steel, bar stock, rails. Most steel mills have rolling mill divisions that convert the semi-finished casting products into finished products. There are many types of rolling processes, including ring rolling, roll bending, roll forming, profile rolling, controlled rolling.
The invention of the rolling mill in Europe may be attributed to Leonardo da Vinci in his drawings. The earliest rolling mills in crude form but the same basic principles were found in Middle East and South Asia as early as 600 BCE. Earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in 1590; these passed flat bars between rolls to form a plate of iron, passed between grooved rolls to produce rods of iron. The first experiments at rolling iron for tinplate took place about 1670. In 1697, Major John Hanbury erected a mill at Pontypool to roll'Pontypool plates'—blackplate; this began to be rerolled and tinned to make tinplate. The earlier production of plate iron in Europe had been in forges, not rolling mills; the slitting mill was adapted to producing hoops and iron with a half-round or other sections by means that were the subject of two patents of c. 1679. Some of the earliest literature on rolling mills can be traced back to Christopher Polhem in 1761 in Patriotista Testamente, where he mentions rolling mills for both plate and bar iron.
He explains how rolling mills can save on time and labor because a rolling mill can produce 10 to 20 or more bars at the same time. A patent was granted to Thomas Blockley of England in 1759 for the rolling of metals. Another patent was granted in 1766 to Richard Ford of England for the first tandem mill. A tandem mill is one. Rolling mills for lead seem to have existed by the late 17th century. Copper and brass were rolled by the late 18th century. Modern rolling practice can be attributed to the pioneering efforts of Henry Cort of Funtley Iron Mills, near Fareham, England. In 1783, a patent was issued to Henry Cort for his use of grooved rolls for rolling iron bars. With this new design, mills were able to produce 15 times more output per day than with a hammer. Although Cort was not the first to use grooved rolls, he was the first to combine the use of many of the best features of various ironmaking and shaping processes known at the time, thus modern writers have called him "father of modern rolling."
The first rail rolling mill was established by John Birkenshaw in 1820, where he produced fish bellied wrought iron rails in lengths of 15 to 18 feet. With the advancement of technology in rolling mills, the size of rolling mills grew along with the size of the products being rolled. One example of this was at The Great Exhibition in 1851, where a plate 20 feet long, 3 ½ feet wide, 7/16 of an inch thick, weighing 1,125 pounds, was exhibited by the Consett Iron Company. Further evolution of the rolling mill came with the introduction of three-high mills in 1853 used for rolling heavy sections. Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening; the starting material is large pieces of metal, like semi-finished casting products, such as slabs and billets. If these products came from a continuous casting operation the products are fed directly into the rolling mills at the proper temperature.
In smaller operations, the material must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces; as the material is worked, the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling. Hot-rolled metals have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than 20 mm thick have some directional properties. Non-uniform cooling will induce a lot of residual stresses, which occurs in shapes that have a non-uniform cross-section, such as I-beams. While the finished product is of good quality, the surface is covered in mill scale, an oxide that forms at high temperatures, it is removed via pickling or the smooth clean surface process, which reveals a smooth surface.
Dimensional tolerances are us
In materials science, a dislocation or Taylor's dislocation is a crystallographic defect or irregularity within a crystal structure. The presence of dislocations influences many of the properties of materials; the theory describing the elastic fields of the defects was developed by Vito Volterra in 1907. The term'dislocation' referring to a defect on the atomic scale was coined by G. I. Taylor in 1934; some types of dislocations can be visualized as being caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead they bend around the edge of the terminating plane so that the crystal structure is ordered on either side; this phenomenon is analogous to the following situation related to a stack of paper: If half of a piece of paper is inserted into a stack of paper, the defect in the stack is noticeable only at the edge of the half sheet. The two primary types of dislocations are edge dislocations and screw dislocations.
Mixed dislocations are intermediate between these. Mathematically, dislocations are a type of topological defect, sometimes called a soliton. Dislocations behave as stable particles: they can move around, but maintain their identity. Two dislocations of opposite orientation can cancel when brought together, but a single dislocation cannot "disappear" on its own. Two main types of dislocations exist: screw. Dislocations found in real materials are mixed, meaning that they have characteristics of both. A crystalline material consists of a regular array of atoms, arranged into lattice planes. One approach is to begin by considering a 3D representation of a perfect crystal lattice, with the atoms represented by spheres; the viewer may start to simplify the representation by visualising planes of atoms instead of the atoms themselves. An edge dislocation is a defect where an extra half-plane of atoms is introduced midway through the crystal, distorting nearby planes of atoms; when enough force is applied from one side of the crystal structure, this extra plane passes through planes of atoms breaking and joining bonds with them until it reaches the grain boundary.
A simple schematic diagram of such atomic planes can be used to illustrate lattice defects such as dislocations.. The dislocation has two properties, a line direction, the direction running along the bottom of the extra half plane, the Burgers vector which describes the magnitude and direction of distortion to the lattice. In an edge dislocation, the Burgers vector is perpendicular to the line direction; the stresses caused by an edge dislocation are complex due to its inherent asymmetry. These stresses are described by three equations: σ x x = − μ b 2 π y 2 σ y y = μ b 2 π y 2 τ x y = μ b 2 π x 2 where μ is the shear modulus of the material, b is the Burgers vector, ν is Poisson's ratio and x and y are coordinates; these equations suggest a vertically oriented dumbbell of stresses surrounding the dislocation, with compression experienced by the atoms near the "extra" plane, tension experienced by those atoms near the "missing" plane. A screw dislocation is much harder to visualize. Imagine cutting a crystal along a plane and slipping one half across the other by a lattice vector, the halves fitting back together without leaving a defect.
This is similar to the Riemann surface of the complex logarithm. If the cut only goes part way through the crystal, slipped, the boundary of the cut is a screw dislocation, it comprises a structure in which a helical path is traced around the linear defect by the atomic planes in the crystal lattice. The closest analogy is a spiral-sliced ham. In pure screw dislocations, the Burgers vector is parallel to the line direction. Despite the difficulty in visualization, the stresses caused by a screw dislocation are less complex than those of an edge dislocation; these s
Johann Bauschinger was a mathematician and professor of Engineering Mechanics at Munich Polytechnic from 1868 until his death. The Bauschinger effect in materials science is named after him, he was the father of astronomer Julius Bauschinger. "Zur Geschichte der Wissenschaft vom Holz: Johann Bauschinger 1834–1893", Holz als Roh- und Werkstoff, 30: 239, 1972, doi:10.1007/BF02617596. Mughrabi, H. "Johann Bauschinger, pioneer of modern materials testing", Materials Forum, 10: 5–10
A chemical substance is a form of matter having constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. Chemical substances can be chemical compounds, or alloys. Chemical elements may not be included in the definition, depending on expert viewpoint. Chemical substances are called'pure' to set them apart from mixtures. A common example of a chemical substance is pure water. Other chemical substances encountered in pure form are diamond, table salt and refined sugar. However, in practice, no substance is pure, chemical purity is specified according to the intended use of the chemical. Chemical substances exist as solids, gases, or plasma, may change between these phases of matter with changes in temperature or pressure. Chemical substances may be converted to others by means of chemical reactions. Forms of energy, such as light and heat, are not matter, are thus not "substances" in this regard.
A chemical substance may well be defined as "any material with a definite chemical composition" in an introductory general chemistry textbook. According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. But, there are exceptions to this definition; the chemical substance index published by CAS includes several alloys of uncertain composition. Non-stoichiometric compounds are a special case that violates the law of constant composition, for them, it is sometimes difficult to draw the line between a mixture and a compound, as in the case of palladium hydride. Broader definitions of chemicals or chemical substances can be found, for example: "the term'chemical substance' means any organic or inorganic substance of a particular molecular identity, including – any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature". In geology, substances of uniform composition are called minerals, while physical mixtures of several minerals are defined as rocks.
Many minerals, mutually dissolve into solid solutions, such that a single rock is a uniform substance despite being a mixture in stoichiometric terms. Feldspars are a common example: anorthoclase is an alkali aluminum silicate, where the alkali metal is interchangeably either sodium or potassium. In law, "chemical substances" may include both pure substances and mixtures with a defined composition or manufacturing process. For example, the EU regulation REACH defines "monoconstituent substances", "multiconstituent substances" and "substances of unknown or variable composition"; the latter two consist of multiple chemical substances. For example, charcoal is an complex polymeric mixture that can be defined by its manufacturing process. Therefore, although the exact chemical identity is unknown, identification can be made to a sufficient accuracy; the CAS index includes mixtures. Polymers always appear as mixtures of molecules of multiple molar masses, each of which could be considered a separate chemical substance.
However, the polymer may be defined by a known precursor or reaction and the molar mass distribution. For example, polyethylene is a mixture of long chains of -CH2- repeating units, is sold in several molar mass distributions, LDPE, MDPE, HDPE and UHMWPE; the concept of a "chemical substance" became established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate. He deduced; this is now known as the law of constant composition. With the advancement of methods for chemical synthesis in the realm of organic chemistry. However, there are some controversies regarding this definition because the large number of chemical substances reported in chemistry literature need to be indexed. Isomerism caused much consternation to early researchers, since isomers have the same composition, but differ in configuration of the atoms. For example, there was much speculation for the chemical identity of benzene, until the correct structure was described by Friedrich August Kekulé.
The idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers. An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmuted into another element through a nuclear reaction; this is so, beca
In materials science, fatigue is the weakening of a material caused by applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading; the nominal maximum stress values that cause such damage may be much less than the strength of the material quoted as the ultimate tensile stress limit, or the yield stress limit. Fatigue occurs when a material is unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands, interfaces of constituents in the case of composites, grain interfaces in the case of metals. A crack will reach a critical size, the crack will propagate and the structure will fracture; the shape of the structure will affect the fatigue life. Round holes and smooth transitions or fillets will increase the fatigue strength of the structure; the American Society for Testing and Materials defines fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs.
For some materials, notably steel and titanium, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit, endurance limit, or fatigue strength. Engineers have used any of three methods to determine the fatigue life of a material: the stress-life method, the strain-life method, the linear-elastic fracture mechanics method. One method to predict fatigue life of materials is the Uniform Material Law. UML was developed for fatigue life prediction of aluminium and titanium alloys by the end of 20th century and extended to high-strength steels, cast iron. In metal alloys, for the simplifying case when there are no macroscopic or microscopic discontinuities, the process starts with dislocation movements at the microscopic level, which form persistent slip bands that become the nucleus of short cracks. Macroscopic and microscopic discontinuities as well as component design features which cause stress concentrations are common locations at which the fatigue process begins.
Fatigue is a process that has a degree of randomness showing considerable scatter in identical samples in well controlled environments. Fatigue is associated with tensile stresses but fatigue cracks have been reported due to compressive loads; the greater the applied stress range, the shorter the life. Fatigue life scatter tends to increase for longer fatigue lives. Damage is cumulative. Materials do not recover. Fatigue life is influenced by a variety of factors, such as temperature, surface finish, metallurgical microstructure, presence of oxidizing or inert chemicals, residual stresses, scuffing contact, etc; some materials exhibit a theoretical fatigue limit below which continued loading does not lead to fatigue failure. High cycle fatigue strength can be described by stress-based parameters. A load-controlled servo-hydraulic test rig is used in these tests, with frequencies of around 20–50 Hz. Other sorts of machines—like resonant magnetic machines—can be used, to achieve frequencies up to 250 Hz.
Low cycle fatigue is associated with localized plastic behavior in metals. Testing is conducted with constant strain amplitudes at 0.01–5 Hz. 1837: Wilhelm Albert publishes the first article on fatigue. He devised a test machine for conveyor chains used in the Clausthal mines. 1839: Jean-Victor Poncelet describes metals as being'tired' in his lectures at the military school at Metz. 1842: William John Macquorn Rankine recognises the importance of stress concentrations in his investigation of railroad axle failures. The Versailles train wreck was caused by fatigue failure of a locomotive axle. 1843: Joseph Glynn reports on the fatigue of an axle on a locomotive tender. He identifies the keyway as the crack origin. 1848: The Railway Inspectorate reports one of the first tyre failures from a rivet hole in tread of railway carriage wheel. It was a fatigue failure. 1849: Eaton Hodgkinson is granted a "small sum of money" to report to the UK Parliament on his work in "ascertaining by direct experiment, the effects of continued changes of load upon iron structures and to what extent they could be loaded without danger to their ultimate security".
1854: Braithwaite reports on common service fatigue failures and coins the term fatigue. 1860: Systematic fatigue testing undertaken by Sir William Fairbairn and August Wöhler. 1870: Wöhler summarises his work on railroad axles. He concludes that cyclic stress range is more important than peak stress and introduces the concept of endurance limit. 1903: Sir James Alfred Ewing demonstrates the origin of fatigue failure in microscopic cracks. 1910: O. H. Basquin proposes a log-log relationship for S-N curves, using Wöhler's test data. 1940: Sidney M. Cadwell publishes first rigorous study of fatigue in rubber. 1945: A. M. Miner popularises Palmgren's linear damage hypothesis as a practical design tool. 1952: W. Weibull An S-N curve model. 1954: The world's first commercial jetliner, the de Havilland Comet, suffers disaster as three planes break up in mid-air, causing de Havilland and all other manufacturers to redesign high altitude aircraft and in particular replace square aperture