Ductility is a mechanical property that describes the extent in which solid materials can be plastically In materials science, deformation is a change in the shape or size of an object due to an applied force. This can be a result of tensile forces, compressive (pushing) forces, shear, bending or torsion (twisting). Deformation is often described as strain deformed without fracture The word fracture is often applied to bones of living creatures, or to crystals or crystalline materials, such as gemstones or metal. Sometimes, in crystalline materials, individual crystals fracture without the body actually separating into two or more pieces. Depending on the substance which is fractured, a fracture reduces strength or inhibits.

In materials science Materials science is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering. This science investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It includes elements of applied physics and chemistry, ductility specifically refers to a material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar concept, refers to a material's ability to deform under compressive stress; this is often characterized by the material's ability to form a thin sheet by hammering or rolling. Ductility and malleability do not always correlate with each other; for instance, gold is both ductile and malleable, but lead is only malleable.^[1] Commonly, the term "ductility" is used to refer to both concepts, as they are very similar.

Scientific fields

Geology

In Earth science Earth science , is an all-embracing term for the sciences related to the planet Earth. It is arguably a special case in planetary science, the Earth being the only known life-bearing planet. There are both reductionist and holistic approaches to Earth sciences. The formal discipline of Earth sciences may include the study of the atmosphere, oceans the brittle-ductile transition zone The brittle-ductile transition zone is the strongest part of the Earth's crust. For quartz and feldspar rich rocks in continental crust this occurs at an approximate depth of 13–18 km . At this depth rock becomes less likely to fracture, and more likely to deform ductilely by creep. This happens because the brittle strength of a material is is a zone, at an approximate depth of 15 km (9 mi) in continental crust The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in contrast to the oceanic crust, called sima due to its, at which rock In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids becomes less likely to fracture and more likely to deform ductilely. In glacial A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice. The corresponding area of study is called glaciology ice Ice, technically, is one of the 15 known crystalline phases of water. In non-scientific contexts, the term usually means ice Ih, which is known to be the most abundant of these solid phases. It can appear transparent or opaque bluish-white colour, depending on the presence of impurities or air inclusions. The addition of other materials such as this zone is at approximately 30 m (100 ft) depth. It is still possible for material above a brittle-ductile transition zone to deform ductilely, and possible for material below to deform brittly. The zone exists because as depth increases confining pressure increases, and brittle strength increases with confining pressure whilst ductile strength decreases with increasing temperature. The transition zone occurs at the point where brittle strength exceeds ductile strength.

Materials science

Ductility is especially important in metalworking Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewellery. It therefore includes a correspondingly wide range of skills, processes, and tools, as materials that crack or break under stress cannot be manipulated using metal forming processes, such as hammering A hammer is a tool meant to deliver an impact to an object. The most common uses are for driving nails, fitting parts, forging metal and breaking up objects. Hammers are often designed for a specific purpose, and vary widely in their shape and structure. The usual features are a handle and a head, with most of the weight in the head. The basic, rolling Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair of rolls. There are two types of rolling process, flat and profile rolling. In flat rolling the final shape of the product is either classed as sheet (typically thickness less than 3 mm, also called "strip") or plate (typically, and drawing Drawing is a metalworking process which uses tensile forces to stretch metal. It is broken up into two types: sheet metal drawing and wire, bar, and tube drawing. The specific definition for sheet metal drawing is that it involves plastic deformation over a curved axis. For wire, bar, and, tube drawing the starting stock is drawn through a die to. Malleable materials can be formed using stamping Stamping includes a variety of sheet-metal forming manufacturing processes, such as punching using a machine press or stamping press, blanking, embossing, bending, flanging, and coining. This could be a single stage operation where every stroke of the press produce the desired form on the sheet metal part, or could occur through a series of stages or pressing A servomechanism press, also known as a servo press or a 'electro press, is a press driven by an AC servo motor. The torque produced is converted to a linear force via a ball screw. Pressure and position are controlled though a load cell and an encoder. The main advantage of a servo press is its low energy consumption; its only 10-20% of other, whereas brittle metals and plastics A plastic material is any of a wide range of synthetic or semi-synthetic organic amorphous solids[citation needed] used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce costs. Monomers of plastic are either natural or synthetic must be molded A mold or mould is a hollowed-out block that is filled with a liquid like plastic, glass, metal, or ceramic raw materials. The liquid hardens or sets inside the mold, adopting its shape. A mold is the opposite of a cast. The manufacturer who makes the molds is called the moldmaker. A release agent is typically used to make removal of the hardened/.

High degrees of ductility occur due to metallic bonds Metallic bonding is the electromagnetic interaction between delocalized electrons, called conduction electrons and gathered in an "electron sea", and the metallic nuclei within metals. Understood as the sharing of "free" electrons among a lattice of positively-charged ions , metallic bonding is sometimes compared with that of, which are found predominantly in metals and leads to the common perception that metals are ductile in general. In metallic bonds valence shell An electron shell may be thought of as an orbit followed by electrons around an atom nucleus. Because each shell can contain only a fixed number of electrons, each shell is associated with a particular range of electron energy, and thus each shell must fill completely before electrons can be added to an outer shell. The electrons in the outermost electrons The electron is a subatomic particle carrying a negative electric charge. It has no known components or substructure, and therefore is believed to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton. The intrinsic angular momentum of the electron is a half integer value in units of ħ, which means that are delocalized and shared between many atoms. The delocalized electrons In chemistry delocalized electrons are electrons in a molecule that are not associated with a single atom or to a covalent bond. Delocalized electrons are contained within an orbital that extends over several adjacent atoms. Classically, delocalized electrons can be found in conjugated systems of double bonds and in aromatic and mesoionic systems allow metal atoms to slide past one another without being subjected to strong repulsive forces that would cause other materials to shatter.

Ductility can be quantified by the fracture strain , which is the engineering strain In continuum mechanics, deformation or strain is the change in the metric properties of a continuous body B in the displacement from an initial placement κ0 to a final placement κ(B). A change in the metric properties means that a curve drawn in the initial body placement changes its length when displaced to a curve in the final placement. If at which a test specimen fractures during a uniaxial tensile test In continuum mechanics, the concept of stress, introduced by Cauchy around 1822, is a measure of the average amount of force exerted per unit area of a surface within a deformable body on which internal forces act . In other words, it is a measure of the intensity or internal distribution of the total internal forces acting within a deformable. Another commonly used measure is the reduction of area at fracture q.^[2]

The following list ranks metals from the greatest ductility to least: gold Gold is a chemical element with the symbol Au (from Latin: aurum, "shining dawn", hence adjective, aureate) and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial, silver Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal. The metal occurs naturally in its pure, free form (native silver), as an alloy with gold and other metals, and, platinum Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements. A dense, malleable, ductile, precious, gray-white transition metal,, iron Iron is the most common element in the earth as a whole, and the fourth most common in the Earth's crust. It is produced as a result of stellar fusion in high-mass stars, and it is the heaviest stable element produced by stellar fusion because the fusion of iron is the last nuclear fusion reaction that is exothermic. Iron is the most widely used, nickel Nickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements that exist around room temperature, the other three being iron, cobalt and gadolinium, copper Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a pinkish or peachy color. It is used as a thermal conductor, an electrical conductor, a building material, and a, aluminium Aluminium (UK: /ˌæljʉˈmɪniəm/ AL-yew-MIN-ee-əm) or aluminum (US: /əˈluːmɨnəm/ ( listen) ə-LOO-mi-nəm) is a silvery white member of the boron group of chemical elements. It has the symbol Al and its atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is the most abundant metal in the Earth's crust,, zinc Zinc , also known as spelter, is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2. Zinc is the 24th most abundant element in the, tin Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead, like the two possible oxidation states +2 and +4. Tin is the 49th most abundant element and has, with 10 stable isotopes, the largest, and lead Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed to air. Lead has a shiny chrome-silver luster when it is melted into a.^[1] The malleability of the same metals are then ranked from greatest to least: gold, silver, lead, copper, aluminium, tin, platinum, zinc, iron, and nickel.^[1] The ductility of steel Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten. Carbon and other elements act as a hardening agent, preventing varies depending on the alloying constituents. Increasing levels of carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with 12C and 13C being stable, while 14C is radioactive, decaying with a half-life of decreases ductility. Many plastics and amorphous solids An "amorphous solid" is a solid in which there is no long-range order of the positions of the atoms. . Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous solid, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous, such as Play-Doh, are also malleable.

Ductile-brittle transition temperature

The ductile-brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature of a metal represents the point at which the fracture energy passes below a pre-determined point (for steels typically 40 J^[3] for a standard Charpy impact test The Charpy impact test, also known as the Charpy v-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's toughness and acts as a tool to study temperature-dependent brittle-ductile transition. It is widely applied in). DBTT is important since, once a material is cooled below the DBTT, it has a much greater tendency to shatter on impact instead of bending or deforming. For example, zamak 3 Zamak is a family of alloys with a base metal of zinc and alloying elements of aluminium, magnesium and copper. Zamak alloys are part of the zinc aluminium alloy family; they are distinguished from the other ZA alloys because of their constant 4% aluminium composition. The name zamak is an acronym of the German names for the metals of which the exhibits good ductility at room temperature but shatters at sub-zero temperatures when impacted. DBTT is a very important consideration in materials selection when the material in question is subject to mechanical stresses. A similar phenomenon, the glass transition temperature Glass transition or vitrification refers to the transformation of a glass-forming liquid into a glass, which usually occurs upon rapid cooling. It is a dynamic phenomenon occurring between two distinct states of matter , each with different physical properties. Upon cooling through the temperature range of glass transition (a "glass, occurs with glasses and polymers, although the mechanism is different in these amorphous materials.

In some materials this transition is sharper than others. For example, the transition is generally sharper in materials with a body-centered cubic The cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals (BCC) lattice than those with a face-centered cubic The cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals (FCC) lattice. DBTT can also be influenced by external factors such as neutron radiation Neutrons may be emitted during either spontaneous or induced nuclear fission, nuclear fusion processes, very high energy reactions such as in the Spallation Neutron Source and in cosmic ray interactions, or from other nuclear reactions such as the historically significant reaction. Neutron radiation was discovered as a result of observing a, which leads to an increase in internal lattice defects Crystalline solids have a very regular atomic structure: that is, the local positions of atoms with respect to each other are repeated at the atomic scale. These arrangements are called crystal structures, and their study is called crystallography. However, most crystalline materials are not perfect: the regular pattern of atomic arrangement is and a corresponding decrease in ductility and increase in DBTT.

The most accurate method of measuring the BDT or DBT temperature of a material is by fracture testing. Typically four point bend testing at a range of temperatures is performed on pre-cracked bars of polished material. For experiments conducted at higher temperatures dislocation activity increases. At a certain temperature dislocations shield the crack tip to such an extent the applied deformation rate is not sufficient for the stress intensity at the crack-tip to reach the critical value for fracture (K_iC). The temperature at which this occurs is the ductile-brittle transition temperature. If experiments are performed at a higher strain rate more dislocation shielding is required to prevent brittle fracture and the transition temperature is raised.

Nuclear power plant reactor pressure vessel embrittlement

One important ductility concern is the embrittlement Embrittlement is a loss of ductility of a material, making it brittle. Various materials have different mechanisms of embrittlement of nuclear power plant Nuclear power is produced by controlled nuclear reactions. Commercial and utility plants currently use nuclear fission reactions to heat water to produce steam, which is then used to generate electricity reactor vessels In a nuclear power plant, the reactor vessel is a pressure vessel containing the coolant and reactor core. It is a device for containing and controlling a chemical reaction. The chemical process enables to convert raw material into final product under given pressure and temperature. During the reaction it becomes necessary to remove excess heat in.^{[citation needed]} Neutron radiation Neutrons may be emitted during either spontaneous or induced nuclear fission, nuclear fusion processes, very high energy reactions such as in the Spallation Neutron Source and in cosmic ray interactions, or from other nuclear reactions such as the historically significant reaction. Neutron radiation was discovered as a result of observing a causes embrittlement of some materials, neutron-induced swelling Neutron-induced swelling is the increase of volume and decrease of density of materials subjected to intense neutron radiation. Neutrons impacting the material's lattice rearrange its atoms, causing buildup of dislocations, voids, and Wigner energy. Together with the resulting strength reduction and embrittlement, it is a major concern for, and buildup of Wigner energy The Wigner effect , also known as the discomposition effect, is the displacement of atoms in a solid caused by neutron radiation. Any solid can be affected by the Wigner effect, but the effect is of most concern in neutron moderators, such as graphite, that are used to slow down fast neutrons. The material surrounding the moderator receives a much^{[dubious – discuss]}, thus affecting the nil ductility temperature of the vessel's metal. This effect is now rigorously scrutinized by the operators, including by periodic testing of metal samples located within the reactor pressure vessel. The vessel's nil ductility temperature is likely to be the limiting factor in plant life, at least for pressurized water reactors (PWR).^{[4][unreliable source?]}

Periodically, all thermal power plants, including nuclear power plants, are shut down for refueling and maintenance. Nuclear power plants use schedules of approximately 18 months between outages, as these are called, for PWRs, and 24 months for boiling water reactors (BWRs). At this time, the reactor pressure vessel is cooled down from above 600 °F (316 °C) (for PWRs) or above 545 °F (285 °C) (for BWRs) to ambient temperatures, the same as in any thermal power plant experiencing a maintenance outage, such as coal, natural gas, oil, geothermal, or solar thermal power plants, though other thermal power plants often have much sharper temperature gradients. This cooling down and warming up afterward creates temperature gradients and thus induced stresses between the different components and areas of the reactor. As the reactor gets older, neutron radiation causes embrittlement and it is desirable that the stresses remain below a certain value.

To ensure that neutron embrittlement does not cause the RPV to go out of specification, numerous material samples of the same material that the RPV was made out of are located within the RPV for retrieval at every outage. These samples are then tested to analyze their ductility. Lack of ductility within the bounds of the vessel specification, the limits of the nameplate of the vessel, and/or the bounds specified within the ASME Boiler and Pressure Vessel Code would require that a mechanical engineer and/or a nuclear engineer be sought to advise as to the situation, and what corrective actions, if any, should be taken, such as modification of plant operations protocols (longer periods of heat-up or cool-down) or eventually temporary shutdown of the plant for replacement of the RPV, an expensive task.

See also

• Deformation
• Work hardening, which reduces ductility

References

1. ^ ^{a b c} Rich, Jack C. (1988), The Materials and Methods of Sculpture, Courier Dover Publications, p. 129, ISBN 0486257428, http://books.google.com/books?id=hW13qhOFa7gC .
2. ^ G. Dieter, Mechanical Metallurgy, McGraw-Hill, 1986, ISBN 978-0070168930
3. ^ John, Vernon. Introduction to Engineering Materials, 3rd ed.(?) New York: Industrial Press, 1992. ISBN 0831130431.
4. ^ Oldest operating US nuclear power plant shut down

External links

• Ductility definition at engineersedge.com
• DoITPoMS Teaching and Learning Package- "The Ductile-Brittle Transition

Categories: Continuum mechanics | Deformation

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