Image:Al tensile test.jpg The local necking and the cup and cone fracture surfaces are typical for ductile metals.]] Image:Cast_iron_tensile_test.JPG with very low ductility.]] Ductility is a mechanical property that describes the extent in which solid materials can be Deformation (engineering) deformed without fracture In materials science ductility specifically refers to a materials ability to deform under tensile stress; this is often characterized by the materials ability to be stretched into a wire. Malleability a similar concept, refers to a materials ability to deform under compressive stress; this is often characterized by the materials 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. Commonly, the term "ductility" is used to refer to both concepts, as they are very similar.

Scientific fields

Geology

In Earth science the brittle-ductile transition zone is a zone, at an approximate depth of in continental crust at which rock (geology) becomes less likely to fracture and more likely to deform ductilely. In glacial ice this zone is at approximately 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

Image:Kanazawa Gold Factory.jpg Ductility is especially important in metalworking as materials that crack or break under stress cannot be manipulated using metal forming processes, such as hammer ng, rolling (metalworking) and drawing (metalworking) Malleable materials can be formed using Stamping (metalworking) or Machine press ng, whereas brittle metals and plastic must be molding (process) High degrees of ductility occur due to metallic bond , which are found predominantly in metals and leads to the common perception that metals are ductile in general. In metallic bonds valence shell electron are delocalized and shared between many atoms. The delocalized electron 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 $\backslash varepsilon\_f$, which is the engineering Strain (materials science) at which a test specimen fractures during a uniaxial tensile test Another commonly used measure is the reduction of area at fracture $q$.G. Dieter, Mechanical Metallurgy McGraw-Hill, 1986, ISBN 978-0070168930 The following list ranks metals from the greatest ductility to least: gold silver platinum iron nickel copper aluminium zinc tin and lead The malleability of the same metals are then ranked from greatest to least: gold, silver, lead, copper, aluminium, tin, platinum, zinc, iron, and nickel. The ductility of steel varies depending on the alloying constituents. Increasing levels of carbon decreases ductility. Many plastics and amorphous solid , such as Play-Doh are also malleable.

Ductile-brittle transition temperature

Image:Ductility.svg 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 JJohn, Vernon. Introduction to Engineering Materials 3rd ed.(?) New York: Industrial Press, 1992. ISBN 0831130431. for a standard Charpy impact test . 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 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 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 (BCC) lattice than those with a face-centered cubic (FCC) lattice. DBTT can also be influenced by external factors such as neutron radiation which leads to an increase in internal lattice defect 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 of nuclear power reactor vessel .lt;!--Are there more critical ductility concerns, like those involving bridges, perhaps? People could actually be killed by something like a bridge collapsing.--> Neutron radiation causes embrittlement of some materials, neutron-induced swelling and buildup of Wigner energy Dubious|dateNovember 2009}}, thus affecting the nil ductility temperature of the vessels metal. This effect is now rigorously scrutinized by the operators, including by periodic testing of metal samples located within the reactor pressure vessel. The vessels nil ductility temperature is likely to be the limiting factor in plant life, at least for pressurized water reactors (PWR).http://www10.antenna.nl/wise/index.html?http://www10.antenna.nl/wise/369/3628.html Oldest operating US nuclear power plant shut down] 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 (for PWRs) or above (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 (engineering) *Work hardening which reduces ductility

References

External links

* http://www.engineersedge.com/material_science/ductility.htm Ductility definition at engineersedge.com] * http://www.doitpoms.ac.uk/tlplib/ductile-brittle-transition/index.php DoITPoMS Teaching and Learning Package- "The Ductile-Brittle Transition] Category:Continuum mechanics Category:Deformation als:Duktilität ar:مطيلية bs:Duktilnost ca:Ductilitat ca:Mal·leabilitat cs:Duktilita da:Duktilitet de:Verformbarkeit de:Duktilität es:Ductilidad es:Maleabilidad eo:Duktileco fa:چکشخواری fa:شکل‌پذیری fr:Malléabilité fr:Ductilité gl:Ductilidade hr:Duktilnost io:Duktila is:Teygjanleiki is:Mótanleiki it:Malleabilità it:Duttilità ms:Kebolehtempaan nl:Ductiliteit no:Duktilitet pl:Ciągliwość pl:Kowalność pt:Ductilidade simple:Ductility simple:Malleability sk:Ťažnosť sl:Duktilnost sr:Дуктилност sv:Duktilitet vi:Độ dẻo ur:طروقیہ zh:延性 ru:Ковкость