Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires", without fracture. Ductility is the most important parameter to consider in metal forming operations such as rolling, extrusion, and drawing. Examples of highly ductile metals are silver, gold, copper, and aluminium. The ductility of steel varies depending on the alloying constituents. Increasing levels of carbon decreases ductility.
Ductility can be quantified by the fracture strain , which is the engineering strain at which a test specimen fractures during a uniaxial tensile test. Another commonly used measure is the reduction of area at fracture .
In materials science the ductile-brittle transition temperature (DBTT), nil ductility temperature (NDT), or nil ductility transition temperature of a material represents the point at which the fracture energy passes below a pre-determined point (for steels typically 40 J 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 3 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. See the section on glass transition temperature for a related discussion.
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 defects and a corresponding decrease in ductility and increase in DBTT.
The most accurate method of measuring the BDT or DBT temperature or 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 (KiC). 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.
Every year a PWR is opened and needs to be cooled down for maintenance works. This cooling down and warming up afterwards creates temperature differences and great stresses in between the different components of the reactor. As the reactor gets older, neutron radiation causes embrittlement and the stresses must be below a certain value. Thus cooling down and warming up must be done slower, but is still possible.
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