Ductility

Ductility is a technological property of matter that indicates the ability of a body or material to deform plastically under load before breaking, which may be expressed as percent elongation or percent area reduction from a tensile test — for example, the ability to withstand plastic deformations. A body is much more ductile, the higher the achieved deformation before breaking.

Ductility can also be defined as the ability of a material to be reduced in thin threads (while similarly, malleability is the ability of a material to be reduced in thin sheets). The two characteristics can be co-present having some factors in common, but this is not necessary, gold is both ductile and malleable, lead is malleable but not very ductile. The materials that most have this property are metals. Property opposed to ductility is fragility, or the inability to deform under load, thus resulting in sudden rupture (also called brittle failure).

Properties

The materials that most benefit from this property are metals. In order of decreasing ductility the most common can be listed in the following order: gold, silver, platinum, iron, nickel, copper, aluminum, zinc, tin and lead.

Property opposite to ductility is brittleness, that is the inability to deform under load, thus reaching sudden rupture (also called brittle rupture).

Indicators of ductility are the percent elongation and percent striation.

Ductility is affected by temperature, in particular it decreases with decreasing temperature. For this reason, even ductile materials (particularly metals with body-centered cubic lattices) can become brittle when exposed to frost or otherwise low temperatures.

This property is also related to the age of the material and loading cycles. In general, it tends to reduce as the material ages and with use.

Ductility of a structural element

The ductility of a structural element is a much sought-after property because, in view of the economic losses and social costs of a structural collapse, an element capable of deforming significantly under load makes it possible to intervene with restoration measures or, in the worst case, to save oneself before collapse. The ductility of a structural element depends on the local ductility and the size of the element.

Structural ductility, especially in earthquake engineering, is a mechanical property no less important than strength. An earthquake-resistant building, in fact, must be able to dissipate energy, through the development, precisely, of inelastic deformations, during the seismic event and to maintain, after the seismic event, a margin of safety against collapse.

Structural ductility can be achieved by causing damage to the building without having sudden (brittle) collapses, so as to dissipate energy. There are two mechanisms that can occur:

  • plasticized columns;
  • plasticized beams.

Structural ductility is evaluated by considering the displacement at the top of the building when the structure is damaged, compared to the displacement at the elastic limit, that is, when the displacement is reversible.

The ductility of a structure, especially for construction in seismic zones, is one of the most important mechanical properties and is exploited in the hierarchy of strengths.

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