The deformation of materials is the description of how the parts are deformed and objects. This has importance for the shaping (parts manufacturing), design (part strength in normal use) and security (protection of persons in case of accident).

The ability of a part to deform or to resist deformation depends on two things:

• the shape of the room;
• the nature of the material and its preparation.

The influence of the shape of the part is studied in continuum mechanics (MMC). The influence of the material is the part that concerns us here.

Material is characterized by three pairs of adjectives:

• flexible / rigid  : this characterizes the elasticity, the ability of the object to deform elastically , in a reversible manner;
• hard / ductile  : this characterizes the ductility , the ability of the object to deform plastically , irreversibly;
• brittle / tough: this characterizes the ability to resist crack propagation, to absorb the energy of a shock.

A hard to resist crack propagation and impact: plastic deformation absorbs some of the strain energy and thus delays the rupture. This is illustrated in the fable of La Fontaine The Oak and the Reed (although in the fable, the resistance of the reed can be attributed more to its elasticity, it illustrates the difference between “hard” and “resistant”).

The notion of “strength” and “resistance” is more complex and depends on the use of the object. A material that deforms easily (soft and / or ductile) is resistant to breakage; break easily, but not a good mechanical strength. A rigid and hard has a good mechanical strength (the room holds its shape, its dimensions when the requests), but breaks easily. Other concepts come into play, such as heat resistance, mass, corrosion…