(mechanics) The property of a solid body whereby it undergoes a permanent change in shape or size when subjected to a stress exceeding a particular value, called the yield value.
Sci-Tech Encyclopedia: Plasticity
The ability of a solid body to permanently change shape (deform) in response to mechanical loads or forces. Deformation characteristics are dependent on the material from which a body is made, as well as the magnitude and type of the imposed forces. In addition to plastic, other types of deformation are possible for solid materials.
One common test for measuring the plastic deformation characteristics of materials is the tensile test, in which a tensile (stretching) load is applied along the axis of a cylindrical specimen, with deformation corresponding to specimen elongation. The load is converted into stress; its units are megapascals (1 MPa = 106 newtons per square meter) or pounds per square inch (psi). Likewise, the amount of deformation is converted into strain, which is unitless. The test results are expressed as a plot of stress versus strain. See also Stress and strain.
Typical tensile stress-strain curves have been calculated for metal alloys and polymeric materials. For both materials, the initial regions of the curves are linear and relatively steep. Deformation that occurs within these regions is nonpermanent (nonplastic) or elastic. This means that the body springs back to its original dimensions once the stress is released, or that all of the deformation is recovered. In addition, stress is proportional to strain (Hooke's law), and the slope of this linear segment corresponds to the elastic (Young's) modulus. See also Elasticity; Hooke's law; Young's modulus.
Plastic (permanent) deformation begins at the point where linearity ceases such that, upon removal of the load, not all deformation is recovered (the body does not assume its original or stress-free dimensions). The onset of plastic deformation is called yielding, and the corresponding stress value is called the yield strength. After yielding, all deformation is plastic and, until fracture, the curves are nonlinear. This behavior is characteristic of many metal alloys and polymeric materials. The concept of plasticity does not normally relate to ceramic materials such as glasses and metal oxides (for example, aluminum oxide). See also Plastic deformation of metal.
(Cited from: http://www.answers.com/Plasticity)
