Plastic Deformation:

Plastic deformation or plastic strain is a dimensional modification which does not disappear while the initiating stress is removed. It is commonly accompanied through a few elastic strains.

The phenomenon of elastic strain and plastic deformation in a material are known as plasticity and elasticity, correspondingly.

At room temperature, most metals have some elasticity that manifests itself as soon as the slightest stress is applied. Commonly, they also possess little plasticity, but this might not become apparent until the stress has been raised appreciably. The magnitude of plastic strain, while it does appear, is likely to be much greater than in which of the elastic strain for a given stress increment. Metals are likely to exhibit less elasticity and more plasticity at elevated temperatures. A few pure unalloyed metals (notably aluminum, copper and gold) show little, if any, elasticity while stressed in the annealed (heated and then cooled slowly to avoid brittleness) condition at room temperature, other than do exhibit marked plasticity.  A few unalloyed metals and several alloys have marked elasticity at room temperature, other than no plasticity.

The state of stress just before plastic strain starts to appear is called as the proportional limit, or elastic limit, and is described through the stress level and the corresponding value of elastic strain. A proportional limit is expressed in pounds per square inch.  To load intensities beyond the proportional limit, the deformation contains of elastic and plastic strains both.

As mentioned previously in this lesson, strain measures the proportional dimensional modify along with no load applied. Like  values  of  strain  are  simply  determined  and  only  cease  to  be sufficiently accurate while plastic strain becomes dominant.

While metal experiences strain, its volume remains constant. Thus, if volume remains constant as the dimension changes on one axis then the dimensions of at least one other axis have to modify also. Another must decrease if one dimension increases. There are a few exceptions. For instance, strain hardening includes the absorption of strain energy within the material structure that results in an increase in one dimension without an offsetting decrease in other dimensions. That causes the density of the material to decrease and the volume to increase.

The material will elongate on the axis of the load (perpendicular to the tensile stress plane) if a tensile load is applied to a material as described in Figure. On the other hand, if the load is compressive, the axial dimension will decrease, as described in Figure. A corresponding lateral contraction or expansion must occur if volume is constant.  This lateral change will bear a fixed relationship to the axial strain. A relationship, or ratio, of lateral to axial strain is known as Poisson's ratio after the name of its discoverer.  It is commonly symbolized through.