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In the use of metal for mechanical engineering purposes, a given
state of stress usually exists in a considerable volume of the material.
Reaction of the atomic structure will manifest itself on a macroscopic scale.
Therefore, whenever a stress (no matter how small) is applied to a metal, a
proportional dimensional change or distortion must take place.
Such a proportional dimensional change (intensity or degree of
the distortion) is called strain and is measured as the total elongation
per unit length of material due to some applied stress.The equation below
illustrates this proportion or distortion.
where:
e = strain (in./in.)
d = total elongation (in.)
L = original length (in.)
Types of Strain
Strain may take two forms; elastic
strain and plastic deformation.
Elastic Strain
Elastic strain is a
transitory dimensional change that exists only while the initiating stress is
applied and disappears immediately upon removal of the stress. Elastic strain is
also called elastic deformation. The applied stresses cause the atoms in a
crystal to move from their equilibrium position. All the atoms are displaced the
same amount and still maintain their relative geometry. When the stresses are
removed, all the atoms return to their original positions and no permanent
deformation occurs.
Plastic Deformation
Plastic deformation (or plastic
strain) is a dimensional change that does not disappear when the initiating
stress is removed. It is usually accompanied by some elastic strain. The
phenomenon of elastic strain and plastic deformation in a material are called
elasticity and plasticity, respectively. At room temperature, most metals have
some elasticity, which manifests itself as soon as the slightest stress is
applied. Usually, they also possess some plasticity, but this may not become
apparent until the stress has been raised appreciably. The magnitude of plastic
strain, when it does appear, is likely to be much greater than that 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 when
stressed in the annealed (heated and then cooled slowly to prevent brittleness)
condition at room temperature, but do exhibit marked plasticity. Some unalloyed
metals and many alloys have marked elasticity at room temperature, but no
plasticity.
The state of stress just before
plastic strain begins to appear is known as the proportional limit, or elastic
limit, and is defined by the stress level and the corresponding value of elastic
strain. The proportional limit is expressed in pounds per square inch. For load
intensities beyond the proportional limit, the deformation consists of both
elastic and plastic strains. As mentioned previously in this chapter, strain
measures the proportional dimensional change with no load applied. Such values
of strain are easily determined and only cease to be sufficiently accurate when
plastic strain becomes dominant.
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