STRAIN
DOE-HDBK-1017/1-93
Properties of Metals
Strain may take two forms; elastic strain and plastic deformation.
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 (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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