What is Elastic Strength?
To the user and the designer of
machines or structures, one significant value to be determined is a
limiting stress below which the permanent distortion of the material
is so small that the structural damage is negligible and above which
it is not negligible. The amount of plastic distortion which may be
regarded as negligible varies widely for different materials and for
different structural or machine parts.
In connection with this limiting stress
for elastic action, a number of technical terms are in use; some of
them are
1. Elastic Limit. The greatest
stress which a material is capable of withstanding without a
permanent deformation remaining on release of stress. Determination
of the elastic limit involves repeated application and release of a
series of increasing loads until a set is observed upon release of
load.
Since the elastic limit of many
materials is fairly close to the proportional limit, the latter is
sometimes accepted as equivalent to the elastic limit for certain
materials. There is, however, no fundamental relation between elastic
limit and proportional limit. Obviously, the value of the elastic
limit determined will be affected by the sensitivity of apparatus
used.
2. Proportional Limit. The
greatest stress which a material is capable of withstanding without a
deviation from proportionality of stress to strain. The statement
that the stresses are proportional to strains below the proportional
limit is known as Hooke’s Law. The numerical values of the
proportional limit are influenced by methods and instruments used in
testing and the scales used for plotting diagrams.
3. Yield Point. The lowest
stress at which marked increase in strain of the material occurs
without increase in load. If the stress-strain curve shows no abrupt
or sudden yielding of this nature, then there is no yield point. Iron
and low-carbon steels have yield points, but most metals do not,
including iron and low-carbon steels immediately after they have been
plastically deformed at ordinary temperatures.
4. Yield Strength. The stress at
which a material exhibits a specified limiting permanent set. Its
determination involves the selection of an amount of permanent set
that is considered the maximum amount of plastic yielding which the
material can exhibit, in the particular service condition for which
the material is intended, without appreciable structural damage.
A set of 0.2% has been used for several
ductile metals, and values of yield strength for various metals are
for 0.2% set unless otherwise stated. The yield strength is generally
used to determine the elastic strength for materials whose
stress-strain curve in the region pr is a smooth curve of gradual
curvature.
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