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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