Tensile vs. Yield Strength

Thursday, May 7, 2015

The tensile strength of a material is the maximum amount of stress that it can be subjected to before failure.

There are three typical definitions of tensile strength:
  •  Yield strength - The stress a material can withstand without permanent deformation.
  •  Ultimate strength - The maximum stress a material can withstand.
  • Breaking strength - The stress coordinate on the stress-strain curve at the point of rupture
In particular, Yield strength, or the yield point, is defined as the stress at which a material begins to plastically deform. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent and non-reversible.

The deformation can be measured in the following ways:
  • Elastic Limit - The lowest stress at which permanent deformation can be measured. This requires a complex iterative load-unload procedure and is critically dependent on the accuracy of the equipment and the skill of the operator.
  • Proportional Limit - The point at which the stress-strain curve becomes non-linear. In most metallic materials the elastic limit and proportional limit are essentially the same.
  •  Offset Yield Point (proof stress) - Due to the lack of a clear border between the elastic and plastic regions in many materials, the yield point is often defined as the stress at some arbitrary plastic strain (typically 0.2%). This is determined by the intersection of a line offset from the linear region by the required strain. In some materials there is essentially no linear region and so a certain value of plastic strain is defined instead. Although somewhat arbitrary this method does allow for a consistent comparison of materials and is the most common.

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