Yield point, also known as yield strength or elastic limit, is an important value to consider when selecting a material for design and building applications, particularly when there are significant loads or stresses being applied. An example of how the value is used is when building a structure with steel beams: it becomes necessary to know how much stress and weight the beams can handle to build a sound structure. Yield point is also a key factor in the processing of metals, which typically involves exposing the metal to high stresses during the manufacturing process.
In design applications, the yield strength is often used as an upper limit for the allowable stress that can be applied. It is especially important in material applications that require precise dimensional tolerances to be maintained in the presence of high stresses and loads. Yield point is typically measured in pounds per square inch (psi) or Newtons per square meter, also known as pascals (Pa).
Stress and Strain
Most matter has a predictable and measurable relationship between the stress applied and the resulting strain, or deformation, that occurs. This relationship can be plotted in a stress-strain curve, which generally shows the yield point. The yield point defines the stress that will cause an onset of permanent deformation to occur in a material.
- Elastic strain: A material that is exposed to tensile, or pulling, stress will experience strain and elongate, resulting in a dimensional change. At low levels of stress, this strain can be reversible. This means that, after the stress is removed, the material may return to its original dimensions. This is known as elastic strain.
- Plastic strain: When the applied stress exceeds the yield point, a material will deform to the point where it can no longer return to its original dimensions once the load is removed. This is referred to as plastic deformation or plastic strain, which is the result of permanent displacement of atoms within the material.
Ductile and Brittle Materials
Yield point is most commonly used with ductile materials. If an object or material is ductile, it will become quite deformed before it actually fractures. Ductility is a measure of how much deformation occurs before complete failure. These materials, such as steel and aluminum, can experience a significant amount of plastic deformation before such a breakdown.
Brittle materials, such as concrete and glass, have very low elasticity and usually exhibit little or no plastic deformation before failure. For this reason, brittle matter does not have a yield point, and tends to fail immediately after a critical value of stress has been exceeded.