Yield strength refers to an indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of the elastic limit. In engineering structural design, yield strength is very important. For example, when designing a component, it must support the force incurred during use, and the component must not deform plastically. In other words, a material with sufficient yield strength should be selected.
Since it represents the upper limit to forces that can be applied without causing permanent deformation, the yield strength is often used to calculate the maximum permissible load in a mechanical part. For various materials, a variety of yield criteria have been established.
However, when a material is stressed, it deforms in a way that can be recovered. A material’s yield strength is the stress after which its deformation becomes plastic. Any deformation caused by a stress greater than the yield strength is irreversible. Because of the linearity of elastic deformation, yield strength is also known as the maximum stress that can be achieved with no deviation. Stress and strain have a proportional relationship. Wide deformations can be observed beyond this point with little to no increase in the applied load. Yield strength is expressed in terms of N/m2 or pascals.
Knowledge of a material’s yield strength is very important when designing components, since it usually represents the upper limit of the load that can be applied. Yield strength is very important for controlling many materials’ production techniques, such as forging, rolling or pressing.
A tensile test is used to assess a material’s yield strength. The test results are plotted on a stress-strain curve. The yield strength of a material is the stress under which the stress-strain curve deviates from proportionality.
In design applications, the yield strength is often used as an upper limit for the allowable stress that can be applied to a material. It is especially important in material applications that require precise dimensional tolerances to be maintained in the presence of high stresses and loads. By altering dislocation density, impurity levels and grain size (in crystalline materials), the yield strength of the material can be fine-tuned. For materials without a clear distinct yield point, yield strength is usually stated as the stress at which a permanent deformation of 0.2% of the original dimension will result, known as the 0.2% yield stress.
The value of yield strength is important in the construction of structures, such that the structures are able to perform in the elastic region under normal servicing conditions. However, when faced with unexpected impact loads such as explosions, fires or natural disasters such as earthquakes, the plastic region of the material becomes crucial, as a large portion of the energy being absorbed by the materials under such circumstances is mainly contributed by the plastic region. As such, having a higher toughness implies that the materials are able to withstand such unforeseen loading for a longer period of time so as to allow more time for safety measures to be carried out.
In general, yield strength increases with strain rate and decreases with temperature. When the latter is not the case, the material is said to exhibit a yield strength anomaly, which is typical for super-alloys and leads to their use in applications requiring high strength at high temperatures. The yield strength anomaly is also referred to as the “yield stress anomaly.” It is usually quite difficult to determine the exact point at which yielding begins, which is why in practice a proof stress is used. This is where the stress is thought to give a certain amount of strain and this is used in defining the yield strength.