Yield strength is an indication of the maximum stress that can be induced 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 structure design, yield strength is very important. For example, when designing a component, it needs to support the forces generated during use, and the component must not be plastically deformed. In other words, a material with sufficient yield strength should be selected.
Since it represents the upper limit of the force that can be applied without causing permanent deformation, yield strength is often used to calculate the maximum allowable load in mechanical parts. For various materials, various yield criteria have been established.
However, when a material is stressed, it deforms in a recoverable manner. The yield strength of a material is the stress at which it becomes plastic after deformation. Any deformation caused by stresses greater than the yield strength is irreversible. Due to the linearity of elastic deformation, the yield strength is also known as the maximum stress that can be reached without deviation. Stress and strain are proportional. Wide deformations can be observed after this point with little increase in the applied load. Yield strength is expressed in N/m2 or Pascals.
When designing components, it is important to know the yield strength of a material, as it often represents the upper limit of the load that can be applied. Yield strength is very important in controlling the production techniques of many materials, such as forging, rolling or pressing.
Tensile testing is used to evaluate the yield strength of a material. The test results are plotted on a stress-strain curve. The yield strength of a material is the stress at which the stress-strain curve deviates from proportionality.
In design applications, yield strength is often used as an upper limit on the allowable stress that can be exerted on a material. This is especially important in material applications that require tight dimensional tolerances to be maintained under high stress and load. By varying dislocation density, impurity levels, and grain size (in crystalline materials), the yield strength of a material can be fine-tuned. For materials without a defined yield point, the yield strength is usually expressed as the stress that produces a permanent deformation of 0.2% of the original size, called the 0.2% yield stress.
The value of the yield strength is important in the construction of a structure so that the structure can function in the elastic region under normal operating conditions. However, when faced with unexpected shock loads such as explosions, fires or natural disasters such as earthquakes, the importance of the plastic region of the material becomes self-evident, since in this case most of the energy absorbed by the material is mainly composed of plastic regional contribution. So having more toughness means the material can withstand such unforeseen loads for a longer period of time, allowing more time for safety measures to be enforced.
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 yield strength anomalies that are typical of superalloys and lead to their use in applications requiring high strength at elevated temperatures. Yield strength anomalies are also referred to as "yield stress anomalies". It is often difficult to determine the exact point at which yielding begins, which is why proof stress is used in practice. This is where stress is considered to produce a certain amount of strain, which is used to define yield strength.
