What is the difference between yield stress and proof stress?

Yield stress is the stress at which a material begins to deform plastically, meaning it will not return to its original shape when the stress is removed.

Proof stress is the stress at which a material exhibits a specified amount of plastic deformation, typically 0.2% strain. This is the most commonly used measure of a material's yield strength.

The yield stress and proof stress are often used interchangeably, but there is a slight difference between the two. The yield stress is the point at which the material starts to deform plastically, while the proof stress is the point at which the material has deformed plastically by a specified amount.

The difference between the yield stress and proof stress is typically small, and it is often not important in engineering applications. However, there are some cases where the difference is important. For example, the proof stress is used to design structures that must not deform plastically, such as bridges and buildings.

The yield stress and proof stress can be determined by performing a tensile test on a material. In a tensile test, a specimen of the material is stretched until it breaks, and the stress-strain curve is recorded. The yield stress is the point on the stress-strain curve where the material starts to deform plastically, while the proof stress is the point on the stress-strain curve where the material has deformed plastically by a specified amount.