Definition - What does Proof Stress mean?
A proof stress is a level of stress at which a material undergoes plastic deformation. More specifically, the proof stress is often defined as the point when the material undergoes an amount of plastic deformation equal to 0.2 percent.
Proof stress is an important material characteristic to be aware of during the material selection process. It is necessary to make sure that the proof stress of the material is not exceeded by the stresses in the application if the material must not be allowed to deform. If the proof stress is exceeded then plastic deformation will occur and could ultimately result in fracture of the material.
Proof stress is also known as the offset yield stress.
Corrosionpedia explains Proof Stress
Proof stress is also called offset yield stress because it is the stress required to make the material "yield."
Proof stress should not be confused with ultimate stress. The proof stress is the stress at which the material deforms, whereas the ultimate stress is the maximum stress that a material can withstand prior to failure. Nor should the proof stress be misinterpreted as elastic deformation, which occurs when stresses are large enough to stretch atomic bonds but small enough that when it is removed the material returns to its original shape and size. The proof stress of a material is large enough to not only stretch atomic bonds, but also to break those bonds and cause the atoms to move across dislocations; this causes permanent deformation.
The proof stress is the yield stress calculated at a given offset. Typically this offset is defined as the stress that a material has undergone when it has deformed by 0.1% or 0.2%. The offset is used because some materials have an exact yield stress that is difficult to discern. The offset yield stress, or proof stress, is more readily measured than the yield stress for these types of materials.