Definition of proportional limit
Proportional limit In the stage of elastic deformation, the maximum stress that the metal material undergoes is proportional to the stress and strain, which is called the proportional limit. When the stress is less than the proportional limit, the stress-strain relationship is linear, and the material is in the elastic phase. Because the proportional limit is difficult to measure, it is often expressed by the stress value where a very small amount of plastic deformation occurs, which is called the prescribed proportional limit, expressed by σp, and the unit is Pascal.