Glossary | Proportional Limit
Once the applied stress exceeds the proportional limit (pt A) of an elastoplastic material, the linear stress strain response transitions to a non-linear relationship. If the stress applied to a material never exceeds its proportional limit, then the specimen will return to its original length when all forces are removed. If the applied stress exceeds the proportional limit, then the material will have a permanent set and will not return to its original length. The stress at which the onset of permanent deformation occurs is referred to as the proportional limit and is indicated as point A in the figure. The portion of the stress strain diagram prior to the proportional limit is also defined as the elastic region and beyond the proportional limit the plastic region.