Limit of Proportionality

The stress (load divided by original area of cross section of the test piece) at which the strain (elongation per unit of gauge length) ceases to be proportional to the corresponding stress. If the load is removed for any stress up to this point, the material will spring back, or assume its original dimensions. The limit is usually determined from a load-elongation diagram, obtained by plotting extensometer readings and is the stress at which the load-elongation line ceases to be straight.