The standard outcome of a tensile test is a plot of the nominal stress against the nominal strain. The former is the force divided by the original sectional area and the latter is the change in length (along the loading direction) divided by the original length. The true stress acting is the force divided by the current sectional area. After a finite (plastic) strain, under tensile loading, this area is less than the original area, as a result of the lateral contraction needed to conserve volume, so that the true stress is greater than the nominal stress.
In practice, it is quite common to present only the nominal plot, and several common procedures for extraction of key parameters are based only on inspection of such curves. To convert from the true form of the stress-strain curve to the tensile nominal, the following analytical curves have been used, noting that these equations are appropriate up to the UTS.
