Stress

Applied Force normalized by the Area of the sample over which the
force acts. The diagram shows the configuration for a sample in
uniaxial tension. The sample deforms under the application of the
stress and its cross section changes in area. Two different areas
may be used to normalize the applied force, F: the initial area, A ₀ ,
or the instantaneous area, A _i .

Engineering Stress = Nominal Stress = (Applied Force / Initial Area) = (F / A ₀ )

True Stress = (Applied Force / Instantaneous Area) = (F / A _i )