During the calculation of longitudinal stress, we say that stress is equal to the magnitude of applied force divided by area of cross section.
However, is this area of cross section, the cross-sectional area of the non- deformed body, or is it the cross-sectional area of the deformed body?
I would think that since we are trying to calculate the 'effect' of restoring force, we would consider the deformed area. A diagram in Feynman-Leighton-Sands Loc: 38-1, Volume 2 also seems to support this. However most questions regarding this do not offer the Poisson's ratio of the material, making it impossible to calculate the stress.