Planar extension tests are carried out on thin specimens which are constrained from deforming in one direction.  For planar extension in the  directions with the  direction constrained, the principal stretches are .  From incompressibility .  Hence .
Therefore,
The left Cauchy–Green deformation tensor can then be expressed as
If the directions of the principal stretches are oriented with the coordinate basis vectors, we have
The engineering strain is .  The engineering stress is