 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
If formula_40, we have
Therefore,
The engineering strain is formula_43.The engineering stress is

For equibiaxial extension in the formula_33 and formula_46 directions, the principal stretches are formula_47.From incompressibility formula_35.